Defecation – Involuntary and Voluntary Bowel Movement Control

What is defecation?

Defecation is the process of passing out stool (feces) through the anus. This eliminates waste material from the rectum and colon. The process of defecation should be painless, regular and to a certain degree, it is under voluntary control.

The rectum usually does not contain feces so as not to trigger local reflexes that can initiate defecation. The structure of the lower bowel is such that the rectum  is able to remain empty of feces. This is achieved by a weak sphincter at the  at the juncture between the sigmoid colon and rectum. In addition, the sharp angle between the sigmoid colon and rectum also provides resistance to feces entering the rectum without nervous control.

Two anal sphincters and its nerve supply ensures that feces do not pass out in a continuous manner. The internal anal sphincter is composed of smooth muscle arranged in a circular manner. It is not under voluntary control. The external anal sphincter is composed of striated muscle, which surrounds the internal sphincter and extends beyond it. This is under voluntary control. Both these sphincters remain constricted all of  the time and is supported by the pelvic floor muscles.

Involuntary Defecation

Usually, the first part of defecation is involuntary and triggered by the defecation reflexes. This may occur for a number of reasons higher up the gut but to a large degree it is influenced by the fecal content contained within the colon. Refer to the Defecation Reflex article for more information on all the autonomic reflexes that affect bowel movements.These reflexes are isolated within the enteric nervous system and spinal reflexes traveling between the lower bowel and conus medullaris of the spinal cord.

Once the defecation reflexes are triggered, peristaltic activity of the descending and sigmoid colon increases, feces enter the rectum and the internal anal sphincter relaxes. It results in the urge to defecate. Conscious control of the external sphincter will then complete the defecation process.

Voluntary Defecation

The peristaltic waves of the intrinsic reflex coupled with the distention of the rectum due to the entry of feces results in the desire to defecate. This will not occur without the relaxation of the external anal sphincter that is controlled by nerve fibers in the pudendal nerve which is part of the somatic nervous system. This means that it is under voluntary control and can be influenced by the conscious mind and to some extent, even by the subconscious mind. It partly explains the reason for diarrhea or constipation due to emotional stress – fear, anger, depression and anxiety. Furthermore, impulses from the brain that cannot reach the conus medullaris of the spinal cord will also affect the voluntary component of defecation.

The defecation reflexes also have other effects outside of the gastrointestinal tract. It triggers a deep breath and closure of the glottis. While holding the breath, the abdominal wall muscle contract increasing the pressure within the abdominal activity. Simultaneously, the pelvic floor muscles relax and reduces the pressure on the anus thereby allowing the feces to pass through. This is known as the Valsalva maneuver.

Conversely, pushing the diaphragm downwards by taking a deep breath and contracting the abdominal wall muscles can increase intra-abdominal pressure and trigger the defecation reflexes. This allows most people to defecate at will even without the urge to have a bowel movement. This ‘unnatural’ stimulation of the defecation reflex requires a degree of straining and is not as effective as when the reflexes occur naturally. Repeatedly ignoring the natural urge to have a bowel movement can diminish the effectiveness of the defecation reflexes and result in constipation.

Understanding the normal process of defecation is crucial in attempting to identify the process behind functional bowel disorders like inflammatory bowel syndrome (IBS) whether it is diarrhea or constipation-predominant.

Related Article

  1. Defecation Reflex and Other Reflexes that Affect Bowel Activity

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