Enteric Nervous System

Overview

• Sometimes not included as part of the autonomic nervous system.
• Contains as many neurons as the spinal cord.
• Functions like a "gut brain".

Key Components

• Myenteric Nerve Plexus

  • Regulates motility.
  • Located between inner circular and outer longitudinal layers of muscle.
  • Known as the Auerbach plexus.

• Submucosal Nerve Plexus

  • Regulates secretions.
  • Found in the submucosa.
  • Known as the Meissner's plexus.

Layers of the Alimentary Canal

1. Mucosa

   • Epithelium
   • Lamina propria (areolar connective tissue)
   • Muscularis mucosa

2. Submucosa

   • Areolar and some dense connective tissue.
   • Contains glands and the submucosal plexus.

3. Muscularis Externa

   • Inner circular layer: acts like a sphincter, forms constriction rings.
   • Outer longitudinal layer: opens lumen in front of bolus.
   • Contains the myenteric plexus.

4. Serosa/Adventitia

   • Serosa (intraperitoneal; has mesentery)
   • Adventitia (retroperitoneal; anchored to posterior abdominal wall)

Function and Regulation

• Intrinsic System: Operates independently.

  • Responds to stretch and chemical stimuli.
  • Stretch receptors: Located in muscularis externa, activate myenteric plexus.
  • Chemoreceptors: Respond to chemical changes (e.g., fatty acids, glucose).

• Mechanism of Action

  • Stretch activates ascending fibers (oral side) to contract circular muscle (via acetylcholine, substance P) and inhibit longitudinal muscle (via VIP, nitric oxide).
  • Descending fibers (aboral side) relax circular muscle and contract longitudinal muscle.

Effects of Chemoreception

• Stimulates submucosal plexus.
• Can increase gland secretions and blood vessel dilation for absorption.
• Releases hormones like secretin, cholecystokinin (CCK), glucose insulinotropic peptide (GIP).

Enteric Hormones

• Secretin: Stimulates bile production, pancreatic bicarbonate.
• CCK: Stimulates gallbladder contraction, enhances liver bile production, stimulates pancreatic enzymes, relaxes sphincter of Oddi.
• GIP: Stimulates insulin production, inhibits gastric motility.

Extrinsic Modulation

• Sympathetic Nervous System

  • Inhibits motility, secretions, absorption.
  • Constricts blood flow and certain sphincters.
  • Involves prevertebral ganglion (e.g., celiac, superior mesenteric).

• Parasympathetic Nervous System

  • Stimulates motility, secretions, absorption.
  • Relaxes sphincters.
  • Mainly via the vagus nerve (intramural ganglion function).

Reflexes

• Short Reflexes: Local reflexes involving intrinsic plexuses.
• Long Reflexes: Involve CNS, such as vagal reflex and enterogastric reflex.
  • Vagal Reflex: Afferent and efferent via vagus nerve, affects myenteric and submucosal plexus.
  • Enterogastric Reflex: Regulates gastric emptying by activating sympathetic responses.