Notes
Motility in the Large Intestine
Sections
Definitions
Haustral contractions are a type of segmentation that further mixes chyme.
Peristalsis moves chyme through the remainder of the GI tract. Peristalsis is the uni-directional propulsion of digested food forward through the digestive tract; it is the result of rhythmic, alternating contractions of the circular and longitudinal smooth muscle layers causes peristalsis.
Mass movements push undigested contents and waste forward for elimination.
Haustral contractions
We draw the large intestine, and label the cecum, ascending, transverse, descending, and sigmoid colons, and the rectum.
Notice the "segmented" appearance of the colon, which is due to sac-like haustra that form along its length; label one of these (singular is haustrum).
We cut a window into the ascending and transverse colon to show the contractions of the haustra; we indicate the smooth muscle layer and show some chyme in the colon.
The chyme produces haustral distention; in response, the smooth muscle of the haustrum contracts.
This haustral contraction propels the chyme into the next haustrum, where the process of distension and contraction occurs.
The function of haustral contractions is to mix the chyme in the large intestine, and to bring the chyme in close contact with the intestinal wall to maximize water and electrolyte absorption.
Haustral contractions occur every 30 minutes or so.
In the descending and sigmoid colons, haustral contractions further concentrate the chyme, which is becoming fecal waste.
Mass Movements
Mass movements, aka, giant migrating complexes, are slow, powerful contractions of the large intestine that move undigested waste to the rectum for defecation via the anus.
These contractions are much like stronger and sustained peristaltic contractions; they occur three to four times per day. They are triggered by the gastrocolic reflex, which begins when food enters the stomach.
In our diagram, we draw the large intestine and label its regions (cecum, ascending, transverse, descending, and sigmoid colons, and the rectum).
We cut a window into the transverse and distal colons; we show the layer of smooth muscle and highlight the circular layer;
Use arrows to show that, in response to the gastrocolic reflex, the circular layer of the transverse colon contracts to move undigested waste towards the rectum.
Unlike peristalsis, the circular remains contracted for some time following its initial trigger mass movement.
The fecal waste moves down the descending colon and into the sigmoid colon toward the rectum as the result of prolonged circular layer contraction.
Mass movements propel undigested waste towards the rectum in preparation for elimination (defecation).