Digestive System Lecture: Layers of the GI Tract

Overview

• The gastrointestinal (GI) tract comprises four layers, each playing a role in digestion and absorption.
• The elementary canal includes structures such as the pharynx and esophagus.
• The canal forms a continuous tube open to the environment at both ends.

Four Layers of the GI Tract

1. Mucosa (deepest layer)

   • Involved in secretion and absorption.
   • Comprised of:
     • Epithelium: Contains cells for absorption and secretion (simple columnar cells) or protection (stratified squamous cells).
     • Lamina propria: Loose fibrous connective tissue.
     • Muscularis mucosa: Thin layer of smooth muscle aiding in contractions.

2. Submucosa

   • Contains blood and lymphatic vessels for absorbing digestive contents.
   • Includes exocrine glands releasing contents into the lumen via ducts.
   • Managed by the submucosal plexus (nerves).

3. Muscularis

   • Consists of two layers of smooth muscle (outer longitudinal and inner circular) except in the stomach.
   • Controlled by nerve plexuses aiding in digestive tract movements.

4. Serosa (most superficial layer)

   • Continuous with the mesenteries.
   • Contains blood vessels, lymphatics, and nerves.
   • Secretes fluid to reduce abdominal cavity friction.

Peritoneum

• A large continuous sheet of serous membrane in the abdominopelvic cavity.
• Consists of:
  • Parietal Peritoneum: Lines abdominal pelvic cavity walls.
  • Visceral Peritoneum: Covers organs.

Mesentery

• Projection of the parietal peritoneum.
• Allows intestine movement and acts as a pathway for vessels and nerves.
• Prevents abnormal coiling or strangulation.

Innervation of the Digestive System

• Controlled by the autonomic nervous system (ANS) with different divisions:
  • Enteric Nervous System: Located entirely within the GI tract walls, regulates digestive function independently of the CNS.
  • Parasympathetic Nervous System: Promotes GI functions during rest via vagal and pelvic nerves.
  • Sympathetic Nervous System: Inhibits GI function during stress or exercise via sympathetic ganglia.