Enteric Nervous System (ENS) – Study Notes

Overview

• ENS is sometimes grouped under the Autonomic Nervous System (ANS) but is unique → often called the “gut brain”.

• Contains as many neurons as the entire spinal cord.

• Can function autonomously but is also modulated by sympathetic and parasympathetic systems.

• Governs GI motility, secretions, absorption, and blood flow.

Histology & Plexuses

Layers of the Alimentary Canal (from lumen outward):

1. Mucosa

   • Epithelium (varies by organ: e.g., esophagus = stratified squamous; intestines = simple columnar).

   • Lamina propria (areolar connective tissue).

   • Muscularis mucosae (thin smooth muscle).

2. Submucosa

   • Areolar + dense connective tissue.

   • Glands and Submucosal (Meissner’s) Plexus → controls secretions & blood flow.

3. Muscularis externa

   • Inner circular smooth muscle.

   • Outer longitudinal smooth muscle.

   • Myenteric (Auerbach’s) Plexus between layers → regulates motility.

   • Stomach exception: has 3 layers (oblique, circular, longitudinal).

4. Serosa (mesothelium + CT) → intraperitoneal.

   • If retroperitoneal → Adventitia instead.

ENS Plexus Functions

• Submucosal (Meissner’s) Plexus:

  • Controls glandular secretions and blood vessel dilation.

• Myenteric (Auerbach’s) Plexus:

  • Coordinates motility (peristalsis & segmentation).

  • Ascending fibers: stimulate contraction behind bolus.

  • Descending fibers: relax in front of bolus.

Reflex Arcs

• Stretch receptors (distension) → activate myenteric plexus → peristalsis.

  • Oral side (behind bolus): Circular muscle contracts (ACh, Substance P). Longitudinal muscle relaxes.

  • Aboral side (in front of bolus): Circular muscle relaxes (VIP, NO). Longitudinal muscle contracts (ACh, Substance P).

• Chemoreceptors (sense fatty acids, glucose, peptides, acid) → activate submucosal plexus:

  • Stimulates glands (e.g., bicarbonate secretion).

  • Dilates blood vessels → ↑ absorption.

  • Stimulates enteroendocrine cells (hormones):

    • S cells → Secretin → ↑ bile synthesis, ↑ pancreatic bicarbonate.

    • I cells → CCK → gallbladder contraction, ↑ pancreatic enzyme secretion, relax sphincter of Oddi.

    • K cells → GIP → ↑ insulin secretion, ↓ gastric motility.

Autonomic Modulation

Sympathetic (T5–L2, thoracolumbar outflow):

• ↓ Motility, ↓ secretions, ↓ absorption.

• Constricts blood vessels (↓ splanchnic flow).

• Constriction of sphincters (exception to general inhibition).

• NT: Norepinephrine.

Parasympathetic (Vagus + pelvic splanchnic nerves S2–S4):

• ↑ Motility, ↑ secretions, ↑ absorption.

• Relaxes sphincters.

• Preganglionic fibers synapse directly in ENS plexuses → intramural ganglia.

Reflex Types

• Short reflexes: entirely within ENS (receptor → plexus → effector).

• Long reflexes: involve CNS or prevertebral ganglia.

  • Vago-vagal reflex: afferent vagus → dorsal motor nucleus → efferent vagus → ENS.

  • Enterogastric reflex: duodenal stretch/acid → sympathetic prevertebral ganglia → ↓ gastric emptying (pyloric sphincter constriction).

Key Points

• ENS = intrinsic control, but modulated by ANS.

• Submucosal plexus = secretions & blood flow.

• Myenteric plexus = motility.

• Sympathetic = brakes, Parasympathetic = gas.

• Hormones (Secretin, CCK, GIP) fine-tune digestion.