Motility of the Small Intestine

• Enterocytes (absorptive cells) make up most of the surface area of the small intestine and have microvilli on their apical surfaces to increase absorption.
• The small intestinal mucosa is covered by a single layer of columnar epithelial cells, which are responsible for absorbing nutrients.
• What is the functional significance of the ongoing activity of enteric inhibitory motor neurons (IMNs) to intestinal circular muscle?

  It maintains a state of relaxation in the muscle, facilitating smooth propulsion of food along the digestive tract.
• How do enteric inhibitory motor neurons prevent excessive muscle tension in the intestinal circular muscle?

  By continuously releasing inhibitory neurotransmitters at the neuromuscular junctions.
• What neurotransmitters do enteric inhibitory motor neurons release to induce relaxation of the intestinal circular muscle?

  Nitric oxide (NO) and vasoactive intestinal peptide (VIP).
• What role do enteric inhibitory motor neurons play during peristalsis?

  They are activated in the segment of the intestine just ahead of the food bolus, causing relaxation of the circular muscle in that area.
• How do enteric inhibitory motor neurons accommodate distension in the intestine?

  They are activated to further relax the circular muscle when the intestinal lumen is distended by food.
• What is the effect of disrupted inhibitory motor neuron activity on the intestinal circular muscle?

  It may lead to overly contracted muscle, causing difficulty in moving food through the digestive tract.
• What are the characteristics of intestinal motility during the absorptive phase compared to the post-absorptive phase?

  • Absorptive Phase:
  • Characterized by segmentation (localized contractions).
  • Frequent and irregular contractions for nutrient absorption.
  • Post-Absorptive Phase:
  • Characterized by the migrating motor complex (MMC).
  • Cycles of quiescence followed by strong propulsive contractions.
• What is the primary function of segmentation during the absorptive phase?

  To mix chyme with digestive enzymes and facilitate absorption.
• How does the migrating motor complex (MMC) function during the post-absorptive phase?

  It exhibits a migrating wave of strong contractions that propel contents distally along the intestine.
• Contrast the effects of parasympathetic and sympathetic nervous activity on small intestinal motility.

  • Parasympathetic Activity:
  • Stimulates and increases motility.
  • Promotes "rest and digest" functions.
  • Sympathetic Activity:
  • Inhibits and decreases motility.
  • Prepares the body for "fight or flight."
• What neurotransmitter is primarily associated with parasympathetic stimulation of the small intestine?

  Acetylcholine.
• What happens to small intestinal motility during stressful situations?

  It is slowed down due to sympathetic nervous system activation.
• How does distension of the colon affect small intestinal motility?

  • Retrograde Reflexes:
  • Alters normal motility after eating.
  • Endoluminal Pressure:
  • Can decrease mucosal and mural perfusion, causing ischemic pain.
  • Lumen Narrowing:
  • Increases tone of circular muscle cells, affecting propulsion and mixing.
• What is the peristaltic reflex as described by the Law of the Intestine (Bayliss and Starling)?

  Stimulation/distension by a bolus induces contraction above and relaxation below the point of stimulation.
• What is the role of interstitial cells of Cajal in the GI tract?

  • Act as pacemaker cells.
  • Generate electrical slow waves that initiate rhythmic contractions for peristalsis.
  • Allow for coordinated movement of food through the digestive system.
• What is the consequence of the frequency gradients of electrical slow waves in the intestinal tract?

  They allow for coordinated movement of food, with slower waves in the stomach and faster waves in the small intestine.
• What happens to the ileocecal sphincter when pressure increases in the ileum?

  It relaxes, allowing chym to move into the cecum.
• What is the gastroileal reflex?

  It is the relaxation of the ileocecal sphincter due to increased ileal pressure, triggered by food in the stomach.
• What stimulates the gastroileal reflex?

  Distension of the stomach by food intake.
• What is the effect of the gastroileal reflex on the ileum?

  It leads to increased motility in the ileum.
• What mechanism underlies the gastroileal reflex?

  Neural pathways that connect the stomach to the ileum, allowing coordinated movement of chyme.
• Goblet cells secrete mucus onto the lumenal side of enterocytes, protecting them from abrasion during peristalsis.
• Paneth cells produce lysozyme, an enzyme that destroys bacteria.
• The ME consists of smooth muscle fibers arranged in three longitudinal bands called taeniae coli.
• The small intestinal epithelium has two layers: an outer layer called the muscularis externa (ME) and an inner layer called the lamina propria (LP).
• Tight junctions prevent leakage between epithelial cells.
• Why is the ongoing activity of enteric inhibitory motor neurons significant to the intestinal circular muscle?

  The ongoing activity of the enteric inhibitory motor neurons (IMNs) to intestinal circular muscle is functionally. significant because it maintains a state of relaxation in the muscle, allowing for smooth propulsion of food along the digestive tract by facilitating timely relaxation ahead of a bolus during peristalsis.