Innervation Of The Gastrointestinal Tract

Extrinsic innervation of the GI tract is provided by both the parasympathetic and sympathetic systems. Together with the enteric or intrinsic nervous system, they make up the autonomic nervous system. The innervation of the GI tract is referred to as autonomic because we are unaware of its activities and have no conscious control over the functions it regulates.

Parasympathetic Innervation

Down to the level of the transverse colon, parasym-pathetic innervation to the GI tract is supplied by the vagus nerve. The pelvic nerve innervates the descending colon, sigmoid colon, rectum, and anal canal (Fig. 2). The striated muscle of the upper third of the esophagus and the external anal sphincter receive cholinergic innervation from the vagus and pelvic nerves, respectively. Preganglionic fibers arise from cell bodies within the medulla of the brain (vagus) and the sacral region of the spinal cord (pelvic). These predominantly choliner-gic fibers synapse with ganglion cells located in the enteric nervous system (Fig. 3). Thus, vagal activity can affect secretion, motility, or the release of hormones, as indicated in Fig. 3. The mediator at the target cells is generally acetylcholine, but this is not always the case. In many cases, the chemical mediator has not been identified.

It is also important to realize that the vagus is a mixed nerve in which approximately 75% of the fibers are afferent. Receptors in the mucosa and smooth muscle relay information back to higher centers via the vagus and pelvic nerves (Fig. 3). This afferent information may trigger a reflex whose efferent limb is also present in the vagus nerve. Reflexes of this type are called vagovagal reflexes.

Sympathetic Innervation

Unlike the parasympathetic system, preganglionic fibers from the sympathetic system synapse outside of the GI tract in prevertebral ganglia (Fig. 2). Preganglionic, cholinergic efferent fibers from the cord synapse in four major ganglia: the celiac, superior mesenteric, inferior mesenteric, and hypogastric. Postganglionic adrenergic fibers from these ganglia innervate the cells of the myente-ric and submucosal plexuses. Elements from the enteric system then innervate smooth muscle, secretory, and endocrine cells. Some blood vessels, certain smooth muscle cells, and the muscularis mucosae receive direct postganglionic sympathetic innervation (Fig. 3). Approximately 50% of the fibers present in sympathetic nerves to the gut are afferent. Thus, information is also relayed from the gut to the spinal cord.

Intrinsic Innervation

Intrinsic innervation of the gut is provided by the enteric nervous system consisting primarily of the networks formed by the submucosal and myenteric plexuses. The elements of the enteric nervous system not only relay information to and from the gut via the parasympathetic and sympathetic systems, but also relay information along the gut. In other words, local or intramural (contained within the wall) reflexes can produce a response in one part of the tract after a stimulus to another part even in the absence of all extrinsic innervation (Fig. 3).

Many different chemicals act as neurocrines within the enteric nervous system. Acetylcholine is released by most of the extrinsic preganglionic fibers and acts on neurons within the prevertebral ganglia and enteric nervous system. Most of the endings of the postganglionic

FIGURE 2 The extrinsic branches of the autonomic nervous system. (A) Parasympathetic. Dotted lines indicate cholinergic innervation of the striated muscle in the esophagus and external anal sphincter. Solid lines indicate afferent and preganglionic innervation of the remaining GI tract. (B) Sympathetic. Solid lines denote the afferent and preganglionic efferent pathways between the spinal cord and the prevertebral ganglia. C, celiac; SM, superior mesenteric; IM, inferior mesenteric. Dashed lines indicate the afferent and postganglionic efferent innervation.

FIGURE 2 The extrinsic branches of the autonomic nervous system. (A) Parasympathetic. Dotted lines indicate cholinergic innervation of the striated muscle in the esophagus and external anal sphincter. Solid lines indicate afferent and preganglionic innervation of the remaining GI tract. (B) Sympathetic. Solid lines denote the afferent and preganglionic efferent pathways between the spinal cord and the prevertebral ganglia. C, celiac; SM, superior mesenteric; IM, inferior mesenteric. Dashed lines indicate the afferent and postganglionic efferent innervation.

sympathetic neurons release norepinephrine, which also acts on neurons within the enteric system. The enteric nervous system itself contains many different mediators. These include acetylcholine, nitric oxide, vasoactive intestinal peptide (VIP), enkephalins, serotonin, and substance P. Our knowledge of the localization of neuro-crines is far from complete, and in many instances a single neuron may release more than one mediator.

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Responses

  • Cesarina
    Why should the G I T be innervated?
    4 months ago

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