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 homeostasis is maintained.
The afferent innervation of the autonomic nervous system is divided into two main systems:
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 recognized that additional afferent inputs from the brain and the target viscera themselves modulate the sympathetic and parasympathetic activity.
Autonomic afferent organization
In the autonomic nervous system, there are two afferent neurons, connecting the CNS to the target organs. These preganglionic and postganglionic nerve cells are joined by a synapse within a ganglion. The cell body of the preganglionic neuron is always in the CNS. Conversely, the cell bodies of the postganglionic neurons are in PNS ganglia, usually located:
- sympathetic
- parasympathetic
- walls of the end organs
The major preganglionic neurotransmitter in the parasympathetic system is acetylcholine (ACh) and in the sympathetic system is norepinephrine (norepinephrine). The major parasympathetic and sympathetic postganglionic neurotransmitter is norepinephrine except for sweat glands which are cholingeric parasympathetic afferents.