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Updated: Jun 10 2021

Gastrointestinal Contractions

  • Overview
    • 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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