Motility in the Large Intestine
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Haustral Contractions
(Definition): Slow, segmenting movements that further mix chyme.
- About every 30 minutes.
- Occur in haustra: small pouches caused by the teniae coli (longitudinal smooth muscle ribbons that run along outside the entire length of the colon). Because they are shorter than the large intestine, the large intestine tucks between the teniae and form sacs
- Primarily occur in ascending and transverse colons.
- Produced by contractions of smooth muscle layer
Steps
1. Chyme fills a haustrum
2. Distension in the haustrum.
3. Smooth muscle layer contracts
4. Contractions move chyme into the next haustrum and subsequent haustra, where the sequence begins again.
Note that haustral contractions play a relatively minor role in propelling fecal waste through the large intestine; their main function to further mix waste.
Contractions also bring chyme in close contact with the large intestine mucosal layer to maximize water and electrolyte absorption
- Hasutral contractions also occur in the descending and sigmoid colon to further concentrate stored fecal waste prior to elimination.
Mass Movements
(Definition): slow, but powerful contractions of the large intestine that move undigested waste to the rectum for defecation via the anus.
- Much like stronger and sustained peristaltic contractions.
- 3-4 times a day.
- Mainly in the transverse, descending, and sigmoid colons.
- Produced by circular layer (smooth muscle) contractions
Steps
1. Undigested waste in the transverse colon.
2. Triggered by the gastrocolic reflex (initiated following ingestion of a meal when food enters the stomach causes its distension)
3. Circular layer contracts in the transverse colon
4. Contractions move waste towards the rectum.
Note: Unlike peristalsis, the circular remains contracted for some time following its initial trigger mass movement.
5. Fecal waste moves down the descending colon and into the sigmoid colon toward the rectum
#Prepares for elimination (defecation)
Peristalsis.
(Definition) uni-directional propulsion of digested food forward through the digestive tract.
- Peristaltic contractions continue from the upper GI and the small intestine to gradually move undigested waste through the remainder of the GI tract.
- Rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.
Steps
1. Circular layer contracts behinds the chyme, decreases the diameter of the large intestine to propel chyme forward.
Longitudinal layer relaxes.
2. Longitudinal layer contracts, which shortens the small intestine and decreases the distance the chyme must travel.
Circular layer relaxes.
3. Circular layer contracts and pinches the large intestine further distally, which propels chyme forward through the large intestine towards the rectum.
Longitudinal layer relaxes.
Learn about dilation of the colon, which can occur when the enteric ganglion are absent or when inflammation impairs stool movement through the colon (megacolon).