Cephalic Phase

The cephalic phase of gastric regulation refers to control by the brain via the vagus nerve. As previously discussed, various conditioned stimuli can evoke gastric secretion. This conditioning in humans is, of course, more subtle than that exhibited by Pavlov's dogs in response to a bell. In fact, just talking about appetizing food is sometimes a more potent stimulus for gastric acid secretion than the actual sight and smell of food!

Activation of the vagus nerve (1) stimulates the chief cells to secrete pepsinogen; and (2) indirectly stimulates the parietal cells to secrete HCl. The vagus nerve endings directly stimulate G cells to secrete gastrin and the ECL cells to secrete histamine.

Table 18.6 The Three Phases of Gastric Secretion

Phase of Regulation

Description

Cephalic Phase

1. Sight, smell, and taste of food cause stimulation of vagus nuclei in brain

2. Vagus stimulates acid secretion a. Indirect stimulation of parietal cells (major effect)

b. Stimulation of gastrin secretion (lesser effect)

Gastric Phase

1. Distension of stomach stimulates vagus nerve; vagus stimulates acid secretion

2. Amino acids and peptides in stomach lumen stimulate acid secretion a. Direct stimulation of parietal cells (lesser effect)

b. Stimulation of gastrin secretion; gastrin stimulates acid secretion (major effect)

3. Gastrin secretion inhibited when pH of gastric juice falls below 2.5

Intestinal Phase

1. Neural inhibition of gastric emptying and acid secretion a. Arrival of chyme in duodenum causes distension, increase in osmotic pressure b. These stimuli activate a neural reflex that inhibits gastric activity

2. In response to fat in chyme, duodenum secretes a hormone that inhibits gastric acid secretion

The gastrin secreted by G cells enters the systemic circulation and is carried back to the stomach, where it also stimulates the ECL cells to release histamine. Histamine, in turn, activates H2 histamine receptors on the parietal cells to stimulate acid secretion (fig. 18.30).

This cephalic phase continues into the first 30 minutes of a meal, but then gradually declines in importance as the next phase becomes predominant.

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