The Sympathetic Nervous System Arises from the Thoracolumbar Spinal Cord
The sympathetic nervous system generally has short pre- ganglionic and long postganglionic axons. Preganglionic axons of the sympathetic nervous system leave the spinal cord by way of the ventral roots of the first thoracic through the third or fourth lumbar spinal nerves (Figure 13-2).
For this reason, the sympathetic nervous system is often called the thoracolumbar system. The preganglionic axons pass through the ventral root and then a communicating branch (ramus) to enter the paravertebral sympathetic ganglion chain (also called the ‘'sympathetic trunk"), where most synapse with a postganglionic neuron (Figure 13-3). The ganglion chain actually extends from cervical to sacral regions. The postganglionic axons then extend to one of the hollow visceral organs or reenter the spinal nerves to extend to more distal structures such as blood vessels, sweat glands, and hair erector muscles of the body
FIGURE 13-3 Synaptic organization of preganglionic (Pre) and postganglionic (Post) neurons of the sympathetic and parasympathetic divisions of the autonomic nervous system. RCt Ramus communicans (communicating branch). (From GanongWF: Review of medical physiology, ed 13, Norwalk, Conn, 1987, Appleton & Lange.)
wall. A few preganglionic axons pass through the paravertebral ganglia, forming splanchnic nerves that synapse with postganglionic neurons in prevertebral ganglia, usually named for neighboring blood vessels. Postganglionic neurons of the prevertebral ganglia also innervate hollow visceral organs.
The adrenal medulla is a special variation on this anatomical theme. A few sympathetic preganglionic axons extend all the way to the adrenal medulla, where they synapse with rudimentary postganglionic neurons that make up the adrenal medullary secretory cells. These vestigial postganglionic neurons secrete their transmitter substance directly into the circulating blood. The transmitter substance, acting as a true hormone, is carried by the blood to all tissues of the body.