3.4 Motility of the Small and Large Instestins

• What are the primary functions of the small intestine?

  DIGESTION and ABSORPTION of dietary nutrients
• What are the two classes of small-intestinal motor activity?

  • Churning or Mixing
  • Propulsion of bolus of luminal contents
• How is churning accomplished in the small intestine?

  By segmental, nonpropulsive contractions
• What does churning mix with luminal contents?

  Pancreatic, biliary, & small intestinal secretions
• When does churning occur in the small intestine?

  Following eating
• What results from the contraction of circular muscle in segments during churning?

  Mixing of luminal contents
• How is propulsion accomplished in the small intestine?

  By propagated, peristaltic contractions
• What is the result of propulsion in the small intestine?

  Caudad movement of intestinal luminal contents
• What are the two outcomes of caudad movement in the small intestine?

  • Absorption at distal sites of intestine
  • Elimination in stool
• What happens during the contraction of circular muscle in the propulsive segment?

  It contracts while longitudinal muscle relaxes
• What occurs in the downstream receiving segment during propulsion?

  Relaxation of circular muscle & contraction of longitudinal muscle
• What type of muscle participates in both churning and propulsion?

  Circular smooth muscle
• What are the four primary functions of the large intestine?

  1. Absorbs fluid & electrolytes
  2. Absorbs short-chain fatty acids
  3. Acts as a reservoir for colonic content
  4. Eliminates contents under voluntary control
• What is the primary function of the proximal part of the colon?

  Fluid & electrolyte absorption
• What occurs in the proximal colon besides fluid absorption?

  Bacterial fermentation takes place
• What is the primary function of the distal portion of the colon?

  Final desiccation and storage of colonic material
• What are the two types of motor activity in the proximal colon?

  • Nonpropulsive segmentation
  • Mass peristalsis
• What generates nonpropulsive segmentation in the proximal colon?

  Slow-wave activity producing circular muscle contractions
• What do segmental contractions in the colon produce?

  Churning and typical appearance of haustra
• What occurs during mass peristalsis in the colon?

  Portion of colonic contents is propelled distally
• What is the primary form of propulsive motility in the colon?

  Mass peristalsis
• What happens to haustra during mass peristalsis?

  They disappear and reappear after completion
• What is the primary motor activity in the distal colon?

  Nonpropulsive segmentation produced by segmental contractions
• What occurs in the distal colon before mass peristalsis?

  Contents are stored
• How is the rectum kept until filled?

  By nonpropulsive segmentation
• What fills the rectum?

  Mass peristalsis of distal end of colon