Gastric Motility

Overview

• Gastric motility
  • functions
    • orad stomach relaxes to receive the food bolus from the esophagus
      • orad stomach = fundus and proximal body 
    • caudad stomach contractions reduce size of the food bolus and mix it with gastric secretions to initiate digestion
      • caudad stomach = distal body and antrum 
    • gastric emptying propels the chyme into the duodenum of small intestines
  • receptive relaxation
    • distension of the lower esophagus by the food bolus produces relaxation and opening of the lower esophageal (cardiac) sphincter and, simultaneously, relaxation of the orad stomach
      • reduces pressure and increases volume of the orad stomach
    • vagovagal reflex 
      • both afferent and efferent limbs of reflex are carried in the vagus nerve
      • afferent fibers deliver sensory information from the periphery to the CNS
        • mechanoreceptors detect distension of the lower esophagus and the orad stomach by the food bolus and relay sensory information to the CNS via afferent fibers of the vagus nerve
      • efferent fibers deliver motor information from the CNS to the periphery
        • CNS relays motor information to the smooth muscle walls of the lower esophagus and the orad stomach via efferent fibers of the vagus nerve
          • postganglionic peptidergic vagal efferent fibers release vasoactive intestinal polypeptide (VIP), which relaxes the lower esophageal (cardiac) sphincter and smooth muscle of the orad stomach
  • mixing and digestion
    • caudad stomach has a thick muscular wall
      • produces contractions necessary for fragmenting and mixing food
      • contractions mix gastric contents and propel contents from the antrum of stomach back into the body of the stomach (“retropulsion”)
        • retropulsion further fragments and mixes gastric contents
  • gastric emptying
    • contractions periodically propel a portion of the gastric contents through the pylorus and pyloric sphincter into the duodenum of the small intestine
    • gastric emptying is closely regulated to allow adequate time for neutralization of gastric acid in the duodenum and adequate time for digestion and absorption
      • H⁺ receptors in the duodenum detect low pH (high H⁺) and act via interneurons in the myenteric (Auerbach’s) nerve plexus to slow gastric emptying
        • ↓ gastric emptying ensures gastric contents are delivered slowly to the duodenum
          • allows adequate time for neutralization of gastric acid in the duodenum by pancreatic HCO₃^–
            • necessary for optimal function of pancreatic enzymes
      • arrival of fatty acids in the duodenum stimulates cholecystokinin (CCK) secretion by I cells in the small intestine to slow gastric emptying
        • ↓ gastric emptying ensures gastric contents are delivered slowly to the duodenum
          • allows adequate time for fatty acid digestion and absorption in the small intestine