Digestion and the Story of Alexis St. Martin

The Story of Alexis St. Martin

• Summer of 1822: French-Canadian fur-trapper Alexis St. Martin was shot in the stomach near Lake Michigan.
• Dr. William Beaumont saved St. Martin and conducted numerous surgeries on him.
• St. Martin was left with a hole (fistula) in his abdominal wall, allowing visibility into his stomach.
• Beaumont conducted 238 experiments on St. Martin, studying his digestion and gastric juices.
• Key discoveries:
  • The stomach uses strong acids and muscular contractions to break down food.
  • Variations in digestibility of different foods.
  • The brain's influence on digestion.
• Beaumont's methods revolutionized physiology.
• St. Martin lived to the age of 83, remaining in good health.

The Digestive Process

• The digestive system disassembles food into molecular forms.
• Mechanisms:
  • Mechanical: Roughing up food, mixing with enzymes, forming a creamy paste.
  • Chemical: Killing harmful invaders.

Stomach's Functions

• Interior Layers: Mucosa, submucosa, muscularis externa, and serosa.
• Special Modifications:
  • Extra smooth muscle layer for mixing.
  • Mucous cells protecting stomach tissues from digestion.
  • Gastric pits leading to gastric glands with secretory cells:
    • Parietal Cells: Release hydrochloric acid.
    • Chief Cells: Secrete pepsinogen, creating pepsin with hydrochloric acid.
    • Enteroendocrine Cells: Regulate hormones like serotonin, histamine, somatostatin, and gastrin.

Neural and Hormonal Regulation

• Cephalic Phase: Sensory input (sight, smell, taste) triggers gastric secretions.
• Gastric Phase: Stretch receptors and chemical changes stimulate digestion.
• Intestinal Phase: Regulates stomach emptying to avoid overloading the small intestine.

Functions of the GI Tract

• Mouth: Stratified squamous epithelium protects against friction.
  • Teeth: 32 teeth for chewing (mastication).
  • Tongue: Repositions food, forming a bolus for swallowing.
  • Salivary Glands: Secrete saliva with enzymes like salivary amylase breaking down starches.
• Esophagus: Smooth muscle-lined tube propelling food to the stomach via peristalsis.
• Stomach: Converts food to chyme, a creamy paste, and acts as a decontamination tank.

Vomiting

• Triggered by overeating, irritants, toxins, or stress.
• Brain influences through fight or flight signals to the stomach.
• Beaumont noted illness/stress impact digestion in St. Martin.

Conclusion

• Mechanical and chemical digestion begins in the mouth and continues in the stomach.
• Stomach’s phases of regulation: cephalic, gastric, and intestinal.