Chapter 16: Gastrointestinal Physiology
Which of the following statements regarding innervation of the gastrointestinal (GI) tract is true?
(A) Sympathetic innervation has a primarily excitatory effect on the GI tract.
(B) Parasympathetic innervation is supplied solely by the vagus nerve and its branches.
(C) The enteric nervous system is composed of a serosal plexus and submucosal plexus.
(D) Local reflex activity occurs in the enteric nervous system in the absence of any sympathetic or parasympathetic innervation.
(E) Most sympathetic fibers terminate in the mucosa.
(D) Innervation of the GI tract can be broadly divided into three categories: sympathetic, parasympathetic, and enteric. Sympathetic innervation is chiefly inhibitory, decreasing motility and secretions; however, sympathetic fibers stimulate contraction of the muscularis mucosa and of certain sphincters. These nerve fibers are typically adrenergic fibers arising from prevertebral and paravertebral plexuses. Few fibers terminate on muscle itself, instead exercising an inhibitory effect by terminating in the submucosal and myenteric plexuses and inhibiting synaptic transmission. Sympathetic fibers also cause vasoconstriction of blood vessels and provide innervation to glandular structures.
Parasympathetic innervation to the preponderance of the GI tract is conveyed through the vagus nerve and its branches. The distal colon, however, receives parasympathetic innervation from the hypogastric plexus. Like sympathetic fibers, parasympathetic fibers mainly terminate in the submucosal and myenteric plexuses. Unlike sympathetic fibers, they have a stimulatory effect on GI motility and secretions.
The enteric nervous system is composed primarily of the submucosal plexus (Meissner’s plexus) and the myenteric plexus (Auerbach’s plexus). These plexuses consist of ganglia connected by unmyelinated fibers (Fig. 16-1). Other, less-well-defined, plexuses also contribute to the enteric nervous system. The enteric nervous system is influenced by sympathetic and parasympathetic innervation but consists of a rich pathway of afferent and efferent nerves such that much of the activity of the gut, including local reflex activity, continues even in the absence of any extrinsic input.
FIGURE 16-1. Enteric nervous system (from Brunicardi FC, Andersen DK, Billiar TR, et al., eds. Schwartz’s Principles of Surgery. 9th ed. New York, NY: McGraw-Hill; 2009: Fig. 26-16. Copyright © The McGraw-Hill Companies, Inc. All rights reserved).
Silbernagl S, Despopoulos A. Color Atlas of Physiology. 6th ed. Stuttgart, Germany: Thieme; 2008:236–237.
Which of the following is a component of the normal swallowing mechanism?
(A) Depression of the soft palate
(B) Posteriosuperior movement of the hyoid bone
(C) Initiation of a peristaltic wave down the ...