Updated: 6/10/2021

Gastrointestinal Contractions

Review Topic
  • Motility 
    • refers to contraction and relaxation of walls and sphincters of gastrointestinal tract
    • fragments and mixes food bolus in preparation for digestion and absorption
    • propels food bolus along gastrointestinal tract
  • Gastrointestinal contractions
    • contractile tissue of gastrointestinal tract is almost exclusively unitary smooth muscle
      • electrically coupled via gap junctions
        • gap junctions permit rapid cell-to-cell spread of action potentials
          • allowing coordinated smooth muscle contraction
      • exceptions include pharynx, upper one third of esophagus, and external anal sphincter
        • contractile tissue of these structures is striated muscle
    • depolarization and contraction of inner layer of circularly-oriented muscularis externa decreases diameter of GI segment
    • depolarization and contraction of outer layer of longitudinally-oriented muscularis externa decreases length of GI segment
    • phasic contractions
      • periodic contractions and relaxations
      • occur in esophagus, antrum of stomach, and small intestine
    • tonic contractions
      • maintain a constant level of contraction ("tone") without regular periods of relaxation
      • occur in lower esophageal (cardiac) sphincter, orad (upper-fundus and proximal body) region of stomach, iliocecal sphincter, and internal anal sphincter
Slow Waves
  • Slow waves
    • unique feature of the electrical activity of gastrointestinal smooth muscle
    • oscillating depolarization and repolarization of membrane potential of smooth muscle cells
    • action potentials occur "on top of" slow wave if membrane potential is depolarized to threshold
  • Origin of slow waves
    • occur at interstitial cells of Cajal
      • "pacemaker" for gastrointestinal smooth muscle
        • slow waves occur spontaneously in interstitial cells of Cajal
        • driving the frequency of slow waves
  • Frequency of slow waves
    • varies along the gastrointestinal tract
      • each part of the gastrointestinal tract has a characteristic frequency
        • stomach
          • 3 slow waves per minute
          • lowest frequency of slow waves in the gastrointestinal tract
        • ileum
          • 8-9 slow waves per minute
        • duodenum
          • 12 slow waves per minute
          • fastest frequency of slow waves in the gastrointestinal tract
    • sets frequency of action potentials which sets the frequency of smooth muscle contractions
  • Mechanism of slow waves
    • depolarization phase
      • cyclic opening of Ca2+ channels
        • inward Ca2+ current depolarizes cell membrane
    • repolarization phase
      • cyclic opening of K+ channels
        • outward K+ current repolarizes cell membrane

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