The autonomic nervous system is generally thought of as those motor neurons which serve visceral effectors. However, a broader view would also say that the autonomic nervous system includes neuronal circuits within the central nervous system, and perhaps also afferent fibres from the viscera.

Structurally and functionally, the autonomic nervous system can be divided into sympathetic and parasympathetic components (figure 1). Sometimes the fibres serving the alimentary canal are thought of as forming a third, distinctive component, the enteric division of the autonomic nervous system.

The sympathetic nervous system is formed by preganglionic neurons located in the thoracolumbar spinal cord. The axons of these neurons project through the ventral roots of the spinal nerves and, by way of the white rami, into the sympathetic ganglia located just anterior and lateral to the spinal column(figure 2). These paravertebral ganglia contain the cell bodies of the postganglionic sympathetic neurons. Hence, there are two major classes of motor neurons in the sympathetic nervous system: preganglionic neurons and postganglionic neurons. The paravertebral sympathetic ganglia are linked and so form what is called the sympathetic chain anterolateral to the spinal column (figure 3). Three major ganglia of the sympathetic chain are located in the neck. These are the superior cervical ganglion, the middle cervical ganglion and the stellate ganglion (previously called the inferior cervical ganglion). From the paravertebral ganglia, postganglionic axons run to somatic structures of the body wall (figure 4). In addition to the paravertebral ganglia, there are major prevertebral ganglia from which postganglionic fibres run to the major viscera. For example, the coeliac ganglion (figure 5) provides innervation to the stomach, liver, gall bladder, pancreas and spleen. The superior mesenteric ganglion (figure 6) provides innervation to the small intestine and large intestine as far as the descending colon. The inferior mesenteric ganglion provides innervation to the descending colon, sigmoid colon and rectum, as well as to the urinary bladder and external genitals.

The parasympathetic nervous system is formed by preganglionic neurons in the brain stem and the sacral cord. Cranial parasympathetic fibres travel in the oculomotor, facial, glossopharyngeal and vagus nerves (figure 7). Sacral parasympathetic neurons travel in the second, third and fourth sacral nerves. Axons from parasympathetic neurons project to ganglia which are generally located relatively close to target organs. Postganglionic neurons then project from the ganglia to the target organ.

The enteric division of the autonomic nervous system innervates the gastrointestinal tract, pancreas and gall bladder. This system contains local visceral afferents as well as interneurons and motor neurons projecting to muscle and blood vessels in the walls of the gastrointestinal tract, and motor neurons to secretory organs in the mucosa. This system provides for local reflex control of motility and secretion. However, the enteric system also receives projections from, and so is influenced by, postganglionic sympathetic and preganglionic parasympathetic fibres.

Centrally, the activity of the autonomic nervous system is regulated principally by the hypothalamus and the nucleus of the solitary tract (nucleus tractus solitarius). The hypothalamus also has a major influence on the activity of the endocrine system, by way of regulation of anterior pituitary function. The nucleus of the solitary tract receives afferent input from the major visceral organs and contains reflex centres which regulate, for example, cardiovascular function and respiration.

The autonomic nervous system influences target organs by the release of a variety of chemical messengers. Two of these are acetylcholine and norepinephrine. All preganglionic neurons use acetylcholine as a neurotransmitter. Acetylcholine is also released by postganglionic parasympathetic neurons and some postganglionic sympathetic neurons. Norepinephrine is the principal postganglionic chemical messenger of the sympathetic nervous system. Norepinephrine and acetylcholine are the two best known neurotransmitters of the autonomic nervous system. Other neurotransmitters as well as vasoactive substances are released by postganglionic autonomic neurons. These substances bind to receptors on the target cells and thereby induce changes in the physiology of the target organs.

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