Regulation and Aging in Gastric Secretion and Mobility

Three Phases of Gastric Secretion and Mobility

1. Cephalic Phase

   • Triggered by the sight, smell, and taste of food.
   • Involves sensory receptors in the head preparing the digestive system.
   • Stimulates the parasympathetic nervous system.
   • Salivary glands stimulated by facial, glossopharyngeal, and vagus nerves.
   • Increases stomach muscle and glandular activity.

2. Gastric Phase

   • Activated by stomach distension and changes in pH detected by stretch and chemoreceptors.
   • Leads to vigorous peristalsis, churning, and glandular secretions.
   • Endocrine influence: Gastrin is released into the bloodstream due to distension and substances like caffeine and protein, increasing gastric secretions and motility.

3. Intestinal Phase

   • Triggered by food moving into the small intestine and detected by stretch receptors and chemoreceptors for fatty acids and glucose.
   • The sympathetic nervous system slows stomach activity and increases intestinal activity.
   • Enterogastric Reflex inhibits gastric motility, increases pyloric sphincter contraction, and decreases gastric emptying.

Hormonal Influences

• Secretin: Stimulates pancreatic juice flow with bicarbonate; inhibits gastric juice secretion.
• Cholecystokinin (CCK): Decreases stomach emptying; stimulates pancreatic enzyme secretion and bile flow.
• Gastric Inhibitory Peptide: Decreases stomach secretions and motility.

Absorption

• Water, electrolytes, and some drugs (e.g., aspirin, alcohol) are absorbed in the stomach.
• Alcohol absorption affected by stomach contents (e.g., presence of fat).

Regulation of Chyme Release

• Controlled by neural and hormonal reflexes.
• Involves gastric emptying regulation through vagus nerve signaling and pyloric valve relaxation.
• Enterogastric Reflex slows stomach release to allow digestion in the small intestine.

Emesis (Vomiting)

• Forceful expulsion of stomach and duodenal contents.
• Triggered by various stimuli (e.g., unpleasant sights, anesthesia, dizziness, drugs).
• Controlled by the medulla, causing muscle contraction and sphincter relaxation.
• Potential consequences include fluid loss and alkalosis.

Aging and the GI Tract

• Decreased secretions and motility, leading to constipation and related issues.
• Loss of muscle tissue strength and tone.
• Common problems include sores, periodontal disease, taste decrease, swallowing difficulty, hernias, gastritis, ulcers, malabsorption, jaundice, cirrhosis, pancreatitis, hemorrhoids, and constipation.
• Increased risk of colon cancer as age advances; regular colonoscopies recommended after age 50.

Goals of the Lecture

• Describe the three phases of digestion.
• Define enterogastric reflex and emesis.
• List changes in the GI tract due to aging.