Gastrointestinal Motility

BER & Smooth Muscle - Gut's Electric Beat

• Basic Electrical Rhythm (BER):
  • Rhythmic slow waves (oscillating membrane potentials) in GI smooth muscle.
  • Generated by specialized pacemaker cells (see ⭐ below).
  • Determine maximum contraction frequency.
  • Frequency varies: Stomach (3/min), Duodenum (12/min), Ileum (8-9/min).
  • Do not directly cause contraction (except stomach); require additional stimuli for spike potentials.
• GI Smooth Muscle Contraction:
  • Unitary type; cells electrically coupled by gap junctions.
  • Contraction occurs when slow wave peak + excitatory stimuli depolarize membrane to threshold.
  • This triggers Ca²⁺ influx and spike potentials (true action potentials) superimposed on slow waves → contraction.

⭐ Interstitial Cells of Cajal (ICCs) are the pacemakers of the gut, generating slow waves (BER).

Regulation of Motility - Motility's Masterminds

• Neural Control:
  • Enteric Nervous System (ENS) - "Gut's brain":
    • Myenteric (Auerbach's) plexus: Motility control. 📌 Auerbach = Action.
    • Submucosal (Meissner's) plexus: Secretion, blood flow. 📌 Submucosal = Secretion.
  • Autonomic Nervous System (ANS):
    • Parasympathetic (ACh): ↑ Motility (Vagus, pelvic nerves).
    • Sympathetic (NE): ↓ Motility.
• Hormonal Control:
  • Excitatory:
    • Motilin (M cells): MMC Phase III initiation.
    • Gastrin (G cells): ↑ Gastric, intestinal motility.
    • CCK (I cells): ↑ Gallbladder contraction; ↓ gastric emptying.
    • Serotonin (5-HT, EC cells): ↑ Motility.
  • Inhibitory:
    • Secretin (S cells): ↓ Gastric emptying & motility.
    • Somatostatin (D cells): Universal GI inhibitor.
    • VIP: Sphincter relaxation; ↓ intestinal motility.

⭐ Motilin, secreted from M cells of the duodenum and jejunum, initiates Phase III of the Migrating Motor Complex (MMC) during fasting.

Regional Motility - The GI Transit Show

• Swallowing (Deglutition):
  • Phases: Oral (voluntary), Pharyngeal (involuntary reflex, airway protection via epiglottis & vocal cords, apnea), Esophageal (involuntary peristalsis).
  • UES & LES relaxation coordinated.
• Esophageal Motility:
  • UES (cricopharyngeus): prevents aerophagia. LES: prevents GERD.
  • Peristalsis: Primary (swallow-initiated), Secondary (distension-clears refluxate).
• Gastric Motility:
  • Storage: Receptive relaxation (vagovagal), gastric accommodation.
  • Mixing: Antral peristalsis (BER by ICC, ~3/min). Retropulsion.
  • Emptying: Pyloric sphincter control. Factors: ↑volume, ↓fat, ↓acid in duodenum. Liquids > Solids.
  • Migrating Motor Complex (MMC): Interdigestive, motilin-driven, "housekeeping".
• Small Intestine Motility:
  • Segmentation: Mixing, ↑contact time. BER gradient (duodenum ~12/min, ileum ~8/min).
  • Peristalsis: Propulsive.

  ⭐ The 'Law of the Gut' states that peristaltic contraction typically occurs on the oral side of a bolus with relaxation on the anal side, propelling contents aborally.

• Large Intestine Motility:
  • Haustrations (mixing, slow shuttling).
  • Mass movements: Propulsive (1-3/day), gastrocolic & duodenocolic reflexes.
  • Defecation: Rectal distension → internal anal sphincter relaxation (involuntary) → external anal sphincter control (voluntary).

Motility Disorders - Motility Mishaps

• Achalasia Cardia: Failure of LES relaxation, aperistalsis. Dysphagia to solids & liquids.

  ⭐ Achalasia cardia is characterized by impaired LES relaxation and aperistalsis of the esophageal body, often presenting with a 'bird's beak' appearance on barium swallow.

• GERD: ↓LES tone. Heartburn, regurgitation.
• Gastroparesis: Delayed gastric emptying. Nausea, vomiting. Common in diabetes.
• Hirschsprung's Disease: Aganglionic megacolon. Neonatal constipation.
• IBS: Altered bowel habits, abdominal pain. Motility & visceral hypersensitivity issues.

High‑Yield Points - ⚡ Biggest Takeaways

• BER (Basal Electrical Rhythm) by ICCs (Interstitial Cells of Cajal) dictates max contraction frequency: Stomach ~3/min, Duodenum ~12/min.
• MMC (Migrating Motor Complex): Fasting "housekeeper" driven by motilin; feeding abolishes it.
• Peristalsis: ENS (Enteric Nervous System) coordinated; proximal ACh/Substance P (contraction), distal NO/VIP (relaxation).
• Segmentation: Primary mixing movement, prominent in the small intestine.
• Gastrocolic reflex: Food in stomach → ↑ colon motility; involves gastrin and extrinsic nerves.
• Achalasia cardia: LES (Lower Esophageal Sphincter) fails to relax, esophageal aperistalsis; due to loss of myenteric plexus inhibitory (NO/VIP) neurons.