Gastric acid cephalic phase

Gastric secretion and motility are controlled by both nervous and hormonal mechanisms. The vagus initiates the cephalic phase of secretion in response to the sight or smell of food, before food is eaten. The gastric phase occurs when food enters the stomach and is controlled both by intrinsic nerve reflexes in the stomach wall and released gastrin. The third, or intestinal, phase of secretion is coordinated by nervous and hormonal mechanisms to limit the release of the chyme from the stomach into the duodenum and reduces further acid secretion. 

The cephalic phase of digestion stimulates only a fraction of the maximum possible levels of gastric and pancreatic secretions. This phase does not seem to produce a rise in the levels of gastrin and CCK. Cephalic stimulation of the pancreas, as mediated by the vagus nerve, provokes release of pancreatic enzymes into the small intestine. Cephalic stimulation of the parietal cells, as mediated by the vagus nerve, provokes release of gastric acid into the lumen of the stomach- In humans, the cephalic phase does not seem to result in release of bicarbonate into the lumen of the small intestine. 

The following experiment describes the release of gastric acid during the cephalic phase. The study involved human subjects who volunteered to have a tube inserted into a nostril and pushed down into the stomach, a standard procedure for collecting stomach juices. The subjects were instructed to be silent, to discuss sports, or to discuss food. Samples were removed at 15-minute inter als during the course of the experiment. The concentration of hydrogen ions (H" ) in the samples was used to calculate the rate of acid release. 

Feldman, M and Richardson, C. T. (1986). Rnic of thought, sight, smell, and taste of food in the cephalic phase of gastric acid secretion in humans Qi irocnlero ogy 90,428-433. 

The basal rate of hydrochloric acid secretion varies diurnally, being highest in the evening and lowest in the morning. After ingestion of a meal, the rate of acid secretion in the stomach increases. The three phases of increased acid secretion in response to food are the cephalic phase (before food reaches the stomach), the gastric phase (elicited by the presence of food in the stomach), and the intestinal phase (elicited by input from the duodenum and upper jejunum). 

Gastric acid secretion is stimulated by food-related signals that stimulate the release of acid from specialized cells within the stomach. This secretory process has been divided into three phases cephalic, gastric, and intestinal, with each phase leading to different amounts of acid being secreted (Table 3.3). 

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