Visceral Efferent Pathways
Unlike the afferent component, the efferent component of the visceral nervous system, sometime referred to as the autonomic nervous system, is arranged in two divisions, sympathetic and parasympathetic, distinguished by morphology, pharmacology, and physiology.
The final conducting pathway of both divisions, unlike that of the somatic system, consists of two motor neurons in succession: the first has its cell body, or perikaryon, within the central nervous system, and the second is located within a peripheral ganglion (Fig. 8.53). The two successive motor neurons are most frequently distinguished as the preganglionic and the postganglionic neurons and together are equivalent to the lower motor neuron of the somatic system.The parasympathetic and the sympathetic divisions of the autonomic nervous system are distinguished anatomically by the nonoverlapping distribution of the preganglionic neurons within the central nervous system. The cell bodies of the preganglionic neurons of the sympathetic division are located within the lateral (visceral efferent) column of the spinal cord gray matter, between the first thoracic and middle lumbar segments (with some interspecific variation) (see Fig. 8.75). The cell bodies of the sympathetic postganglionic neurons are found in paravertebral ganglia of the sympathetic chain or in the subvertebral ganglia on the aorta; both groups are relatively close to the spinal cord.
The cell bodies of the parasympathetic preganglionic neurons are restricted to the brainstem; to the nuclei of the oculomotor, facial, glossopharyngeal, and vagus nerves; or to the lateral gray matter columns of the sacral segments of the cord (see Fig. 8.74). The cell bodies of parasympathetic postganglionic neurons are located within small ganglia in close proximity to or actually incorporated within the walls of the organs they supply.
The neurotransmitter released onto visceral smooth muscle by postganglionic sympathetic neurons is norepinephrine and that by postganglionic parasympathetic neurons is acetylcholine; both are released with a host of neuropeptides. The two divisions therefore react differently to autonomic agonist and antagonist drugs.
In spite of the nonoverlapping arrangement of sympathetic and parasympathetic preganglionic neurons, the two systems have broadly similar distributions with respect to visceral organ innervation and are frequently described as antagonist: one inhibits while the other stimulates a particular activity. This rule is less absolute than was once supposed, and their roles are better regarded as collaborative. The more diffuse anatomy of the peripheral sympathetic nerves (which are described later) and the use of norepinephrine as a transmitter indicate the more general effects produced by sympathetic activity, in contrast to those of parasympathetic activity, which are often local, affecting single specific functions.
FIG. 8.53 Comparison of the organization of the visceral (black) and the somatic (red) nervous systems at the thoracolumbar level of the spinal cord. Afferent fibers are indicated by interrupted lines, efferent fibers by solid lines. The postganglionic sympathetic fibers are indicated by alternating black and stippled lines. 1, Dorsal root ganglion; 2, ventral root; 3, dorsal branch of spinal nerve; 4, ventral branch of spinal nerve; 5 and 6, white (preganglionic) and gray (postganglionic) communicating branches, often fused; 7, sympathetic trunk with ganglia; 8, prevertebral ganglion; 9, gut; a, somatic afferent fibers; b, visceral afferent fibers; c, somatic efferent fibers; d, visceral efferent fibers (preganglionic sympathetic); e, postganglionic sympathetic (to peripheral structures); f, postganglionic sympathetic (to abdominal organs).