Regulation of pepsinogen secretion by the peptic cells in the oxyntic glands is much less complex than regulation of acid secretion; it occurs in response to two types of signals: (1) stimulation of the peptic cells by acetylcholine released from the vagus nerves or fromthe gastric enteric nervous plexus, and (2) stimulation of peptic cell secretion in response to acid in the stomach. The acid probably does not stimulate the peptic cells directly but instead elicits additional enteric nervous reflexes that support the original nervous signals to the peptic cells. Therefore, the rate of secretion of pepsinogen, the precursor of the enzyme pepsin that causes protein digestion, is strongly influenced by the amount of acid in the stomach. In people who have lost the ability to secrete normal amounts of acid, secretion of pepsinogen is also decreased, even though the peptic cells may otherwise appear to be normal.
Gastric secretion is said to occur in three “phases” (as shown in Figure 64–7): a cephalic phase, a gastric phase, and an intestinal phase.
Cephalic Phase. The cephalic phase of gastric secretionoccurs even before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food, and the greater the appetite, the more intense is the stimulation. Neurogenic signals that cause the cephalic phase of gastric secretion origi-nate in the cerebral cortex and in the appetite centers of the amygdala and hypothalamus. They are trans-mitted through the dorsal motor nuclei of the vagi and thence through the vagus nerves to the stomach. This phase of secretion normally accounts for about 20 per cent of the gastric secretion associated with eating a meal.
Gastric Phase. Once food enters the stomach, it excites (1) long vagovagal reflexes from the stomach to the brain and back to the stomach, (2) local enteric reflexes, and (3) the gastrin mechanism, all of which in turn cause secretion of gastric juice during several hours while food remains in the stomach. The gastric phase of secretion accounts for about 70 per cent of the total gastric secre-tion associated with eating a meal and therefore accounts for most of the total daily gastric secretion of about 1500 milliliters.
Intestinal Phase. The presence of food in the upperportion of the small intestine, particularly in the duo-denum, will continue to cause stomach secretion of small amounts of gastric juice, probably partly because of small amounts of gastrin released by the duodenal mucosa.
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