Regulation of Digestion

The activities of the digestive system are regulated by both hormones and neural reflexes. Four important hormones and their effects upon target cells follow.

·Gastrin is produced by enteroendocrine cells of the stomach mucosa. Effects include


oStimulation of gastric juice (especially HCl) secretion by gastric glands.
oStimulation of smooth muscle contraction in the stomach, small intestine, and large intestine, which increases gastric and intestinal motility.
oRelaxation of the pyloric sphincter, which promotes gastric emptying into the small intestine.


·Secretin is produced by the enteroendocrine cells of the duodenal mucosa. Effects include


oStimulation of bicarbonate secretion by the pancreas, which neutralizes the acidity of chyme when released into the duodenum.
oStimulation of bile production by the liver.
oInhibition of gastric juice secretions and gastric motility, which, in turn, slows digestion in the stomach and retards gastric emptying.


·Cholecystokinin (CCK) is produced by enteroendocrine cells of the duodenal mucosa. Effects include


oStimulation of bile release by the gallbladder.
oStimulation of pancreatic juice secretion.
oRelaxation of the hepatopancreatic ampulla, which allows flow of bile and pancreatic juices into the duodenum.


·Gastric inhibitory peptide (GIP) is produced by enteroendocrine cells of the duodenal mucosa and causes the inhibition of gastric juice secretion and gastric motility, which, in turn, slows digestion in the stomach and retards gastric emptying.


The second regulatory agent of the digestive system is the nervous system. Stimuli that influence digestive activities may originate in the head, the stomach, or the small intestine. Based on these sites, there are three phases of digestive regulation:
·The cephalic phase comprises those stimuli that originate from the head: sight, smell, taste, or thoughts of food, as well as emotional states. In response, the following reflexes are initiated:


oNeural response. Stimuli that arouse digestion are relayed to the hypothalamus, which, in turn, initiates nerve impulses in the parasympathetic vagus nerve. These impulses innervate nerve networks of the GI tract (enteric nervous system), which promote contraction of smooth muscle (which causes peristalsis) and secretion of gastric juice. Stimuli that repress digestion (emotions of fear or anxiety, for example) innervate sympathetic fibers that suppress muscle contraction and secretion.
oGeneral effects. The stomach prepares for the digestion of proteins.


·The gastric phase describes those stimuli that originate from the stomach. These stimuli include distention of the stomach (which activates stretch receptors), low acidity (high pH), and the presence of peptides. In response, the following reflexes are initiated:


oNeural response. Gastric juice secretion and smooth muscle contraction are promoted.
oHormonal response. Gastrin production is promoted.
oGeneral effects. The stomach and small intestine prepare for the digestion of chyme, and gastric emptying is promoted.


·The intestinal phase describes stimuli originating in the small intestine. These include distention of the duodenum, high acidity (low pH), and the presence of chyme (especially fatty acids and carbohydrates). In response, the following reflexes are initiated:


oNeural response. Gastric secretion and gastric motility are inhibited (enterogastric reflex). Intestinal secretions, smooth muscle contraction, and bile and pancreatic juice production are promoted.
oHormonal response. Production of secretin, CCK, and GIP is promoted.
oGeneral effects. Stomach emptying is retarded to allow adequate time for digestion (especially fats) in the small intestine. Intestinal digestion and motility are promoted.