The submucosal (Meissner's) and myenteric (Auerbach's) plexuses within the wall of the intestine contain 100 million neurons, about as many as are in the spinal cord! These include preganglionic parasympathetic axons, the ganglion cell bodies of postganglionic parasympathetic neurons, postganglionic sympathetic axons, and afferent (sensory) neurons. These plexuses also contain interneu-rons, as does the CNS. Also like the CNS, the enteric nervous system (or enteric brain) contains more glial cells than neurons, and these glial cells resemble the astrocytes of the brain.
Some sensory (afferent) neurons within the intestinal plexuses travel in the vagus nerve to deliver sensory information to the CNS. These are called extrinsic afferents, and they are involved in regulation by the autonomic nervous system. Other sensory neurons—called intrinsic afferents—have their cell bodies in the myenteric or submucosal plexuses and synapse with the interneurons of the enteric nervous system.
Peristalsis, for example, is regulated by the enteric nervous system. A bolus of chyme stimulates intrinsic afferents (with cell bodies in the myenteric plexus) that activate enteric interneurons, which in turn stimulate motor neurons. These motor neurons innervate both smooth muscle cells and interstitial cells of Cajal, where they release excitatory and inhibitory neurotransmitters. Smooth muscle contraction is stimulated by the neurotransmit-ters ACh and substance P above the bolus, and smooth muscle relaxation is stimulated by nitric oxide, vasoactive intestinal pep-tide (VIP), and ATP below the bolus (fig. 18.31).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.