Anatomy of the Autonomic Nervous System

The GI tract is innervated by the autonomic nervous system (ANS). It is called the ANS because, under normal circumstances, people neither are conscious of its activities nor exert any willful control over them. The ANS can be divided into the extrinsic nervous system and the intrinsic, or enteric, nervous system.

The extrinsic nervous system is in turn divided into parasympathetic and sympathetic branches ( Fig. 2.1 ). Parasympathetic innervation is supplied primarily by the vagus and pelvic nerves. Long preganglionic axons arise from cell bodies within the medulla of the brain and the sacral region of the spinal cord. These preganglionic nerves enter the various organs of the GI tract, where they synapse mainly with cells of the enteric nervous system ( Fig. 2.2 ). In addition, these same nerve bundles contain many afferent nerves whose receptors lie within the various tissues of the gut. These nerves project to the brain and spinal cord to provide sensory input for integration. Approximately 75% of the fibers within the vagus nerve are afferent. Thus information can be relayed from the GI tract to the medulla and integrated, and a message can be sent back to the tract that may influence motility, secretion, or the release of a hormone. These long or vagovagal reflexes play important roles in regulating GI functions.

Extrinsic branches of the autonomic nervous system. (A) Parasympathetic. Dashed lines indicate the cholinergic innervation of striated muscle in the esophagus and external anal sphincter. Solid lines indicate the afferent and preganglionic efferent innervation of the rest of the gastrointestinal tract. (B) Sympathetic. Solid lines denote the afferent and preganglionic efferent connections between the spinal cord and the prevertebral ganglia. Dashed lines indicate the afferent and postganglionic efferent innervation. CG , Celiac ganglion; IMG , inferior mesenteric ganglion; SMG , superior mesenteric ganglion.

The integration of the extrinsic (parasympathetic and sympathetic) nervous system with the enteric (myenteric and submucosal plexuses) nervous system. The preganglionic fibers of the parasympathetic synapse with ganglion cells located in the enteric nervous system. Their cell bodies, in turn, send signals to smooth muscle, secretory, and endocrine cells. They also receive information from receptors located in the mucosa and in the smooth muscle that is relayed to higher centers via vagal afferents. This may result in vagovagal (long) reflexes. Postganglionic efferent fibers from the sympathetic ganglia innervate the elements of the enteric system, but they also innervate smooth muscle, blood vessels, and secretory cells directly. The enteric nervous system relays information up and down the length of the gastrointestinal tract, and this may result in short or intrinsic reflexes.

Reprinted with permission from Johnson LR. Essential Medical Physiology , 3rd ed. Philadelphia, Academic Press, 2003.

Sympathetic innervation is supplied by nerves that run between the spinal cord and the prevertebral ganglia and between these ganglia and the organs of the gut. Preganglionic efferent fibers arise within the spinal cord and end in the prevertebral ganglia. Postganglionic fibers from these ganglia then innervate primarily the elements of the enteric nervous system (see Fig. 2.1 ). Few fibers end directly on secretory, absorptive, or muscle cells. Afferent fibers also are present within the sympathetic division. These nerves project back to the prevertebral ganglia and/or the spinal cord. Thus an abundance of sensory information also is available via these nerves.

Elements of the intrinsic , or enteric , nervous system are grouped into several anatomically distinct networks, of which the myenteric and submucosal plexuses (see Fig. 2.2 ) are the most prominent. These plexuses consist of nerve cell bodies, axons, dendrites, and nerve endings. Processes from the neurons of the plexuses do not just innervate target cells such as smooth muscle, secretory cells, and absorptive cells. They also connect to sensory receptors and interdigitate with processes from other neurons located both inside and outside the plexus. Thus pathways within the enteric nervous system can be multisynaptic, and integration of activities can take place entirely within the enteric nervous system (see Fig. 2.2 ), as well as in the ganglia of the extrinsic nerves, the spinal cord, and the brainstem.

Many chemicals serve as neurocrines within the ANS. Several of these chemicals have been localized within specific pathways, and a few have defined physiologic roles. Most of the extrinsic, preganglionic, efferent fibers contain acetylcholine (ACh). This transmitter exerts its action on neurons contained within the prevertebral ganglia and enteric nervous system. Norepinephrine is found in many nerve endings of the postganglionic efferent nerves of the sympathetic nervous system. This transmitter also exerts its effects primarily on neurons of the enteric nervous system. Within the enteric nervous system, ACh, serotonin , vasoactive intestinal peptide (VIP), nitric oxide (NO), and somatostatin have been localized to interneurons. ACh and the tachykinins (TKs) (e.g., substance P ) have been localized to nerves that are excitatory to the muscle; VIP and NO have been localized to inhibitory nerves to the muscle. In many instances, more than one transmitter can be localized to the same nerve. The goals of mapping neural circuits within the extrinsic and enteric nervous system and elucidating their functions are far from complete.

Neurohumoral Regulation of Gastrointestinal Function

Although it is convenient to discuss the ANS and the endocrine/paracrine systems separately, it is important to understand that they do not function independently of one another. Rather, the regulatory systems interact to control secretion, absorption, and motility. Specific examples of such regulation are given in subsequent chapters (see Fig. 8.10 , Fig. 9.7 for examples). The targets, be they secretory, absorptive, or smooth muscle cells, have a certain resting output that is modulated by both neurally released chemicals and humorally delivered chemicals. These chemicals are released from nerve endings and glandular cells in response to various stimuli that act on specific receptors. The sources of these stimuli can be either in the environment or within the body. For example, seeing or smelling appetizing food alters many aspects of GI function, as does the presence of many foodstuffs and products of digestion within the lumen of the stomach and intestine. In many cases, the stimuli result from the secretory and motor functions of the target cells themselves. Whatever their source, these stimuli initiate inputs that are integrated in the neural and endocrine systems to produce outputs that appropriately regulate the functions of the target cells.