General Topography of Autonomic Nervous System (Continued)

The axons of autonomic neurons in the cranial nerve nuclei and sacral spinal segments usually produce effects opposite to those produced by the axons of neurons in the thoracolumbar intermediolateral cell columns. The cranial and sacral groups comprise the parasympathetic system, and the more numerous thoracolumbar groups, the sympathetic system. The neurons of sympathetic and parasympathetic systems are morphologically similar; they are smallish, ovoid, multipolar cells with myelinated axons and variable number of dendrites.

The axons of the autonomic nerve cells in the nuclei of the cranial nerves, in the thoracolumbar intermediolateral columns, and in the gray matter of the sacral spinal segments are termed preganglionic fibers and form synapses in peripheral ganglia. The axons of the ganglion cells are called postganglionic fibers; these unmyelinated axons convey efferent output to the viscera, vessels, and other structures.

The cranial parasympathetic preganglionic fibers form synapses in the ciliary, pterygopalatine, otic, submandibular, cardiac, and celiac ganglia, and in much smaller ganglia in the walls of the trachea, bronchi, and gastrointestinal tract. The corresponding sacral fibers form synapses in the inferior hypogastric (pelvic) plexuses, within the enteric plexuses of the distal colon and rectum, and in the walls of the urinary bladder and other pelvic viscera. Most of the thoracolumbar sympathetic preganglionic fibers synapse in sympathetic trunk ganglia, but some fibers pass through the sympathetic trunk ganglia to form synapses in other ganglia, such as the celiac, mesenteric, and renal.

Parasympathetic relay ganglia are located near the structures innervated or within the walls of hollow organs or solid viscera; therefore parasympathetic post-ganglionic fibers are relatively short. Sympathetic relay ganglia are generally more distant from the structures they innervate, so sympathetic postganglionic fibers are often much longer than their parasympathetic counterparts. Plate 7-2 illustrates the arrangement of the preganglionic and postganglionic fibers to all the important viscera, the positions of the ganglia in which the synaptic relays occur, and the consequent disparities in the lengths of the postganglionic fibers. For example, in the heart, sympathetic preganglionic fibers synapse with the neurons in the superior cervical to the fifth thoracic sympathetic ganglia; the relatively long postganglionic fibers are conveyed to the heart in the cervical and thoracic sympathetic cardiac nerves. The parasympathetic preganglionic fibers reach the heart in the cardiac branches of the vagus nerves and relay in ganglia of the cardiac plexus or in small subendocardial ganglia; their postganglionic fibers are relatively short.

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