The Sympathetic Nervous System

Sympathetic preganglionic neurons are concentrated in the intermediolateral cell column (lateral horn) in the thoracic and upper lumbar segments of the spinal cord (Fig. 11-2). Some neurons may also be found in the C8 segment. In addition to the intermediolateral cell column, groups of sympathetic preganglionic neurons are found in other locations, including the lateral funiculus, the intermediate gray matter, and the gray matter dorsal to the central canal.

The axons of preganglionic neurons are often small myelinated nerve fibers known as B fibers (see 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 T1 to L2. The preganglionic axon may synapse on postganglionic neurons in this ganglion; 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 (Figs. 11-1 and 11-2). A splanchnic nerve is a nerve that innervates the viscera; it contains both visceral afferents and autonomic 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 (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 distinction between white and gray rami is a consequence of the relative content 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.

Generally, the sympathetic preganglionic neurons are distributed to ipsilateral ganglia and thus control autonomic function on the same side of the body. One important exception is that the sympathetic innervation of the intestine and the pelvic viscera is 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 in C8 to T5, whereas those that control the adrenal gland are in T4 to T12.

The Parasympathetic Nervous System

The parasympathetic preganglionic neurons are located in several cranial nerve nuclei in the brainstem, as well as in the intermediate region of the S3 and S4 segments of the sacral spinal cord (Fig. 11-1). The cranial nerve nuclei that contain parasympathetic preganglionic neurons are the Edinger-Westphal nucleus (cranial nerve III), the superior (cranial nerve VII) and inferior (cranial nerve IX) salivatory nuclei, and the dorsal motor nucleus and nucleus ambiguus (cranial nerve X). Postganglionic parasympathetic cells are located in cranial ganglia, including the ciliary ganglion (preganglionic input is from the Edinger-Westphal nucleus), the pterygopalatine and submandibular ganglia (input from the superior salivatory nucleus), and the otic ganglion (input from the inferior salivatory nucleus). The ciliary ganglion innervates the pupillary sphincter and ciliary muscles in the eye. The pterygopalatine ganglion supplies the lacrimal gland, as well as glands in the nasal and oral pharynx. The submandibular ganglion projects to the submandibular and sublingual salivary glands and to glands in the oral cavity. The otic ganglion innervates the parotid salivary gland and glands in the mouth.

Other parasympathetic postganglionic neurons are located near or in the walls of visceral organs in the thoracic, abdominal, and pelvic cavities. Neurons of the enteric plexus include cells that can also be considered parasympathetic postganglionic neurons. These cells receive input from the vagus or pelvic nerves. The vagus nerves innervate the heart, lungs, bronchi, liver, pancreas, and gastrointestinal tract from the esophagus to the splenic flexure of the colon. The remainder of the colon and rectum, as well as the urinary bladder and reproductive organs, is supplied by sacral parasympathetic preganglionic neurons that travel through the pelvic nerves to postganglionic neurons in the pelvic ganglia.

The parasympathetic preganglionic neurons that project to the viscera of the thorax and part of the abdomen are located in the dorsal motor nucleus of the vagus (see Fig. 4-7 E, F) and the nucleus ambiguus. The dorsal motor nucleus is largely secretomotor (it activates glands), whereas the nucleus ambiguus is visceromotor (it modifies the activity of cardiac muscle). The dorsal motor nucleus is secretomotor (it activates glands) and visceromotor (it activates the smooth muscle of the gut), whereas the nucleus ambiguus is visceromotor (it modifies the activity of cardiac muscle). Electrical stimulation of the dorsal motor nucleus results in gastric acid secretion, as well as secretion of insulin and glucagon by the pancreas. Although projections to the heart have been described, their function is uncertain. The nucleus ambiguus contains two groups of neurons: (1) a dorsal group (branchiomotor) that activates striated muscle in the soft palate, pharynx, larynx, and esophagus and (2) a ventrolateral group that innervates and slows the heart (see also Chapter 18).

Visceral Afferent Fibers

The visceral motor fibers in the autonomic nerves are accompanied by visceral afferent fibers. Most of these afferent fibers supply information that originates from sensory receptors in the viscera. The activity of many of these sensory receptors never reaches the level of consciousness. Instead, these receptors initiate the afferent limb of reflex arcs. Both viscerovisceral and viscerosomatic reflexes are elicited by these afferent fibers. Visceral reflexes operate at a subconscious level, and they are very important for homeostatic regulation and adjustment to external stimuli.

The fast-acting neurotransmitters released by visceral afferent fibers are not well documented, although many of these neurons release an excitatory amino acid transmitter such as glutamate. However, visceral afferent fibers also contain many neuropeptides or combinations of neuropeptides, including angiotensin II, arginine vasopressin, bombesin, calcitonin gene—related peptide, cholecystokinin, galanin, substance P, enkephalin, oxytocin, somatostatin, and vasoactive intestinal polypeptide.

Visceral afferent fibers that mediate sensation include nociceptors that travel in sympathetic nerves, such as the splanchnic nerves. Visceral pain is caused by excessive distention of hollow viscera, contraction against an obstruction, or ischemia. The origin of visceral pain is often difficult to identify because of its diffuse nature and its tendency to be referred to somatic structures (see Chapter 7). Visceral nociceptors in sympathetic nerves reach the spinal cord via the sympathetic chain, white rami, and dorsal roots. The terminals of nociceptive afferent fibers project to the dorsal horn and to the region surrounding the central canal. They activate not only local interneurons, which participate in reflex arcs, but also projection cells, which include spinothalamic tract cells that signal pain to the brain.

A major visceral nociceptive pathway from the pelvis involves a relay in the gray matter of the lumbosacral spinal cord. These neurons send axons into the fasciculus gracilis that terminate in the nucleus gracilis. Thus, the dorsal columns not only contain primary afferents for somatic sensation (their main component) but also second-order neurons of the visceral pain pathway (recall that second-order axons for somatic pain travel in the lateral funiculus as part of the spinothalamic tract). Visceral nociceptive signals are then transmitted to the ventral posterior lateral nucleus of the thalamus and presumably from there to the cerebral cortex. Interruption of this pathway accounts for the beneficial effects of surgically induced lesions of the dorsal column at lower thoracic levels to relieve pain produced by cancer of the pelvic organs.

Other visceral afferent fibers travel in parasympathetic nerves. These fibers are generally involved in reflexes rather than sensation (except for taste afferent fibers; see Chapter 8). For example, the baroreceptor afferent fibers that innervate the carotid sinus are in the glossopharyngeal nerve. They enter the brainstem, pass through the solitary tract, and terminate in the nucleus of the solitary tract (see Fig. 4-7, D—F). These neurons connect with interneurons in the brainstem reticular formation. The interneurons, in turn, project to the autonomic preganglionic neurons that control heart rate and blood pressure (see Chapter 18).

The nucleus of the solitary tract receives information from all visceral organs, except those in the pelvis. This nucleus is subdivided into several areas that receive information from specific visceral organs.

The Enteric Nervous System

The enteric nervous system, which is located in the wall of the gastrointestinal tract, contains about 100 million neurons. The enteric nervous system is subdivided into the myenteric plexus, which lies between the longitudinal and circular muscle layers of the gut, and the submucosal plexus, which lies in the submucosa of the gut. The neurons of the myenteric plexus primarily control gastrointestinal motility (see Chapter 26), whereas those in the submucosal plexus primarily regulate body fluid homeostasis (see Chapter 34).

The types of neurons found in the myenteric plexus include not only excitatory and inhibitory motor neurons (which can be considered parasympathetic postganglionic neurons) but also interneurons and primary afferent neurons. Afferent neurons supply mechanoreceptors within the wall of the gastrointestinal tract. These mechanoreceptors form the afferent limb of reflex arcs within the enteric plexus. Local excitatory and inhibitory interneurons process these reflexes, and the output is sent through the motor neurons to smooth muscle cells. Excitatory motor neurons release acetylcholine and substance P; inhibitory motor neurons release dynorphin and vasoactive intestinal polypeptide. The circuitry of the enteric plexus is so extensive that it can coordinate the movements of an intestine that has been completely removed from the body. However, normal function requires innervation by the autonomic preganglionic neurons and regulation by the CNS.

Activity in the enteric nervous system is modulated by the sympathetic nervous system. Sympathetic postganglionic neurons that contain norepinephrine inhibit intestinal motility, those that contain norepinephrine and neuropeptide Y regulate blood flow, and those that contain norepinephrine and somatostatin control intestinal secretion. Feedback is provided by intestinofugal neurons that project back from the myenteric plexus to the sympathetic ganglia.

The submucosal plexus regulates ion and water transport across the intestinal epithelium and glandular secretion. It also communicates with the myenteric plexus to ensure coordination of the functions of the two components of the enteric nervous system. The neurons and neural circuits of the submucosal plexus are not as well understood as those of the myenteric plexus, but many of the neurons contain neuropeptides, and the neural networks are well organized.