Hormonal Control of Gastrointestinal Motility
We discuss the extreme importance of several hormones for controlling gastrointestinal secretion. Most of these same hormones also affect motility in some parts of the gastrointestinal tract. Although the motility effects are usually less important than the secretory effects of the hormones, some of the more important of them are the following.
Gastrin is secreted by the “G” cells of the antrum of the stomach in response to stimuli associated withingestion of a meal, such as distention of the stomach, the products of proteins, and gastrin releasing peptide, which is released by the nerves of the gastric mucosa during vagal stimulation. The primary actions of gastrin are (1) stimulation of gastric acid secretion and(2) stimulation of growth of the gastric mucosa.
Cholecystokinin is secreted by “I” cells in the mucosa of the duodenum and jejunum mainly inresponse to digestive products of fat, fatty acids, and monoglycerides in the intestinal contents. This hormone strongly contracts the gallbladder, expelling bile into the small intestine where the bile in turn plays important roles in emulsifying fatty substances, allow-ing them to be digested and absorbed. Cholecystokinin also inhibits stomach contraction moderately. There-fore, at the same time that this hormone causes emp-tying of the gallbladder, it also slows the emptying of food from the stomach to give adequate time for diges-tion of the fats in the upper intestinal tract.
Secretin was the first gastrointestinal hormone dis-covered and is secreted by the “S” cells in the mucosa of the duodenum in response to acidic gastric juiceemptying into the duodenum from the pylorus of the stomach. Secretin has a mild effect on motility of the gastrointestinal tract and acts to promote pancreatic secretion of bicarbonate which in turn helps to neu-tralize the acid in the small intestine.
Gastric inhibitory peptide is secreted by the mucosa of the upper small intestine, mainly in response to fattyacids and amino acids but to a lesser extent in response to carbohydrate. It has a mild effect in decreasing motor activity of the stomach and therefore slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food products.
Motilin is secreted by the upper duodenum duringfasting, and the only known function of this hormone is to increase gastrointestinal motility. Motilin is released cyclically and stimulates waves of gastroin-testinal motility called interdigestive myoelectric com-plexes that move through the stomach and smallintestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by mechanisms that are not fully understood.
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