Defacation

Motility in Large IntestineChyme that enters the colon continues to be mixed by segmental contractions. A wave of contraction decreases the diameter of a segment of colon and sends a substantial bolus of material forward. Occur 3-4 times a day

Gastrocolic reflex

When food enters the stomach, mass movements occur in the colon primarily by means of gastrocolic reflex.most evident after the first meal of the day and is often followed by the urge to defecate. The gastroileal reflex moves the remaining small intestine contents into the large intestine, and the gastrocolic reflex pushes the colonic contents into the rectum, triggering the defecation reflex.

DefecationThe rectum is usually empty feces forced by mass movements stretching of the rectal wall initiates the defecation reflex sigmoid colon and the rectum contract, the internal anal sphincter to relax, and the external anal sphincter to contract (initially). As feces are forced into the anal canal, messages reach the brain decide whether the external (voluntary) anal sphincter should be opened or remain constricted to stop feces passage temporarily.

If defecation is delayed, the reflex contractions end within a few seconds, and the rectal walls relax.

During defecation, the muscles of the rectum contract to expel the feces.

We aid this process voluntarily by a procedure called Valsalvas maneuver. We also contract the levator ani muscle , which lifts the anal canal superiorly.

This lifting action leaves the feces below the anusand outside the body.

Involuntary or automatic defecation occurs in infants

Defecation, like urination, is subject to emotional influence.

When feces are retained in the colon, either through consciously ignoring a defecation reflex or through decreased motility, continued water absorption creates hard, dry feces that are difficult to expel.

One treatment for constipation is glycerin suppositories. Glycerin attracts water and helps soften the feces to promote defecation