3.5 Tonic and Rhythmic Contractions

• What regulates colonic contractions?

  Myogenic, neurogenic, and hormonal factors
• How is parasympathetic control of the proximal colon mediated?

  By the vagus nerve
• What mediates the control of the descending and rectosigmoid colon?

  Pelvic nerves from the sacral spinal cord
• What can parenteral alimentation provide?


  Sufficient calories for adults and infants
• When is total parenteral nutrition used?

  When oral intake is impossible or undesirable
• What determines smooth muscle activity in the GI tract?

  Integrated effect of slow waves and action potentials
• What occurs during slow-wave activity in smooth muscle?

  Depolarization and increase of intracellular calcium
• What regulates the activities of intestinal smooth muscle contraction?

  Neural and hormonal stimuli
• What is largely a function of intracellular calcium concentration?

  Modulation of intestinal smooth muscle contraction
• How do several agonists regulate intracellular calcium concentration?

  By activating G protein-linked receptors or calcium channels
• What is the function of the lower esophageal sphincter?

  To prevent reflux of gastric contents
• What initiates esophageal muscle contractions?

  Swallowing
• What is primary peristalsis?

  Peristalsis initiated by swallowing
• What is secondary peristalsis?

  Peristalsis elicited by esophageal distention
• What do segmental contractions in the small intestine do?

  Decrease the unstirred water layer for absorption
• How do churning movements occur in the small intestine?

  Through contractions of circular muscle
• How does electrical and mechanical activity differ in fasting and fed states?

  Patterns differ significantly between the two states
• What is the migrating motor complex (MMC)?


  Rhythmic contractions of the small intestine in fasting
• How often do MMCs occur in humans?

  Every 90 to 120 minutes
• What are the four distinct phases of MMCs?

  Quiescent, increasing action potential, peak activity, declining activity
• What can pass from the stomach into the duodenum during the interdigestive period?

  Particles >2 mm in diameter
• What do slow propulsive contractions during MMCs clear from the small intestine?

  Residual content including undigested food and bacteria
• Where do MMCs usually originate?

  In the stomach
• What terminates MMCs?

  Feeding
• What is the major determinant of MMC pattern?

  Hormone motilin
• Where is motilin synthesized?

  In the duodenal mucosa
• What factors induce the fed motor pattern?

  Vagus nerve, caloric content, and food type
• What is the submucosal plexus also known as?

  Meissner's plexus
• Where is the myenteric plexus located?

  Between circular and longitudinal muscle layers
• What type of muscle primarily composes the GI tract?

  Smooth muscle
• What are the exceptions to the smooth muscle composition of the GI tract?

  Striated muscle in UES, upper esophagus, and external anal sphincter