Britannica Sources used in this answer

How can the sight and smell of food trigger gastric juice secretion?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of the human digestive system viewed from the front, showing and labeling organs including the mouth, teeth, tongue, salivary glands, pharynx, esophagus, liver, stomach, gallbladder, pancreas, large intestine, small intestine, appendix, rectum, and anus. — digestion The human digestive system as seen from the front.

The sight and smell of food trigger the secretion of gastric juice through the cephalic phase of gastric secretion. This process is entirely reflex in origin and is mediated by the vagus nerve.1

When sensory stimuli—such as the sight, smell, taste, or even the thought of food—are received, they excite the vagus nerve, which in turn stimulates the gastric glands in the stomach lining to produce and secrete gastric juice. This physiological response prepares the stomach for the arrival of food by initiating the production of digestive substances, including hydrochloric acid and enzymes like pepsin.21

This mechanism was famously demonstrated by the Russian physiologist Ivan Pavlov, who showed that such sensory inputs act as triggers for the digestive system before food even enters the stomach.13