General considerations

The autonomic nervous system (ANS) derives entirely from the neural crest. It consists of the catecholaminergic and non-catecholaminergic cells of the sympathetic nervous system, including both sympathetic ganglia and adrenal chromaffin cells, the cholinergic and non-cholinergic parasympathetic ganglia, and the enteric ganglia. The ANS provides motor innervation to smooth muscles and viscera. Visceral sensory endings are also present in most internal organs from which myelinated and unmyelinated afferent fibers ascend to the spinal cord and brain through various visceral nerves, one of the most important being the vagus nerve which extends over most of the gut. Visceral motor centers are located in the spinal cord, brainstem, and higher centers of most vertebrates. Visceral motor neurons within the CNS are connected to postganglionic neurons with which they form at least a two-neuron visceral motor pathway for the reflex control of all smooth muscles, of the cardiac muscle, and of glandular epithelia.

The sympathetic branch of the ANS includes preganglionic neurons located in the intermediolateral column of the spinal cord along thoracolumbar segments of the axis. Fibers from preganglionic neurons synapse with secondorder neurons located in the paravertebral and prevertebral sympathetic ganglia. The parasympathetic division consists of preganglionic fibers found in cranial nerves III, VII, IX, and X, as well as in sacral nerves that synapse on ganglia localized close to or within the target organ. As a general rule, the preganglionic myelinated axons are therefore much longer, and unmyelinated postganglionic fibers shorter, in the parasympathetic system than in the sympathetic system.