3.10

Learning Objectives

  • Understand digestion regulation through local physical and chemical changes as bolus or chyme moves in the GI tract.
  • Describe processes during the cephalic, gastric, and intestinal phases of digestion.
  • Explain how short and long reflex loops and hormones from enteroendocrine cells regulate secretion, motility, and digestion in the GI tract.

GI Tract Structure

  • Layers of the GI Tract

    • Mucosa: Consists of epithelium, lamina propria, and muscularis mucosae.
    • Submucosa: Contains blood vessels, nerves, and connective tissue.
    • Muscularis: Composed of circular and longitudinal muscle layers.
    • Serosa: Outer layer made of areolar connective tissue.

  • Smooth Muscle Characteristics:

    • Myogenic (autonomous) vs. Neurogenic (dependent on nervous system).
    • Involuntary, under autonomic nervous system (ANS) control.
    • Neurotransmitters include ACh (Acetylcholine) and NE (Norepinephrine).

Stomach Histology

  • Types of gastric cells:

    • Surface Mucous Cells: Secrete mucus.
    • Mucous Neck Cells: Secrete mucus.
    • Parietal Cells: Secrete HCl and intrinsic factor.
    • Chief Cells: Secrete pepsinogen and gastric lipase.
    • G Cells: Secrete the hormone gastrin.

  • Histological Structure:

    • Stomach lined with simple columnar epithelium.
    • Contains gastric pits leading to gastric glands.

Digestion Phases

1. Cephalic Phase

  • Triggered by thoughts, sight, smell, and taste of food.
  • Involves:
    • Stimulation of salivary glands (via parasympathetic impulses along vagus nerve).
    • Initiation of gastric juice secretion by gastric glands.

2. Gastric Phase

  • Initiated by entry of food into the stomach increasing gastric activity.
    • Neural influences: Stretch and chemoreceptors (increased pH) signal bolus entry.
    • Endocrine influences: G cells secrete gastrin, which increases gastric secretions and motility.
    • Periodic release of chyme into the duodenum.

3. Intestinal Phase

  • Entry of chyme into the duodenum slows gastric activity and increases intestinal activity.
    • Neural influences: Stretch receptors and chemical contents activate sympathetic nerves, slowing gastric activity (enterogastric reflex).
    • Endocrine influences: Secretin and cholecystokinin (CCK) are released, decreasing stomach activity and emptying.

Reflexes Regulating Digestion

  • Enterogastric Reflex: Regulates chyme release into the duodenum.
  • Gastroileal Reflex: When the stomach is full, gastrin relaxes the ileocecal sphincter, allowing SI emptying into the LI.
  • Gastrocolic Reflex: Strong peristaltic wave moves contents from the transverse colon into the rectum.
  • Defecation Reflex: Stretch receptors signal the sacral spinal cord, leading to the contraction of the rectum and relaxation of the internal anal sphincter.

Pancreatic Secretions Regulation

  • Secretin: Released in response to acidity in the small intestine, stimulates bicarbonate secretion from pancreas.
  • CCK (Cholecystokinin): Released in response to fats and proteins in SI, stimulates release of digestive enzymes.

Bile Secretion Regulation

  • Secretin: Enhances bile flow rich in bicarbonate from the liver.
  • CCK: Causes the gallbladder to contract and release bile into the duodenum.

Digestive Health Concerns

  • Diarrhea: Rapid transit of chyme causing insufficient water absorption.
  • Constipation: Slow bowel movement with excessive water absorption.
  • Fiber Types:
    • Insoluble Fiber: Speeds up transit time (e.g., wheat bran).
    • Soluble Fiber: Delays gastric emptying and reduces blood cholesterol (e.g., oats, beans).