The process of digestion is composed of three stages – cephalic, gastric, and intestinal – each with a distinct control center. The cephalic phase is the first stage, and it starts even before the food enters the stomach. It is controlled by the central nervous system and is initiated by any food-related sensory stimuli, such as the sight and smell of food, which send signals to the brain. While eating, the taste receptors intensify these signals, which travel to the cerebral cortex and then to the hypothalamus and medulla, where they are processed. These triggers prepare the stomach for digestion.
The neural output travels through the parasympathetic fibers of the vagus nerve, stimulating the mucous, chief, parietal, and G cells in the stomach. The G cells secrete gastrin, which in turn stimulates chief cells to release enzymes and parietal cells to release gastric acid. The mucus secreted by the mucous cells mixes with the secretions of parietal and chief cells, forming gastric juice.
When the food enters the stomach, it triggers the stretch receptors in the stomach wall, activating the long reflex to the medulla. In turn, the medulla responds via the vagus nerve by stimulating gastric glands to increase gastric juice production.
Overall, the cephalic phase lasts over 1-2 minutes and results in the production of gastric juice at rates of about 500 mL/h, or about 2 cups per hour.
Digestion has three overlapping phases — cephalic, gastric, and intestinal — based on the location of their control center.
The cephalic phase begins before food enters the stomach and is controlled by the central nervous system.
Food-related sensory stimuli, including sight and smell, can initiate the cephalic phase.
Additionally, the taste receptors intensify the signals to the brain while eating.
From the cerebral cortex, these stimuli are relayed to the hypothalamus and the medulla.
The neural output is transmitted via the parasympathetic fibers of the vagus nerve, stimulating the mucous, chief, parietal, and G cells in the stomach.
The G cells of gastric glands secrete gastrin, which stimulates the chief cells to release pepsinogen and the parietal cells to release gastric acid.
The mucus from the mucous cells, combined with the secretions of the parietal and the chief cells, forms gastric juice.
Upon entering the stomach, food triggers the stretch receptors in the wall, activating the long reflexes to the medulla.
In turn, the medulla stimulates increased production of gastric juice via the vagus nerve for a few minutes.
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