Stimuli Arising from the Alimentary Canal

For many years it was the opinion of physiologists that the autonomic nervous system was composed mainly of motor (efferent) fibers. However, more recent observations in both animals and human, based on morphological and electrophysiological techniques, have clearly shown that sensory fibers are much more numerous than motor fibers in the physiological regulation of gut homeostasis, visceral motility, and alimentery behavior. Their normal function is related to events accompanying digestion. They signal events related to the mechanical state of the digestive tract (i.e., mechanoreceptors measuring contraction or distension) or the physicochemical properties of the gut contents (i.e., chemoreceptors measuring chemical composition, concentration of nutrients, osmotic pressure, acid and alkaline sensitivity, general chemo-sensitivity). Other polymodal receptors may respond to a broad range of stimuli, and digestive nerve endings appear to exhibit a general sensitivity to neurotransmitters, hormones, and other agents. A disturbance of this control can trigger changes in peristaltic contractions, the coordination of gastrointestinal activity, gastric emptying, and food intake.

These receptors can be stimulated by a diverse range of foreign stimuli of varying intensity. For example, simple overeating or serious obstruction can both cause distension and induce nausea and vomiting by mechanoreceptor stimulation. Similarly, the oral administration of a moderately innocuous solution of sodium chloride or a severe gut infection with staphylotoxin enterotoxin will both stimulate chemorecep-tors to induce nausea and vomiting (see Fig. 2). Stimulation of receptors within the alimentary canal is the single most important cause of the induction of nausea and emesis in the normal population (Fig. 3). The vagus nerve is particularly important in sensory viscerotopic organization. Its afferent components contain general somatic, special visceral (taste), and general visceral sensory fibers, which carry both mechano- and chemoreceptive information from the gut to terminate in the nucleus tractus solitarius. The splanchnic nerves also relay sensory information from the gut. The efferent components of the vagus nerve contain both general and special visceral motor fibers arising in the dorsal motor nucleus of the vagus (DMV) and the nucleus ambiguous, respectively. These medullary systems comprise a final common pathway from the brain to control the activity of the gastrointestinal tract.

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