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.