The cells that give rise to the preganglionic fibers of this division lie mainly in the intermediolat-eral columns of the spinal cord and extend from the first thoracic to the second or third lumbar segment. The axons from these cells are carried in the anterior (ventral) nerve roots and synapse with neurons lying in sympathetic ganglia outside the cerebrospinal axis. Sympathetic ganglia are found in three locations: paravertebral, prevertebral, and terminal.
The 22 pairs of paravertebral sympathetic ganglia form the lateral chains on either side of the vertebral column. The ganglia are connected to each other by nerve trunks and to the spinal nerves by rami communicantes. The white rami are restricted to the segments of the thoracolum-bar outflow; they carry the preganglionic myelinated fibers that exit the spinal cord via the anterior spinal roots. The gray rami arise from the ganglia and carry postganglionic fibers back to the spinal nerves for distribution to sweat glands and pilomotor muscles and to blood vessels of skeletal muscle and skin. The prevertebral ganglia lie in the abdomen and the pelvis near the ventral surface of the bony vertebral column and consist mainly of the celiac (solar), superior mesen-teric, aorticorenal, and inferior mesenteric ganglia. The terminal ganglia are few in number, lie near the organs they innervate, and include ganglia connected with the urinary bladder and rectum and the cervical ganglia in the region of the neck. Small intermediate ganglia lie outside the conventional vertebral chain, especially in the thoracolumbar region, usually proximally to the communicating rami and the anterior spinal nerve roots.
Preganglionic fibers issuing from the spinal cord may synapse with the neurons of more than one sympathetic ganglion. Their principal ganglia of termination need not correspond to the original level from which the preganglionic fiber exits the spinal cord. Many of the preganglionic fibers from the fifth to the last thoracic segment pass through the paravertebral ganglia to form the splanchnic nerves. Most of the splanchnic nerve fibers do not synapse until they reach the celiac ganglion; others directly innervate the adrenal medulla (see below).
Postganglionic fibers arising from sympathetic ganglia innervate visceral structures of the thorax, abdomen, head, and neck. The trunk and the limbs are supplied by the sympathetic fibers in spinal nerves, as described earlier. The prevertebral ganglia contain cell bodies whose axons
FIGURE 6-1 The autonomic nervous system (ANS). Schematic representation of the autonomic nerves and effector organs based on chemical mediation of nerve impulses. Blue, cholinergic; gray, adrenergic; dotted blue, visceral afferent; solid lines, preganglionic; broken lines, postganglionic. In the upper rectangle at the right are shown the finer details of the ramifications of adrenergic fibers at any one segment of the spinal cord, the path of the visceral afferent nerves, the cholinergic nature of somatic motor nerves to skeletal muscle, and the presumed cholinergic nature of the vasodilator fibers in the dorsal roots of the spinal nerves. The asterisk (*) indicates that it is not known whether these vasodilator fibers are motor or sensory or where their cell bodies are situated. In the lower rectangle on the right, vagal preganglionic (solid blue) nerves from the brainstem synapse on both excitatory and inhibitory neurons found in the myenteric plexus. A synapse with a postganglionic cholinergic neuron (blue with varicosities) gives rise to excitation, whereas synapses with purinergic, peptide (VIP), or an NO-generating neuron (black with varicosities) lead to relaxation. Sensory nerves (dotted blue lines) originating primarily in the mucosal layer send afferent signals to the CNS but often branch and synapse with ganglia in the plexus. Their transmitter is substance P or other tachykinins. Other interneurons (white) contain serotonin and will modulate intrinsic activity through synapses with other neurons eliciting excitation or relaxation (black). Cholinergic, adrenergic, and some peptidergic neurons pass through the circular smooth muscle to synapse in the submucosal plexus or terminate in the mucosal layer, where their transmitter may stimulate or inhibit GI secretion.
innervate the glands and smooth muscles of the abdominal and the pelvic viscera. Many of the upper thoracic sympathetic fibers from the vertebral ganglia form terminal plexuses, such as the cardiac, esophageal, and pulmonary plexuses. The sympathetic distribution to the head and the neck (vasomotor, pupillodilator, secretory, and pilomotor) is via the cervical sympathetic chain and its three ganglia. All postganglionic fibers in this chain arise from cell bodies located in these three ganglia; all preganglionic fibers arise from the upper thoracic segments of the spinal cord, there being no sympathetic fibers that leave the CNS above the first thoracic level.
The adrenal medulla and other chromaffin tissue are embryologically and anatomically similar to sympathetic ganglia. The adrenal medulla differs from sympathetic ganglia in that its principal catecholamine is epinephrine (Epi, adrenaline), not NE. The chromaffin cells in the adrenal medulla are innervated by typical preganglionic fibers that release ACh.
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