22.11F: Defecation Reflex

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Defecation is a combination of voluntary and involuntary processes that create enough force to remove waste material from the digestive system.

Learning Objectives

Describe the defecation reflex

Key Points

The rectal ampulla acts as a temporary storage facility for the unneeded digestive material.
A sufficient increase in fecal material in the rectum causes stretch receptors from the nervous system that are located in the rectal walls to trigger the contraction of rectal muscles, the relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external sphincter.
The relaxation of the internal anal sphincter causes a signal to be sent to the brain indicating an urge to defecate.
If defecation is delayed for a prolonged period, the fecal matter may harden and autolyze, and result in constipation.
Once the voluntary signal to defecate is sent back from the brain, the ano-rectal angle decreases, becoming almost straight, and the external anal sphincter relaxes. The rectum contracts and shortens in peristaltic waves, forcing fecal material out of the rectum and down through the anal canal.

Key Terms

defecation: The act or process of voiding feces from the bowels.
rectum: The terminal part of the large intestine through which feces pass.
anal canal: The terminal part of the large intestine, situated between the rectum and anus.
constipation: A state of the bowels in which the evacuations are infrequent and difficult, or the intestines become filled with hardened feces.
autolyze: To destroy itself; to be destroyed by its own enzymes.

EXAMPLES

Constipation is uncomfortable, but it may be a signal that your diet is deficient in fibrous foods (eat more fruits and vegetables) and that you are not drinking enough water, so laxatives are generally not the best way to treat it.

Defecation

For the adult human, the process of defecation is normally a combination of both voluntary and involuntary processes that create enough force to remove waste material from the digestive system.

The rectal ampulla acts as a temporary storage facility for the unneeded material. As additional fecal material enters the rectum, the rectal walls expand. A sufficient increase in fecal material in the rectum causes the stretch receptors from the nervous system, located in the rectal walls, to trigger the contraction of rectal muscles, the relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external sphincter. The relaxation of the internal anal sphincter causes a signal to be sent to the brain indicating an urge to defecate.

This is a diagram of the defecation reflex. The conscious and parasympathetic pathways of the defecation reflex are shown. The conscious pathway goes directly to the external sphincter. The parasympathetic pathways go to the sigmoid colon, the rectum, and internal sphincter.

Defecation reflex: The conscious and parasympathetic pathways of the defecation reflex.

If this urge is not acted upon, the material in the rectum is often returned to the colon by reverse peristalsis where more water is absorbed, thus temporarily reducing pressure and stretching within the rectum. The additional fecal material is stored in the colon until the next mass peristaltic movement of the transverse and descending colon. If defecation is delayed for a prolonged period, the fecal matter may harden and autolyze, resulting in constipation.

Once the voluntary signal to defecate is sent back from the brain, the final phase begins. The abdominal muscles contract (straining), causing the intra-abdominal pressure to increase. The perineal wall is lowered and causes the anorectal angle to decrease from 90 degrees to less than 15 degrees (almost straight), and the external anal sphincter relaxes.

The rectum now contracts and shortens in peristaltic waves, thus forcing fecal material out of the rectum and down through the anal canal. The internal and external anal sphincters, along with the puborectalis muscle, allow the feces to be passed by pulling the anus up and over the exiting feces in shortening and contracting actions.