The autonomic nervous system (ANS) is a subdivision of the peripheral nervous system. The autonomic system provides innervation of the involuntary muscles, i.e. myocardium and smooth muscle, and glands, through which fine control of homoeostasis is maintained.

The afferent innervation of the autonomic nervous system is divided into two main systems:

• parasympathetic nervous system (PaNS)

• sympathetic nervous system (SNS)

The enteric nervous system, which innervates the gut, is also part of the autonomic nervous system, and some consider it to form a third system.

Historically, these three systems were felt to represent the entirety of the autonomic system but it is now recognised that additional afferent inputs from the brain and the target viscera themselves modulate the sympathetic and parasympathetic activity.

Autonomic afferent organisation

In the autonomic nervous system, there are two afferent neurones, connecting the central nervous system to the target organs. These preganglionic and postganglionic nerve cells are joined by a synapse within a ganglion. The cell body of the preganglionic neurone is always in the central nervous system. Conversely, the cell bodies of the postganglionic neurones are in ganglia of the peripheral nervous system, usually located:

• sympathetic

  • sympathetic trunk

  • autonomic plexuses e.g. coeliac plexus

• parasympathetic

  • walls of the end organs

The major preganglionic neurotransmitter in the sympathetic and parasympathetic system is acetylcholine (ACh). The major parasympathetic postganglionic neuotransmitter is acetylcholine, and the major sympathetic postganglionic neurotransmitter is noradrenaline except for sweat glands which have cholingeric sympathetic afferents ³.