Autonomic Reflex Pathways

The autonomic reflex arc is similar to the somatic reflex arc, although, in the somatic arcs, the interneurons and their connections are entirely within the central nervous system (CNS). In the autonomic arcs, the interneurons are within the CNS, but their axons synapse outside the CNS to reach the ganglia in which they terminate. Initially, the autonomic and somatic components of the nervous system develop together, but during the embryonic and fetal phases, groups of nerve cells migrate outward along the spinal nerve roots and form ganglia, such as those of the sympathetic trunks, and more peripheral ganglia, such as the celiac and mesenteric (see Plate 7-13). These migrant cells are efferent autonomic neurons, and in order to maintain their synaptic relationships, the axons of the interneurons must follow them, to reach the autonomic ganglion cells with which they form synapses. These axons are termed preganglionic fibers, whereas the axons of ganglionic neurons lie beyond the ganglia and are called postganglionic fibers.

The preganglionic fibers are myelinated, and when seen together, as in the large groups of sympathetic preganglionic fibers passing from all the thoracic and the upper two lumbar spinal nerves to nearby sympathetic trunk ganglia, they are almost white in color and constitute the white rami communicantes. Afferent myelinated fibers pass through these rami to the spinal nerves and contribute to their whitish appearance. The postganglionic fibers are unmyelinated and appear grayish pink in color when seen in mass. They form the gray rami communicantes connecting each sympathetic trunk ganglion to the adjoining spinal nerves.

One part of a parasympathetic arc (vagal) is illustrated; the efferent preganglionic fibers arise from the dorsal vagal nucleus and reach the walls of the intestine by vagal branches that are part of, and synapse with cells in the ganglia forming the enteric plexus; postganglionic fibers innervate the intestines. The cell bodies of the afferent pseudounipolar neurons are also located in afferent ganglia of the enteric plexus and their central axonal processes travel to the brainstem in the vagal nerve to synapse with the neurons in the dorsal vagal nuclei.

The illustration also shows that sympathetic preganglionic fibers emerge through the anterior root of the thoracic or upper lumbar spinal nerves. They all pass through white rami communicantes to the adjacent sympathetic trunk ganglia. Many of these preganglionic fibers synapse with the cells of the ganglia; others pass upward or downward in the sympathetic trunks to form synapses with neurons in other cervical, lumbar, and sacral ganglia. Still other preganglionic fibers pass through the sympathetic trunk ganglia without relaying and run in splanchnic nerves to end in ganglia, such as the celiac and mesenteric or the adrenal medulla. The postganglionic axons all pass to adjacent spinal nerves as gray rami communicantes; this explains why all spinal nerves have gray rami communicantes, whereas white rami communicantes are limited to the thoracolumbar region. Also shown is the recurrent meningeal sympathetic branch carrying postganglionic fibers to the spinal meninges and the spinal perivascular plexuses.

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