Gastrointestinal Physiology: Motility covers gastrointestinal motility for INI CET (AIIMS PG).
GI Tract Propulsion Patterns:
- Segmentation: Rhythmic, non-propulsive; local mixing of chyme with digestive secretions; dominant in small intestine (every few cm; MMC resets)
- Peristalsis: Propulsive waves; coordinated contraction above and relaxation below bolus; esophageal and intestinal
- Migrating Motor Complex (MMC): Cyclic, slow propulsive waves during fasting (every 90–120 min); clears undigested material; interrupted by feeding
GI Smooth Muscle Electrical Activity:
- Slow waves (basic electrical rhythm — BER): Generated by interstitial cells of Cajal (ICC) — pacemakers in the muscularis externa
- Amplitude: 5–15 mV; Frequency varies by region: Stomach (3/min), Duodenum (12/min), Ileum (8/min)
- Depolarization above threshold → spike potentials (Ca²⁺-dependent action potentials) → trigger contraction
- Tone: Basal contractile state of smooth muscle (important for pressure maintenance)
Swallowing (Deglutition):
- Oral phase (voluntary): Bolus moved to oropharynx by tongue
- Pharyngeal phase (involuntary): Closure of airway (larynx elevates, epiglottis covers) → pharyngeal constrictors contract → upper esophageal sphincter (UES) relaxes → bolus enters esophagus
- Esophageal phase: Primary peristalsis (initiated by swallowing) + secondary peristalsis (distension by residual bolus); LES relaxes during swallowing (VIP + NO mediated); coordinated to allow food passage into stomach
Gastric Motility:
- Fundus: Tone — receptive relaxation (vagally mediated) to accommodate food with minimal ↑intragastric pressure
- Body: Mixing and grinding; antropyloric coordination
- Antrum: Powerful peristaltic contractions → Retropulsion of food against closed pylorus → mechanical breakdown (grinding into <1mm particles)
- Pylorus: Tonic contraction (mostly closed) → allows only small particles (<1mm) to pass into duodenum; regulates gastric emptying rate
- Gastric emptying: Faster for liquids, isotonic solutions, low fat; slower for solids, fatty meals, hypertonic solutions
Intestinal Motility:
- Ileocecal junction: Prevents reflux of colonic contents; relaxes on gastrin; contracts on pentagastrin
- Small intestine: Segmentation contractions (mixing); migrating motor complexes (cleansing between meals)
- Large intestine: Haustrations (non-propulsive, sac-like protrusions) — mixing; Mass peristalsis (propulsive) — 1-3 times/day after meals
Defecation:
- Rectal distension → internal anal sphincter (involuntary — relaxes reflexively) + external anal sphincter (voluntary — must relax for defecation)
- Defecation reflex: Stretch receptors → internal sphincter relaxation + urge to defecate; if socially appropriate → external sphincter voluntarily relaxes
Neural Control:
- Enteric nervous system (ENS): “Little brain” — myenteric (Auerbach’s) plexus (motor + inhibitory) + submucosal (Meissner’s) plexus (secretory + local reflexes)
- Extrinsic innervation:
- Parasympathetic (vagal and pelvic): ↑ motility, ↑ secretions, ↓ sphincter tone (except pylorus and anal sphincters)
- Sympathetic: ↓ motility, ↓ secretions, ↑ sphincter tone, vasoconstriction
- VIP (Vasoactive Intestinal Polypeptide) and NO: Main inhibitory neurotransmitters → cause sphincter relaxation and segmental inhibition
⚡ Exam Tip for INI CET (AIIMS PG): Cholinergic agonists (ACh, muscarinic M3 agonists) → ↑GI motility and secretions; anticholinergics (atropine) → ↓motility (constipation), used as antispasmodics. Dicyclomine (anticholinergic) used for IBS. 5-HT₃ antagonists (ondansetron) block serotonin-induced nausea/vomiting (5-HT₃ receptors in CTZ). 5-HT₄ agonists (metoclopramide, cisapride — withdrawn) → ↑acetylcholine release → ↑GI motility.