The cephalic phase of digestion is the gastric secretion that occurs even before food enters the stomach, especially while it is being eaten. It results from the sight, smell, thought, or taste of food, and the greater the appetite, the more intense is the stimulation. Neurogenic signals that cause the cephalic phase of gastric secretion originate from the cerebral cortex and in the appetite centers of the amygdala and hypothalamus.They are transmitted through the dorsal nucleus of vagus nerve, and then through the vagus nerve to the stomach. This phase of secretion normally accounts for about 20 percent of the gastric secretion associated with eating a meal. Gastric secretion in response to sensory stimuli prepares the gastrointestinal tract for food processing. These cephalic phase responses are both innate and acquired.
This enhanced secretory activity brought on by the thought or sight of food is a conditioned reflex. It only occurs when we like or want food. When appetite is depressed this part of the cephalic reflex is inhibited.
Effects on gastric secretion
The cephalic phase induces ECL cells in the gastric glands to secrete histamine and increase HCl in the stomach. There will also be an influence on G cells to increase gastrin circulation. It will also stimulate Chief cells to release Pepsinogen.
Chain of events (outline)
Thinking of food (i.e. smell, sight) – Stimulates cerebral cortex – Sends messages to hypothalamus – to the medulla oblongata – to parasympathetic nervous system by the vagus nerve – to stomach (Gastric Glands in walls of fundus and body of stomach) – to secrete gastric juice. (nervous system and hormone system)
a. Food enters stomach – stomach stretches and activates stretch Receptors – sends message to the medulla oblongata (vagus nerve) and then back – to stomach by the vagus nerve — Result: parietal cell secrete gastric juice.
b. Chemical Stimuli (i.e. partially digested proteins, caffeine) – directly activates – G cells (Enteroendocrine Cells) located in the pyloric region of the stomach – to secrete gastrin – Gastrin stimulates – Gastric Glands to secrete gastric Juice.