Overview
This lecture explains the regulation of gastric secretions, focusing on the cephalic and gastric phases—covering neural and hormonal stimuli, key cell types, and inhibition mechanisms.
Gastric Secretion Phases
- Gastric secretion has three phases: cephalic, gastric, and intestinal (this lecture covers cephalic and gastric).
- Cephalic phase accounts for about one-third and gastric phase for two-thirds of gastric juice secretion.
Cephalic Phase
- Stimuli: sight, thought, smell, and taste of food stimulate the cerebral cortex.
- Stimulated cortex signals the hypothalamus, then the medulla, activating the vagus nerve.
- The vagus nerve stimulates parietal cells (secrete HCl) and chief cells (secrete pepsinogen) before food enters the stomach.
Gastric Phase
- Triggered by stomach distension (stretch) and presence of food.
- Stretch receptors activate vagovagal reflex (long arc) and enteric neurons (short arc).
- G cells in the antrum are stimulated by partially digested proteins to release gastrin.
- Gastrin stimulates parietal cells (HCl) and chief cells (pepsinogen).
- Protein buffering affects pH; high pH promotes pepsinogen conversion to pepsin.
Regulation and Inhibition
- Parasympathetic (vagus nerve) stimulates, while sympathetic activity (stress, emotion) inhibits gastric secretion.
- Somatostatin (from D cells) is released in response to high proton concentration (low pH) and inhibits gastrin and acid secretion.
- Other inhibitors: prostaglandins (PGE2) reduce HCl via EP3 receptors.
Cellular Mechanisms
- Parietal cells use carbonic anhydrase to form H+ and HCO3−; H+ secreted into stomach, HCO3− into blood (alkaline tide).
- Proton/K+ ATPase (proton pump) enables gastric acid secretion.
- Histamine from ECL cells stimulates acid and pepsinogen release via H2 receptors.
- Chief cell pepsinogen release stimulated by gastrin, acetylcholine, and histamine.
Protective Mechanisms
- Mucus (from foveolar and neck cells) forms the mucosal barrier, protecting the stomach lining from HCl and pepsin.
- Mucus contains water, electrolytes, phospholipids, mucins, and bicarbonate.
Key Terms & Definitions
- Parietal cells — secrete hydrochloric acid (HCl) and intrinsic factor.
- Chief cells — secrete pepsinogen, the inactive form of pepsin.
- G cells — enteroendocrine cells releasing gastrin hormone.
- D cells — secrete somatostatin, inhibiting gastric secretion.
- ECL cells — enterochromaffin-like cells that release histamine.
- Vagus nerve — cranial nerve X, major parasympathetic stimulator of gastric secretion.
- Gastrin — hormone stimulating acid and pepsinogen release.
- Somatostatin — hormone inhibiting gastrin and acid secretion.
- Proton pump (H+/K+ ATPase) — enzyme in parietal cells pumping H+ into the stomach.
- Mucosal barrier — protective mucus layer lining the stomach.
Action Items / Next Steps
- Review the role of the intestinal phase of gastric secretion in the next lecture.
- Study the detailed mechanisms of mucosal barrier protection.