Large Intestinal Motility
Three prominent patterns of motility are observed the colon:
- Segmentation contractions which chop and mix the ingesta, presenting it to the mucosa where absorption occurs. These contractions are quite prominent in some species, forming sacculations in the colon known as hausta.
- Antiperistaltic contractions propagate toward the ileum, which serves to retard the movement of ingesta through the colon, allowing additional opportunity for absorption of water and electrolytes. Peristaltic contractions, in addition to influx from the small intestine, facilitate movement of ingesta through the colon.
- Mass movements constitute a type of motility not seen elsewhere in the digestive tube. Known also as giant migrating contractions, this pattern of motility is like a very intense and prolonged peristaltic contraction which strips an area of large intestine clear of contents.
In periods between meals, the colon is generally quiescent. Following a meal, colonic motility increases significantly, due to signals propagated through the enteric nervous system - the so called gastrocolic and duodenocolic reflexes, manifestation of enteric nervous system control. In humans, the signal seems to be stimulated almost exclusively by the presence of fat in the proximal small intestine. Additionally, distension of the colon is a primary stimulator of contractions.
Several times each day, mass movements push feces into the rectum, which is usually empty. The gastrocolic reflex mentioned above is a stimulus for this. Distension of the rectum stimulates the defecation reflex. This is largely a spinal reflex mediated via the pelvic nerves, and results in reflex relaxation of the internal anal sphincter followed by voluntary relaxation of the external anal sphincter and defecation.
In humans and "house-trained" animals, defecation can be prevented by voluntary constriction of the external sphincter. When this happens, the rectum soon relaxes and the internal sphincter again contracts, a state which persists until another bolus of feces is forced into the rectum.
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