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The various components of the gastrointestinal (GI) tract serve several functions, including digestive, excretory, endocrine, and exocrine. Control of these functions requires neuronal activity from both local and higher centers.

The GI system has a complex collection of highly organized neurons called the enteric nervous system(ENS) located in the intestinal walls (Figure 36–1). The ENS may be considered a third division of the autonomic nervous system, and includes the myenteric plexus and the submucosal plexus. These neuronal networks receive preganglionic fibers from the parasympathetic system as well as postganglionic sympathetic axons. They also receive sensory input from within the wall of the gut. Fibers from the cell bodies in these plexuses travel to the smooth muscle of the gut to control motility. Other motor fibers go to the secretory cells. Sensory fibers transmit information from the mucosa and from stretch receptors to motor neurons in the plexuses and to postganglionic neurons in the sympathetic ganglia. The parasympathetic and sympathetic fibers that synapse on enteric plexus neurons appear to play a modulatory role.

Figure 36–1.

A highly simplified diagram of the intestinal wall and some of the circuitry of the enteric nervous system (ENS). The ENS receives input from both the sympathetic and the parasympathetic systems and sends afferent impulses to sympathetic ganglia and to the central nervous system. Many transmitter or neuromodulator substances have been identified in the ENS. These agents include acetylcholine (ACh), norepinephrine (NE), nitric oxide (NO), neuropeptides (NP), substance P (SP), and serotonin (5-HT). Additional abbreviations: longitudinal muscle layer (LM), myenteric plexus (MP), circular muscle layer (CM), and submucosal plexus (SMP).

Multiple neurotransmitters, neuromodulators, and autocrine factors are present in the GI system. Autacoids are endogenous molecules that have powerful physiologic and pharmacologic effects but do not fall into traditional autonomic or hormonal groups. Two important amineautacoids, histamine (H) and serotonin (5-hydroxytryptamine, 5-HT), and numerous peptide autacoids are present. Both of these amine-autacoids are discussed in previous chapters; histamine in the respiratory system (Chapter 35) and 5-HT in the central nervous system (Chapter 19). Other well-documented autacoids are cytokines (Chapter 32) and prostaglandins (Chapter 34).

As previously discussed (Chapter 35), stimulation of histamine type 1 receptors results in mucous secretion in the respiratory system and contraction of several types of smooth muscle. In contrast, stimulation of histamine type 2 (H2) receptors results in gastric acid secretion in the stomach. Serotonin is produced from the amino acid tryptophan and stored in vesicles in the enterochromaffin cells of the gut as well as neurons in the ENS. In addition to its activity as a central nervous system neurotransmitter (Chapters 12 and 19), 5-HT has a physiologic role as a neurotransmitter in the ENS and perhaps a role as a local hormone that modulates gastrointestinal smooth muscle ...

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