GastroIntestinal Notes

Department Overview

  • Department of Biomedical Sciences

    • College of Veterinary Medicine, Tuskegee University

    • Tuskegee, AL 36088

  • Course: Physiology of the Gastrointestinal Tract

    • Instructor: Ayman I. Sayegh

Canine and Equine Gastrointestinal Disorders

  • Canine Gastritis

    • Normal tissue

    • Inflamed tissue

  • Equine Gastric Ulcer

    • Healthy Stomach

    • Stomach Ulcers

Left Displacement of the Abomasum (LDA)

  • Normal Anatomy

  • Abnormal Anatomy

Overall Course Aim

  • To build a foundational understanding of gastrointestinal (GI) physiology to facilitate comprehension of clinical gastroenterology.

Course Outline

  1. General GI Physiology

    • Definition

    • Divisions of the gut

    • Histology of the gut

    • Control systems

  2. Special GI Physiology

    • Motility

    • Secretion

    • Digestion/Absorption

    • Fermentation

General GI Physiology

Definition

  • The gastrointestinal (GI) tract is defined as a tube-like structure extending from the mouth to the anus, responsible for supplying the body with nutrients, water, and electrolytes through the following functions:

    • Digestion

    • Absorption

    • Secretion

    • Motility

Divisions of the Gut

  • Key Structures:

    • Heart

    • Liver

    • Gall Bladder

    • Pancreas

    • Esophagus

    • Hepatic Portal Vein

    • Stomach (including the regions: Duodenum, Jejunum, Cecum, Ileum, Colon, Rectum)

  • Pathway:

    • Food and water enter via oral cavity through the esophagus into the stomach, followed by the small intestine (duodenum, jejunum), large intestine (cecum, ileum, colon), and exit through the rectum.

Histology of the Gut

  • Layers of the Gut Wall:

    1. Serosa

    2. Muscularis Propria: Outer longitudinal and inner circular muscle layers

    3. Myenteric Plexus: Located between the muscle layers, controls motility

    4. Submucosal Plexus: Controls secretion local to the gut

    5. Submucosa

    6. Muscularis Mucosae

    7. Lamina Propria

    8. Mucosa: Includes epithelial cells (enterocytes and endocrine cells)

    9. Lumen of the gut: Structure housing the contents of the GI tract.

Control Systems of Gastrointestinal Functions

  1. Intrinsic Systems (located in the wall of the gastrointestinal tract):

    • Nerves

    • Endocrine secretions (e.g., Secretin, Gastrin, CCK, GIP, Motilin)

  2. Extrinsic Systems (located outside the wall of the gastrointestinal tract):

    • Vagus nerve (Parasympathetic)

    • Splanchnic nerves (Sympathetic)

The Enteric Nervous System (ENS)

  • Comprising two main plexuses:

    • Myenteric Plexus (Auerbach): Controls motility

    • Submucosal Plexus (Meissner): Controls secretion and local blood flow

  • Functions of the Myenteric Plexus include:

    • Increase tonic contraction

    • Increase rhythmic contraction

    • Increase contraction velocity

    • Inhibition of sphincters

  • Function of Submucosal Plexus: Increase blood flow local to the gut

Ion Characteristics in Nervous Control

Enteric vs. Other Nerves

  • Differences in Action Potentials:

    • Enteric nerves:

    • Ion channels are slow to open/close

    • Spike potentials last 10-40 times longer than activation of other muscles

    • Each spike lasts 10-20 milliseconds

    • Smooth muscle action potentials mainly involve Ca++ influx

    • Other muscles:

    • Involve Na+ influx

Dendritic Structure in Nerves

  • The structure includes:

    • Dendrites

    • Cell body

    • Presynaptic terminals

    • Axon

    • Varicosities

Varicosities and Their Importance

  • Varicosities enable neurotransmitter release to smooth muscle cells via a diffusion mechanism, influencing smooth muscle contraction.

Classification of Enteric Neurons

  • Neuronal types can be classified morphologically, electrophysiologically, and chemically.

  • Morphological types include Dogiel Type 1 (inhibitory motor neurons involved in inducing hyperpolarization in target cells) and others.

Gastric Hormones Overview

Types of Gastrointestinal Hormones

  1. Gastrin

  2. Gastric Inhibitory Peptide (GIP)

  3. Secretin

  4. Cholecystokinin (CCK)

  5. Motilin

Functions of Gastrointestinal Hormones

  • Hormones are secreted in response to stimuli (e.g., food) and affect various gut functions.

Actions of Gastrointestinal Hormones

  • Gastrin: Stimulates acid secretion

  • GIP: Inhibits acid secretion, stimulates insulin secretion

  • Secretin: Acts as nature's antacid; inhibits gastrin

  • CCK: Stimulates gallbladder contraction and pancreatic secretion, inhibits gastric emptying

  • Motilin: Is involved in regularizing gut motility during fasting periods.