Basics of Gastrointestinal Motility

I. Esophageal Motility

Upper Esophageal Sphincter (UES)

• Striated muscle, tonically contracted between swallows
• Resting pressure: 30–150 mm Hg
• Relaxes in coordination with pharyngeal contraction to receive bolus

Lower Esophageal Sphincter (LES)

• Smooth muscle, tonically contracted between swallows
• Innervated by vagal excitatory (ACh) and inhibitory (NO, VIP) fibers
• Resting pressure: 10–45 mm Hg
• Relaxes at swallow initiation and remains relaxed until bolus passes
• Transient LES relaxation (tLESR):
  • Occurs postprandially due to gastric distention
  • Primary mechanism of GERD (90%)

Esophageal Body

• Upper 1/3: striated muscle (somatic vagal cholinergic fibers from nucleus ambiguus)
• Lower 1/3: smooth muscle (preganglionic vagus from dorsal motor nucleus)
• Resting pressure lower than intragastric pressure
• Primary peristalsis: initiated by swallowing; 2–4 cm/s, 4 sec duration, 35–180 mm Hg
• Secondary peristalsis: triggered by distention (refluxate, retained food)
• Tertiary peristalsis: spontaneous, nonperistaltic, low-amplitude contractions

II. Gastric Motility

Functional Regions

• Proximal (fundus, proximal corpus): reservoir, high distensibility, tonic contractions
• Distal (distal corpus, antrum): mixing/grinding, slow waves at 3/min, phasic contractions

Fasting State

• Migrating Motor Complex (MMC): cyclic pattern
• Phase II: mixed lo/hi pressure waves;
• Phase III: antral contractions >40 mm Hg, 3/min, 3–7 min duration with pyloric relaxation to clear residuals
  
• Phase I: antral quiescence

Fed State

• Proximal relaxation:
  • Receptive: triggered by swallowing
  • Adaptive: sustained via mechanoreceptors stimulated by food bolus
    
• Fundic slow tonic contractions regulate pressure and transfer solids
• Slow waves from corpus to pylorus move solids
• Antral contractions grind particles to <1 mm for pyloric passage
• Manometry: irregular amplitudes, ↑ motility index (13.67–15.65), influenced by meal content

III. Small Intestinal Motility

Fasting State (MMC: 90–180 min cycles)

• Phase I: quiescence
• Phase II: irregular contractions
• Phase III: rhythmic peristalsis from stomach to ileum
  • Antrum: 3/min (>40 mm Hg)
  • Small intestine: 11–12/min (>20 mm Hg)
  • “Housekeeper” function and marker of neuromuscular integrity

Fed State

• MMC interrupted by nutrient ingestion
• Fed pattern: random bursts for mixing and absorption
• Begins 5–10 min after meal; peaks at 10–20 min
• Response influenced by meal composition (fat > CHO > protein)

IV. Colonic Motility

• Irregular patterns alternating between inactivity and contractions
• Diurnal variation: ↓ during sleep, ↑ upon waking
• Segmental contractions: arrhythmic, mixing contents
• Propagated contractions:
  • Low-amplitude: <50 mm Hg, unclear role
  • High-amplitude (HAPCs): >60 mm Hg, >10 sec, >30 cm propagation
  • Originate in proximal colon; occur 4–6×/day, post-meal, morning, pre-defecation
• Rectal motor complexes: 2–4/min at night, >5 mm Hg, >10 min duration
• Fasting: low-amplitude non-propagating segmental contractions; HAPCs may occur on waking
• Fed: gastrocolonic reflex
  • ↑ motility index >15% after eating
    
  • Triggered within minutes; lasts up to 3 hours
  • Fat > CHO influence response
• Presence of gastrocolonic reflex and HAPCs = normal neuromuscular integrity

V. Anorectal Motility

Continence Mechanisms

• Internal anal sphincter (IAS): smooth muscle, 75–85% of intra-anal pressure (33–90 mm Hg)
• External anal sphincter (EAS): striated muscle, 15–25% of intra-anal pressure (81–276 mm Hg)

Defecation Physiology

• Rectal distention causes reflex relaxation of the internal anal sphincter and transient contraction of the external anal sphincter. This is called the Rectoanal inhibitory reflex (RAIR) → IAS relaxation + EAS contraction
• Indepedent of the spinal reflex
  
• RAIR absent in Hirschsprung disease (no inhibitory ganglion cells)
  
• Sampling reflex: allows differentiation of gas/liquid/solid by specialized receptors in the anal canal
  
• Decision to defecate:
  • Yes: Valsalva, forward peristalsis, anorectal angle widening, IAS/EAS relaxation
  • No: EAS contraction, maintain acute anorectal angle, reverse peristalsis, rectal accommodation