Updated: 4/4/2015

Large Intestinal Motility

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Overview
  • Large intestinal motility
    • functions
      • fecal material moves from cecum through colon, to rectum, and to anal canal
        • colon includes ascending, transverse, descending, and sigmoid colons
      • fecal material is destined for excretion
    • segmentation contractions
      • mix intestinal contents of large intestine
      • occur in cecum and proximal colon
      • contractions are associated with characteristic sac-like segments called haustra in large intestine
    • mass movements
      • move intestinal contents of large intestine over long distances 1-3 times per day
      • propel fecal material into rectum
        • fecal material in distal colon is semi-solid and moves slowly
          • a consequence of colonic water absorption occurring in distal colon
    • defecation
      • rectosphincteric reflex
        • as retcum fills with fecal material, smooth muscle wall of rectum contracts and internal anal sphincter relaxes
          • defecation does not occur
            • external anal sphincter, composed of striated muscle, is under voluntary control
              • external anal sphincter is tonically contracted
        • once rectum fills to 25% capacity, there is an urge to defecate
          • when convenient to defecate, external anal sphincter is relaxed voluntarily
          • smooth muscle wall of rectum contracts and generates sufficient pressure
            • fecal material is forced out through anal canal
      • gastrocolic reflex
        • distension of stomach by food → ↑ motility of colon → ↑ frequency of mass movements in large intestine
 

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