Control of the Sensorimotor Functions of the Gastrointestinal Tract
The motor function of the gut is controlled by the extrinsic sympathetic and parasympathetic fibers and by the intrinsic nervous system, a vast enteric brain with approximately the same number of neurons as are found in the mammalian spinal cord. The sympathetic nervous system stimulates sphincters and relaxes nonsphincteric muscle. In contrast, the parasympathetic nervous system stimulates nonsphincteric muscle. In the enteric nervous system, there are hard-wired neural circuits that serve to facilitate certain responses, such as the interdigestive migrating motor complex during fasting and the response of the gut to feeding. The "law of the intestine" is the peristaltic reflex and, in recent years, the neurotransmitters and mediators primarily involved in this reflex have been the object of intense study. The excitatory components of the reflex that result in orad contraction are acetylcholine and substance P; descending inhibition in the aborad region is mediated predominantly by vasoactive intestinal peptide, and nitric oxide. The sensory arm of the peristaltic reflex involves a number of transmitters, including calcitonin gene-related peptide and serotonin (5HT). Some of the major advances in pharmacotherapy are based on modulation of these transmitters.
Sensation of the gut is mediated through a 3-order neuron chain; the initial transducer of sensation is the enteroendocrine cell, which is both a mechanical and chemical transducer.[1,2,3] Visceral signals result in reflexes that modulate motor and secretory functions and may not always project to the conscious brain because they synapse with efferent pathways in the prevertebral ganglia and in the spinal cord. Pain is projected through spinal afferents that course along but are not sympathetic fibers, entering the cord in the dorsal horn. The dorsal horn neuron is a center for modulation of sensation. There is a balance between ascending information and descending modulation from the brainstem that determines the level of central projection of incoming signals from the dorsal horn neuron. The cerebral hemispheres determine the level of perception and autonomic responses following gut stimuli. There are centers of vigilance in the prefrontal cortex, and the limbic system and brainstem reflexes change the levels of satiety and modulate autonomic responses (pulse and blood pressure) to gut stimulation.
Medscape General Medicine. 1999;1(3) © 1999
Cite this: Functional Gastrointestinal Disorders: Novel Insights and Treatments - Medscape - Nov 01, 1999.