secretion, regulation and motility

Regulation of the GI Tract

Enteric Nervous System

  • The enteric nervous system (ENS) is composed of the submucosal plexus and the myenteric plexus.

  • Together, they regulate the activity and contraction of smooth muscle within the layers of the GI tract.

  • Functions:
      - Mixes the contents within the lumen.
      - Ensures forward movement of the contents at appropriate times.

Hormonal Regulation

  • Hormones play a crucial role in digestion, released by the stomach and small intestine. Key hormones include:
      - Gastrin: Secreted by the stomach.
      - Cholecystokinin (CCK): Released by the duodenum and jejunum of the small intestine.
      - Secretin: Also released from the small intestine.

Phases of GI Tract Control

The control of the GI tract occurs in three phases:

  1. Cephalic Phase
       - Triggered by the central nervous system.
       - Sensations such as thought, sight, or smell of food stimulate salivation and prepare the stomach.
       - G cells in the stomach release gastrin, initiating gastric phase.

  2. Gastric Phase
       - Originates from within the stomach.
       - Gastrin prepares the stomach for digestion upon food intake.
       - Gastrin promotes secretion of gastric juices, including hydrochloric acid and pepsinogen.

  3. Intestinal Phase
       - Initiates when chyme enters the small intestine.
       - Hormones secreted include secretin and CCK to facilitate digestion and nutrient absorption.

Short-Term Regulation of Food Intake

  • Mechanisms of short-term regulation include:
      - Orexogenic Factors: Signal hunger. Example: Ghrelin released when the stomach is empty.
      - Satiety Factors: Signal fullness. Examples include:
        - Insulin: Released in response to elevated blood glucose levels.
        - CCK: Released when fats are present in the intestine.

  • Neural input from mechanoreceptors and chemoreceptors also affects satiety.
      - Stomach overstretch signals fullness to the brain.

Gastric Secretions and Regulation

  • Cephalic Phase Stimulations: Stimuli such as smelling or thinking about food increase gastric acid and pepsinogen secretion.

  • Parasympathetic Nervous System: Enhances activity and gastric acid secretion through gastrin release from G cells.

  • Stimuli for secretion include:
      - Presence of proteins, peptides, and amino acids in the stomach.
      - Stretching of the stomach from food intake.

Inhibition of Gastric Secretion

  • Movement of chyme to the small intestine reduces stomach stretching and gastrin release.

  • Increased acidity (pH below 2) also inhibits gastrin secretion.

Intestinal Phase Activation

  • The intestinal phase begins with:

  • Increased osmolarity due to nutrients in the small intestine.

  • Presence of fat and amino acids stimulates specific hormonal responses:
      - Secretin: Released in response to acidic chyme, stimulates the pancreas to release bicarbonate to neutralize acidity.
      - Cholecystokinin (CCK): Released due to fats and amino acids, stimulates pancreatic enzymes and bile release.

Bile Release

  • CCK Actions:
      - Stimulates acinar cells to produce lipases and proteases for fat and amino acid breakdown.
      - Triggers bile release from the gallbladder for fat emulsification.

  • Secretin Actions:
      - Causes the liver to release bile but in lower amounts compared to gallbladder stimulation.

Motility and Its Regulation

  • GI motility is primarily due to smooth muscle contraction in the:
      - Muscularis mucosa
      - Muscularis externa

  • Functions of smooth muscle movement include:
      - Mixing and propulsion of GI contents.

Basic Electrical Rhythm

  • Smooth muscle depolarization leads to contraction:
      - Smooth muscle has a basic electrical rhythm characterized by slow spontaneous depolarizations.
      - Depolarization frequency is increased by the parasympathetic nervous system and decreased by the sympathetic nervous system.
      - Prolonged depolarization above a certain threshold results in more action potentials and stronger contractions.

Types of Contractions

  1. Peristalsis
       - Rhythmic contractions cause forward movement through the GI tract, involving coordination of longitudinal and circular muscle contractions.

  2. Segmentation
       - Local contractions that mix intestinal contents, rather than moving it forward, by alternating contractions of circular muscles, producing a churning effect.

  • Both peristalsis and segmentation are governed by waves of contraction in response to the basic electrical rhythm of smooth muscle activity.