Gastric Secretions: Cephalic and Gastric Phases

Overview

• Gastric secretion has three phases: cephalic, gastric, and intestinal.
• This lecture focuses on the cephalic and gastric phases.
• Cephalic phase accounts for about 1/3 of gastric juice secretion.
• Gastric phase accounts for 2/3 of gastric acid secretion.

Cephalic Phase

• Begins before food enters the stomach.
• Stimuli for Cephalic Phase:
  • Sight of food.
  • Thought of food.
  • Smell of food.
  • Taste of food.
• Pathway:
  • Stimuli activate the cerebral cortex.
  • Information sent to the hypothalamus and medulla.
  • Activation of the dorsal nucleus of Vagus (DNV).
  • Vagus nerve stimulates gastric secretions.
  • Targets parietal cells (secrete HCl) and chief cells (secrete pepsinogen) in stomach.

Inhibition of Cephalic Phase

• Activation of the sympathetic nervous system (e.g., stress) inhibits gastric secretions.

Gastric Phase

• Occurs when food is in the stomach.
• Stimuli for Gastric Phase:
  • Distension of stomach (stretch receptors).
  • Partially digested proteins stimulate G cells to release gastrin.
• Pathway:
  • Stretch receptors activate the vagus nerve.
  • Gastrin stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen.
  • Gastrin operates via CCK2 receptors on parietal cells and CCK1 receptors on chief cells.

Inhibition of Gastric Phase

• Sympathetic nervous system activation inhibits gastric secretions.
• Somatostatin is released in response to low pH (high proton concentration) and inhibits gastrin release.

Mechanism of HCl Production

• Parietal cells use CO2 and H2O to form H2CO3, which dissociates into protons and bicarbonate.
• Protons secreted into stomach lumen via a proton pump.
• Bicarbonate enters the bloodstream, causing an alkaline tide.
• Chloride ions secreted into the stomach lumen, forming HCl with protons.

Additional Factors Affecting Secretion

• Histamine: Stimulates via H2 receptors.
• Prostaglandins (PGE2): Inhibit via EP3 receptors.
• Acetylcholine: Stimulates via M3 receptors.
• Somatostatin: Inhibits via SST receptors.

Protection of Stomach Lining

• Mucosal Barrier:
  • Formed by foveolar cells and mucous neck cells.
  • Contains water, electrolytes, phospholipids, mucin proteins, and bicarbonate.
  • Protects against damage from HCl and pepsin.

Conclusion

• This lecture covered the cephalic and gastric phases of gastric secretion.
• Importance of neural and hormonal regulation on gastric juice production.
• Future lectures will cover the intestinal phase.