Synaptic Plasticity

A synapse is a point of communication between two neurons. In the majority of synapses -- the chemical as opposed to electrical type -- the neurons come into close apposition but remain separated by a small gap, the synaptic cleft. A signal is passed across the cleft via a chemical messenger or neurotransmitter, which is released into the cleft from one side and detected by receptors on the other. The receptors then in some way transduce the signal into some change in the receiving cell. For example, one common mechanism is the opening of a channel in the membrane that allows selected ions to flow into or out of the cell, changing its internal chemical make-up, which then leads, perhaps quite indirectly, to changes in its behaviour.

These synapses are generally very asymmetric, with nearly all the transmitting machinery concentrated on one side of the cleft and the receptors clustered on the other. The signalling thus largely occurs in one direction. The neuron that does the releasing is called the presynaptic cell; the receiving one is post-synaptic.

This synaptic asymmetry reflects a larger-scale asymmetry in the neurons themselves. These are effectively little biochemical integrating machines that take in a large number of inputs and generate from them a single principal output signal. The inputs are taken from all over the cell, but in particular from a complex branching formation of dendrites, where the bulk of the receiving side of synapses are found. This integrated signal the neuron produces -- an ongoing pattern of rapid voltage changes known as action potentials -- travels down a specialised output cable called the axon to the terminals or boutons that form the sending side of synapses.