A.R.
Gardner-Medwin (UCL), N.A. Curtin, (Imperial),
P. Tatham (UCL) © 1990-2001

** Words in science are often used in different ways from
ordinary English. Completely different meanings even occur in different
branches of physiology, e.g. for the word "accommodation". As a
student,
it is easy to despair! But the meanings of words are very precise in
science: if you don't understand them, or if you use them casually, you
won't be able to study efficiently, and you won't get good marks. This
glossary is for browsing in, testing yourself, and learning about words
that catch your eye. Stop and read wherever you may have doubts or
interest. Also of course it is a place to look up words you come across
and don't understand. But it is very selective. You need both an
English
dictionary and a scientific or medical dictionary too. Many words that
you think you understand OK, you may find aren't used quite how you
thought! This glossary concentrates on words that often cause
confusion,
especially in the basic sciences (physics, chemistry and mathematics)
that are the foundation of physiology. You don't need to understand
everything, but the more you understand, the easier it will be to learn
more. You are often expected in exams to explain the meanings of words:
practise doing it! **A short glossary of Medical
Terms - Dissected, Defined and Explained is also available.

*Suggestions, corrections,
additions: Click
or email to a.gardner-medwin <a> ucl.ac.uk*

**ab- **Away from.

**ablation **Removal.

**abscissa **RELATED: X, Y axes CONVERSE: ordinate ♦ X-axis,
horizontal axis. It is usually simpler and clearer to say 'horizontal
axis', since most people have to stop to think which axis 'abscissa'
means. As an aid to memory: Abscissa-Ordinate, X-Y, Horizontal-Vertical
are all in alphabetical order.

**absolute refractory period **The time within a refractory
period during which no stimulus, however big, can elicit a second
action
potential from an excitable tissue.

**absorption **Transfer of substances into the body or into
the
blood, usually across an epithelial surface. E.g. absorption through
the
skin or from the gut, or re-absorption of substances from the lumen of
tubules in the kidney.

**absorption spectrum **Graph of the proportion of incident
light (or other radiation) absorbed by a substance as a function of
wavelength, frequency or energy.

**acceleration **Rate of change of velocity. Note that all
parts of a rotating object are accelerating (except on the axis), even
if the rotation is steady: the acceleration is 4π^{2}f^{2}r
where f is the frequency of rotation and r is the radius from the axis.
Accelerations are sometimes expressed relative to the free acceleration
due to gravity (1g=9.81 m.s^{-2}). Acceleration is detected in
man largely by the labyrinths.

**acclimatization, acclimation **The process of adaptation to
a
new environment, for example with altered ambient temperature, pressure
or (with aquatic animals) salinity. Acclimation is sometimes used
specifically to mean adaptation to a single changed factor (as under
laboratory conditions).

**accommodation **RELATED: adaptation ♦ A process of
adjustment
to new conditions. Used in several senses: 1. Attenuation or absence of
a response when a stimulus is applied slowly (e.g. failure to detect a
smell when it builds up slowly, or failure of a slow depolarisation to
give an action potential). NB this is distinct from adaptation, but is
often seen in the same systems. 2. Adjustment of focus of the eye.

**accumulation **Progressive build-up of a substance, etc.
Cumulative toxins are ones that are absorbed but not excreted, for
which
it is the total dose over a prolonged period that matters rather than
the level of exposure at any one time.

**accuracy of measurement **RELATED: resolution ♦ The limits
of
confidence in a parameter, based on its measurement. Accuracy is
limited
firstly by the ability of a measuring system to distinguish different
conditions (resolution) and secondly by its ability to relate the
conditions correctly to a standard (calibration). The two
considerations
must not be confused. A high resolution instrument giving stable
readings and sensitive to small changes may nevertheless be completely
inaccurate if not adequately calibrated. Accuracy cannot usually be
greater than the resolution of a measurement, except in special
circumstances(e.g. where a digital reading flicks between adjacent
values and it may be possible to conclude that the value is close to an
incremental step).

**acid **CONVERSE: base ♦ An aqueous solution with a pH less
than 7.0. A molecule or ion that can dissociate to release a proton (H^{+}).
For example, H_{2}CO_{3} and H_{2}PO_{4}^{-}
are both acids.

**acidosis **CONVERSE: alkalosis ♦ Abnormally low pH of the
blood. An acidotic condition may be capable of being restored to normal
acid-base balance by increasing respiratory ventilation to reduce the CO_{2}
content of the body (respiratory acidosis), or it may require retention
or injection of HCO_{3}^{-} (metabolic acidosis).

**acid-base balance **Control of the factors influencing the
pH
of the blood. The main factors under homeostatic control are the
excretion of CO_{2}, HCO_{3}^{-} and NH_{4}^{+}.
Disturbances are lessened in the short term by numerous pH buffer
systems. Influences affecting blood pH are altered absorption,
metabolism and respiration, and disturbances affecting the control
systems.

**acoustic **RELATED: auditory ♦ Related to sound or sound
waves.

**action potential **ABBREV: spike ♦ A transient voltage
change
recorded inside or close to a nerve, skeletal muscle or other excitable
cell, due to a characteristic change of membrane potential. Action
potentials occur spontaneously or as all-or-none or frequency coded
reactions to stimulation. The rising (depolarizing) phase is due to
voltage dependent membrane channels allowing passive influx of Na^{+}
or Ca^{++}. The falling phase is due to passive K^{+}
efflux. Sodium spikes are brief (ca. 1-2 ms long); calcium spikes are
more prolonged. Extracellularly recorded action potentials may be small
(<1mV), while the voltage changes across the membrane may be
>100mV. A compound action potential is an extracellular recording of
the combined effect of action potentials in many cells.

**activation **The process of initiating or increasing
activity
of some sort. In muscle physiology, 'activation' usually means the
induction of action potentials in the cell membrane (= excitation)
rather than the initiation of tension (NB the link between the two is
'excitation-contraction coupling').

**activation energy **RELATED: Arrhenius plot ♦ The energy
required to achieve an intermediate state in a chemical reaction. A
high
activation energy lowers the rate of a reaction in proportion to e^{-Ea/RT}
according to Arrhenius' equation. A catalyst reduces the activation
energy.

**active **CONVERSE: passive, inactive ♦ 1. Requiring energy:
e.g. active transport vs. passive transport. 2. Doing something: e.g.
an
active nerve fibre, undergoing action potentials. 3. (Rather vague
&
confusing, best avoided) vigorous, important: e.g. carbon dioxide plays
an active role in respiration. 4. Effective: E.g. an active ingredient.

**active site **The part of a cell or of a receptor that binds
to a substrate.

**active transport **RELATED: carrier mediated transport ♦ Net
movement of a substance across a membrane from a lower to a higher
concentration or (in the case of ions) against an electrochemical
gradient. This requires energy, and can occur by linkage of carrier
mediated transport to a process providing energy, such as the
hydrolysis
of ATP or the downhill movement of another substance.

**activity **RELATED: concentration ♦ 1. The state of doing
something: E.g. action potentials, secretion, movement according to
tissue. 2. (In chemistry) a parameter similar to concentration, that
determines the rate at which reactions take place. The ratio of
activity
to concentration is the 'activity coefficient', usually less than 1
because of molecular interactions in the solution that interfere with
reactions.

**acuity **Ability to identify fine detail in a sensory
pattern. Visual acuity depends on light level, and is highest at the
fovea. It is usually measured clinically by the ability to discriminate
letters on a Snellen Chart: an acuity of 6/24 means that an individual
can just read letters at 6m distance that a person with nominally
normal
acuity could just read at 24m. Somatosensory acuity varies markedly
over
the body surface and may be measured by the separation required to
identify that the skin is being touched at two points.

**acute **CONVERSE: dull, chronic ♦ Sharp, severe,
short-lived,
or of sudden onset. Rather ambiguous. E.g. acute pain, acute phase of a
response (usually a part of a response that is large and short-lived).
An 'acute experiment' is one in which an animal is anaesthetised at the
start and eventually killed before recovering consciousness.

**AC-coupled **RELATED: DC-coupled, filter, off-scale ♦ A
recording that is sensitive to changes in the measured parameter, but
not to its steady level. Useful if the changes are of interest, and a
large or variable background level means that the record would
otherwise
go 'off-scale'. The transition between what is and is not recorded is
characterised by a 'cut-off frequency', below which signals are
severely
attenuated (by more than a factor of 2 in energy). Some AC-coupled
circuits are characterised by a 'time constant', where the return to
baseline following a step change of input level is exponential.
AC-coupling introduces distortion of signals that have frequency
components around or below the cut-off frequency.

**ad- **Towards, beside.

**adaptation **Decline of a response while a stimulus is
maintained constant after onset. Distinguish carefully from both
accommodation and habituation. Light and dark adaptation are the
processes of adjustment of the eye to different light levels (whether
fast or slowly changing).

**adequate stimulus **The form of stimulation that normally
elicits a specific reaction, such as an action potential or a reflex
response. ??

**adrenaline, epinephrine (USA) **A hormone released together
with noradrenaline from the adrenal medulla, especially in conditions
of
stress. Acts at both alpha and beta adrenergic receptors, and mimics
some of the effects of sympathetic nerve activation.

**adrenergic **Related to adrenaline or noradrenaline.
Adrenergic fibres release noradrenaline, while alpha and beta
adrenergic
receptors bind adrenaline, noradrenaline and other adrenergic agonists.

**adult **Sexually and/or emotionally mature.

**aequorin **A natural protein extracted from jellyfish
(Aequorea aequorea) that emits light in the presence of ATP and Ca^{++}
ions. Used experimentally to measure Ca^{++} concentrations
within cells.

**aerobic **CONVERSE: anaerobic ♦ Involving the use of oxygen.
Aerobic conditions are ones in which O_{2} is present.

**aetiology, etiology (USA) **The cause of a disease.

**affect **RELATED: effect ♦ 'Affect' and 'effect' often cause
trouble. 'Affect' is a verb meaning 'to alter', e.g. 'salt affects the
taste of food'. 'Effect' is a noun meaning 'consequence', e.g. 'salt
has
many physiological effects'. The simplest plan is to learn & stick
to these meanings. However, 'effect' can be a verb too, meaning 'to
produce', e.g. 'salt effects a reduction in urine flow'. Note that it
is
true to say 'salt affects urine production' but untrue to say 'salt
effects urine production' (it does exactly the opposite!). Be careful!
A
last complication is that 'affect' is a technical noun in psychology,
meaning 'mood'. It is even true that lithium salts have effects that
affect affect: clinical depression is an affective disorder for which
lithium can provide effective treatment, though unfortunately it seldom
effects a permanent cure.

**affective **Relating to emotion or mood.

**afferent **CONVERSE: efferent ♦ Travelling or conveying
something towards a structure, or relating to its inputs. Afferent
nerves or action potentials are usually those travelling towards the
central nervous system. Afferent vessels carry fluids towards a
specified organ or tissue.

**afferent arteriole **Arterioles carrying blood towards a
structure organ, such as the kidney glomeruli, in which there is
arteriolar control of both afferent and efferent vessels.

**affinity **The strength of binding of two chemicals, or a
chemical or ion to a receptor or enzyme. Affinity of a receptor for an
agonist may be expressed by the agonist concentration at which half of
the receptors are bound to agonist molecules (the dissociation constant
for the complex: KD), or by -log_{10} of this quantity, or by
its
reciprocal.

**afterdischarge **Action potentials occurring after cessation
of a stimulus or a voluntary action.

**agonist **CONVERSE: antagonist ♦ Something that assists or
mimics an action. For example, agonist muscles pull parts of the
skeleton in the same direction. Agonist drugs or chemicals bind to the
same receptors, producing the same effects.

**algorithm **A set of rules for solving a problem. These are
often expressed in a computer program.

**alimentary **Relating to food, or the gastro-intestinal
tract.

**all-or-none **RELATED: threshold, positive feedback
CONVERSE:
graded ♦ A response that appears only if the strength of a stimulus
exceeds some threshold level, but whose amplitude and characteristics
do
not depend on the stimulus, e.g. an action potential, or a sneeze. The
sneeze is more familiar: pepper either does or doesn't lead to a
sneeze.
A little pepper doesn't lead to a 'small' sneeze, though it may lead to
fewer sneezes. This isn't to say that sneezes are always identical: you
can stifle a sneeze and, for example, reduce the noise it makes. These
alterations are nothing to do with the stimulus that caused the sneeze,
however. Exactly parallel things are true for action potentials.
All-or-none responses usually involve some form of 'positive feedback'.

**alpha radiation **ABBREV: α-rays ♦ Helium nuclei emitted
during disintegration of some radioactive elements.

**altitude **Height above sea level. This affects
physiological
processes largely through the reduced atmospheric pressure, which falls
exponentially by ca. 12% per 1000m rise (18% per 5000ft). Oxygen
percentage in inspired air remains constant (ca. 21%). Effects of
altitude are often studied by simulation in a chamber with reduced
pressure. Respiration is affected noticeably at 4000m (13,000ft) and
severely at 6000m (20,000ft). Reduced PO_{2} leads to
hyperventilation, which partially maintains arterial PO_{2} but
lowers arterial PCO_{2}.

**alveolar air **Air that is or has been in the alveoli.
Samples of alveolar air can be taken at the end of expiration after
dead-space air has been expelled (end-tidal samples). Alveolar air is
normally approximately in equilibrium with arterial blood, and
therefore
has approximately the same partial pressures of O_{2} and CO_{2}
as arterial blood.

**alveolus **The terminal air sacs in the lungs, where gas
exchange takes place with the blood. Typically 150-300µm in
diameter in man (ca. 5nl volume).

**ambi- **Both.

**ambient **Relating to the environment of an animal. E.g.
ambient temperature, pressure. Note that the conditions for a
particular
tissue may be different from the ambient conditions.

**ampere **ABBREV: A, amp ♦ Unit of electrical current flow.

**amphi- **Both.

**amplification **RELATED: gain CONVERSE: attenuation ♦
Increase in the size of a signal. This may be active (requiring energy,
as in an electrical amplifier) or passive, as in the amplification of
movement that may be achieved where a muscle pulls on a lever.
Quantitatively, 'amplification' is the ratio of the two signals.

**amplitude **Magnitude; how big something is. It can often be
defined in several different ways, and it is important to specify what
your definition is. For example, the 'amplitude' of a sinusoidal
waveform y=a.sin(wt) may mean the coefficient 'a' in this expression,
or
it may mean the 'peak-to-peak amplitude' (i.e. difference in height
between the maxima and minima in the waveform (=2a), or it may mean the
'root mean square (r.m.s.) amplitude' (=0.71a). The same thing goes for
measurements you make of a response amplitude, for example an action
potential: always say how you are measuring it (e.g. 'from the resting
potential to the positive peak').

**an- **Without, lacking.

**ana- **Up, increased.

**anaesthesia **RELATED: paraesthesia, analgesia ♦ The absence
of sensation. This may be general anaesthesia, in which case the
subject
is unconscious, or local anaesthesia affecting sensation from just a
part of the body. Local anaesthesia may be due to influence of an
anaesthetic drug or to nerve trauma, etc..

**analgesia **Absence of pain sensation.

**analogue **RELATED: graded ♦ 1. Continuously variable. An
analogue parameter can have any value within a range, e.g. body
temperature. This is as opposed to a digital parameter, which can only
have certain (perhaps integer) values, e.g. the number of limbs an
animal has. Analogue numbers are approximated in computers by digital
numbers. 2. A drug similar to another drug or to a hormone, etc.

**analytical **RELATED: numerical ♦ Analytical methods for
solving mathematical problems employ mathematical techniques to arrive
at explicit solutions, which can then be used in more or less general
instances. Numerical methods, by contrast, use computers to solve
problems in specific cases.

**aneurysm **An abnormally bulging part of a vessel, usually
an
artery. NB nothing to do with nerves: derives from ana= up + eurys=
wide.

**Ångstrom **ABBREV: Å ♦ An old-fashioned unit of
length: 10^{-10}m or 0.1nm.

**angular velocity **Rate of change of orientation of a
rotating body, usually measured in radians.s^{-1} (in which
case
the angular velocity is 2π ♦ frequency of rotation.

**anion **CONVERSE: cation ♦ Negative ion. NB negative ions
move towards an anode (positive electrode) in solution: hence the
seemingly confusing nomenclature.

**anisotropy **Having different properties in different
directions.

**annulus **Ring shaped. The zone between two concentric
circles.

**anodal block **Prevention of propagation of action
potentials
in a region close to an anode, due to hyperpolarization.

**anode **CONVERSE: cathode ♦ A positive electrode. In
solution, cations move away from and anions towards an anode, both
contributing to current flow away from the anode.

**anode break stimulation **Initiation of action potentials
close to an anode (where the cell is hyperpolarised during current
flow), occurring after cessation of the current flow.

**anomalous **Unexpected, different from the main trend of the
results or from theoretical expectation.

**anoxia **A condition without oxygen.

**antagonist **CONVERSE: agonist ♦ Something having the
opposite effect. E.g. a chemical opposing the action of another
chemical, or a muscle pulling in the opposite direction.

**ante- **CONVERSE: post- ♦ Before. E.g. antenatal= before
birth.

**anterior **CONVERSE: posterior ♦ Near or nearer to the front.

**anti- **RELATED: contra- CONVERSE: ortho- ♦ Opposite,
against. E.g. antidromic= opposite direction to normal.

**antilog **RELATED: logarithm ♦ The inverse function of a
logarithm. It is nearly always used for the inverse of logarithms to
the
base 10. In this case antilog(x)=10^{x}.

**antrum **Cavity. Narrow duodenal end of the stomach
(pylorus).

**aqueous **Associated with water.

**arc **1. Part of the circumference of a circle. 2. A reflex
arc.

**Arrhenius plot **A plot of a reaction rate against 1/T,
where
T is the absolute temperature. This is usually linear, according to
Arrhenius' equation, with a slope of -Ea/R where Ea is the activation
energy and R the gas constant.

**artefact, artifact **Something arising incidentally from the
way an experiment is carried out, and not related to the main aim of
the
experiment. 'Control' experiments are often helpful in trying to
identify artefacts.

**arterial pressure **The pressure within an artery, relative
to ambient atmospheric pressure. This varies during the pulse between
systolic pressure (maximum) and diastolic pressure (minimum). It also
varies along the length of arteries due to the effects of hydrostatic
pressure and (especially in small or obstructed arteries) the
resistance
of the arteries to flow. Standard clinical measurements are normally
made in a large artery at the same height as the heart.

**arteriole **Narrow terminal portion of an artery, leading to
the capillaries. Resistance vessel. Major site of control of blood flow
to tissues, via innervation, chemosensitivity and thermal sensitivity
of
the smooth muscle of the arteriole walls. Constriction increases the
resistance to flow in the arterioles. Note that it does not directly
increase the pressure within the arterioles (a common misapprehension),
since they are open at both ends. Indeed, constriction generally
reduces
pressure at the distal (capillary) end of the arterioles, because blood
flow through the capillaries is reduced and the capillary pressure
therefore becomes closer to venous pressure.

**arteriosclerosis **Loss of elasticity of arteries, usually
due to ageing and atherosclerosis.

**arteriovenous **Relating arteries and veins. The
arteriovenous pressure difference is the net driving force that causes
blood to flow through a tissue.

**artery **Elastic, thick walled vessels carrying blood at
relatively high pressure away from the heart. Small arteries are
contractile, due to smooth muscle present in the tunica media. The
elasticity is important in accommodating the blood ejected from the
heart during systole without excessive rise in pressure.

**-ase **Enzyme that breaks down a substance.

**asymptote **A straight line that is approached but never
quite reached by a graph, usually as you go far along one or other
axis.
Not all functions give asymptotes. For example, the graph of y=1/x
approaches both axes asymptotically; the graph of y=x+1/x approaches
the
y axis and the line y=x. Neither y=x^{2} nor y=sin(x) have
asymptotes. Try sketching these graphs, with their asymptotes where
applicable.

**asynchronous **Out of synchrony, or not occurring in
relation
to an ongoing rhythm, such as the cardiac cycle.

**atmospheric air **Composition of fresh air varies little,
apart from water content (0-7%). Oxygen is 21%, N_{2} 78% and CO_{2}
0.03% of dried air. The CO_{2} content can be regarded as zero
for all practical purposes in animal physiology. The total pressure
(ca.
10.1kPa, 760mmHg at sea level) is the sum of the partial pressures of
constituent gases, including water vapour pressure (= relative humidity
♦ saturated water vapour pressure).

**atomic weight **Average mass of the naturally occurring
isotopes of an element, relative to H as 1 or O as 16. The atomic
weights of the commonest biologically important elements are: H 1.0, C
12.0, N 14.0, O 16.0, Na 23.0, Mg 24.3, P 31.0, S 32.1, Cl 35.5 K 39.1,
Ca 40.0

**attenuation **RELATED: amplification, gain CONVERSE:
amplification ♦ Reduction in size or effect. Sounds are attenuated with
distance. The changes in blood pressure during the pulse are attenuated
in small arteries. A recording device may have an attenuator to allow
you to vary the gain (or overall amplification).

**atto- **ABBREV: a RELATED: SI units ♦ One million million
millionth, 10^{-18}.

**auto- **Self.

**autonomic **RELATED: parasympathetic, sympathetic ♦ Relating
to the autonomic nervous system. This innervates smooth muscle, glands
and visceral organs, which are not normally under voluntary control.
Subdivided principally into the sympathetic and parasympathetic
efferent
systems. Autonomic reflexes are reflexes that act through these
efferent
systems; their afferent pathways may be either the same as pathways
that
subserve conscious perceptions (as with salivation) or they may be
different (as with baroreceptor reflexes). The afferent pathways are
not
distinctive in any anatomical way, and are not usually described as
'autonomic' except by association with particular reflex actions.

**autoradiography **Localization of radioactive material in
tissue by the apposition of film or incorporation of photographic
emulsion in the histological processing of tissue.

**autoregulation **A process by which an organ directly
regulates changes in its vascular resistance so as to control its blood
flow, without involving the central nervous system or systemic hormonal
influences. This can produce a relatively flat portion of the graph of
flow against arterial pressure.

**average **Mean.

**Avogadro's number **6.0×10^{23} g^{-1}.
The number of atoms or molecules having a mass, in grams, equal to
their
atomic or molecular weight.

**Avogadro's principle **Equal volumes of different gases
contain, under the same conditions of temperature and pressure, the
same
number of molecules. A corollary of more direct physiological relevance
is that the percentage breakdown of a gas mixture by volume (e.g. 21% O_{2}
in normal air) is the same as its percentage breakdown by the number of
moles or molecules of the constituent gases. One mole of gas at 1atm
pressure occupies 24 litres at 20^{o}C (22.4 litres at 0^{o}C).
Thus 1 litre of air contains 42mmol of combined constituents at 20^{o}C,
including ca. 9mmol of O_{2}.

**axon **A nerve fibre. May be myelinated or unmyelinated. A
nerve is made up of many axons, Schwann cells (supporting cells) and a
sheath.

**balance **A balance control on a measuring instrument (e.g.
an oscilloscope amplifier) allows you to adjust the internal circuitry
of the instrument for ideal operation. To adjust the balance control of
an oscilloscope, connect the input of the amplifier to earth and
gradually increase the sensitivity from minimum. With no input (zero
volts) you should expect the recording (i.e. spot or trace height) not
to change with increasing sensitivity. If it does change, adjust the
'balance' and try again from minimum sensitivity. Once you have done
this, you should not use the balance control to make adjustments if a
recording goes off-scale: you can usually use a 'shift' or 'offset'
control.

**band pass **RELATED: filter, high pass, low pass ♦ A type of
filter that lets through a range of frequencies and cuts both higher
and
lower frequencies. A high pass and a low pass filter acting in series
will achieve this, though it may also be achieved in a single circuit.
A
band pass filter with a narrow pass band is equivalent to a 'resonant'
filter.

**bandwidth **RELATED: frequency response ♦ The range of
frequencies that a system will respond to, or that is passed by a
filter.

**bar graph **RELATED: X-Y graph, histogram ♦ A graph in which
the observations (normally along the vertical axis) are shown as bars
rather than as points. This is preferred if the horizontal axis,
separating different experimental conditions, is qualitative rather
than
quantitative. For example, measurements may be made with and without a
drug or a lesion, or in patients and control subjects. A histogram is a
special kind of bar graph, where the horizontal axis is quantitative.

**-baric SUF **Relating to pressure. E.g. hyperbaric= above
atmospheric pressure.

**base **In an expression of the form y=a^{b}, 'a' is
the base and 'b' is the exponent. Correspondingly, logarithms are
defined in relation to a specific 'base'. In the expression y=a^{b},
'b' is the logarithm to the base 'a' of 'y'. In chemistry a 'base' is a
molecule or ion that will bind hydrogen ions.

**baseline **RELATED: control ♦ Period of recording before a
stimulus is given. Recordings in an experiment are usually useless
unless there is some baseline recording, showing reasonably stable
measurements. A response is only worth considering if it is larger
than,
or at least different from, baseline fluctuations. Sometimes it is
impossible to get baseline measurements: for example, you usually
cannot
make baseline measurements on patients before they get sick. Then you
have to rely on control subjects (e.g. people who aren't sick) to check
that they don't show the same changes when you start to make
measurements on them.

**basic science **Research carried out without the immediate
aim of applying the results in a technological or clinical context.
Discoveries that lead on to enormously important applications are often
made purely as a result of the scientific curiosity of a researcher
(e.g. Faraday's discovery of the principles of electric power
generation, or Fleming's discovery of penicillin). Much debate takes
place over the question of whether progress in the useful application
of
science is best encouraged by putting money into applied or basic
research.

**billion **A number best avoided! It usually means 10^12 or 10^{12}
in the UK and 10^9 or 10^{9} in the USA and in financial
circles.
Call it a thousand million or a million million.

**binary **RELATED: digital, decimal, octal, hexadecimal ♦
Having only 2 possible values. Numbers are represented in computers as
sequences of binary elements: e.g. 25=11001, corresponding to the fact
that 25= 1×16 + 1×8 + 0×4 + 0×2 + 1×1.
This is essentially similar to the familiar decimal convention in which
'25' means 2×10 + 5×1. Fractions can also be represented in
binary, e.g.: 0.625(decimal) = 0.101 (binary), i.e. 1×0.5 +
0×0.25 + 1×0.125.

**birefringence **Optical anisotropy of a material leading to
a
different refractive index for light polarized in different directions.

**blind **RELATED: double blind ♦ An experiment in which
either
the experimenter or the subject is not aware of some aspect of the
experimental conditions. For example, an assay might be done 'blind',
with the person carrying it out not aware of the source of individual
samples. This helps to eliminate 'bias', i.e. the possibility of
preconceived ideas of the experimenter affecting the results.

**block **1. In physiology and pharmacology, 'block' usually
means to stop something happening (e.g. a local anaesthetic blocks
action potentials in nerve fibres), or to diminish (by any of a number
of ways) the effectiveness of a drug, e.g. curare blocks the action of
acetylcholine at the nerve-muscle junction. 2. An experiment with
trials
carried out 'in a block' means the trials are close in time, or not
interspersed with other trials under different conditions.

**boundary conditions **RELATED: differential equations ♦ The
conditions that determine the particular form of solution that a
differential equation will have. Sometimes these are conditions at a
physical boundary, e.g. the edge of a piece of tissue. Sometimes they
are conditions at a particular time or place, as when there is a sudden
local disturbance. For example, the diffusion equation applied to a
region of tissue has different solutions depending on whether the
surface is permeable and washed by a solution free of the diffusing
substance (c=0 at the surface), or is impermeable to the substance
(implying zero flux and concentration gradient perpendicular to the
surface).

**buffer **RELATED: pH ♦ A buffer mechanism makes the effects
of a disturbance less than they would otherwise be. Particular mixtures
of chemicals can buffer the concentration of an ion, for example H^{+}
or Ca^{++}. A buffer for pH is a mixture of the protonated and
unprotonated forms of a weak acid (e.g. H_{2}PO_{4}^{-}
and HPO_{4}^{--}. Addition of acid or base causes
conversion between these two forms, binding (or releasing) hydrogen
ions. This keeps the pH relatively constant. Buffers are most
effective,
for a given total concentration, when the pH is approximately equal to
the pK of the reaction: the two forms of the buffer are then in equal
(and therefore quite high) concentration.

**bug **A mistake or problem, usually in a computer program.
To
'debug' is to remove such mistakes, while a 'debugger' is a program
that
helps you to do this.

**bulk flow **Flux of a solution as a whole, carrying all its
solutes.

**ca. **=circa About.

**cable equation **The partial differential equation governing
the spread of current and voltage along electric cables and cell
membranes. If V is disturbance of membrane potential from rest, T the
membrane time constant and Λ the electrotonic length constant for the
cell, then dV/dt=(Λ^{2}d^{2}V/dx^{2}-V)/T (away
from the site of a disturbance). Solutions of this equation are set out
in Jack, Noble & Tsien (1975), for various boundary conditions and
for cells coupled together to form syncytia. One important situation is
where there is a local but relatively long lasting disturbance (e.g. at
a synapse): the resulting shift of membrane potential falls off
exponentially with an exponential length constant of Λ.

**cable theory **Mathematical description of the purely
passive
spread of electrical current in a cell. Similar to the theory necessary
for understanding the properties of very long telephone cables.

**calci- **Relating to calcium, E.g. hypercalcaemia

**calculus **In mathematics, the methods for handling
gradients, derivatives and integrals. Anatomically, a stone or hard
deposit in tissue.

**calibration **RELATED: accuracy ♦ Comparison of a
measurement
with a known standard. A set of standards are often required to
construct a calibration curve: a graph of the output of a measuring
system against the known values. An accurately linear measuring system
permits interpolation between just two standards.

**calorie **Unit of energy, particularly heat. 1 calorie= 4.18
Joules. The 'Calorie' of nutrition, in a non-scientific context, is
unfortunately usually 10^{3} calories. However, 'kcal' on a
food
packet will correctly mean 10^{3} cal, not 10^{6} cal.
Best to avoid this unit (not strictly part of S-I units) when possible,
and use Joules.

**cannula **RELATED: catheter ♦ A tube inserted into a vessel
(usually a blood vessel) through its wall, so that solution can be put
into or withdrawn from the vessel.

**capacitance **Physiological 'capacitance vessels' in the
circulation are ones that can contain an extra volume of blood with
little or no increase in pressure (mainly the veins, which are like
floppy tubes). Electrical 'capacitance' is related: electrical charge
is
like an amount of electrical substance, and voltage is like pressure.
The capacitance between places A and B is the amount of charge you can
shift from A to B per unit of voltage change between A and B. A large
capacitance (measured in farads: = Coulombs/Volt) means you need a lot
of charge to get much of a voltage shift. Myelinated nerve fibres
conduct faster, largely because the myelin reduces the capacitance of
the axon.

**cardiac **Relating to the heart, or (as in 'cardiac
sphincter') to the top end of the stomach.

**carrier **RELATED: channel ♦ In the context of membrane
transport, a protein that binds a specific substance and by changing
conformation transports the substance to the other side of the
membrane.
A carrier is very much like an enzyme. Carriers are involved in
facilitated diffusion, in which the net flux is always down an
electrochemical gradient, and also in active transport. Some carriers
move two substrates at once.

**catabolism **Metabolism involving the destruction or
disappearance of tissue. Fat stores, and ultimately muscle tissue, are
catabolised in starvation.

**catheter **RELATED: cannula ♦ Tube inserted into a narrow
opening so that fluid can be removed or introduced. You might sometimes
insert a catheter through a cannula.

**cation **CONVERSE: anion ♦ Positive ion.

**caudal **CONVERSE: rostral ♦ Toward the tail end of the
animal.

**causal factor **Something that contributes to the
probability
of occurrence of something else, but does not always cause it.

**centi- **ABBREV: c RELATED: SI units ♦ One hundredth; 10^{-2}.
Seldom used, except in the unit cm (=10mm).

**cerebral **Relating to the brain.

**cervical **Relating to the neck, or the neck of the uterus
(womb).

**cgs **RELATED: mks ♦ A system of units based on the
centimetre, gram and second. This used to be the commonest standard in
the UK, but was replaced some years ago by the SI system, which uses
mks
units.

**channel **RELATED: permeability, carrier ♦ In membrane
transport, a protein that spans the cell membrane and allows substances
(usually ions) to move across the membrane. A conformational change in
the protein is not involved for each ion that moves, unlike with a
carrier. Much higher fluxes can occur than with carriers, and channels
are mainly responsible for the permeability of most membranes. Many
channels are gated, i.e. can be opened or closed, e.g. as a result of
binding of transmitters, hormones or intracellular messengers, or as a
result of changes of membrane potential. Different channels also have
different selectivities for the ions they let through. Single isolated
ion channels can be studied with 'patch' techniques.

**chaos **RELATED: turbulence ♦ A recently developed branch of
mathematics, dealing with systems that obey well defined equations, but
that are nevertheless intrinsically unpredictable in some respects. The
solutions of the equations are very sensitive to the initial starting
conditions, so that conditions that are very similar (perhaps within
the
resolution of any measuring system) can lead, after a short time, to
completely different behaviour. It may be possible to predict the
behaviour in statistical terms (e.g. the mean velocity of turbulent
fluid flow), but not in detail. There are potential applications of
chaos theory to the physiological understanding of irregular cardiac
rhythms and neural networks.

**chronic **RELATED: tonic CONVERSE: acute ♦ Continuous, or
existing for a long time.

**circa **ABBREV: ca.) ♦ Approximately.

**circadian rhythms **RELATED: nocturnal, diurnal ♦ Cyclic
patterns of behaviour or of tissue activity (e.g. hormone secretion)
that repeat with a period of approximately 24 hours, even when
organisms
are isolated from the normal daily environmental changes.

**clamp **To keep a parameter constant, or change it in a
controlled way to a required value during an experiment. Usually the
process involves a control system with negative feedback, which
automatically adjusts the parameter if there is a discrepancy from its
set value. A 'voltage clamp' circuit is used to control the membrane
voltage of a cell in order to study the effects of changes in membrane
voltage. It usually requires two microelectrodes in the cell: one to
measure the membrane potential, and the second to pass current into the
cell to alter the membrane voltage (though sophisticated circuits can
sometimes use a single electrode for both purposes).

**coefficient **This is usually a number by which something is
multiplied in an equation. For example, the area of a circle is
proportional to the square of the radius, with a coefficient π (i.e.
A=π×r^{2}). In much the same way, it is often used for a
quotient (or ratio) of two things, expressing some characteristic
property of a substance or an observation. For example the coefficient
of variation of a measurement is its standard deviation divided by its
mean value.

**cofactor **A chemical in addition to the substrates
(reactants) that must bind to an enzyme for a reaction to take place.

**compartment **A definable part of the total volume of a
tissue or organism. E.g. blood plasma, intracellular or interstitial
fluid compartments.

**competitive **In considering the binding of substrates to
receptors, enzymes or carriers, this means that two or more related
substances are similar enough to bind to the same site. Consequently,
the extent of binding of one will be reduced if the other is also
present and binding to some of the sites. This can lead to competitive
inhibition of a reaction or of the normal physiological effects
produced
by substrate-receptor binding. Many drugs ('competitive antagonists')
act in this way.

**compiler **RELATED: interpreter ♦ A computer program that
translates instructions written in a high level computer language into
a
form that can be used directly by a specific type of computer. Once a
program has been written, the compiler only has to be used once to
generate the 'machine code', usually in the form of an 'executable
file'
that can then be used without the compiler. The translation process may
take quite a long time, and must be repeated when mistakes are
corrected
in the original program or when changes are required. In these
respects,
the translation process is quite different from the use of an
'interpreter'.

**complex **A 'complex' is a combination of several things,
for
example anatomical structures that occur together, or signs and
symptoms
that occur together in a disease.

**complex numbers **ABBREV: x+iy RELATED: vector ♦ A way of
thinking about 'vectors' (i.e. arrows) on a 2-dimensional plane as if
they were represented by single (but 'complex') numbers. In an ordinary
sense it takes 2 numbers to characterise an arrow: either its length
(r)
and orientation (θ), or the displacements it produces in the x & y
directions. It turns out that the 'complex' expressions x+iy and
r×e^{iθ}), where 'i' is an 'imaginary number' that gives
-1 when squared, follow exactly the same rules as vectors when added
and
multiplied. In handling vectors and sine waves (which correspond to the
height of a rotating vector) it is often convenient to use these
complex
formulae and avoid the need to think in geometrical terms.

**compliance **ABBREV: RELATED: stiffness ♦ A measure of how
easily something 'complies' with a force applied to it. For muscles
tendons, etc., it is the increase in length divided by the increase of
force applied to the structure (i.e. the gradient of the graph of
length
plotted against force). For the lungs, compliance is the increase of
volume divided by the increase of pressure applied at the mouth to
inflate the lungs. Stiffness is the reciprocal of compliance.

**concentration **ABBREV: RELATED: activity ♦ The quantity of
a
solute per unit volume. It may be measured in g/l, mols/l, kg/m^{3},
g/100ml, etc., always with dimensions mass/volume. Physiological
concentrations are most often expressed in mols/l (i.e. the 'molar
concentration' or 'molarity'). The unit of molar concentration is
1mol/l, also written as 1M. Note that it is quite wrong to express a
concentration either as 1mol, or 1M/l. These are NOT units of
concentration! Occasionally concentrations are expressed as %, or parts
per million (ppm), or g/kg. These ratios usually mean the weight of
solute per unit weight of solution, or (for a gas) the volume fraction
within a mixture. '%' may also mean g/100ml.

**condenser **In electricity: old-fashioned word for a
capacitor.

**conductance **The reciprocal of resistance. The unit of
electrical conductance, the reciprocal ohm, is also called a 'mho' or a
'Siemens' (S). It is often more convenient to think about conductance
than resistance. For example, the conductance of a membrane is
proportional to its area and is characterised by a 'specific membrane
conductance' in S.m^{-2}.

**conductivity **The reciprocal of resistivity. Units: Ω^{-1}m^{-1}
or S.m^{-1}.

**confidence limits **RELATED: significance ♦ The range of
values within which you are 'confident' that the true result lies,
after
doing an experiment. The mean result that you obtained will normally be
around the middle of the range. Depending on how variable your
individual results were, and how many results you obtained, you may
need
to assign a large or small range of uncertainty around this mean. '95%
confidence limits' indicate that if the true mean is outside these
limits, the probability that your result would have come out as far
away
as it did from the true mean is less than 5%.

**constant **RELATED: variable ♦ A parameter that is always,
and everywhere, the same within the experimental conditions under
consideration, but whose value may be relevant to the results. For
example the temperature of the Ringer solution might be a constant
throughout a series of experiments on isolated muscle: when you
describe
the results you should state what it is, since it very likely might be
relevant. In order to establish whether and how it is relevant, you
would have to treat it as a variable in another set of experiments.
'Physical constants' (e.g. the charge on an electron or the speed of
light) cannot be treated as variables since so far as we know they
always have the same values.

**constriction **CONVERSE: dilation ♦ Narrowing of a vessel.
E.g. vasoconstriction = narrowing of blood vessels, usually due to
contraction of smooth muscle.

**contra- **RELATED: anti- ♦ Against, opposite

**contraction **CONVERSE: relaxation ♦ State of activity of a
muscle in which it may produce force and/or shorten, depending on the
mechanical constraints on it. Note that 'contraction' in ordinary
English means 'getting smaller'. In muscle physiology the word
'shortening' is reserved for this, and 'contraction' may occur even
while a muscle is lengthening. Note also that 'activation' in muscle
physiology is also different from contraction, referring specifically
to
the electrical activation of the membrane.

**control **RELATED: control system, control experiment ♦ Many
physiological systems are 'control' systems, maintaining homeostasis of
some tissue parameters. 'Control' here means regulation, just as you
might control the temperature in your room. A control experiment, or
'control' for short, has a different meaning. 'Control' here means
'check'. It doesn't take this meaning in ordinary English, unlike in
some other languages (where the gasman might 'control' your meter). A
control experiment is one designed to check that a result is not due to
an incidental aspect of your protocol. For example, if you dissolve a
drug in oil to administer it, you should do some 'controls' to check
that oil alone does not have some effect.

**cooperative **A process in which different individual
elements do not act independently, but produce a bigger effect in
combination than the sum of their effects on their own. Chemical
reactions exhibiting 'cooperativity' give a sigmoid (S-shaped) curve
when product concentration is plotted against reactant concentration.

**correlation **RELATED: regression ♦ A systematic
relationship
between two variables. If A tends to be high when B is high, it is a
positive correlation, while if A is high when B is low it is a negative
(or inverse) correlation. A correlation need not be perfect. Thus in
the
adult population there is a positive correlation between height and
weight, even though the taller of two people is not invariably the
heavier. A correlation coefficient indicates the degree of consistency
of the correlation. Uncertainty and misunderstanding sometimes surround
the issue whether a correlation indicates a causal relationship. If
stress is correlated with early mortality, is it perhaps because stress
is also correlated with smoking?

**cortex **The skin, or outer part of an organ (e.g. the
cerebral cortex or adrenal cortex).

**coulomb **The unit of electrical charge. 1 Amp = 1 Coulomb
flowing per second

**countercurrent **A mechanism in which fluids flowing in
opposite directions interact by transport between them of heat or
solutes, etc.. Such mechanisms are important in many animals for the
conservation of heat in limbs exposed to extreme cold (where the
transfer is purely passive), and in the kidney for the concentration of
solutes in excreted fluids (where the transport of solutes involves
active transport).

**cranial **Relating to the skull (cranium).

**critical experiment **RELATED: crucial experiment ♦ An
experiment in which the outcome disproves a hypothesis, or in principle
could disprove a hypothesis. It is of course true that science advances
through experiments that support hypotheses. However, experiments that
merely support a hypothesis and that do not disprove a different
hypothesis that was otherwise plausible, are of less value: they are
not
'critical'.

**crucial experiment **RELATED: critical experiment ♦ An
experiment for which two plausible hypotheses give clearly distinct
predictions. It may be better than a critical experiment, because it
may
separate the two hypotheses. If you think that muscle fatigue (i.e. a
drop in maximum voluntary tension) might be due to either a failure of
the nerve-muscle junctions or a failure in the muscle cells, then a
crucial experiment would be to measure the tension due to direct muscle
stimulation, before and during the fatigue. If you stimulated instead
the nerve to the muscle, this would be a critical but not a crucial
experiment (i.e. it could prove both hypotheses wrong, but it couldn't
tell them apart).

**CT scan **RELATED: CAT ♦ Computed tomography, or computed
axial tomography. An X-ray technique in which an image of the internal
structure of tissue is reconstructed, not just a shadow picture as in
an
ordinary X-ray.

**current **RELATED: charge ♦ An electric current is the rate
of flow of charge in a circuit. Its units are Amps (A). Moving charges
may be positive (e.g., in tissue, Na^{+}, K^{+} ions)
or
negative (e.g. Cl^{-} ions or (in metals) electrons). The
DIRECTION of current flow is the direction that positive charges are
moving, if there are any present (as there always are in an ionised
solution). Negative charges move in the opposite direction. The
DIRECTION of current in a resistor or in tissue is always from +ve to
-ve (unless the current is coupled to a chemical reaction, as in a
battery, or a form of ACTIVE TRANSPORT providing energy, when charge
may
be moved "uphill" towards places with a more positive voltage.

**curvilinear **A non-linear relation, i.e. one for which the
graph is curved in at least part of its range. E.g., a sigmoid relation.

**data **The results of an experiment or investigation.
Strictly, it is the plural of 'datum', and you should say for example
'These data show...'. In practice, the word 'datum' is hardly ever used
and it is probably more common to say 'This data shows...'.

**database **RELATED: keyword ♦ A computer program for keeping
track of large numbers of related records, allowing them to be sorted,
ordered, and selected on the basis of information contained in them.

**deca- **ABBREV: da RELATED: SI units ♦ Prefix meaning ten
times, as opposed to deci- meaning one tenth. Very easily confused and
very seldom used in physiology.

**deci- **ABBREV: d RELATED: SI units ♦ One tenth; 10^{-1}.
Seldom used. The SI system of units encourages units based on multiples
of 10^{3}. A litre is 1dm^{3}. Old-fashioned rulers
sometimes mark a decimetre (=100mm). Don't talk about it!

**decibel **ABBREV: dB) ♦ A unit of intensity of sound, or
other forms of energy. It is a logarithmic unit, which compresses the
huge range of audible sound intensities into a range from 0-100dB or
thereabouts. 10dB corresponds to an increment of 1 in the log of the
sound energy, or a factor of 10 in the energy itself. The sound
intensity corresponding to 0dB is nominally the threshold of hearing.
This is one of the few units where the prefix 'deci-' is used. The
primary unit (1bel=10dB) is seldom used at all.

**default **What is to be understood if you don't specify
something different. This word has become common through computer
usage,
where software will often use its own ('default') values for parameters
unless you take steps to change them. Some scientific words like
'percentage' or 'amplitude' have default meanings in particular
contexts, that can nevertheless be altered. It is usually good practice
not to assume a default meaning, since it may not be universally
accepted.

**definition **The precise meaning of a word or phrase. The
meaning may depend on context: see e.g. 'control'. Scientific and
physiological meanings often differ from those in an ordinary
dictionary. Where there is ambiguity, the context should make the
meaning clear. Ambiguities are a plague in science, as in everything
except the subtler arts! If you are asked for a definition, give (a)
one
or more paraphrases of what the word or phrase can mean, (b) brief
accounts of the contexts in which it might be used, with examples, (c)
if appropriate, an equation relating it to other concepts (e.g. for
'peripheral resistance'). Don't write an essay on 'Everything you need
to know about X, except what it is'.

**degrees of freedom **ABBREV: d.f. RELATED: statistical test
♦
The number of things that can be varied independently. These may be
anatomical (e.g. the shoulder has 3 d.f. of movement), or physiological
(e.g. the properties of the eye mean that you have just 3d.f. in mixing
colours that can be distinguished in high light levels and only 1d.f.
in
low light levels). In mathematics, a linear relation has 2d.f. (e.g.
slope and height, or 2 points it must pass through). In statistics, the
no. of d.f. is usually 1 or 2 less than the no. of variables that go
into your calculation, often because the result (e.g. a standard
deviation) would be unaffected if you were to use the (n-1) differences
from the first sample, instead of the (n) individual measurements.

**delta **ABBREV: ΔX, δX, dX RELATED: differential ♦ The Greek
letter delta is often used to indicate a change in a parameter.

**dendrite **A branching, tree-like structure. Most commonly,
the part of a neuron that receives synaptic contacts.

**denominator **RELATED: numerator ♦ The bottom line of a
quotient. B in the expression A/B. It is seldom necessary to use the
word: certainly avoid it if you might confuse it with 'numerator'. You
could call it 'the bottom line of the expression' to avoid ambiguity.

**dependence **CONVERSE: independence ♦ 'Dependence' of A on B
means either simply that A affects B (e.g. skin temperature depends on
environmental temperature) or, in some contexts, that B cannot exist
without A (e.g. consciousness depends on adequate cerebral blood flow).
Note that either relationship is a 'dependence', not a 'dependency'
(which is usually a small island off the coast somewhere! - i.e.
something that is dependent).

**dependent **'Dependent' and 'independent' variables are
tricky concepts. In many experiments you vary something (an independent
variable) and measure another (dependent) variable. When you draw a
graph (e.g., say, cell volume against osmolarity of your solution), you
always plot the independent variable horizontally and the dependent
variable vertically. In some experiments, both variables may be
strictly
'dependent': you might measure red cell volume and blood osmolarity in
a
lot of different subjects. You could plot this relation either way. You
would need extra experiments, or a clear argument in 'discussion', to
establish whether a correlation reflected a dependence of one variable
on the other.

**depression **RELATED: inhibition CONVERSE: potentiation,
facilitation ♦ A decrease in the activity of tissue or the amplitude of
a response. If the decrease is due to the involvement of structures
(e.g. particular synapses) that appear to have the specific function of
causing the depression, then it is usually called 'inhibition'.
'Spreading depression' is a disturbance that causes a profound but
transient depression of the excitability of nerve cells in local
regions
of the central nervous system. 'Clinical depression' is a mental
condition characterized by extreme sadness, a component of several
clinical syndromes.

**derivative **ABBREV: dy/dx RELATED: partial derivative
CONVERSE: integral ♦ The gradient of the graph of a function: dy/dx is
the gradient of y plotted against x. The 'second derivative' (d^{2}y/dx^{2})
is the gradient of the graph that you get if you plot dy/dx (the 'first
derivative') against x. If you know the equation for a graph, you can
usually write down an equation for its derivative (i.e. 'differentiate'
the equation). If you know the shape of a graph, you should always be
able to sketch the graph of the derivative. In chemistry, a derivative
is a compound of similar structure that can be made from another
compound.

**derivatives (simple functions) **RELATED: differentiate ♦ If
you need to differentiate an equation, then you need to know the rules
for differentiation, plus the derivatives of some simple functions. The
most important examples are: If y=x, then dy/dx=1. If y=x^{2},
then dy/dx=2x. If y=1/x, then dy/dx=-1/x^{2}. NB these are
really all special cases of a single rule: if y=x^{n}, then
dy/dx=nx^{n-1}). If y=e^{x}, then dy/dx=e^{x}.
If y=ln(x), then dy/dx=1/x. For functions of angles, the derivatives
are
simplest if the angles are measured in radians: if y=sin(x), then
y=cos(x); if y=cos(x), then dy/dx=-sin(x).

**describe **RELATED: results, discussion ♦ 'Describe the
results' is a common beginning to an exam question. You should
understand how this is quite different from 'Discuss the results'. The
same distinction holds between the RESULTS and DISCUSSION in a
scientific paper. 'Describe' means give a clear account of just the
facts in front of you (which may include graphs or tables). Indicate
how
the data were obtained, and draw attention to the amplitude and time
course of any effects or correlations that are demonstrated
(identifying
dependent and independent variables), and try to assess which features
look likely to be reproducible (either by eye or with the help of
statistical tests). Try to lay a constructive base for a discussion.

**determine **Two distinct, but straightforward, meanings: (1)
To measure. (e.g. 'Determine the pH of the blood.' means 'Take blood
and
use a pH meter!'. (2) To influence and control. 'What determines the pH
of the blood?' asks for an account of the role of respiration and the
kidney in acid-base physiology; it does not invite the one line answer
'a pH meter'.

**diastole **RELATED: relaxation, filling CONVERSE: systole ♦
The part of the cardiac cycle during which the ventricular myocardium
relaxes, pressure falls and the chamber fills with blood. 'Diastolic
pressure' is the arterial pressure at the end of diastole, i.e. the
lowest arterial pressure during the cycle.

**differential **ABBREV: δX or dX RELATED: delta ♦ A change in
a parameter. In mathematics it is a very small (actually infinitesimal)
change. Gradients (=rates of change, = derivatives) are the ratios of
differentials for two parameters plotted against each other, hence the
terminology dy/dx. The word 'differential' is sometimes used
incorrectly
for 'derivative'. A 'differential diagnosis' is, more
straightforwardly,
the process of distinguishing two clinical conditions that have similar
symptoms.

**differential equation **RELATED: boundary conditions ♦ An
equation involving a derivative, or rate of change, of a function. For
example, in many situations following a disturbance from a normal
steady
state the rate of return towards the steady state is proportional, at
each instant, to the size of the disturbance: dy/dt=-ay. This gives
rise
to an exponential solution: y=Ae^{-at}, with a time constant
1/a. A partial differential equation relates partial derivatives with
respect to two or more parameters (e.g. time and position). Important
examples of partial differential equations in physiology are the
diffusion equation (Fick's second law) and the cable equation.

**differentiate **RELATED: derivative CONVERSE: integrate ♦ To
find the derivative of a graph, e.g. by making a sketch, by calculation
with a computer, or (if you know the equation of the graph) by using
the
rules of differentiation. There are 3 rules: (1) If functions add, then
their derivatives add: i.e. if y=f(x)+g(x) then dy/dx=df/dx+dg/dx. (2)
If two functions are multiplied, y=f(x)×g(x) then
dy/dx=f×dg/dx+g×df/dx. (3) If y is a function of a
function,
i.e. y=f(u) where u=g(x), then dy/dx=(df/du)×(dg/dx). Rule 3 is
best understood by example: if y=(1+x^{2})^{3}, then
this can be written y=u^{3} where u=1+x^{2}. Therefore
dy/dx=(3u^{2})×(2x)= 6x(1+x^{2})^{2}.
These rules, plus knowledge of the simple derivatives, enable you to
differentiate any function.

**differentiation **The process of finding the derivative
(gradient) of a mathematical function. The process by which cells
acquire different properties during development.

**diffusion **The process by which molecules get from one
place
to another (or across a membrane) by random thermal motion.
'Facilitated' diffusion is where a special molecular mechanism in a
membrane (a 'carrier') allows diffusion of certain molecules that would
not otherwise get through: rather like a parent who lifts children over
a fence.

**diffusion coefficient **The constant that appears in either
of Fick's two laws of diffusion. It has dimensions L^{2}T^{-1}.
Typical values for small molecules and ions in free solution are of
order 10^{-9}m^{2}s^{-1}. Typical values for
diffusion of gaseous molecules are roughly 10^{4} times larger;
hence diffusion can be an important physiological mechanism in the gas
phase (for example for fluxes of O_{2} and CO_{2}) over
much larger distances than in solution.

**diffusion equation **The partial differential equation
governing the diffusion of substances. Fick's second law of diffusion.
It states that dc/dt=D(d^{2}c/dx^{2}+d^{2}c/dy^{2}+d^{2}z/dz^{2}),
where c is concentration and D is the diffusion coefficient. In vector
notation it is dc/dt=D∇^{2}c. It has the same form as the
equation for conduction of heat, so solutions of the diffusion equation
with particular boundary conditions (see e.g. Crank: Diffusion) can
often be obtained by reference to the equivalent solutions in a
textbook
of heat theory (Carslaw & Jaeger: Conduction of Heat in Solids).
Solutions are often gaussian or error functions.

**digital **RELATED: binary CONVERSE: analogue ♦ A system
(e.g.
a computer or measuring system) that uses numbers that can only have
discrete values (i.e. a limited number of specific values). A digital
oscilloscope is convenient because it can store and manipulate data,
but
in some ways it is less satisfactory than an analogue oscilloscope,
because it makes measurements at discrete time intervals and may
therefore miss a signal of interest.

**dilatation, dilation **CONVERSE: constriction ♦ Opening up
of
a hollow or tubular structure, such as a blood vessel or the pupil of
the eye. The form 'dilation' is apparently incorrectly formed from
Latin, but seems to be rising in popularity (there is no distinction in
meaning).

**dimension **RELATED: unit ♦ The relation between the
fundamental physical quantities that correspond to a parameter. E.g.
velocity has dimension length/time; pressure has dimension force/area,
or mass/(length x time^{2}). Quantities with particular
dimensions may be measured in different units (e.g. velocity in m/s or
km/hr). The units must themselves have the correct dimensions however.
In order to compare the size of two parameters, they must have both the
same dimensions and the same units.

**discrete **Separate, unmixed (e.g. discrete compartments in
the body). Taking one of certain specific values (e.g. discrete
frequencies at which a piano string will resonate: the fundamental and
its harmonics).

**discuss **RELATED: describe ♦ Read first about 'describe'.
You may be asked to discuss something specific, but in general a
'discussion' sets out firstly what you think you can conclude from the
results. This may not be anything very definite, but you should at
least
be able to generate some ideas (hypotheses) that could explain the
data.
You may also see that some ideas you might have had are ruled out by
the
data: this is the often the most constructive kind of conclusion in
science. Put this all down. Discuss how you might distinguish the
different hypotheses with extra experiments or with more information.
In
general, try to clarify the implications and limitations of the data,
and relate it to what you already know.

**dissect **RELATED: isolate ♦ To cut. To remove a piece of
tissue or a cell from some larger structure. To separate or cut through
part of a structure to expose some element of it.

**distal **CONVERSE: proximal ♦ Far (or farther) from some
implied point. The distal parts of a limb are those far from the body.
Distal parts of a neuron are those far from either the central nervous
system (when talking about peripheral nerves) or from the cell body of
the neuron. (NB occasionally this can cause confusion: for example, the
term should not be used without clarification when talking about the
dorsal roots, where the two directions are opposite.)

**distortion **RELATED: filter ♦ A change from normal, usually
in relation to physical shape or the time course of a signal. A tissue
may be distorted by pushing on it. A waveform may be distorted by
passing it through a filter. A filter may be useful for cutting out
noise or interference even if it does distort signals somewhat.

**distribution **In mathematics, this is the way a set of
points are scattered along one axis or throughout the plane of a graph.
A 'probability distribution' is the way they would be scattered if you
took very many samples. It can be expressed by a histogram, showing how
many points lie in each interval, or by a probability density function
(essentially the same thing, for very many points and very small
intervals). Data for analogue variables often has a 'normal' or
'gaussian' (bell-shaped) distribution, which can be characterised by a
single parameter: its standard deviation. In physiology, 'distribution'
also means the range of places in a tissue where you will find a
particular substance.

**dorsal **CONVERSE: ventral ♦ Relating to the back.

**dose-response curve **A graph of the response to an applied
drug, plotted against drug concentration or dosage.

**double blind **RELATED: blind ♦ An experimenter in which
both
subject and experimenter are unaware of the conditions relating to each
individual subjects. For example, neither patient nor doctor may know
whether the patient is taking a drug being tested or a 'placebo'.
Information about which condition relates to each subject is kept
separately and used eventually at the end of the study to analyse the
results.

**drift **RELATED: trend ♦ A gradual, continuous change in a
parameter. Often this is an unwanted change due to an instability in a
physiological system or a gradual change in the properties of a
measuring instrument (e.g. 'baseline drift').

**drive **RELATED: trigger ♦ A structure is said to 'drive'
another if events in the first trigger events in the second. For
example, the pacemaker region drives the cardiac cycle in other parts
of
the heart. 'Drive', as a noun, usually relates to a state of an animal
in which its behaviour is directed to satisfying a particular need,
such
as hunger, sex, etc..

**-dromic SUF **Relating to direction. E.g. antidromic.

**drug **A chemical that affects biological tissues. Often the
usage is restricted to chemicals that are used clinically or for
research purposes, or ones that are abused socially. Nutrients, toxins,
and chemicals one is particularly fond of, tend not to be called
'drugs'.

**dyne **The cgs unit of force: 10^{-5} newtons.

**dys- **Abnormal, painful. E.g. dyspnea=breathless.

**earth **RELATED: = ground (esp. in USA) ♦ An electrical
concept. You can't measure a voltage at one place. You can only measure
voltage differences. It is convenient therefore to have a constant
reference, relative to which you measure all voltages in a circuit or a
tissue. This is called 'earth'. It is usually connected to the chassis
of equipment and to the outer part (the 'screen') of screened cables.
For safety reasons, and also to reduce interference, the reference
points are usually connected to the ground and to metal parts of the
building; if not, the reference is said to be 'floating'.

**-ectomy SUF **RELATED: -tomy ♦ Cutting and removing. E.g.
lobectomy= removing a lobe of an organ.

**effect **RELATED: affect ♦ See 'affect' for a clarification
of the difference in meaning of these two words.

**efficiency **The ratio of how effective something is to the
maximum effectiveness it could possibly have. The thermodynamic
definition is the ratio: work done by a system / available free energy.

**e.g. **RELATED: i.e. ♦ Distinguish carefully between "e.g."
and "i.e.". It can give quite the wrong meaning if you muddle them.
"e.g." means "example given" (strictly, "*exempli gratia*" in
Latin). "I.e." means "that is" ("*id est*" in Latin). If you mean
to say *You can get drinks (e.g. beer) from the bar.* and you
actually say *You can get drinks (i.e. beer) from the bar.* then
it
would imply that the bar only provides beer.

**Einthoven's triangle **A standard way of analyzing the
electrocardiogram, in which the electrodes are assumed for simplicity
to
be at the corners of an equilateral triangle. The axis of the heart is
taken as the vector that would give the observed signals when projected
onto the sides of this triangle.

**electrical axis **See axis of the heart.

**electrocardiogram **ABBREV: EKG or ECG) ♦ Voltage changes
recorded with electrodes on the skin, due to the electrical events of
the cardiac cycle. Standard electrodes are placed on the left (L) and
right (R) arms and on the left leg or foot (L). The right leg is
earthed. Additional (precordial) electrodes may be used on the front of
the chest. Three standard connections of leads are usually made, to
record the differences in voltages: I=L-R, II=F-R, III=F-L. Alternative
'augmented' configurations are also sometimes used (aVR, aVL, aVR). The
principal components of the EKG signal are the P wave (due to atrial
depolarization), the QRS complex (ventricular depolarization) and T
wave
(ventricular repolarization).

**electroencephalogram **ABBREV: EEG) ♦ Voltage changes due to
the brain, recorded from electrodes on the scalp.

**electrolyte **A salt that dissociates into ions especially
when dissolved in water. An electrolyte may be *strong *e.g.
NaCl
or *weak* e.g. an amino acid. Any electrolyte in solution will
conduct electricity. All tissue fluids contain electrolytes. Strong
solutions of strong electrolytes (e.g. 3M NaCl) have a high
conductivity
(i.e. low resistivity).

**electromyogram **ABBREV: EMG) ♦ Voltage changes due to
muscle, recorded from electrodes on the skin over the muscle.

**electroneutrality **The principle of electroneutrality
states
that the total charges on positive and negative ions in a solution must
be equal and opposite. This leads to a great paradox in physiology and
physical chemistry: how can this be true if at the same time we talk
about fluxes of just one kind of ion (e.g. sodium ions) between the
solutions on two sides of a membrane? The answer is that
electroneutrality is not always exact. However, unbalanced fluxes (even
big enough to produce large shifts of membrane potential, say 100mV)
are
so small that the violation of electroneutrality would be quite
unmeasurable with chemical techniques (corresponding typically to a
concentration ratio of 1.0001:1).

**electro-oculogram **ABBREV: EOG) ♦ Voltage changes due to
movements of the eyes, recorded from electrodes above and below, or on
either side of, the eyes.

**embarrass **Physiologically, an organ or a function is
'embarrassed' if it is prevented from operating normally. For example,
putting your shoulder over the back of a chair can embarrass the
circulation to your arm.

**embolism **RELATED: air embolism ♦ Blockage of an artery by
an embolus. Emboli in the venous circulation generally lodge in the
lungs (pulmonary embolism). Arterial (systemic) emboli can be more
serious, sometimes producing stroke, myocardial infarction or gangrene
depending on the site of obstruction.

**embolus **Abnormal fragments of material carried in the
circulation, e.g. a blood clot (thrombus), fat, air (as a bubble) or
foreign bodies.

**e.m.f. **The potential difference between the terminals of
an
electrochemical cell or battery when no current is drawn from it. Units
are Volts.

**endo- **CONVERSE: epi-, exo- ♦ Within. E.g. endogenous=
arising within a tissue.

**enzyme **A protein that catalyses (enhances the rate of) a
chemical reaction. Its action involves binding of the substrate or
substrates (reactants) and a conformational change in the enzyme itself.

**epi- **RELATED: peri- CONVERSE: endo- ♦ Above, surrounding.
E.g. epicardium= layer of tissue immediately surrounding the heart
(part
of the pericardium), epidermis= outer parts of the skin.

**epinephrine **See adrenaline.

**equilibrium** RELATED: partial pressure ♦ A condition in
which
there is no tendency for a substance to move spontaneously in either
direction across a boundary. A solution in contact with a gas mixture
will be in equilibrium when there is no net flux of gas molecules
across
the interface, and the *partial pressures* of the gases in
solution
are then the same as in the gas phase. An ion is at equilibrium across
a membrane if there is no passive flux of the ion (though NB there
might be *active transport* of the ion species, requiring energy,
even at equilibrium. A third example is osmotic pressure: absence of
passive water flux across a semi-permeable membrane (selectively
allowing water across) requires that any tendency for water to diffuse
(due to an *osmolarity* difference) is balanced by a pressure
difference: this equilibrium defines the *osmotic pressure difference*
across the membrane.

**equilibrium potential **ABBREV: Veq(x) RELATED: Nernst
equation, resting potential ♦ The value of the membrane potential of a
cell at which there would be electrochemical equilibrium for a
specified
ion. In general in a particular cell the equilibrium potential is
different for different ions. If the membrane potential is made equal
to
Veq for a particular ion, then there is no net passive flux of that ion
even if there are open channels that are permeable to it. In nerve
cells, Veq(K) is close to the negative resting potential, while Veq(Na)
is positive). The equilibrium potential can be calculated from the
Nernst Equation.

**error bars **RELATED: standard error, confidence limits ♦
Graphs are often drawn with bars at each point, as well as the mean
measurement. These indicate a measure of the variability of the data
that has contributed to the mean. Usually the bars extend 1 standard
error above and below the mean, though it should always be stated
exactly what the bars are, since they may be standard deviations or
confidence limits. If they are standard error bars, then a useful rule
of thumb is that if the error bars overlap for 2 sets of measurements,
then the means for these measurements are certainly not significantly
different. If they are separate, but not far apart, you would need to
do
a statistical test to see if the difference was significant

**error function **RELATED: gaussian ♦ Error functions are
integrals of the gaussian and closely related functions, i.e. the area
under a part of the bell-shaped curve. You can't write down an exact
expression for calculating these, but you can look them up in tables:
e.g. tables of the areas under the graph of a standard normal
distribution, also known as the 'cumulative' normal distribution.

**eu- **RELATED: ortho- ♦ Normal. E.g. eupnea = normal
breathing.

**evoked **An 'evoked response' is a response following a
brief
applied stimulus. In neurophysiology it is often a voltage change, also
called an 'evoked potential' or an 'event related potential'.

**excise **RELATED: dissect ♦ To cut out and remove.

**excitability **RELATED: threshold ♦ The ease with which a
cell can be made to respond to a stimulus, for example by giving an
action potential. The term is usually used loosely for comparison (e.g.
'low extracellular calcium concentration increases the excitability of
neurons'). It is not usually defined as a quantitative parameter,
though
it is inversely related to 'threshold'.

**excitation **RELATED: stimulation ♦ Stimulation above a
threshold level for producing a response, particularly an action
potential response of nerve or muscle cells (excitable cells).

**exo- **RELATED: epi- CONVERSE: endo- ♦ Outside. E.g.
exoskeleton= structural support on the outside, rather than the inside
of tissue, as in many invertebrates; exogenous= something not normally
found in the tissue.

**expectation **RELATED: mean, average ♦ In an ordinary sense
this is what you expect in a situation, given your assumptions and
theories. In a mathematical sense, the expectation of a parameter is
the
mean value that it would have if you repeated a large number of
observations (more strictly, the limit for an infinite number of
observations). This can sometimes be calculated as the sum of products
of all possible values of the parameter with the probability of
obtaining each value. The expectation for the outcome of throwing a
dice
is 3.5 (1/6 + 2/6 ...+ 6/6). The expectation for the height of a
subject
selected at random is the mean for the population from which he is
selected.

**experiment **RELATED: critical, crucial, exploratory
experiments ♦ A procedure for finding out something new or for
demonstrating a known fact. In physiology, 'an experiment' sometimes
refers to a procedure carried out on a single animal or sample of
tissue, sometimes to a set of procedures including control
observations,
etc., carried out on many animals or samples.

**explicit **CONVERSE: explicit ♦ An equation involving a
parameter (x) is explicit for x if it is written in the form 'x equals
some expression not involving x'. For example, if you have a quadratic
equation: ax^{2}+bx+c=0, the quadratic formula for solution of
such an equation enables you to solve the equation explicitly.

**exploratory experiment **RELATED: critical experiment ♦ An
experiment carried out to find out what happens in a particular
situation, without any clearly formulated hypothesis at the start. For
example, if you discovered a new type of cell in the body, but had no
idea about its function, you might study the effects on it of several
known hormones. Much research is of this general type. It provides
information that may generate hypotheses and eventually support or
disprove ideas.

**exponent **RELATED: logarithm ♦ The power to which a number
is raised: if y=x^{a}, then a is the exponent. If a=1,2,3,
etc.,
the meaning is simple: x is multiplied by itself 'a' times. Note that x^{a}
× x^{b} is x^{a+b}. This rule allows us to
generalise to negative and fractional exponents. Firstly, x^{0}
must be 1, since the rule requires that x^{0} × x^{a}
=x^{a}. Secondly, x^{-a} × x^{a} = x^{0}
(=1), so x^{-a} is the reciprocal of x^{a}. Third, x^{1/a}
is the a'th root of x, since if you multiply it by itself a times, you
get x^{1}. Lastly, x^{b/a} is the a'th root of x^{b},
by the same argument. You should become completely familiar with the
first 2 or 3 of these rules!

**exponential constant **RELATED: time constant (τ), space
constant(Λ), rate constant, half-life ♦ This is a common way to express
how fast an exponential function falls or rises. In graphical terms, if
you draw a tangent to an exponential curve at any point, it crosses its
baseline exactly one exponential constant to left or right. It is the
reciprocal of a *rate constant* (*rate of change* / *displacement
from steady value*. For a declining exponential, the time constant
is the time it would take to reach the final steady value if the rate
of
decline didn't slow down. An exponential curve actually falls (or
rises)
to 1/e (×0.368) or ×e (×2.718) in this time, so an
equivalent definition is the time for the value (relative to its steady
value) to change to 37% or 272% of its current value (1.44 ×
'half-life' or 'doubling time').

**exponential function **ABBREV: e^{x}, exp(x)
RELATED:
natural logarithm ♦ The function y=e^{x}, where 'e' is a
specific number (approx. 2.718) always called just 'e'. The exact value
of 'e' cannot (like π) be written down. You can get as good an
approximation as you like by calculating the value of (1+1/n)^{n}
with n large enough. Try this on a calculator! This definition of 'e'
(as the limit of the expression as n → ∞) arises naturally from the way
'e' is introduced in the entry on the 'exponential constant'. The
function ex is especially important because its gradient is always
exactly equal to its value. In other words, it is a solution of the
differential equation dy/dx=y. This makes it the simplest exponential
relation to handle mathematically.

**exponential relation **A relation is exponential if one
parameter changes by a constant factor every time another changes by a
constant amount. It has the form y=a×bx. This means that for 1
unit shift along the x axis, y gets multiplied by a constant factor
'b'.
The graph is increasing or decreasing, depending on whether b>1 or
b<1. [ NB, note the important difference from a power law
(y=a×x^{b}).
In an exponential relation the variable parameter x appears in the
exponent.] There is really only one shape of exponential curve, since
all graphs of the form y=a×bx can be made equivalent by changes
of
scale or reversing the axes. An exponential relation gives a straight
line if you plot log(y) against x, for example on log-linear graph
paper.

**exponentials: examples **Rising exponentials are seen in
unrestricted population growth (e.g. bacteria, which might double their
population each hour) or growth of capital in a bank account (8%
interest per annum corresponds to a rate constant for increase of
0.077/ann, a doubling time of 9 years, or a time constant of 13 years).
Decreasing exponentials are common in physiology: disturbances often
decline by an approximately constant factor per unit time, for example
membrane potential disturbances with a time constant of a few ms after
stimulation, or heart rate over minutes after exercise. The size of a
synaptic potential declines with distance from the synapse along a
dendrite approximately exponentially, with a space constant that might
typically be 0.5mm.

**exsanguinate **To let out much or all of the blood from an
animal.

**extra- **CONVERSE: intra- ♦ Outside. E.g. extravasation=
something getting out of blood vessels.

**e.g. **'Example given'. 'For example'. Don't muddle this
with
'i.e.', which implies that what follows is always true, not just an
example of what might be true. If you do muddle these, the reader may
be
led to think you have quite the wrong idea.

**facilitation **RELATED: potentiation CONVERSE:
embarrassment,
inhibition ♦ A process that makes something easier or increases its
effectiveness. For example, a bronchodilator can facilitate breathing.
An influence is said to facilitate a response if the response to a
constant stimulus is increased while the influence is applied. For
example, clenching of the fists often facilitates the knee-jerk tendon
reflex (the 'Jendrassik manoeuvre'). 'Potentiation', on the other hand,
is an increase that outlasts the influence that causes it.

**factor **Something that influences or induces the response
of
a tissue. The term 'factor' is often used when very little is known
about an influence: for example, vasodilation might in some situation
be
caused by a neural or hormonal factor, or by a physical factor such as
a
change of temperature. 'Factor analysis' is a statistical technique for
identifying the combinations of known variables that are the best
predictors of a measured parameter (e.g. of IQ).

**factorial **ABBREV: n!) ♦ Product of all the integers from 1
to n. 1!=1, 2!=2, 3!=6, 4!=24, etc.. For large numbers, a reasonable
approximation is (n/e)n.

**farad **ABBREV: F) ♦ Unit of electrical capacitance. The
capacitance of a membrane is proportional to its area, and is typically
about 1uF.cm^{-2}.

**faraday **ABBREV: F) ♦ The amount of electrical charge per
mol of univalent positive ions: 96,500 coulombs/mol, i.e. about 10^{5}
coulombs/mol. You need to know this whenever you are thinking about how
much of a concentration change is associated with electrical changes,
as
with current flowing across cell membranes. This is the 'F' of the
quantity RT/F that comes into the Nernst equation. Note that the
abbreviation F is the same as for a 'farad'. This should not lead to
confusion, since a faraday is a quantity, while a farad is a unit, but
you do need to be careful.

**fatigue **A reduction in a response as a result of repeated
or prolonged stimulation or activity. In the context of muscle
function,
it usually means a reduction in the amount of force that the muscle can
produce.

**feedback **RELATED: control ♦ Information sent to an earlier
stage in a multistep process. 'Negative feedback' has effects that
reduce the subsequent feedback signal, or reduce its rate of increase.
This usually has a control function, keeping a parameter in the system
constant. For example, the activity of the heart raises the arterial
blood pressure. If the pressure rises above a controlled level, signals
are sent via baroreceptors and the brain back to the heart, leading to
a
decrease in heart rate. 'Positive feedback' produces changes that lead
to a further increase in the feedback signal. This can lead to runaway
changes in the system, and usually to an all-or-none response (e.g. an
action potential).

**femto- **ABBREV: f RELATED: SI units ♦ One thousand million
millionth. 10^{-15}. The smallest fraction of a unit you
normally come across in physiology. For example, the small magnetic
fields produced by brain activity are measured in femto-Tesla (fT).

**fibrillation **RELATED: atrial flutter ♦ Repetitive high
frequency spontaneous activation or contraction of a muscle,
particularly cardiac muscle. Quivering. Atrial fibrillation is faster
than atrial flutter (>300/min) and more irregular. Ventricular
fibrillation is similar, and arises frequently from electrocution or a
myocardial infarct: it is normally lethal within a few minutes if not
arrested (e.g. by electric shock from a defibrillator) or unless blood
flow is sustained by cardiac massage.

**filter **RELATED: low, high, band pass; notch filter ♦ An
electrical circuit or structure that passes some things and prevents
passage of others. Electrical filters usually pass and 'cut' particular
frequency components of a signal. They may be 'passive' (not requiring
a
power source) or 'active' (requiring a power source). Physiological
systems, such as the kidney glomeruli, sometimes act as physical
filters, passing water and some solutes to produce a 'filtrate' or
'ultrafiltrate' (the latter including only small molecules). A substance is
'freely filtered' if (as with inulin in the kidney) it is in about the same
concentration in the filtrate as in the filtered solution.

**fixation **Chemical treatment of a tissue to precipitate
proteins and ensure that cell structures will not change subsequently.
This is the first procedure in the preparation of specimens for
microscopy. Fixatives (e.g. formaldehyde, glutaraldehyde) kill cells
and
may introduce artefacts (distortions of normal structure).

**fluorescent **Capable of emitting light at one wavelength
when light of another wavelength is absorbed. The wavelength of the
emission is always longer (more towards the red end of the spectrum)
than the wavelength of the exciting light. Fluorescent dyes are useful
for marking cells into which they are injected, or for labelling
receptors to which they bind. Some dyes exhibit fluorescence that
changes with pH or pCa, and these can be used as indicators to measure H^{+}
or Ca^{2+} concentrations.

**flux **RELATED: net flux ♦ Rate of movement of a substance
or
solute. Movement may be in both directions at once, as with molecules
diffusing within solution or across a membrane, in which case the 'net
flux' is the difference between the rates in each direction separately
(the 'unidirectional fluxes'). Unidirectional fluxes are often measured
with the help of radioactive tracers, 'labelling' the solute on one
side. In an equilibrium situation there is no net flux, unless active
transport is involved. Flux is sometimes measured as the total amount
moving per second (e.g. into a cell), sometimes as the amount moving
through unit area per second. The units should make it clear which is
used (e.g. mol.m^{-2}.s^{-1}).

**form **RELATED: time course, profile ♦ The shape of a graph,
for example of the time course of a parameter. The form of a
relationship is considered to be independent of the scale on which the
graph is drawn, on either axis. Thus the time courses for the recovery
from tachycardia (high heart rate) and hyperthermia (high temperature)
after exercise have the same form (both roughly exponential), even
though the restoration of normal heart rate may be much faster.

**fractional change **RELATED: proportional, percentage ♦ The
fraction by which a parameter has changed, i.e. (new value - old
value)/(old value). Fractional changes are often multiplied by 100 to
give 'percentage changes'.

**frequency components **RELATED: sinusoidal ♦ The waveform of
any signal can be broken down into a combination of frequency
components: individual sinusoidal waves, each with its own frequency.
The signal is the sum of these frequency components. This is called
'Fourier analysis'. The reason for choosing this particular breakdown
of
a signal is that any linear system produces a very simple response to a
sinusoidal signal: an output that is sinusoidal at the same frequency,
with just an amplitude and a phase to be determined. The response to an
arbitrary signal can then be predicted from a simple description of its
response to different frequencies. Signals may be broken down into
spatial as well as temporal frequency components.

**frequency response **RELATED: transient response ♦ The range
of frequencies to which an instrument will respond. All instruments
have
an upper limit to their frequency response. For example, pens can
follow
signals up to ca. 100Hz, while oscilloscopes may follow up to 1MHz or
more. The lower limit is 0Hz in a 'DC-coupled' instrument or a higher
frequency in an 'AC-coupled' instrument. Physiological systems may also
have a frequency response: e.g. the response of the human auditory
system is ca. 30Hz-20kHz. Since the response of any system falls off
gradually with frequency, in instrument assessment the frequency for a
'3dB cut' is usually specified (i.e. when the energy in the signal
falls
to 50%, or the amplitude to 71% of normal).

**function **In mathematics, this is merely a relationship
between 2 or more variables, usually expressible as an equation. In
physiology, the statement 'A is a function of B' (where A and B are two
parameters) implies not only that you could sketch a graph of A against
B, but that there is a specific causal relationship: changes in B cause
changes in A (NB changes in A may or may not cause changes in B). For
example, urine production is a function of alcohol concentration in the
blood (but not vice versa). Ventilation rate is a function of arterial
CO_{2}
concentration, and also vice versa; but note that the graphs of these
two functional relations would be completely different (with opposite
slopes).

**gain **RELATED: amplifier ♦ Amplification.

**gate **The verb 'to gate' means to switch something on or to
allow something through under controlled circumstances. A stimulator
may
be 'gated', i.e. generate pulses only when a voltage is applied at a
specific ('gate') input. Ion channels may be 'voltage-gated' (i.e. open
only if the membrane voltage falls within a specific range) or
'receptor-gated' (i.e. opened or closed by the binding of a substrate
to
a receptor associated with the channel molecule).

**gaussian **RELATED: normal distribution, error functions ♦
The function corresponding to the normal distribution, giving a
bell-shaped curve. The equation is y = a exp(-0.5x^{2}), where
x
is the actual deviation from the mean divided by the 'standard
deviation'. The same function arises in the mathematics of diffusion:
if
something spreads from an initially highly concentrated point source,
the subsequent bell-shaped distributions are gaussian or closely
related
functions, depending on the exact geometry involved (e.g. whether it
can
spread in one or more directions).

**-genic SUF **Giving rise to. E.g. anxiogenic= leading to
anxiety.

**-genous SUF **Arising from. E.g. exogenous= something that
comes from outside the body or tissue.

**giga- **ABBREV: G RELATED: SI units ♦ One thousand million.
10^{9}.
For example, a 'patch' electrode that is well sealed to a biological
membrane may have a resistance (between the inside of the electrode to
the bath solution) of several giga-ohms.

**glyc- **Relating to glucose or glycogen.

**Goldman equation **RELATED: Nernst equation ♦ If a membrane
is only permeable to one type of ion that is present on either side,
that ion will move across the membrane carrying charge, until the
membrane potential adopts the Nernst equilibrium potential for that
ion.
If it is permeable to several ions, perhaps present in different
concentrations and with different permeabilities, then the membrane
potential will adopt a value intermediate between the different
equilibrium potentials: a weighted average, biassed towards the values
for ions with high permeability and high concentration. It is more
important to understand the concepts in these terms than to remember
the
Goldman equation, which gives a reasonable approximation to the
results.
For Na+,K+ and Cl- it can be written, with permeabilities P_{Na+},
etc. and concentrations Na_{i}, Na_{o}, etc.:

V_{m} = -RT/F log_{e} ( (P_{Na+}Na_{i}+
P_{K+}K_{i}+ P_{Cl-}Cl_{o}) / (P_{Na+}Na_{o}+
P_{K+}K_{o}+ P_{Cl-}Cl_{i}) )

To gain some insight into the equation, see what happens to V_{m}
when any one of the 3 permeabilities becomes much bigger than all the
others (as P_{Na+} does in an action potential). It should of
course (and does!) reduce to the appropriate Nernst Equation.

**graded **CONVERSE: all-or-none ♦ Capable of having any one
of
a continuous range of values. A physiological response is 'graded' if
variations in the strength of a stimulus produce variations in the
amplitude of a response: a graph of response against stimulus strength
is a smooth curve, unlike the graphs for all-or-none or stepped
responses. Instruments often have both 'graded' and 'stepped'
(click-position) controls for varying their settings, for example the
amplitude or frequency controls on a stimulator.

**gradient **ABBREV: dy/dx RELATED: slope ♦ The ratio of
changes in one parameter to changes in another. The gradient (or slope)
of y plotted against x corresponds to the steepness of the graph, i.e.
the amount it moves up in a small segment, divided by the amount it
moves horizontally. The dimensions and units of the gradient are those
of y divided by those of y. A linear relation has a constant gradient.
A
curvilinear relation has a varying gradient.

**ground **RELATED: earth ♦ USA term for 'earth' in an
electric
circuit.

**gustatory **Relating to taste.

**g-i tract **Gastrointestinal tract, from the mouth to the
anus.

**habituation **

**haemostasis **Blood clotting.

**half life **ABBREV: t_{1/2} RELATED: time constant ♦
The length of time for something (most commonly a rate of radioactive
decay) to fall to 50% of its current value. If the half life is
constant
whatever the current value, then the decline is exponential. The half
life is then ln(2) (=0.69) times the exponential time constant for the
decline.

**hardware **RELATED: software ♦ The actual equipment
(particularly a computer) that you use. These days, the usefulness of
computer-based equipment often depends more on the quality of the
software than on the hardware. The modern equivalent of 'A bad workman
always blames his tools' is perhaps 'A frustrated user should blame the
software, not the hardware'.

**Hebb synapse **A synapse or hypothetical synapse in which
strengthening of excitatory action occurs following conditions in which
presynaptic firing contributes to the successful firing of the
postsynaptic cell (following a hypothesis for a learning mechanism put
forward by DO Hebb, 1949).

**hecto- **ABBREV: h RELATED: SI units ♦ One hundred, 10^{2}.
Hardly ever used.

**hepato- **Related to the liver.

**hertz **ABBREV: Hz) ♦ Unit of frequency. 1Hz = 1 cycle or
repetition per second. E.g. Mains frequency is 50Hz (60Hz in the USA).
A
filter might pass only frequency components above 2kHz. A nerve might
be
stimulated at 10Hz. SI recommendations say that you should only use Hz
for sinusoidal components of a signal. People seem usually to ignore
this, however. Strictly according to this rule you should say you are
stimulating at 10 pulses per sec, or 10s^{-1}, unless you are
using a sinusoidal stimulus current (which would be rather unusual).

**hetero- **CONVERSE: homo- ♦ Different, varied.

**high pass **RELATED: low pass, filter ♦ A type of filter
that
lets through high frequency components and removes (or 'cuts') low
frequencies.

**histogram **RELATED: bar graph ♦ A bar graph in which the
area of each bar shows the number of observations within a particular
range on the horizontal axis. The range is indicated by the bar width.
This is a good way of showing the distribution of values obtained for a
parameter. Values within a range (sometimes called a 'bin' or a 'cell')
are pooled and counted together, so that the detailed information about
precise values is lost and doesn't confuse the overall picture of the
distribution.

**homeo- **RELATED: = homo-, iso- ♦ Same. E.g. homeostasis.

**homo- **RELATED: = homeo-, iso- CONVERSE: hetero- ♦ Same.
Equivalent to homeo-, in words that come from Latin instead of Greek.
E.g. homogeneous (homogenous in USA) = having a constant property
everywhere.

**homogeneous **RELATED: isotropic ♦ Having the same
properties
at every place. Distinguish carefully from 'isotropic'. For example,
the
structure of contractile proteins within a skeletal muscle cell is
homogeneous, but not isotropic. Any place in the cell has similar
characteristics to any other, but these characteristics are very
different in different directions.

**hormone **RELATED: endocrine ♦ A chemical that is released
from cells and transported to its target organ in the blood.

**humoral **Relating to body fluids.

**hyper- **RELATED: iso-, normo-, ortho- CONVERSE: hypo- ♦
Above normal. E.g. hypertonic = more concentrated than normal, tending
to make cells shrink; hyperventilate = breathing more then normal).

**hyperbolic **A relation for which the graph is a hyperbola,
most commonly one of the form y=a/x (i.e. an inverse proportional
relation) or y=a/(x+b)+c (which is the same curve shifted on both axes,
as in Hill's equation for the relation between force and velocity in
muscle tissue).

**hypo- **RELATED: iso-, normo-, ortho- (all meaning roughly
'normal' or 'equal' CONVERSE: hyper- ♦ Below, less than normal. Used as
a prefix, e.g. hypotonic = less concentrated than isotonic. Hypoxic=
less oxygenated than normal. Hypodermic= below the skin.

**hypothesis **RELATED: null hypothesis, theory ♦ A
suggestion;
part of a theory; a postulate. Many of the explanations in
physiological
textbooks for how things work are really hypotheses. They may be widely
accepted, or sometimes rather uncertain and controversial. It is seldom
possible in science to prove that a hypothesis is correct. You should
always treat scientific explanations with a healthy scepticism,
regarding them as in principle capable of being proved wrong or
requiring some modification in the light of new experiments. If you are
asked for a hypothesis to explain some data, it is always a good idea
to
think of 2 hypotheses: you may then be able to devise a crucial
experiment to distinguish them.

**i.e. **RELATED: e.g. ♦ i.e. means 'That is, ...' (Latin: id
est). This introduces either a rephrasing of what has gone before, or
something that is strictly implied by it. Distinguish it carefully from
'e.g.'. Examples: Salt can cause natriuresis (i.e. sodium excretion in
the urine). Hypothermia can stop respiration, i.e. ultimately cause
death.

**impedance **RELATED: resistance, reactance ♦ Equivalent to
the resistance of a circuit, but under more general conditions where
the
ratio of voltage and current depends on the frequency of the signals.
Often it is used more or less interchangeably with "resistance": e.g.
you might read that the input impedance of an amplifier is 10MΩ.
Strictly, though, impedance should be quoted for a particular
frequency:
e.g. >10MΩ at <50kHz. If the impedance varies with frequency (as
it will do if capacitative effects are significant) then there will be
a
phase difference between the voltage and the current changes. Impedance
is often then expressed as a complex number (q.v.). The symbol Z is
often used for impedance: Z=R+iX where R is the resistance and X the
effective reactance in series.

**implicit **CONVERSE: explicit ♦ An equation involving a
parameter x may imply that x has a particular value, but not enable you
to calculate it by direct (explicit) application of any formula. The
value of x is then 'implicit'. For example, the equation x=e^{-x}
clearly has a solution (as a sketch will show), but cannot be solved
explicitly. The solution (x= ca. 0.675) can be found by iteration, for
example by using the Newton-Rhapson method to derive the iterative
formula x'=2/(1+e^{x}) to get a new estimate x' from an
existing
estimate x.

**impulse **Action potential.

**in parallel **CONVERSE: in series ♦ Two things that happen
at
the same time, or are arranged side by side. The brain can often do
several tasks at once, in parallel. Muscle fibres produce more force if
they are arranged in parallel.

**in series **RELATED: serial CONVERSE: in parallel ♦ Two
things that happen one after the other, or that are arranged physically
in a row. Resistors in series are connected end to end, giving a total
resistance that is the sum of the individual values.

**in situ **RELATED: in vivo, in vitro ♦ In the normal
position
in the body, or tissue.

**in vitro **RELATED: culture CONVERSE: in vivo ♦ Literally,
in
glass. Often refers to a procedure carried out on cells or tissue
isolated from the body and maintained in a tissue bath.

**inch **Approximately 25.4mm.

**independent **RELATED: dependent ♦ See 'dependent' for a
discussion of dependent and independent variables. The concepts of
'independent measurements' and 'independent samples' are also
important.
If you carry out 15 blood counts, but 5 of them were on blood from the
same subject, then you do not have 15 independent measurements. The 5
are more likely to be similar to each other than to the counts from
other subjects. The samples were not 'independent'. It would be
incorrect to calculate a standard deviation and to treat it as a
measure
of the variation of blood counts between individuals. The validity of
statistical tests often depends on your samples or measurements being
independent.

**indication **A sign, hint, or suggestion. In clinical
terminology: a circumstance that suggests that a particular therapeutic
regime or diagnostic test is merited (as opposed to a
'contra-indication': a circumstance that suggests that a course of
action might be unwise).

**indifferent **An 'indifferent' electrode is one used as a
reference for measuring voltages at one or more different places, or
one
used when stimulating with a different electrode to pass the return
current from the tissue.

**infarct **A non-functioning, or totally dead, region of
tissue (often resulting from ischaemia).

**infinitesimal **RELATED: differential ♦ The limit of very
small quantities, analogous to the reciprocal of 'infinity'.In one
sense, an infinitesimal quantity is simply zero. However, you may be
interested in the ratio of two things as they become infinitesimal,
when
it is not helpful to think of them as zero.

**infinity **ABBREV: ∞ RELATED: limit CONVERSE: infinitesimal
♦
The limit of very large values. Infinity is not a number, and it can be
confusing to think of it as such. You can usually rephrase any
statement
about infinity in the form 'As x tends to infinity (i.e. gets larger
and
larger), y increases without limit', or 'As x tends to infinity, y
tends to a particular value (i.e. gets closer and closer to this
value).'

**infra- **RELATED: sub-, hypo- CONVERSE: supra- ♦ Below. E.g.
infra-orbital= below the orbit (eye socket).

**infusion **A continuous slow injection over a long period.
Sometimes referred to colloquially as a 'drip', because a drip chamber
is often used to check that the infusion is proceeding normally.

**inhibition **RELATED: depression CONVERSE: excitation,
facilitation ♦ An influence that reduces activity in a tissue or
reduces
the response produced by a stimulus. Synaptic inhibition can operate
either by hyperpolarizing a cell (subtracting from the effects of
excitation) or by reducing the amount of depolarization caused by
excitation (diminishing the excitatory influence, with no effect unless
excitation is taking place). The second situation can arise with
'presynaptic' inhibition, and also as a result of a postsynaptic
conductance increase that opens channels that tend to keep the membrane
potential close to the resting potential. If the decrease of a response
outlasts the influence that brings it about, it is usually called
'depression'.

**innervate **One tissue is innervated by another if it
contains terminals of axons arising from cell bodies in the second
tissue. These might be synaptic terminals or sensory terminals (as in
the sensory innervation of the skin from cells in the dorsal column
nuclei). The term is essentially anatomical, and does not relate to the
processes of activation or synaptic transmission.

**input **CONVERSE: output ♦ An influence or a physical route
into a structure, that is capable of affecting it. The influence may be
the physical entry of something (e.g. the input of food into the
stomach) or the passage of information (e.g. action potentials in the
innervation of the stomach). Cables connecting pieces of equipment
usually have clearly defined input and output ends: signals or electric
power are usually conveyed in one specific direction, which you should
identify to make sense of the wiring.

**input impedance, input resistance **The impedance (or
resistance) between the input terminals of an amplifier. Amplifiers for
measuring voltages without disturbing the conditions they are measuring
should have a very high input impedance, so that whatever voltage the
source generates, very little current is drawn by the amplifier. Input
resistances of 10^{12}Ω are easily achieved with modern
components, though at frequencies of physiological interest (e.g.
10kHz)
the input impedance will be much less because the input behaves like a
capacitor in parallel with a resistor.

**integral **ABBREV: ∫ y dx) ♦ The area under a curve. A
'definite integral' of a function f(x) is the area under the curve
y=f(x) between two specified values of x, in some cases ±
infinity. An 'indefinite integral' is itself a function of (x): the
area
under the curve to the left of (x). An indefinite integral includes an
arbitrary constant, since the left hand limit of the area is not stated
in the definition. An indefinite integral is in a sense the opposite of
a derivative. If I(x)=∫f(x)dx, then f(x)=dI(x)/dx. Thus the integrals
of
functions can sometimes (not always) be written down by using backwards
the rules for derivatives. Integration can always be carried out
numerically on a computer.

**inter- **RELATED: intra- ♦ Between. Be careful to
distinguish
'inter-' from 'intra-'. For example, 'intercellular' means between
cells. It may therefore refer to the extracellular space, or to
junctions between cells.

**interpolate **RELATED: extrapolate ♦ Estimate the value of a
function between two or more known points, either by assuming a linear
relationship between the two nearest points (linear interpolation) or
by
fitting a more complex function.

**interpreter **RELATED: compiler ♦ A program that translates
instructions written in a high level computer language into a form that
can be executed by the computer, at the time the program is being used.
The interpreter must be used every time the high level program is used
(unlike a 'compiler'). An interpreter is slow in execution compared
with
a compiler, because it repeats the translation process every time an
instruction is used. Computers are often so fast that this is not
important. Interpreters can save time if a program needs to be changed
frequently, because no 'compiling' is needed after each change. The
commonest 'interpreted' language is BASIC.

**interstitial **RELATED: extracellular ♦ Relating to the
interstices, or spaces in between things (usually cells). The
interstitial space is the extracellular compartment.

**intra- **RELATED: inter- CONVERSE: extra- ♦ Inside.
'Intracellular' means inside cells, quite different from
'intercellular'.

**invasive **CONVERSE: non-invasive ♦ An 'invasive' procedure
involves putting something into the tissue under study, removing a
sample of it ('biopsy'), or performing some surgery. Radiation, X-rays,
etc. are usually regarded as 'invasive' because the invisible
penetration by particles or waves is capable of doing damage.

**inverse **Two things are inversely related if one goes up
when the other goes down. For example, pH and hydrogen ion
concentration
are inversely related. If the relation is not strict (as it is in this
case, where there is an equation that always relates the two things
exactly), then it is more usual to talk about a 'negative correlation'
than an 'inverse relationship'.

**inverse function **ABBREV: INV) ♦ This is in a sense the
'opposite' of a function. If y=f(x), then x=INVf(y). For example, the
inverse of x^{2} is x^{0.5} or sqrt(x), while
INVlog(x)=10x. Sometimes (e.g. on calculators) the shorthand
terminology
f^{-1}(x) is used, but this is potentially very confusing since
it could also mean 1/f(x): avoid this terminology. In trigonometry, the
expressions arcsin(x), arccos(x) are used for the inverse functions of
sin(x) and cos(x). The 'inverse' of x is sometimes used to mean 1/x,
but
this is best avoided since the word 'reciprocal' means this and is
unambiguous (e.g. conductance is the reciprocal of resistance, rather
than the inverse of resistance).

**ipsi- **RELATED: auto- CONVERSE: contra- ♦ Same, self.
Ipsilateral= on the same side.

**ischaemia **RELATED: hypoxia, anoxia ♦ Cessation of blood
flow. This is a normal condition in some tissues, for example in many
muscles during maximal voluntary contractions. If continued for too
long, the products of metabolism build up, substrates become depleted,
and temporary or irreversible loss of function (infarction) may result.

**iso- **RELATED: hypo-, hyper- ♦ Having the same value, or
property. E.g. isotonic, isomer

**isolated **RELATED: in vitro ♦ A tissue separated from its
normal inputs. It might be kept in the body for study ('in situ') or it
might be studied 'in vitro'. An 'isolated stimulator' is an electrical
stimulator in which neither of the output terminals is connected to
earth; this is useful for reducing the current during a stimulus that
may flow through a recording device, causing a 'stimulus artefact'.

**isotropic **RELATED: homogeneous ♦ Having the same
properties
in all directions. Distinguish carefully from 'homogeneous'.

**iteration **A step that is repeated many times in trying to
achieve a goal (e.g. to solve an equation, or straighten a picture on a
wall). If the procedure is convergent, the steps eventually make almost
no further difference, and you are close to a solution. If it is
divergent, the steps get further from the goal, and the procedure may
be
useless. For example, you can find sqrt(2) by starting with any guess
(except zero) and using the iteration formula (x/2+1/x) to alter the
current estimate (x) again and again. (This is an example of the widely
useful Newton-Rhapson formula for iterations to solve an equation of
the
form f(x)=0: x'=x-f(x)/(df/dx).)

**i-m **Intramuscular (usually referring to an injection site).

**i-p **Intraperitoneal. An injection given into the abdominal
cavity (belly).

**iso- **equal, same, uniform

**isotonic **RELATED: isosmolar ♦ (1) Having the same tonicity, or the same osmotic
effect on cells as the fluid in their normal environment. This is not
necessarily the same as having the same osmolarity (see tonicity) because some solutes
contributing to the osmolarity of a solution may be permeant and
equilibrate across the membrane, and so have no effect on cell volume,
or only a transient effect. (2) In muscle physiology, an isotonic
contraction is one during which the tension is constant (contrast isometric) .

**-itis SUF **Inflammation of.

**i-v **Intravenous (an injection or infusion into a vein).

**joule **ABBREV: J RELATED: calorie ♦ The SI unit of energy.
For mechanical energy it is equivalent to a 1 Newton force moving 1
meter distance. One 'watt' (power) is 1J/s. 1J=10^{7} ergs (cgs
unit of energy).

**kal- **Relating to potassium (Latin = Kalium, hence symbol
K). E.g. hypokalaemia= low blood potassium. Distinguish carefully from
calci-.

**keyword **RELATED: database ♦ A word used to gain access to
information in a database; an index entry.

**kilo- **ABBREV: k RELATED: SI units ♦ One thousand. 10^{3}.
E.g. 1kg = 1000g

**labile **CONVERSE: stable ♦ Transient, short-lived, or
fragile.

**laminar flow **RELATED: vector, turbulence ♦ A pattern of
flow of a fluid (e.g. blood) in which the velocity and direction of
movement vary smoothly as you go from one place to another. A graph of
velocity against position would be a smooth curve, usually increasing
monotonically from the walls to the middle of a vessel. You can
describe
the flow in terms of laminae (layers) of fluid flowing steadily past
each other. The physical laws governing such flow are simple (e.g.
Poiseuille's Law).

**language **RELATED: program ♦ A computer language is a set
of
rules for expressing instructions to a computer. Examples are BASIC,
FORTRAN, PASCAL, 'C', 'Machine Language' or 'Assembler'. A 'low level
language' (e.g. the machine language for a specific computer) is one in
which each instruction carries out very simple operations, and many
instructions are required. A 'high level language' includes very
powerful instructions that lead to complex sets of operations. High
level languages must be translated into low level instructions before
they are used: this is usually carried out automatically by programs
called 'compilers' or 'interpreters'.

**latency **A time delay between a stimulus and the beginning
of a response. Don't use the term 'latency' when you are talking about
the time to the peak of a response. The word literally means 'hidden'
time: the time after the stimulus when there is still no sign that any
response is going to occur.

**lateral **On one or other side of the body. Far (or farther)
from the midline.

**leak **In membrane physiology, a leak is a passive flux of
ions by a mechanism that is either artefactual and undesired (e.g.
through damaged membrane around a site of electrode penetration) or
relatively unselective between different species of ions.

**lesion **RELATED: trauma ♦ A region of injury. The verb 'to
lesion' is sometimes used for the deliberate induction of tissue damage
for experimental purposes.

**ligand **A chemical or ion that binds to another chemical.

**ligate **RELATED: ligature ♦ To tie closed, for example, a
blood vessel or duct from a gland.

**limit **A mathematical 'limit' is a process in which some
parameter gets closer and closer to a specified value, but doesn't
actually reach it. Sometimes a function is not defined actually at the
value that is approached. For example, if y=x/sin(x), the value of y is
not straightforwardly defined at x=0 since both the top and bottom of
the expression for y are then zero. In the limit, as x approaches zero,
y nevertheless gets closer and closer to the value 1. 'Integration
limits' are the values on the x axis between which the area under a
graph is calculated.

**line **RELATED: mains ♦ USA term for 'mains'.

**linear **RELATED: proportional ♦ A relation between two
parameters, for which the graph is a straight line. The equation
relating them is of the form y=a+b×x. Note that 'linear' is not
the same as 'proportional'. A proportional relationship is always
linear, since its graph is a straight line through the origin. A linear
relation is not necessarily a proportional one (i.e. the line doesn't
necessarily go through the origin).

**logarithm **ABBREV: log(x) RELATED: natural logarithm ♦
Logarithms strictly are to a certain 'base', which should be specified.
If none is specified, the base is 10, while for 'natural logarithms' it
is 'e'. Log(x) (to base 10) is the exponent (or power) to which 10 must
be raised to give the number itself. Thus if y=log(x), then x=10^{y}.
For example, log(10)=1, log(100)=2, log(20)=1.3010, etc. Logarithm
tables used to be useful because they simplified complex
multiplications
and divisions. With calculators and computers, you won't need them.
However, logarithms are enormously useful and important in the
presentation of data (see logarithmic scales) and you should know by
instinct that if log(X)=-6.2, then X is between 10^{-7} and 10^{-6}.

**logarithmic scale **Graphs are sometimes plotted with
distance along one or both axes proportional to the logarithm of a
variable, rather than to the variable itself. There are two reasons.
One
is to separate data points that would otherwise be crammed near one
axis. The other is because some relationships give a straight line
plotted in this way. For example, an exponential relationship can
become
linear if you use a vertical log scale, while a power law becomes
linear
if you use two log scales. You can buy log-linear and log-log graph
paper to save you having to calculate the logs. You can identify a log
scale by seeing that the values change by a constant factor
(e.g.×10) when you move a constant distance.

**lyse **To break, dissolve or destroy. Often refers to cells
bursting, for example, due to a hypotonic extracellular solution.

**macro- **CONVERSE: micro- ♦ Large. E.g. macrophage= a large
cell that engulfs particles. In computer jargon, a 'macro' is a single
command that initiates a complex set of commands.

**macroscopic **CONVERSE: microscopic ♦ Relating to
large-scale
features of something

**magnetic resonance **ABBREV: MRS, MRI RELATED: NMR ♦
Magnetic
resonance. A technique for studying the internal structure and
chemistry
of tissue by observing its properties in a high magnetic field. MR
spectroscopy (MRS) gives information about the different chemicals
present. Imaging techniques (MRI) give images of the structures that
differ in their chemistry, which can reveal tumours, dead tissue, etc.

**mains **RELATED: line ♦ The domestic electricity supply.
Usually 220-240V, 50Hz in Europe and 110V, 60Hz in the USA.

**mains hum **RELATED: noise, interference ♦ A type of
interference that may be picked up during electrical recording, from
mains operated circuits and cables nearby. It has the frequency of the
mains, and may therefore be identified by seeing whether it has a
period
of 20ms (16.7ms in the USA). It is usually sinusoidal, but may include
higher frequency components ('harmonics') giving a jagged appearance to
the waveform on an oscilloscope, especially if fluorescent lights are
the source of the interference. If the signal is fed to a loudspeaker,
it sounds like a deep hum.

**malignant **RELATED: cancer CONVERSE: benign ♦ A condition
that gets worse if not treated. Particularly refers to a tumour that
invades and destroys other tissues.

**maximum **CONVERSE: minimum ♦ The largest value that a
parameter can have (e.g. the maximum age of a particular species), or a
value on a graph that is larger than values on either side (E.g. levels
of growth hormone often have several maxima during the day: i.e. the
graph of concentration against time has several peaks).

**mean **RELATED: average ♦ The sum of a set of quantities,
divided by how many there are. It is nonsensical to take the mean of a
set of quantities that are not in the same units. For a 'weighted'
mean,
each quantity is multiplied by a coefficient, or 'weight' before
summing, and the result is divided by the sum of the weights. If the
weights are all equal, then the weighted mean is the same as an
ordinary
mean. For example, a chemical's concentration in excreted urine will be
the weighted mean of the concentrations produced by each kidney,
weighted in proportion to the urine flow rates produced by each kidney.

**mechanism **RELATED: why ♦ The sequence of events, steps and
interactions involved in a process. When a physiologist asks why
something happens, he usually wants to know the mechanism.

**median **RELATED: quartile, percentile ♦ The value within a
distribution that is exceeded by half of the data points. The 50%
percentile.

**medium **RELATED: culture ♦ Nutrient fluid suitable for
growth or maintenance of cells or tissue *in vitro*.

**mega- **ABBREV: M RELATED: SI units ♦ One million. 10^6 or 10^{6}.

**megaly SUF **Large. E.g. megalomania = delusion of grandeur.

**membrane potential **ABBREV: V_{m} Voltage
difference
between the inside and outside of a membrane. V_{m} is normally
taken as the *inside* voltage relative to the outside, taken as
the
reference: V=0. So V_{m} is normally negative at rest (the
resting potential). Note that some cells (e.g. in endothelia involved
in
transport, V_{m} may be different on different faces, and
positive on some.

**messenger **RELATED: second messenger ♦ A chemical released
in one place, that has an influence on processes in another place, e.g.
a hormone, transmitter, or intracellular messenger. Its role is
essentially to act as a signal, conveying information.

**metabolism **The chemical reactions that occur in the body.
Chemicals ingested or manufactured in the body are either metabolised,
excreted or accumulated.

**metabolite **A product of metabolism of nutrients or of a
particular specified substance.

**meter SUF **Measure, or measuring instrument. E.g.
manometer=
instrument for measuring vacuum or pressure.

**micro- **ABBREV: u (Gr mu) RELATED: SI units ♦ One
millionth:
10^{-6}. E.g. 1000 microvolts = 1mV.

**micron **ABBREV: um) ♦ An old-fashioned word for a
micrometre
(1um). It used to be abbreviated as 1u, but this is confusing and
strongly discouraged within SI units. The unit 'mu' (meaning
milli-micron, or 1nm) was used by spectroscopists, and is also
discouraged.

**micturition **Urination.

**milli- **ABBREV: m RELATED: SI units ♦ One thousandth. 10^{-3}.
For example, the commonest unit of voltage used in physiology is the
millivolt (mV). Note that the abbreviation (m) is the same as that for
a
metre. This does not lead to confusion if you use units correctly. If
'milli-' is used, it always precedes a symbol for an ordinary unit, and
never stands on its own. Separate units that are multiplied together
should always be separated by a dot (period). Thus 1ms = 1 millisecond;
1m.s = 1 meter second.

**minimal **RELATED: minimum, maximal ♦ Something that is
almost small enough to have no effect. Distinguish from 'minimum',
which
means the smallest value under consideration or (in mathematics) one
that is smaller than its neighbours. For example: Standing up leads to
a
transient fall in blood pressure, but this usually has minimal effects
on brain function. At its minimum, the blood pressure may have dropped
by 40mm Hg.

**minimum **CONVERSE: maximum ♦ The lowest value a parameter
can have (e.g. the minimum water temperature in which a person can
survive), or a value on a graph that is lower than points on either
side.

**mks **RELATED: SI, cgs ♦ System of units based on metres,
kilograms & seconds

**mmHg **The unit of pressure most commonly used for measuring
blood pressure. 1mmHg is approximately 133 Pa (pascals). The unit is
retained, despite not being an SI unit, because the commonest and most
accurate instrument for measuring blood pressure is a mercury
sphygmomanometer, which gives a direct reading in mmHg.

**mode **RELATED: mean ♦ The most common, or most probable
value in a distribution. Seldom a useful concept in physiology, though
it is often about the same as the mean or median.

**modulator **A chemical that influences (decreases or
increases) the response to another stimulus.

**molality **RELATED: molarity ♦ A way of expressing the
ratios
of the amounts of substances in solution. The molality of a solute is
the number of mols of solute per kg of solvent. In physiology, dealing
with aqueous solutions, this is for most practical purposes almost the
same as the number of mols per litre of solution (i.e. the 'molar'
concentration of the solution). 'Molarity' is the simpler and more
commonly used concept, though there are some advantages in relation to
thermodynamics in expressing concentrations as 'molalities'. Also it is
useful where a significant fraction of fluid volume is non-aqueous -
e.g. lipid or protein.

**molarity **ABBREV: mol/l, M RELATED: concentration ♦ A
common
way of expressing concentrations. The unit of molarity is the mol/l,
often abbreviated as 'M', or 'molar'. A solution having a concentration
of 0.2M (=200mM) contains 0.2mol of solute in each litre of volume.
That
is the same as 0.2mmol in each ml, etc.. The molar concentration is the
concentration in g/l divided by the molecular weight of the solute.

**mol, mole **RELATED: molarity ♦ A unit of quantity of a
substance. If the molecular weight of a substance is X, then 1mol is X
grams. For example, the MW of NaCl is 48.5 (=23+35.5), and therefore
1mol of NaCl is 48.8g. A 1M solution would therefore contain 48.8g of
NaCl in 1litre. You can usually find the molecular weight of a compound
(or 'formula weight', e.g. for a hydrated crystalline compound such as
MgSO_{4},7H_{2}O) by looking on the label, to save you
going to a set of chemical tables to work it out.

**monotonic **A relation whose gradient has the same sign at
all points, at least within a specified range. Linear, exponential and
sigmoid relations are all monotonic. The relation between rate of
sweating and body temperature is monotonic within the physiological
range. That between mental performance and body temperature is not
monotonic (performance declines at both high and low temperatures).

**morbid **Diseased. Distinguish 'morbidity' (the extent to
which something, for example an infection, causes disease) from
'mortality' (the extent to which it causes death).

**motility **Movement, particularly of cells or parts of cells.

**motor **Associated with movement, or production of force,
e.g. motor nerves, vasomotor tone (action potentials producing a steady
contraction of blood vessels).

**M-current **Outward K^{+} current through channels
that are opened by prolonged depolarisation and closed by acetylcholine
acting at muscarinic receptors. Possibly important in control of
adaptation and excitability of several types of nerve cell.

**nano- **ABBREV: n RELATED: SI units ♦ One thousand
millionth.
10^{-9}.

**natri- **Relating to sodium (Latin = Natrium, symbol Na).
E.g. Natriuretic factor = something leading to extra sodium in the
urine.

**natural logarithm **ABBREV: ln(x) RELATED: exponential
function ♦ Logarithm to the base 'e' (approx. 2.718). If y=ln(x), then
x=e^{y}. A natural logarithm is 2.303 times the ordinary
logarithm of the same number: ln(x)=2.303log(x). It is 'natural'
because
in certain respects it is mathematically simple. For example, ln(1+δ)
where δ is a small fraction (<<1) is approximately equal to δ
(whereas log_{10}(1+δ) = 0.43 δ ). The gradient or derivative
of
ln(x) is 1/x. Hence the integral of 1/x is ln(x).

**negligible **RELATED: minimal ♦ Capable of being neglected,
not worth considering. Less than minimal. Note that 'negligible' may
only be meaningful in a clearly defined context. What can be neglected
from one point of view may be important in another context. E.g. a
negligible alcohol consumption in relation to its effect on behaviour
may not be negligible if you are trying to study alcohol metabolism.

**nephro- **RELATED: renal ♦ Relating to kidney. E.g.
nephritis= inflammation of kidney.

**Nernst equation **RELATED: equilibrium potential, Goldman
equation ♦ This is the equation to calculate the equilibrium potential
for a particular ion, i.e. the voltage difference across a membrane
that
would be just big enough to counter any net flux of that ion due to a
concentration gradient. If ion X (e.g. Na^{+}) has
concentrations C_{i} and C_{o} on the inside and
outside
of the membrane, and a charge *z* (e.g. *z*=1 for Na^{+},*z*=-1
for Cl^{-}), then the equilibrium potential on the inside
(relative to the outside) is:

V_{eq(X)} = - RT/(zF) log_{e}(C_{i}/C_{o})

where R is the gas constant, T the absolute temperature and F Faraday's
constant. At 20^{o}C the value of RT/F is 25mV. Converting the
log_{e} to log_{10} (multiplying by 2.303), the
expression becomes V_{eq(X)} = -58mV /z log_{10}(C_{i}/C_{o}).
At 37^{o}C the constant is 61mV instead of 58mV.

**neural network **A system of neurons that are coupled
together in such a way as to process information. For example, a
network
might produce activity in one particular neuron only when a certain
class of patterns is presented ('classification' or 'feature
detection'), or it might generate a pattern of outputs (perhaps leading
to a coordinated movement) whenever a simple input is received. The
study of the behaviour of neural networks (real nervous systems,
simulated nervous systems and artificial systems resembling neurons in
some respects) is a large area of information science that bridges
neuroscience, computer science, physics and engineering.

**newton **SI unit of force, that which would accelerate a 1kg
mass by 1ms^{-2}. The force that gravity exerts (at sea level)
on a mass of about 102g. [It is reputed to be a coincidence that this
is
close to the mass of a 17th century Lincolnshire apple, said to have
triggered an understanding of such matters.]

**NMR **RELATED: Magnetic resonance ♦ Nuclear magnetic
resonance. The same as 'magnetic resonance'. The technique is based on
the physics of the nuclei within certain atoms. The word 'nuclear' has
recently tended to be dropped from the terminology in hospitals,
because
it is reputed to scare patients! The technique has nothing to do with
radioactivity, however: which is presumably what patients might be
scared of.

**nomogram **A graphical device for solving equations. A chart
is prepared specially for solving a particular problem. You draw on
this
a straight line between points corresponding to known values, and you
find the unknown value by reading where your line crosses one of the
prepared lines on the chart. For example, the pH of the blood is
affected, according to quite complex formulae, both by the chronic
condition of a patient (reflected by 'metabolic' acidosis or
alkalosis),
and by the current rate of ventilation and CO_{2} levels. If
you
know the pH and the PCO_{2}, you can use a nomogram to find the
degree of metabolic acidosis directly.

**non-invasive **CONVERSE: invasive ♦ A technique that doesn't
require entry into the tissue being studied, or damage to it. Many
recent medical advances have arisen through the development of
non-invasive techniques that assist diagnosis (e.g. Magnetic resonance).

**non-parametric test **A statistical test that does not
require assumptions about the probability distributions of the measured
parameters. It may use, for example, just the rank order of a set of
data. Such tests are much used in psychology.

**normal **RELATED: physiological, pathological ♦ 1. Not a
sign
of disease or trauma, or a risk factor for disease. E.g. A normal range
of blood pressure or body weights. Definitions of 'normal' are often
arbitrary or contentious: where is the borderline of 'unusual
behaviour'
and 'mental illness'? 2. A chemical unit meaning 'containing 1M H^{+}
or OH^{-} ions'. 3. Perpendicular.

**normalise **To divide values for some measured variable by
another parameter, in order to take account of some known relationship.
For example, if you wanted to see if male and female muscle can produce
the same force, you might measure the forces produced by muscles in men
and women. Since you know, however, that a larger muscle can produce a
larger force, you would have to normalise the data with measurements of
the cross-sectional area of the muscles (and perhaps the lever ratios).
Otherwise you might come to the wrong conclusion because of differences
in the size and geometry of muscles in men and women.

**normo- **RELATED: eu- ♦ Normal. E.g. normocapnic= having
normal CO_{2} levels.

**notch filter **RELATED: band pass filter ♦ An electrical
filter that cuts out a narrow range of frequencies. Most commonly used
to cut out mains hum (at a well defined frequency), while passing other
signals of interest.

**noxious **A stimulus that is painful or unpleasant.

**null hypothesis **RELATED: statistical test, significance
limits ♦ A hypothesis that there is no real effect underlying a
difference suggested by data, and that the apparent effect is due to
random variations within the data. A null hypothesis must be clear, and
in some circumstances may have to be quite precise and quantitative.
For
example, if you know from large surveys that the mean male-female
height
difference between adult siblings in England is 12cm and you make a
small survey in France that gives a mean difference of 15cm, your null
hypothesis might be that the difference in France is really 12cm. In
this way you could test whether there is a significant difference
between the two populations in this respect.

**numerator **RELATED: denominator ♦ The top line of a
quotient. A in the expression A/B. Avoid the word unless you are quite
sure not to confuse it.

**numerical aperture **(N.A.) RELATED: resolving power, depth
of focus ♦ A quantity that describes the light gathering power and the
resolving power of a lens. The N.A. is greater with a larger diameter
lens, and also if the medium between the object and the lens has a high
refractive index (as with an oil immersion microscope objective). This
increases the resolving power (i.e. means that smaller objects can be
distinguished) but reduces the depth of focus of the lens (so that
objects slightly out of the focal plane are blurred). In a camera, the
"f-stop" number (e.g. f16) is 1/N.A.: a small f number (e.g. f1.8)
gives
you lots of light, but little depth of focus.

**numerical methods **RELATED: analytical ♦ Methods for
solving
mathematical problems in specific cases (with known parameters), by
using calculations and approximations. Usually this requires a
computer.
For example, you can integrate any function numerically; there are some
functions that you cannot integrate analytically (e.g. exp(-x^{2})).

**observation **A measurement, or observation of a qualitative
feature. Observations of the responses to skin trauma might include
changes of skin colour, swelling, changes of heart rate, vocalisation,
etc.

**occlusion **A blockage in for example, a blood vessel. The
response to one stimulus is sometimes said to 'occlude' the response to
another if the response to both stimuli presented together is less than
the sum of the two separately. This occurs in the nervous system, for
example, where each stimulus induces an all-or-none response in some of
the same neurons.

**oedema **RELATED: = edema (US) ♦ Swelling

**offset **RELATED: shift ♦ An 'offset' control on a recording
instrument allows you to add or subtract an adjustable steady signal at
the input to the instrument. This has a similar effect to a 'shift'
control (operating at the output), in that it shifts the record up or
down. The amount of the shift increases in proportion to the
sensitivity
of the instrument, however. An offset allows you to but it shifts the
record by an amount that

**off-scale **A measurement that is beyond the range that is
satisfactorily registered by a measuring instrument. This may lead to
disappearance of the record (as usually on an oscilloscope) or to
'saturation' of the record so as to give a flat recording, as on a pen
recorder. In these circumstances it may be possible to use a 'shift' or
'offset' control to bring the recording on-scale. Alternatively, it may
be convenient and satisfactory to use 'AC-coupling' of the signal.

**Ohm's law RELATED: resistance, conductance, current, potential
difference ♦ This states that current through a material is
proportional
to the potential difference (or voltage) across it: I=V/R **where R
is a constant called the electrical resistance. This relation can also
be written as V=IR or R=V/I. Ohm's law holds closely for most ordinary
materials, like metal wires and ionic solutions (electrolytes). It
doesn't hold for many special electrical components like rectifiers, or
for membrane channels, or even for ordinary wires if you pass so much
current through them that they get hot (e.g. light bulbs): these things
have a different resistance under different conditions.

**olfactory **Relating to smell.

**operator **A mathematical symbol indicating an operation.
Some operators are 'unary', i.e. act only on one number. For example, -
and ! indicate respectively the negative and the factorial of a number.
Some are 'binary', acting on two numbers, e.g. +,× (or *),/,^
indicating addition, multiplication, division and exponentiation.
Conventional formulae often use the relative position of numbers to
indicate operations (e.g. superscripts to indicate exponentiation).
Formulae cannot easily be put into a computer in this form, so
conventions are adopted that use symbols all on one line (e.g. /, ^,
and
* instead of × to avoid confusion). Use brackets whenever there
might be ambiguity without brackets: e.g. 3/(2*x), not 3/2*x.

**order of magnitude **A useful but imprecise expression. Be
careful with it. 1. Used to indicate that quantities are so different
in
size that the difference itself rules out some hypotheses. E.g. 'The
size of a large mammal is a quite different order of magnitude from the
distances over which diffusion can take place'. 2. Sometimes people use
'order of magnitude' to mean 'roughly a factor of ten'. E.g. 'an
elephant (2000kg) is about 5 orders of magnitude heavier than a shrew
(ca. 15g)'. 3. Mathematicians use 'order' in a more precise way, to
indicate that a ratio tends asymptotically to zero or 1 under specific
conditions.

**ordinate **ABBREV: Y axis RELATED: abscissa ♦ The vertical,
or Y axis, usually used for plotting a dependent variable in an
experiment.

**ortho- **Straight, normal. E.g. orthodontics = straightening
of teeth.

**orthogonal **Perpendicular.

**-osis SUF **Diseased condition involving the preceding part
of the word. E.g. Psychosis, tuberculosis.

**osmolarity **ABBREV: osmol/l RELATED: tonicity ♦ This is the
sum of the molar concentrations of all the ions or molecules in
solution. 150mM NaCl, 100mM Na_{2}SO_{4} and 300mM
sucrose each have the same osmolarity, 300mosmol/l. The osmolarity is a
measure of how many particles are in solution. If two solutions are
separated by a membrane that is permeable to water, water will diffuse
into the solution with greater osmolarity (i.e. more particles in
solution), unless there is a pressure difference to oppose this
movement.

**output **RELATED: projection CONVERSE: input ♦ A structure
leading from a system or something (a chemical, signal or a form of
energy) produced by the system.

**overshoot **RELATED: undershoot, transient ♦ Part of a
response to a continuing stimulus, in which the response transiently
exceeds the level at which it finally settles. Note that it is not
really the converse of an undershoot.

**paired comparison **RELATED: student's t test ♦ A comparison
in which measurements under one condition are paired in some way with
measurements under another. This is to ensure that apart from the
deliberate difference in conditions, each pair of measurements has as
much as possible in common. For example, the paired measurements might
be made on the same subject. It isn't always possible, however, to use
the same subjects twice: for example, you may be comparing different
treatments for a disease. In such cases you may be able to 'match'
subjects to some extent (e.g. for similar age, sex and symptomatology).
Differences between the paired measurements are calculated, and
statistical tests carried out on the mean difference.

**para- **Alongside, resembling. E.g. E.g. parathyroid= gland
beside the thyroid, paraesthesia= abnormal (but not absent) sensation,
paramedical= alongside medicine. Can also mean a defence against
something (e.g. parasol), and in chemistry two positions opposite each
other on a molecule.

**paradigm **An idealised example. The logic that defines a
difficult concept, for example how one defines 'anaemia', 'alertness'
or
'clinical depression'.

**parameter **RELATED: quantity, unit, dimension ♦ The value
of
a quantity, including its units.

**partial derivative **RELATED: derivative ♦ The derivative
with respect to one parameter (with the others fixed), of a function
that depends on two or more parameters. For example, the concentration
of a chemical in solution depends in general on position and time. It
has partial derivatives with respect to time (i.e. the rate of change
of
concentration at each place) and with respect to the three directions
in
space (i.e. the concentration gradients in each direction at any one
time). A partial differential equation (the diffusion equation) relates
these derivatives.

**partial pressure in a gas mixture** RELATED: vapour pressure
♦
The portion of the total pressure of a gas mixture that is due to a
particular constituent. The total pressure is the sum of the partial
pressures, and the proportions are the same as the proportions of the
quantities of gas, either by numbers of molecules or moles, or by
volume
(measured at a fixed pressure). For example, normal expired air
contains
(after drying) about 5% CO_{2} by volume (5% of the molecules
are
CO_{2}). The CO_{2} partial pressure (P_{CO2})
is therefore about 0.05 of an atmosphere (38 mmHg or 5.0 kPa). Since
part of the pressure in the alveoli (about 47 mmHg or 6.3 kPa) is water
*vapour
pressure* the CO_{2} partial pressure in the alveoli would
be
less: 0.05 of (760-47) mmHg = 36 mmHg, or 0.05 of (101-6.3) kPa = 4.7
kPa

**partial pressure in a solution** RELATED: partial pressure in
a
gas mixture ♦ The partial pressure of a gas (like O_{2} or CO_{2})
in a solution is the partial pressure in a gas mixture *in
equilibrium with* the solution. It is *not* directly related
to
how much of the gas is in the solution. Under normal conditions,
arterial blood has P_{O2} = 100 mmHg and P_{CO2}
= 40 mmHg, close to the values in the alveolar gas mixture with which
it
has equilibrated. At these normal partial pressures, the amount of O_{2}
and CO_{2} in the blood may vary depending on how many red
cells
and much haemoglobin the blood contains.

**passive **RELATED: electrochemical gradient CONVERSE: active
♦ A process or event that doesn't require energy, or that doesn't
require the active participation of some system. It is often wise to
clarify the senses in which 'active' and 'passive' are being used. For
example, it is perfectly correct to say that an action potential
(representing neural 'activity') is a purely passive event, in that it
involves nothing but the passive movement of ions. In the context of
membrane transport, passive movement is transport down an
electrochemical gradient. Passive movement of a limb is movement
induced
elsewhere, not by contraction of the muscles that normally act on the
limb.

**pathogen **An organism or chemical that causes disease.

**pathological **Diseased, disordered, or abnormal.

**pathophysiology **Study of the function of tissue under
pathological conditions.

**pCa **RELATED: pH ♦ A way of expressing very low calcium
concentrations, by analogy with pH. pCa=-log_{10}(molar Ca
concentration). E.g. pCa 5 means [Ca^{2+}] = 10^{-5}M.

**peak **CONVERSE: trough ♦ A point on a graph that is higher
than those on either side (i.e. a 'maximum').

**percentage **ABBREV: % RELATED: proportional ♦ A quantity
expressed relative to a reference as 100%. It is most commonly used to
describe changes. Note carefully that a '150% increase' means a final
value 2.5 times the original, while an 'increase to 150%' means a final
value 1.5 times the original. Unless otherwise stated, the initial
value
immediately before the change is always the reference. Thus if a
parameter falls by 40% and then rises by 40%, its final value is 84% of
the initial value, not 100%. There is obviously a risk of
misunderstanding in this kind of expression, so it is usually best to
make it exactly clear what you mean, e.g.: 'The parameter fell to 60%
and then rose to 84% of its initial value'.

**percentile **The X% percentile within a distribution is the
value below which X% of the data points lie.

**perception **Awareness of something, especially a sensation
due to a sensory input. There are many sensory inputs that do not lead
to conscious perceptions (e.g. afferents from arterial baroreceptors
and
chemoreceptors). Curiously, subjects can deny any perception, or any
knowledge at all of a stimulus, yet be able to identify accurately some
of its properties when forced to guess what they are (e.g. in the
phenomenon of 'blindsight' in patients who have total lesions of the
primary visual cortex).

**perfuse **RELATED: superfuse, infuse ♦ To pass fluid
through.
E.g. an isolated salivary gland might be perfused with saline.

**peri- **RELATED: epi- ♦ Surrounding, around. E.g.
perineurium= sheath around nerve, perinatal= around the time of birth.

**perinatal **Around the time of birth.

**permeability **RELATED: diffusion, transport ♦ Passage of a
substance by diffusion across a membrane. A membrane is said to be
'permeable' to a substance; the substance is said to be 'permeant'. The
permeability of a membrane only allows a 'passive' net flux to take
place, i.e. in the direction down the electrochemical gradient of the
permeant substance. If net movement occurs in the other direction, the
substance must be subject to 'active transport'. Quantitatively, the
'permeability coefficient' for an uncharged substance, or for an ion
under conditions when the membrane potential is zero, is the flux per
unit area, divided by the concentration difference between the two
sides
of the membrane (dimensions: LT^{-1}).

**personal computer **ABBREV: PC) ♦ A computer used by just
one
person, as opposed to a computer that is connected to many 'terminals'
for separate users, or a set of computers that are all connected
together in a 'network'. An 'IBM-compatible' PC is a computer on which
you can use the same software as on a common type of PC made by IBM.

**pH **RELATED: pCa ♦ A measure of acidity. pH = - log_{10}(molar^{H+}
concentration). Note (a) that it is a logarithmic measure, so a change
of 1 unit in pH corresponds to a 10-fold change in ^{H+}
concentration, and 0.3 units to a 2-fold change in concentration. (b)
it
is an inverse measure: low pH corresponds to high ^{H+}
concentration and high acidity. It is easy to remember that pH=7
corresponds to 10^{-7} M and pH=8 to 10^{-8} M. Normal
plasma pH, ca. 7.4, is of course in between.

**phage SUF **Eating, engulfing. E.g. phagocyte.

**phase **RELATED: time course, amplitude ♦ 1. One of the
parts
of the time course of an event. For example, the rising or falling
phase
of an action potential; the recovery phase after exercise. 2. For a
sinusoidal function: a parameter that indicates where you are on the
waveform, relative to when the waveform crosses the axis in a
positive-going direction (phase=0). This is measured as an angle:
2π´t/T radians or 360×t/T degrees, where t is the time
relative to the crossover and T is the period. A 'phase shift' is the
amount you have to shift one sinusoidal waveform relative to another
(of
the same frequency) to make them have the same crossovers.

**photomultiplier tube **An extremely sensitive light sensing
device that transduces a weak light signal into an electrical signal.

**physiological **CONVERSE: unphysiological ♦ Characteristic
of
the normal workings of the tissue. If an experimental condition or a
type of stimulation is said to be 'physiological', this means that it
is
within the range of conditions normally experienced by the tissue
within
the animal. Experiments under 'unphysiological' conditions are often
enormously helpful in understanding how the tissue works. For example,
study of the force produced by muscle cells when stretched farther than
the skeleton will normally permit them to be stretched (i.e. beyond
their 'physiological range') were instrumental in establishing the
sliding filament theory of muscle contraction.

**-physis SUF **Growth. E.g. hypophysis= outgrowth under the
brain, i.e. pituitary.

**pi **π = 3.142 (approx.) For reference: Circumference of a
circle = 2π´r. Area of circle = π´r^{2}. Surface of
sphere = 4π´r^{2}. Volume of sphere=(4/3)×p´r^{3}.
Volume of cone = (1/3)π´r^{2}×h.

**pico- **ABBREV: p RELATED: SI units ♦ One million millionth.
10^{-12}.

**pixel **The smallest unit capable of having a range of
colours or shades, in a picture made up of dots. The picture on a
computer screen usually consists of at least 640x400 pixels.

**placebo **An inactive chemical given, as a control, to a
subject in an experiment. New drugs cannot be claimed to be effective
unless they are better than a placebo in a controlled, preferably
'blind', study. Many clinical conditions show a surprising improvement
in many subjects following administration of a placebo (a 'placebo
effect'). It has been suggested that the size of a placebo effect is a
measure of the social and psychological skills of a doctor, since
undoubtedly these play a part in therapy, and with a placebo there is
nothing else to help.

**plasma **RELATED: serum ♦ The extracellular fluid of the
blood, in which the cells are suspended. If blood is prevented from
clotting, the cells will settle out leaving the relatively clear,
yellowish plasma.

**plasticity **The capacity to be modified after the changes
associated with development have occurred.

**plateau **A horizontal part of a graph, where the vertical
parameter ceases to be influenced along with changes in the horizontal
parameter. Many dose-response curves exhibit a plateau at high drug
concentrations, because the concentration is more than enough to
interact fully with all the receptors practically all of the time.

**pool **Pooled data is data analysed together, ignoring some
differences in conditions, or in how it was gathered. For example, you
might look at the effects of a heavy meal on exercise performance,
including both male and female subjects because you thought there might
be an interesting difference. If you then found no significant
difference between males and females, you might pool the data from all
the subjects to get the best estimate of the effects of the meal itself.

**post- **After, behind. E.g. post-ischaemic hyperaemia =
increase of blood flow after a cessation of blood flow.

**posterior **Near or nearer to the back end or tail.

**potential difference (p.d.) **RELATED: voltage,
electrochemical potential ♦ You can think of this as the driving force
that is tending to push electric charge from one place to another
through an electric conductor. It is like a more familiar temperature
difference, which makes heat flow through a thermal conductor.
Potential
difference is measured in volts (V). Energy is dissipated (i.e. turned
into heat or work) when a charge moves between places with a p.d.: 1
Joule is dissipated if 1 Coulomb moves between places with a p.d. of 1
Volt. Conversely, it takes this much energy to move a unit of charge
(1 C) in the opposite direction. Thus 1 V =
1 Joule/Coulomb.

**potentiation **RELATED: facilitation CONVERSE: depression ♦
An increase in the power of something; a persistent increase in the
amplitude of a response. The increase may last for a few milliseconds
after the cause of potentiation, or for many days (for example, the
potentiation of muscle strength that results from training, or the
'long
term potentiation' of synaptic strength that may be involved in
memory).
'Facilitation', on the other hand does not outlast whatever induces it.

**power **In mathematics: = 'exponent'. In physics and
physiology, it also means rate of doing work or rate of expending
energy
(measured in Joules.s^{-1}, or Watts).

**power law **RELATED: logarithm ♦ A relationship in which one
parameter is proportional to the other raised to a fixed power:
y=a×x^{p}. If you study animals of different sizes, you
will find that many of their parameters scale with size according to an
approximate power law. For example if x is body mass, metabolic rate
and
heart rate follow approximate power law relationships with p=0.72 and
p=-0.25 respectively. (Note that the relation for heart rate is an
inverse relation: heart rate is faster in a mouse than an elephant.)
You
can verify whether something is a power law relationship by plotting
log(y) against log(x): you get a straight line with a slope of (p) if
you have a power law: log(y)=log(a)+p×log(x).

**prandial **Relating to a meal.

**preparation **A part of an animal, tissue, or cell, made
ready for experimentation.

**pressure **RELATED: partial pressure ♦ The degree to which a
fluid pushes on boundary surfaces. It is measured as force per area, in
Pascals (Pa: 1Nm^{-2}). Alternative units are millimetres of
mercury (mmHg: 133 Pa), standard atmospheres (760 mmHg or 101 kPa) or
bar (100 kPa). Often, (e.g. when talking about arterial presure)
"pressure" really means "relative pressure" - the excess above the
external atmospheric pressure. Pressure differences between parts of a
fluid, unless caused and balanced by gravity, cause fluid flow. The
total pressure of a gas mixture (e.g. air) is the sum of *partial
pressures* of its constituents.

**principal **RELATED: principle ♦ Distinguish these
carefully.
"Principal" means chief - either as an adjective (*principal features*)
or as a noun (*school principal*). As an aid to memory, think of a
pal who is a prince and principal of a principality!

**principle **RELATED: principal ♦ Distinguish these
carefully.
A "principle" is a basic idea or mechanism. *New medical principles*
would mean advances in medicine. *New medical principals* would
mean a clearout of senior staff!

**process **(1) An event or set of events. (2) A long thin
branch of a cell (e.g. the dendrites and axon of a neuron).

**profile **A graph plotted against position, showing the
distribution of a parameter. For example, you might plot the profile of
temperature along a diameter of the arm.

**prognosis **The predicted course of a disease or disorder.

**program **RELATED: language ♦ A set of instructions that can
be obeyed by a computer. Programs must be written in a specific
'language'. Computers are of no value without programs. Sophisticated
programs like wordprocessors or spreadsheets can allow the user to
perform complex tasks by giving commands in a form that is easier to
understand than a programming language. The word 'program' in relation
to computers is usually spelt in this way even in England, where the
ordinary word 'programme' (as in a 'programme of work') is spelt
differently.

**prophylactic **A drug or a preventative measure taken to
avoid disease or infection.

**proportional **RELATED: linear, fractional change,
percentage
♦ A relationship between two parameters in which they always change
together by the same factor: if you double X, you double Y, etc. They
change 'in proportion'. The equation is of the form y=a×x, for
which the graph is a straight line through the origin. An inverse
proportional relationship is one in which doubling X halves Y, etc.,
i.e. y=a/x. The graph for this is a hyperbola. A 'proportional change'
in a parameter is the same as a 'fractional change', i.e. (new value -
old value)/(old value).

**proteinuria **Presence of protein in urine, sometimes
indicative of kidney or heart disorders.

**protocol **The plan for an experiment.

**pylorus **The narrow end of the stomach (pylorus) leading to
the duodenum.

**Q _{10} **The coefficient often used to express the
temperature dependence of chemical, biological or physical processes.
It
is the factor by which the rate of the process increases when there is
a
10oC rise in temperature. The Q

**quadratic **RELATED: linear ♦ A relation of the form y=ax^{2}+bx+c.
The last two terms are a general linear function. The first makes it
curvilinear, in fact in the shape of a parabola. This is one of the
simplest non-linear functions that you may use to try to get a better
fit to some data than by using a straight line.

**quadratic formula **The formula for solving a quadratic
equation. If ax^{2}+bx+c=0, then x=(-b±sqrt(b^{2}-4ac))/(2a)

**qualitative **RELATED: quantitative ♦ Relating to the nature
or qualities of something, not just to the values of parameters
associated with it. For example: 'It is uncertain whether there are
qualitative, not just quantitative, differences between the brains of
monkeys and those of humans.'

**quantitative **RELATED: qualitative ♦ Relating to the value
of a parameter.

**quantity **RELATED: parameter ♦ Something you can, or in
principle could, measure.

**quartile **RELATED: median ♦ A quartile is the value within
a
distribution, beyond which only 25% of the data lies. A distribution
has
upper and lower quartiles, the 25% and 75% percentiles.

**quotient **RELATED: denominator ♦ One parameter divided by
another. If the parameters have dimensions (i.e. are not simple
numbers), then the dimensions and units of the quotient are those of
the
top line (numerator) divided by those of the bottom (the denominator).
E.g. 'peripheral resistance' is the quotient of arterio-venous pressure
difference and blood flow: its units are those of pressure (Pascals)
divided by those of flow (ml/min), i.e. Pa.ml^{-1}.min.

**q.v. **quod vide (LATIN) = "which see": Refers you to
something else you could look up, usually a definition (q.v.) in a
dictionary or glossary.

**radian **A unit of angle. 1 radian = 57.3 degrees (180 deg /
π). The radian is a natural unit. The distance along the curve of a
circle is simply radius × angle subtended (in radians). If x is
in
radians, sin(x)=x for small angles (within 5% for angles less than 30
degrees, or 0.5 radians). If x is in radians, the gradient of the graph
y=sin(x) is cos(x).

**rate constant **RELATED: time constant ♦ If the rate of
change of a quantity is proportional to its present value, then the
'rate constant' is the proportionality constant: rate of change = rate
constant × value. The resulting graph of the quantity against
time
is exponential, with a 'time constant' equal to the reciprocal of the
rate constant. The dimensions of a rate constant are time^{-1}.
In complex dynamic systems, in which several influences may tend to
cause changes in a parameter (e.g. in complex chemical reactions), each
influence may be separately characterised by a 'rate constant'; the
graphs of the parameters (e.g. concentrations) in such situations are
not generally exponential curves.

**rate of change **Gradient. Most often but not always refers
to the gradient of something plotted against time. You might talk of
the
rate of change of sweat production with temperature.

**ratio **RELATED: quotient ♦ The numerical relationship
between two things, regardless of units. It is nonsense to talk about
the ratio of two quantities unless they have the same dimensions. E.g.
The ratio of males to females in the population is 48 : 52, or 1 :
1.08.
People sometimes give just one number for a ratio. If you do, it must
be
the quotient of the first thing divided by the second. It would be very
confusing to say the ratio of males to females was 1.08 in the above
situation. In general, a ratio expressed with one number is likely to
elicit the question: 'Which way round?'

**reactance (capacitative) **The equivalent of resistance, but
for a capacitor instead of a resistor. Its units are ohms. Current will
only flow in and out of a capacitor if you apply an ALTERNATING voltage
to it. Therefore, its reactance is infinite for a steady voltage (i.e.
if the frequency f=0). The higher the frequency, the lower the
reactance: X = 1/(2πfC) where X is the reactance (in Ω), C the
capacitance (in Farads) and f the frequency (in Hz). Reactance is the
ratio of the peak voltage to the peak current (X=V/I) just like Ohm's
Law, though the voltage and current are 90degrees out of phase.

**receptor **(1) A binding site, usually on a cell membrane,
for a particular chemical or class of chemicals (ligands). Some
functional property of the cell is usually altered depending on whether
the binding site is occupied or empty (e.g. an ion channel may be
opened
or shut). (2) A sense organ or sensory cell: a structure that is
affected by changes in the external or internal environment of an
organism and that produces signals (usually action potentials) that
influence other parts of the organism.

**reciprocal **ABBREV: x^{-1} RELATED: inverse ♦ The
reciprocal of x is the quantity that when multiplied by x gives 1. This
may be a simple number, or it may be a quantity with dimensions, or a
unit. For example electrical conductance is the reciprocal of
resistance
and is measured in reciprocal ohms (also known as mhos, siemens).

**recording **A continuous measurement, usually plotted as a
graph against time.

**rectifier **An electrical device that has a low resistance
for current in one direction and a high resistance in the other
direction. Often a semiconductor diode.

**recurrent **In the nervous system, 'recurrent collaterals'
are branches of axons that come back into a structure from which they
originated. Likewise, 'recurrent inhibition' means inhibition of a set
of cells that results from activity in the cells, amounting to a form
of
negative feedback. 'Recurrent' also has the ordinary meaning of
something that repeats again and again.

**reflex **A response to a stimulus in which the signals are
relayed by neurons, and are not normally subject to voluntary control.

**reflex arc **The set of structures involved in a reflex:
usually afferent and efferent nerves and one or more sets of synapses.

**refractive index **A quantity (n) that describes the ability
of a transparent material to refract (bend) light.

**refractory period **A period in which a cell or tissue is
unresponsive to stimuli (absolute refractory period) or has a raised
threshold (relative refractory period) following a preceding period of
activity.

**regression **RELATED: correlation ♦ The line of closest fit
to a graph exhibiting a correlation. This is usually a straight line
('linear regression'), though sometimes a quadratic, exponential, or
other curve may be fitted if the data shows a clear relationship that
departs from a straight line. The criterion for 'closest fit' is
usually
that the sum of the squares of the deviations of all the points from
the
line (either horizontally or vertically) is a minimum. If a correlation
is not statistically significant, then a regression line is not, in
general, of any interest and it is a bit naughty to plot it since this
may suggest there is a relationship evident in the data.

**regulate **RELATED: control ♦ To control, in the sense of
keeping something constant (e.g. body core temperature) or constantly
adjusted to suit current requirements (e.g. fluid excretion in the
kidney).

**relationship **RELATED: correlation ♦ Two quantities are
related if a graph shows that knowledge of one enables you to predict,
to some extent, the value of the other. The line of the graph must,
over
the range considered, be neither horizontal nor vertical. The
relationship may be strict (if the graph is a definite line through all
the points) or statistical (if the points are scattered, and the
prediction is simply a statement about the probability distribution of
the second parameter). A statistical relationship is usually called a
'correlation'.

**relative humidity **Water vapour pressure as a percentage of
saturated water vapour pressure. In comfortable conditions about
50-70%.
Relative humidity increases as air is cooled, until it reaches 100%
(dewpoint), when condensation occurs.

**relative refractory period: **see refractory period

**renal **Relating to the kidney.

**resistance (electrical) **RELATED: Ohm's law ♦ A measure of
how much something (e.g. a wire or a resistor) "resists" the passage of
electric current. The bigger the resistance, the bigger the voltage
(potential difference) needed to produce a given current: R=V/I. Units:
ohms or Ω (for V in volts and I in amps). For many objects and
circuits,
R is nearly or exactly constant. In other words, the current is
proportional to the potential difference. This is a statement of Ohm's
Law. But note that Ohm's Law (q.v.) does not hold for everything.

**resistance vessels **The arterioles and small arteries,
which
provide the greatest resistance to blood flow of all the vessels
through
which the blood flows in sequence through a tissue. Consequently, the
biggest drop of pressure is along the resistance vessels.

**resistivity **RELATED: conductivity ♦ An intrinsic property
of an electrical conductor. It is the resistance of a section with unit
cross-sectional area and unit length. Thus the resistance along the
cytoplasm of a length L of an axon with cross-sectional area A and
resistivity &ro; would be &ro;L/A. Units of resistivity are
Ω.m.
The resistivity of tissue fluids is typically 0.5-2Ω.m, while for
copper
metal it is 0.02mΩ.m, i.e. 10 million times less. Conductivity is
1/resistivity (units Ω^{-1}m^{-1} or Siemens/m).

**resolution **1. The degree of detail contained in a visual
image (e.g. expressed by the number of pixels). 2. The acuity or
resolving power of a sensory system or an instrument. 3. The
termination
of an acute phase of a disease, especially a stage at which
inflammation
disappears.

**resolution of measurement **RELATED: accuracy ♦ Ability of a
measurement to distinguish slightly different conditions. Resolution
may
be limited by the size of incremental steps in the measurement (e.g. a
digital pH meter may give readings to 2, 3, or 4 decimal places) or by
inconsistencies in behaviour. It may be possible to identify the
sources
of some inconsistencies and reduce them (e.g. by placing the instrument
in a standard state immediately prior to a measurement). Limitations
due
to fluctuations that are truly random can usually be improved by a
factor SQRTn by averaging n independent measurements. Resolution is
just
one of the factors that limits the accuracy of a measurement.

**resolving power (resolution) of a microscope **The minimum
distance between two objects at which they appear as separate objects
when viewed through a microscope.

**respiration **(1) The act of breathing. (2) metabolic
reactions involving oxygen as a reactant.

**response **RELATED: stimulus ♦ A change caused by a stimulus.

**retrograde **Moving backwards or in an abnormal direction or
(as in retrograde axonal transport) in the opposite direction to
something else (the action potentials).

**Reynold's number **ABBREV: R RELATED: turbulence ♦ A
dimensionless number that characterises the transition from laminar to
turbulent fluid flow in a particular geometrical situation. For flow in
a straight tube, R = q×p/(π´r×n) where q= volume flow
rate, r= radius, p= fluid density, n= viscosity. If R is greater than a
critical value (ca. 1000) then the flow becomes turbulent. Transition
from laminar flow to turbulence might occur because of an increase of q
(in a tube with constant r) or a decrease of r under conditions with
constant q. Thus turbulence in the aorta (giving a 'heart murmur') may
occur above a critical cardiac output, or when there is narrowing
('stenosis') of the aorta.

**-rhea SUF **Flowing, running. E.g. diarrhea='flowing
through'.

**Ringer solution **RELATED: Krebs solution, ♦ A simple saline
solution that is sufficiently similar to the normal environment of a
tissue that the tissue continues to behave fairly normally when
immersed
in it. Named after Sidney Ringer, who discovered that calcium ions are
an essential component of the physiological environment of cardiac
cells. Appropriate Ringer solutions vary for different tissues and
different species, and according to the needs of a particular
experiment. Ringer solutions are usually not bubbled with a gas mixture
containing CO_{2}, and therefore normally have a lower than
normal bicarbonate concentration to ensure a physiological pH.

**risk factor **Something statistically associated with an
undesired occurrence, such as a disease or accident. This may be a
causal factor (e.g. cigarette smoking in relation to lung cancer) or
something that suggests the possible existence of a causal factor (e.g.
having a relative who suffers from a genetically related disease).

**root **CONVERSE: power ♦ The nth root of x is the number
that, multiplied by itself n times, gives x. This is x raised to the
power 1/n.

**root mean square **ABBREV: rms) ♦ A measure of the typical
size of a quantity, regardless of whether it is positive or negative.
Take all the values. Square them. Take the mean of the squares. Then
take the square root of the mean: this is the rms value. The rms value
has the same dimensions as the original quantity, and may therefore be
measured in the same units. E.g. the rms value of the mains voltage in
the UK is 220-240V. The mean voltage is zero (because it is negative
for
half the time) and the peak values are ± ca. 320V.

**saline **RELATED: Ringer solution ♦ A solution of salts,
usually one suitable for bathing or injecting into tissues. The
simplest
saline for clinical and mammalian use is 0.9% NaCl (approx. 150mM),
which is roughly isosmotic with cells. More elaborate solutions for
maintaining tissues in good physiological condition are often named
after scientists who worked out their formulae, e.g. Ringer, Krebs.

**scalar **RELATED: vector ♦ A quantity that simply has a
magnitude, and no direction in space. For example: concentration is a
scalar, flow or flux is a vector.

**science citation ind **ABBREV: SCI RELATED: Index medicus ♦
A
reference publication in the library. Useful if you want to find
published literature about a specific topic. It lists all the papers
referred to ('citations') in everything that is published during the
year. It is particularly useful if you want to find recent work
following up something published earlier: you can expect the recent
work
to refer to the older work, and therefore to be traceable. Available on
computer operated compact disc (CD ROM).

**-scope SUF **Instrument that allows viewing of something.

**selectivity **RELATED: specificity ♦ Membranes and ion
channels exhibit 'selectivity', or selective permeability: they have a
higher permeability for certain substances or ions.

**serum **RELATED: plasma ♦ The fluid that separates from
blood
when it clots. Approximately equivalent to the plasma without much of
its dissolved proteins.

**shift **RELATED: balance, offset ♦ A 'shift' or 'position'
control on an amplifier allows you to add or subtract an adjustable
voltage at the output of the amplifier so as to bring the recording
on-scale or to a convenient height. The size of the shift is usually
independent of the sensitivity of the amplifier.

**SI units **Standard International system of units. This is
based mainly on metres, kg, seconds, amps, degrees Kelvin. Larger and
smaller units are named in multiples of 1000. Units larger than the
standard are named kilo-, Mega-, Giga-, Tera-. Units smaller than the
standard are named milli-, micro-, nano-, pico-, femto-, atto-. The
prefixes deci-, centi-, deca-, hecto- are separate from the 10^{3}
sequence, and are used rather seldom. You should certainly know the
main
sequence from Mega- down to pico-, and their abbreviations: M, k, m,
µ, n, p. You don't often need the other prefixes in physiology.

**sigmoid **A curved graph that always rises as you move to
the
right, but for which the gradient increases at first and then
decreases.
If you sketched a graph of the gradient, this would rise and then fall.
This is a common shape of graph in biology, for example for
dose-response curves, stimulus-response curves and for chemical
reactions that involve cooperative phenomena, such as the oxygen
dissociation curve for haemoglobin. The word means S-shaped, but it's
really more like the shape of an integral sign.

**sign **RELATED: symptom ♦ Evidence that a physician can
obtain, suggesting a disorder, that is not necessarily evident to the
patient. Some symptoms and signs are not related in any obvious or well
understood way to the nature of the disease.

**signal **The waveform of a parameter that varies in time. In
experiments, physiological signals are often converted into voltages
(electric signals) that can be conveniently processed and recorded.

**significance level **ABBREV: P RELATED: null hypothesis,
statistical test ♦ The probability that, assuming a null hypothesis,
you
would get by chance an effect as large as was seen in a particular
experiment. This is the end result of doing a statistical test. It is
usually expressed, for example, as P<0.01, along with details of the
test. Usually if P>0.05 a result is described as 'not significant'
(n.s.) , meaning that there isn't much certainty at all that the result
was not a chance one. There is no universal answer to what is an
'acceptable' significance level. If it would challenge firm ideas to
reject the null hypothesis, then you must weigh the probability that
these ideas are wrong against the probability P that your results might
be due to chance.

**sine function **ABBREV: sin(x) RELATED: cosine, tangent ♦
The
sine of an angle is the opposite side to that angle in a right angled
triangle, divided by the hypotenuse (longest side). This can easily be
generalised to negative angles and angles greater than 90deg (π/2
radians) by thinking about an arm that sweeps around from the origin of
a graph, making a steadily increasing angle (A) with the horizontal
axis: it always intersects a circle of unit radius around the origin at
a height of sin(A) and at a horizontal displacement cos(A), generating
two sinusoidal functions.

**sink **CONVERSE: source ♦ See 'source'.

**sinusoidal waveform **RELATED: frequency components ♦ A
repetitive waveform with the equation y=a×sin(w×t+p). It is
often but not necessarily a function of time (t), as is described here.
'a' is the amplitude, 'w' the angular frequency (in radians/sec), and
'p' the phase (in radians or degrees). The signal fluctuates between +a
and -a, with a repeat interval (or 'period') of 2π/w sec. The
'frequency' (i.e. how many times the waveform repeats itself per
second)
is w/(2π) Hz. The phase 'p' doesn't affect the shape of the waveform,
but shifts it along the time axis. Note that a cosine function, e.g.
cos(wt), is simply a sinusoidal function with a phase shift:
cos(wt)=sin(wt+π/2).

**sketch **You understand what it is to sketch an object: to
illustrate its main features without necessarily producing a perfectly
neat drawing with exactly straight lines and precise quantitative
relationships. If you are asked to sketch a graph, much the same thing
is expected. It should be clear whether lines slope up or down, which
way they are curved (if at all), and where lines intersect. If
quantitative information is important (e.g. you might want to indicate
maximum and minimum values or normal values), mark a number against the
points: don't try to plot exact axes. Try sketching the heights of
people as a function of age, marking puberty and any other ages
important in the context.

**skewed **A distribution or profile that is not symmetrical
about its mean.

**slope **Gradient.

**smoothing **RELATED: filter, frequency components ♦ A
process
for removing the high frequency components of a graph. This is often
useful if the original data contains a lot of random fluctuation that
obscures some consistent relationship. A histogram can be smoothed by
using larger bin widths. Other graphs can be smoothed by averaging
adjacent points, or by taking a weighted mean of several points around
each value on the horizontal axis. Smoothing leads to 'distortion',
since sharp peaks in the graph become smaller and broader.

**software **RELATED: database, spreadsheet, wordprocessor ♦
Computer programs. These may be highly specific to a task (e.g. for
running a particular piece of equipment), or they may be very general
purpose aids to doing jobs (e.g. wordprocessors). Whether it is easy to
use a computer depends on the software, not on the computer.

**solve **RELATED: explicit, implicit ♦ To 'solve' an equation
means to find the value that a parameter must have for the equation to
be correct. It is always possible to write an equation involving just a
single variable x in the form f(x)=0. A 'solution' is then a value of x
for which the graph of y=f(x) meets or crosses the x-axis. In general
there may be one, or many, or no solutions depending on the shape of
the
graph. If there are solutions, you can always find them by plotting a
graph. Sometimes it is possible to find a formula for an 'explicit'
solution. If there are several (n) unknown variables to be found, then
you need at least n equations to find all the unknowns and you cannot
always solve the problem graphically.

**soma **Body. E.g. cell soma= cell body; somato-sensory =
sensation on the surface of the body (touch, etc.).

**source **CONVERSE: sink ♦ In analysing changes that occur
around electrically active cells, a current 'source' is a region where
current flows out across the membranes of active cells and a 'sink' is
where it flows in. It is sometimes possible to infer the 'current
source
density' from measurements of voltage change at a number of sites
within
the tissue.

**space **RELATED: compartment ♦ A fluid compartment (e.g.
'extracellular space'), or the equivalent volume that a simple fluid
compartment would have to have to account for the distribution of a
chemical. For example, sucrose usually remains largely in extracellular
space, so the 'sucrose space', i.e. the amount of sucrose in the tissue
divided by the sucrose concentration with which it is equilibrated, may
serve as an estimate of the extracellular space fraction.

**space constant **ABBREV: Λ (Greek lambda) RELATED: time
constant, cable theory ♦ Parameters may decline exponentially with
distance, as well as with time. For example, if you give a weak
stimulus
to a long thin cell electrically at one place, then the resulting
change
of voltage across the membrane falls off exponentially as you go
further
away (in either direction). The space constant is the normal way to
measure the degree of spread: the distance for the change to fall to
36.8%, exactly equivalent in other respects to a time constant.

**specific **(1) Relating to a particular one, or a small
group
of things. A specific drug, inhibitor, etc., is one that performs
exclusively, or almost exclusively, the function described. 'Specific'
binding of a substance in a tissue is binding to a receptor that is
selective, usually with high affinity, for that substance. 'Specific'
also may relate specifically to a biological species (e.g. a 'specific
name'). (2) A parameter that characterises a property of a substance or
tissue, irrespective of how much is actually present. 'Specific
membrane
resistance' is the resistance of unit area of membrane (in ohm.m^{2}),
and 'specific conductance' the resistance per unit area (in mho.m^{-2}).

**spike **RELATED: impulse ♦ A brief transient deflection on a
record. An action potential is often referred to as a spike, especially
when recorded on a slow trace, so that details of the timecourse of the
individual action potentials are not apparent.

**spreadsheet **RELATED: database, wordprocessor ♦ A general
purpose computer program to help with calculations and (usually)
drawing
graphs. A quite revolutionary tool for scientists (and for business),
which enables you to do elaborate calculations and to lay the results
out clearly, without learning to write programs.

**standard **1. Something having a parameter with a known
fixed
value, used for calibration purposes, e.g. a standard battery (with a
known voltage) or a standard solution with known concentration. 2. A
way
of assessing something according to an agreed rule, e.g. a standard
deviation.

**standard deviation **ABBREV: s.d. RELATED: variance,
standard
error ♦ This is the commonest measure of the variability of a quantity.
It is the square root of the variance, and is usually more convenient
than the variance because it has the same dimensions as the quantity
itself (and therefore can be measured in the same units). For example,
if you measured urine production in 20 of your colleagues, you might
express the results in the form 230 ± 35 ml/hr (mean ±
s.d., n=20). Generally about 70% of the values will fall within
±
1 s.d. of the mean (i.e. in this case, between 195 and 265 ml/hr).

**standard error of mean **ABBREV: s.e.m. RELATED: standard
deviation, error bars ♦ An estimate of how much in error the mean of a
set of measurements is likely to be, compared with the true mean you
would get with very many measurements. It is the estimated standard
deviation of the mean. With a small number (n) of measurements, the
s.e.m. is larger. It is equal to the estimated standard deviation of
the
measurements (which tells how variable they themselves are) divided by
sqrt(n). When you do an experiment, you may not be particularly
interested in the variability of the original data (i.e. in the s.d.),
but if you calculate a mean you should always be interested in how
uncertain that is (i.e. in the s.e.m.). Calculators or spreadsheets do
most of the work for you.

**stasis **Constancy; cessation of movement. E.g. homeostasis,
haemostasis.

**statistical test **RELATED: null hypothesis, significance
level ♦ A procedure to estimate the probability that an observed
experimental result (or one even stronger) might have been obtained by
chance, on the assumption of a 'null hypothesis'. A test may in general
be 'one-tailed' or 'two-tailed'. A one-tailed test gives the
probability
that the result would be as large or larger in the observed direction.
A
two-tailed test gives the probability (often just twice as great) that
it be as large in either direction. If you predicted the effect in
advance, and that is how the experiment turned out, then a 1-tailed
test
is appropriate. In other circumstances, a 2-tailed test should be used.

**std. dev. of functions **If a quantity (y) is calculated
from
parameters x_{1},x_{2},etc., then it may be possible to
calculate the standard deviation (sd) of y from the sd's of the other
parameters. With only one parameter (i.e. y=f(x)), then
sd(y)=sd(x)×|df/dx|, where |df/dx| is the absolute value of the
gradient of y plotted against x. If y=a×x^{p} (a power
law), then the sd expressed as a percentage (sd%) is simple:
sd%(y)=p×sd%(x). For example, a 3% sd in the diameters of spheres
corresponds to a 9% sd in their volumes. With 2 or more parameters,
with
statistically independent variations, the variance V(y)=Σ_{i}(V(x_{i})(df/dx_{i})^{2}).
These formulae are not always exact, but are good approximations for
small variations.

**std. dev. of sums & products **Two special cases of the
calculation of the standard deviation (sd) of functions are common and
important. If a quantity y is calculated from variable parameters x_{1},x_{2},..,
whose variations are statistically independent, then simple rules
apply. If y is the sum or difference of x_{1},x_{2},..,
then the square of sd(y) is the sum of the squares of the sd's for all
the x_{i}. If y is the product or quotient of x_{1},x_{2},..,
then a similar rule applies but in relation to the sd's expressed as
percentages (sd%): the square of sd%(y) is (approximately) the sum of
the squares of sd% for all the x_{i}, regardless of whether
they
appear on the top or the bottom of the formula for y.

**steady state **RELATED: equilibrium ♦ A state in which all
parameters are constant (i.e. not changing with time). In biological
experiments, this is really a matter of all 'important' parameters
being
considered constant. For example, there may be continuing fluctuations
of concentration or voltage within the tissue. Age is a relevant
parameter in many experiments and is never strictly constant! If you
place a cell in a new solution, its resting membrane potential may
rapidly (within seconds) reach a new steady state, but the
concentrations of chemicals in its cytoplasm may change slowly and take
hours to reach a new steady state. You need to use the concept with
care.

**stimulus **RELATED: response ♦ Strictly 'urging on', this
means something that activates a tissue or process. E.g., arterial CO_{2}
is a stimulus to respiration. Any external intervention applied to a
tissue is described as a stimulus, however, even if the effect is
inhibition or depression of function. This can be confusing: a stimulus
can stimulate (i.e. activate) or inhibit some function. The usage is
sensible in as much as interventions act by inducing some change in the
tissue, which may itself either activate or inhibit particular
processes. The effect of one tissue on another (e.g. nerve on muscle)
is
not 'stimulation' unless it its effect is excitatory. The vagus
certainly does not stimulate the heart.

**stomy SUF **RELATED: -tomy ♦ Mouth. E.g. tracheostomy=
making
an artificial opening (mouth) in the trachea (windpipe). Distinguish
from -tomy (e.g. tracheotomy).

**striated **Striped.

**Student's t test **RELATED: Null hypothesis, significance
level ♦ A very useful statistical test in which you compare the mean
value of an observation with the value that you might expect according
to a 'null' hypothesis. 't' is the difference from the expected value,
divided by the standard error. You need to know the number of 'degrees
of freedom' (= the number of observations, less 1) and you can then
look
up the minimum value of 't' for the required significance level in a
set
of tables. As a rule of thumb, the difference is never significant if
t<2, while t>2.5 will always be significant (P<0.05) unless
you
have very few observations (<6). A more complex 't' test is required
if you want to compare 2 means.

**sub- **RELATED: hypo-, oligo- CONVERSE: super- ♦ Under,
beneath. E.g. subnormal, sub-lingual (under the tongue).

**subject **Human experimental animal. A patient participating
in an experiment or a clinical trial is a subject; a normal subject
participating in an experiment, even in a hospital, will not appreciate
being called a patient!

**summation **ABBREV: Σ (Greek Sigma) ♦ 1. Mathematically,
this
means to add a set of numbers: Sigma(x) means the sum of all the
numbers
like x. For example, in calculating a line of closest fit (a
'regression') you choose the parameters in a function f(x) so as to
minimise Sigma((y-f(x))^{2}). 2. Physiologically, summation
means the addition of influences that arrive at a cell either at
separate times ('temporal summation') or at separate places ('spatial
summation').

**super- **RELATED: supra-, hyper- CONVERSE: sub- ♦ Above.

**superfuse **RELATED: perfuse, infuse ♦ To pass fluid over
something. E.g. the tongue might be superfused with solutions
containing
chemicals with different tastes.

**supra- **RELATED: super-, hyper- CONVERSE: sub- ♦ Above.

**supramaximal **RELATED: maximal, threshold ♦ Above maximal.
A
supramaximal stimulus is more than big enough to have the full effect:
increasing it further will not increase the response. The stimulus is
therefore on the plateau of a stimulus- response graph. This may be
because the stimulus is above the threshold for all of the cells
contributing to a response.

**suture **A stitch made with needle and thread or a metal
clamp, to hold tissue together.

**sweep **The left to right movement of the spot on an
oscilloscope.

**symptom **RELATED: sign ♦ A change noticed by a patient,
that
indicates an underlying disorder. A 'presenting symptom' is one that
leads the patient to seek medical advice: it is not necessarily
indicative of the most serious aspects of the disorder.

**syncytium **A set of cells that are coupled in some way, so
that they behave in certain respects as if their cytoplasm was
continuous. For example, electric current can pass through the gap
junctions between cells in the heart, allowing an action potential to
propagate throughout the heart without synaptic transmission.

**syndrome **A set of symptoms and/or signs that are
characteristic of a particular disorder.

**systole **RELATED: diastole ♦ The time of contraction of the
ventricles of the heart, when arterial blood is expelled into the
aorta.
Systolic pressure is the maximum arterial pressure reaches during the
cycle, at the end of systole.

**tangent **RELATED: gradient, slope ♦ A straight line that
meets a curve and also has the same gradient at the point of contact.

**teleological **Relating to the purpose of a tissue or organ.
A teleological argument whereby, for example, you say 'I know what this
hormone is for, so it must have this effect because that is how it
would
easily achieve its purpose' is sometimes useful in helping to remember
facts or guess at them. But it isn't very reliable, since biological
evolution may not always have led to the same principles of successful
design that you think would be sensible!

**temporal **1. Relating to time. 2. Relating to the temple,
or
side of the head.

**tera- **ABBREV: T RELATED: SI units ♦ One million million, 10^{12}.

**tetanus **RELATED: twitch ♦ In muscle physiology, a
continuous contraction produced by stimuli or action potentials at a
high frequency. A 'fused' tetanus is one in which the frequency is high
enough that responses to individual stimuli cannot be distinguished.
Tetanus (or lockjaw) is also a disease in which sustained muscular
contractions occur. The word is used in neurophysiology to denote high
frequency stimulation, though this is often considered improper since
the origin of the word has to do with the tautness of muscle, not with
the frequency of stimulation.

**-thermic SUF **Relating to temperature.

**threshold **RELATED: excitability ♦ Minimum strength of
stimulus that elicits a response.

**time constant **ABBREV: τ (Greek tau) RELATED: exponential
constant, half life, rate constant ♦ The exponential constant for a
parameter that varies exponentially as a function of time. The equation
for the parameter is of the form y=a.e^{t/τ} or y=a.e^{-t/τ},
depending on whether the exponential is rising or falling. In either
case, 'a' is the value at time t=0. One 'time constant' = 1.443 'half
lives'. The reciprocal of τ is the 'rate constant'. E.g., the time
constant for cooling of a cadaver is (according to Agatha Christie) 3.5
hrs. You should be able to sketch the cooling graph and mark the time
constant on it by eye.

**time course **RELATED: latency, rise time, overshoot,
undershoot ♦ The temporal (i.e. time-related) characteristics of a
response or event. In general, if you are asked to describe the time
course of a response you should consider whether there is a latency,
what the rise time or the time to peak is, whether the response is
transient or maintained, whether there is an overshoot or undershoot,
how long the recovery takes, and whether the effect of the stimulus is
reversible. Not all these factors may be relevant, and there may be
others (e.g. perhaps several peaks in a response). You don't need to
make precise measurements, but you should make approximate quantitative
statements. Try describing, from experience, the time course of effects
of running a mile.

**timebase **RELATED: trigger ♦ The part of an oscilloscope
that moves the spot steadily from left to right. It always has a
control
for varying the speed of movement (the 'sweep' speed) and various
controls for determining when the sweep begins ('trigger' controls).
The
trigger options are usually FREE RUN, i.e. it starts to sweep as soon
as
the last sweep is complete, EXT TRIG, i.e. it starts when a pulse is
fed
through a cable into a socket on the front of the timebase, INT TRIG,
i.e. it starts when the vertical deflection on one of the traces
exceeds an adjustable level, LINE, i.e. it starts at a constant phase
of
the mains voltage supply

**-tomy **RELATED: -ectomy ♦ Cutting. E.g. lobotomy= incising
a
lobe of an organ.

**tonic **CONVERSE: phasic ♦ Continuous. A tonic response is
one that is maintained during the period of a stimulus, i.e. that does
not show complete adaptation. Muscle 'tone' is a continuous level of
contraction, arising usually from tonic activity in the innervating
nerves (e.g. in skeletal muscles and vascular smooth muscles).

**tonicity **RELATED: osmolarity ♦ In physical chemistry, this
is the same as 'osmolarity'. In physiology, it relates specifically to
the behaviour of cells in a solution. It is a measure of the tendency
of
a solution to make cells swell or shrink. An 'isotonic' solution leaves
cells at normal volume. A hypotonic solution (e.g. a more dilute
solution, with lower osmolarity) makes cells swell; a hypertonic
solution makes them shrink. Substances in solution that readily pass
through membranes (e.g. urea) contribute to the osmolarity of a
solution, but not to its tonicity. A 1M urea solution has a higher
osmolarity than blood plasma, but is hypotonic. Water will enter the
cells along with the urea, and make the cells burst.

**toxin **A poisonous or harmful substance. Many useful drugs
have toxic effects.

**transducer RELATED: transduction ♦ Something that converts a
signal (in more rigorous terms: energy or information) into a different
form. For example, the conversion of a light signal into an electrical
signal by a photomultiplier tube. **Sensory receptors are
transducers, though the term is more usually applied to instruments.

**trace **A record from a measuring instrument, in graphical
form.

**transient **RELATED: phasic CONVERSE: maintained ♦ Not
maintained. E.g. 'Cigarette smoking gives a transient feeling of
well-being and contributes to a long term health risk.'

**transmitter **RELATED: modulators ♦ A chemical messenger
released at a synapse by the pre-synaptic cell, and acting on receptors
in the postsynaptic membrane. Transmitters have a rapid local action,
most often acting by opening channels in the postsynaptic membrane. The
same chemicals may also be released as hormones (e.g. noradrenaline) or
as modulators.

**transport **In the context of membrane physiology, (verb) to
move across a membrane.

**trauma **RELATED: lesion ♦ A painful, harmful, or
destructive
event, or the wound caused by such an event. The word is used in
relation to both physical and psychological damage.

**trend **RELATED: regression, correlation ♦ 1. An approximate
relation, or a generalisation that is true on average but not in every
individual case (as in a correlation). E.g. There is a trend for taller
people to be heavier. There is a trend towards lower calorie food
intake
in the summer. 2. Some people use 'trend' to mean a difference found in
an experiment, that doesn't reach statistical significance. Usually it
is a waste of time to talk about such differences. Certainly it is
dangerous to suggest, by giving them a special name, that they are
worth
paying attention to. The whole idea of a statistical test is to show
whether a difference is too small to be worth paying attention to.

**trigger **RELATED: all-or-none ♦ Something that starts
something else off. E.g. An action potential may be said to be
triggered
by a stimulus that is above threshold; a fit of coughing may be
triggered by a deep breath; the start of the sweep of an oscilloscope
may be triggered by a brief pulse coming from a stimulator circuit. The
essential thing here is that the triggering event doesn't merely affect
what is triggered, but it causes the start of a stereotyped chain of
events. The relation between the trigger and the triggered event is
usually all-or-none. The analogy with the trigger of a gun is fairly
obvious.

**trophic **Relating to nutrition or growth. A trophic
influence is one that influences growth or maintenance of a tissue.

**tropic SUF **Related to direction. Turning. E.g.
trophotropic= turning or moving towards food.

**trough **CONVERSE: peak ♦ A point on a graph that is lower
than those on either side (i.e. a 'minimum').

**turbid **Muddy, milky (e.g. a suspension of cells).

**turbulence **RELATED: laminar flow, Reynold's number ♦ A
state in which fluid flow fluctuates markedly from one place to another
and from one time to another. The pattern of flow is then governed by
the physics of 'chaos' (which is not yet well understood) rather than
the physics of laminar flow. Transition from laminar flow to turbulence
often occurs quite suddenly as the rate of flow is increased or as a
constriction is narrowed (e.g. in a cardiac valve). The transition can
sometimes be characterised by a 'Reynold's number'. You can picture it
as the transition from dark to white water in the flow of a river.
Energy is dissipated in additional ways (e.g. by sound), and different
stresses are placed on the walls of a vessel.

**turgid **Swollen, rigid (e.g. a cell in a hypotonic
solution).

**twitch **RELATED: tetanus ♦ The response of a muscle cell to
a single action potential.

**ultra- **Extreme.

**undershoot **RELATED: overshoot ♦ A phase of the time course
of a response in which the measured parameter may cross below the
baseline level and be shifted in the opposite direction from the
initial
response, before final recovery. For example, if you hyperventilate
deliberately, your respiration rate will normally show an undershoot
subsequently. Terminology is rather ambiguous (and should be explained
more clearly) if you are discussing a response in which the initial
effect is a decrease. For example if you deliberately hypoventilate (or
hold your breath), the subsequent hyperventilation is best not
described
as either an undershoot or an overshoot.

**uniform **RELATED: homogeneous ♦ Having the same value,
usually in different places or between different subjects (not
necessarily at different times). E.g. Oedema due to over-drinking is
relatively uniform throughout all tissue compartments. If you do
experiments on students, you usually have a population with very
uniform
ages.

**unilateral **RELATED: ipsi-, contralateral CONVERSE:
bilateral ♦ On one side only.

**unit **RELATED: dimension ♦ The standard to which a quantity
is compared, in assigning it a numerical value. It is useless to give
numerical values to parameters without specifying the units, and
dangerous to compare values without converting them to the same units
(NB if you insist on doing this, you have to bear in mind the relation
between the units as well as the relation between the two values -
unnecessary mental gymnastics). E.g. metre (m) or millimetre (mm) for
length; pascal (Pa= Nm^{-2}) or mm Hg (ca. 133 Pa) for pressure.

**upregulation ** Increase in something (usually the
expression
of a signal or a receptor), in response to a stimulus.

**-uresis **Relating to, or appearing in, the urine. E.g.
diuresis= extra urine, proteinuresis= protein in the urine.

**valid ** Sound, logical. Note that an argument may be
invalid,
even if the conclusion is correct. (The patient may have cancer even
though your reasons for thinking this were invalid.)

**vapour pressure **RELATED: relative humidity ♦ The partial
pressure of a vapour (evaporated liquid) within a gas mixture. When in
equilibrium with a liquid at a particular temperature, the gas has a
vapour pressure equal to the saturated vapour pressure. For water, this
is 6.3kPa (47mmHg) at 37^{o}C and 2.3kPa (18mmHg) at 20^{o}C.

**variable **RELATED: parameter, constant, dependent CONVERSE:
constant ♦ A parameter that you can alter or expect to vary during an
experiment. For example, during an experiment on a human subject, the
inspired oxygen concentration and body core temperature may be
variables, but body height is a constant parameter. In experiments on
several subjects, height becomes a 'variable', that may or may not
affect the results. Variables can be 'dependent' or 'independent'. If
you look at how body temperature is affected by exercise, temperature
is
a dependent variable. If you vary body temperature directly (e.g. in a
hot or cold bath) and look at its effect on mental capacity,
temperature
is the independent variable.

**variance **RELATED: standard deviation ♦ A measure of how
variable a set of numbers is. You first have to think about the
individual differences of all the numbers from the mean value for all
of
them. These differences will be both +ve and -ve, with a mean of zero.
If you square them before taking the mean (making them all positive),
then you get the variance. In other words, it is the mean square
difference from the mean, or strictly the limit that this would
approach
if you take more and more samples. The standard deviation is the square
root of the variance.

**vaso- **Relating to a vessel (i.e. in anatomy, a tube).
Usually relating to a blood vessel (e.g. vasoactive= affecting blood
vessels), but NB vasectomy= removal of a part of the vas deferens.

**vector **RELATED: scalar ♦ A quantity that has direction as
well as magnitude. You can represent it visually as an arrow. For
example, the velocity of blood flow is a vector, having magnitude and
direction everywhere within a blood vessel. If the flow is laminar in a
straight vessel, the direction is everywhere the same, but the
magnitude
falls off towards the vessel walls. At a branch point, the vectors in
different places have different directions. If there is turbulent flow,
the vectors fluctuate somewhat chaotically with both position and time.
You can 'resolve' a vector into components in 3 perpendicular
directions. NB vectors with zero magnitude do not have a defined
direction.

**vector differentiation **ABBREV: grad, div, curl) ♦ 3 types
of vector differentiation are defined. The first is differentiation of
something (such as concentration C) that isn't itself a vector. You can
take the derivative of this with respect to position in each direction
and form a vector from the 3 components: this is grad(C) (=vC),
proportional to the flux of substance that will diffuse in the
concentration gradient. If a vector A is defined at every point (e.g.
flux or velocity of movement) then the 'divergence' div(A) (=v.A)
corresponds to the rate at which moving particles are being added
locally to the system. (NB often v.A=0.) Curl(A) (=vxA) is a vector
corresponding to the circulation (like a vortex) of particles locally.

**vector multiplication **ABBREV: A.B, AxB) ♦ Two types of
vector multiplication are defined. Both are generalisations of the
ordinary multiplication of simple numbers. A 'scalar product' A.B is
the
magnitude of vector A × the component of vector B in the
direction
of A. It arises, for example, if A is a force applied to a point and B
is its displacement: A.B is then the work done during the movement (NB
work is a 'scalar': it doesn't have direction). The 'vector product'
AxB
is twice the area formed by the triangle defined by A and B. This has
direction (perpendicular to the plane of the triangle) as well as
magnitude, i.e. it is a vector. It arises, for example, in calculating
the force due to a current (A) in a magnetic field (B).

**vectorcardiography **A way of analyzing electrocardiogram
signals in which the signals are displayed as a spot moving in two or
three dimensions, corresponding to the changes of both amplitude and
electrical axis of the electrocardiogram during the cycle. The spot
traverses loops, which give a direct visual indication of the
electrical
axis.

**ventilate **RELATED: breathe ♦ To cause air to flow through
the airways.

**vivisection **Literally, means to cut something living.
Refers to experiments on living (including anaesthetised) animals.
Anti-vivisectionists are those who oppose the use of animals for
experiments. Opposition to animal experiments should be distinguished
from opposition to cruelty to animals. Most people who perform animal
experiments for biomedical and veterinary research are wholly opposed
to
cruelty and would argue strongly that their work causes little or no
suffering to animals, and that where suffering is caused (as is
inevitable, for example, in some research on pain) it is more than
balanced by the alleviation of suffering due to the successes of such
research in leading to clinical advances.

**voltage **see potential difference

**waveform **RELATED: time course, profile ♦ The shape of the
graph of a signal plotted against time, showing its timecourse.

**wavelength **The distance between two points of equal phase
on a (sinusoidal) waveform, for example between wave crests. For a
light
wave, wavelength is inversely related to frequency. In the visible
spectrum the shortest wavelengths (ca. 0.4nm) correspond to violet
light
and the longest (ca. 0.8nm) correspond to red light.

**white noise **RELATED: hum ♦ A signal that fluctuates
randomly in time, with a large range of different frequency components.
If you played it through a loudspeaker it would sound like a hiss. All
amplifiers generate a certain amount of such noise. The term 'white'
comes by analogy with white light, which usually contains all different
frequencies (or wavelengths) of light. The energy in white noise is
proportional to the bandwidth. You can learn to distinguish white noise
on an oscilloscope from, for example, sinusoidal 'hum' due to signals
picked up from the mains (50Hz).

**why **A 'why' question in physiology may relate either to
the
mechanism by which something happens ('As a result of what processes?')
or to the role that it plays in affecting other things, i.e. its
consequences ('For what benefit?'). A full answer (e.g. to the question
"Why does peripheral resistance rise when a person stands up?") would
include both aspects: in this case the reflex mechanisms, and also the
consequences if these did not exist. Try to avoid the ambiguity if
possible, and if in doubt (e.g. in an exam) say how you are
interpreting
the question.

**wordprocessor **RELATED: spreadsheet, database ♦ A computer
program used to help in the preparation and editing of text documents.

**χ ^{2} **RELATED: student's t test, significance ♦
Chi
squared test. Often used for testing the significance of the
differences
of two proportions. For example, if 70/100 patients given a new drug
recover, while only 20/50 given a placebo recover, is there significant
evidence that the drug has an effect?