The autonomic nervous system, which is part of the peripheral nervous system, is involved with the control of the involuntary functions of organs such as the heart, blood vessels, the lungs, the bladder, glands and the gastrointestinal tract. There are 3 main divisions of the autonomic nervous system: the sympathetic and the parasympathetic systems which provide a dual mechanism for controlling internal organs and the enteric system which regulates the activity of the gut.

In addition, centres in the central nervous system, particularly the brain stem and the spinal cord, from which peripheral nerves arise, transmit signals to and from autonomically innervated organs.