Cephalic Phase of Digestion

The cephalic phase begins before food enters the stomach and is triggered by food-related sensory stimuli, including sight and smell. These sensory signals travel to the brain, where taste receptors intensify them during eating. The cerebral cortex processes these stimuli and relays them to the hypothalamus and medulla, preparing the stomach for digestion.

The central nervous system controls the cephalic phase through neural pathways. Sensory signals reach the cerebral cortex, hypothalamus, and medulla, which then transmit neural output via parasympathetic fibers of the vagus nerve. This stimulates mucous, chief, parietal, and G cells in the stomach to prepare gastric juice for digestion.

The cephalic phase activates four types of stomach cells: G cells secrete gastrin, chief cells release pepsinogen, parietal cells release gastric acid, and mucous cells produce mucus. These secretions combine to form gastric juice, which prepares the stomach for food digestion and can be produced at rates of about 500 mL per hour.

Gastrin is a hormone secreted by G cells in response to vagus nerve stimulation during the cephalic phase. It stimulates chief cells to release pepsinogen and parietal cells to release gastric acid. Gastrin essentially amplifies the stomach's digestive preparation, ensuring adequate enzyme and acid production before food arrival.

When food enters the stomach, it triggers stretch receptors in the stomach wall, activating long reflexes to the medulla. The medulla responds by stimulating increased gastric juice production via the vagus nerve for several minutes. This reflex reinforces and extends the digestive preparation initiated during the cephalic phase.

Gastric juice is composed of mucus from mucous cells, pepsinogen from chief cells, and gastric acid from parietal cells. These three components combine to create a protective and digestive fluid. The mucus protects the stomach lining while pepsinogen and acid work together to break down proteins.

The cephalic phase lasts approximately one to two minutes and results in gastric juice production at rates of about 500 milliliters per hour, equivalent to roughly two cups per hour. This rapid production prepares the stomach for the incoming food and initiates the digestive process before the gastric phase begins.