Table 5.1 ). However, some are unmyelinated C fibers. They leave the spinal cord in the ventral root and enter the paravertebral ganglion at the same segmental level through a white communicating ramus. White rami are found only from the levels of T1 to L2. The preganglionic axon may synapse on postganglionic neurons in the ganglion at its level of entry; may travel rostrally or caudally within the sympathetic trunk and give off collaterals to the ganglia that it passes; or may pass through the ganglion, exit the sympathetic trunk, and enter a splanchnic nerve to travel to a prevertebral ganglion (see
Figs. 11.1 11.2 ). Splanchnic nerves innervate the viscera; they contain both visceral afferents and autonomic motor fibers (sympathetic or parasympathetic).
Postganglionic neurons whose somata lie in paravertebral ganglia generally send their axons through a gray communicating ramus to enter a spinal nerve (see
Fig. 11.2 ). Each of the 31 pairs of spinal nerves has a gray ramus. Postganglionic axons are distributed through the peripheral nerves to effectors, such as piloerector muscles, blood vessels, and sweat glands, located in the skin, muscle, and joints. Postganglionic axons are generally unmyelinated (C fibers), although some exceptions exist. The names white and gray rami reflect the relative contents of myelinated and unmyelinated axons in these rami.
Preganglionic axons in a splanchnic nerve often travel to a prevertebral ganglion and synapse, or they may pass through the ganglion and an autonomic plexus and end in a more distant ganglion. Some preganglionic axons pass through a splanchnic nerve and end directly on cells of the adrenal medulla, which are equivalent to postganglionic cells.
The sympathetic chain extends from the cervical to the coccygeal levels of the spinal cord. This arrangement serves as a distribution system that enables preganglionic neurons, which are limited to the thoracic and upper lumbar segments, to activate postganglionic neurons that innervate all body segments. However, there are fewer paravertebral ganglia than there are spinal segments because some of the segmental ganglia fuse during development. For example, the superior cervical sympathetic ganglion represents the fused ganglia of C1 through C4; the middle cervical sympathetic ganglion is the fused ganglia of C5 and C6; and the inferior cervical sympathetic ganglion is a combination of the ganglia at C7 and C8. The term stellate ganglion refers to fusion of the inferior cervical sympathetic ganglion with the ganglion of T1. The superior cervical sympathetic ganglion provides postganglionic innervation to the head and neck, and the middle cervical and stellate ganglia innervate the heart, lungs, and bronchi.
In general, the sympathetic preganglionic neurons are distributed to ipsilateral ganglia and thus control autonomic function on the same side of the body. Important exceptions are the sympathetic innervation of the intestines and the pelvic viscera, which are both bilateral. As with motor neurons to skeletal muscle, sympathetic preganglionic neurons that control a particular organ are spread over several segments. For example, the sympathetic preganglionic neurons that control sympathetic functions in the head and neck region are distributed at levels C8 to T5, whereas those that control the adrenal gland are distributed at levels T4 to T12.
The Parasympathetic Nervous System
The parasympathetic preganglionic neurons are found in several of the cranial nerve nuclei of brainstem and in the sacral spinal cord (S3-S4) gray matter (see