ENS-CNS interactions
The gastrointestinal tract is in two way communication with the CNS. Afferent neurons convey information about the state of the gastrointestinal tract. Some of this reaches consciousness, including pain and discomfort from the gut and the conscious feelings of hunger and satiety, which are integrated perceptions derived from the gastrointestinal tract and other signals (blood glucose, for example). Other afferent signals, concerning, for example, the nutrient load in the small intestine, or the acidity of the stomach, do not normally reach consciousness. In turn, the CNS provides signals to control the intestine, which are, in most cases, relayed through the ENS. For example, the sight and smell of food elicits preparatory events in the gastro-intestinal tract, including salivation and gastric acid secretion. This is termed the cephalic phase of digestion. Swallowed food stimulates the pharynx and upper esophagus, eliciting afferent signals that are integrated in the brainstem, and subsequently provide efferent signals to enteric neurons in the stomach that cause acid secretion and increased gastric volume, in preparation for the arrival of the food. At the other end of the gut, signals from the colon and rectum are relayed to defecation centres in the spinal cord, from which a programmed set of signals is conveyed to the colon, rectum and anal sphincter to cause defecation. The defecation centres are under inhibitory control from higher CNS regions, and inhibition that can be released when it is chosen to defecate. The other central influences are through sympathetic pathways, which have been discussed under the sections on control of motility and regulation of fluid exchange and local blood flow, above.
