The cephalic phase of gastric secretion 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 motor nuclei of the vagi 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.
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
Cephalic phase cause ECL cells to secrete histamine and increase HCl acid 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.