Motility in the Small Intestine

Motility in the Small Intestine

Muscular movements of small intestine

Peristalsis

Uni-directional propulsion of digested food forward through the digestive tract.

• Activated following a meal (postprandial).
• Rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.

Key Steps of Peristalsis

1. Circular layer contracts and pinches behind the chyme
   – Decreases diameter of the lumen
   – Propels chyme forward

• Longitudinal layer relaxes around the chyme
  – Dilates the small intestine to easily receive the chyme.

2. Longitudinal layer contracts in front of pinched region
   – Shortens region of the small intestine (much like when we bunch up a tube sock to get our foot into it)
   – This step does not actively move the chyme forward → it shortens the distance the chyme must travel and prepares for its propulsion forward through the small intestine.

3. Circular layer contracts again, pinches the small intestine farther distally
   – Propels chyme further through the small intestine

Migrating Motility Complex (MMC)

Produces specific periodic peristaltic contractions that sweep digested contents, cellular debris, and bacteria through the small intestine during fasting.

• Occur every 90 minutes to two hours (during interdigestive periods - when a meal has been digested and absorbed).
• Regulated by the candidate digestive hormone motilin.

Segmentation

Alternating contractions of the circular layer between intestinal segments to breakdown food and mix chyme.

• Activated following a meal (postprandial).

Key Steps of Segmentation

1. Mixture of chyme enters the small intestine, surrounded by digestive juices: duodenal secretions, pancreatic secretions, and bile

2. Circular layer segmentally contracts → chyme separated into these segments

3. Circular layer contracts and pinches the middle of these segments to re-segment them.
   – Chyme from each segment moves in both directions (forward and backward) from the contraction site, into the other segments.

4. Circular layer contracts again in the original site and again re-segments the small intestine.
   – Chyme from the different segments mix.
   – Also mix chyme with the duodenal and pancreatic secretions and bile.

• The circular layer promotes stationary mixing of partially digested chyme with small intestine and pancreatic secretions.

Other key features of segmentation:

• Actively brings digested chyme into close contact with intestinal epithelium → efficient absorption.
• Does not contribute to the movement of chyme down the small intestine.
• Contributes to mechanical digestion of food → further breakdown of ingested macromolecules to simpler, absorbable forms.
• Segmentation contractions cease following meal absorption.