What triggers defecation reflex?

Answer 1

The internal anal sphincter is governed by involuntary control, so it is reflexive. It is what triggers the feeling that you have to defecate. However, the external sphincter is under voluntary control. So, if you feel that you have to defecate, you can continue to hold your external sphincter shut, until the urge to defecate diminishes. Then, when it is convenient to do so, the internal sphincter again activates the urge to defecate. The anal sphincter has an internal involuntary part, and an external voluntary part. The internal sphincter has an involuntary reflex which gives your the urge to defecate. If it is not convenient to defecate, then the external sphincter, which is under voluntary control, keeps the anus shut, and after a while, the urge to defecate can subside until a later time when it is convenient to defecate.

Answer 2

Fecal continence is maintained by two sphincters that surround the anal canal. The internal anal sphincter is an extension of the muscularis externa, and thus is smooth muscle innervated by enteric neurons found in the myenteric plexus. The external anal sphincter is skeletal muscle that can be consciously controlled, and is innervated by somatic efferent neurons. The figure is a schematic sagittal view of the lower rectum and anus showing the anal sphincters.

Another skeletal muscle that contributes to the maintenance of fecal continence is the puborectalis muscle. This muscle loops around the rectum like a sling, pulling it forward to create a more acute angle between the rectum and the anal canal (the anorectal angle). During defecation, there is conscious relaxation of the puborectalis muscle, which creates a more open anorectal angle and allows for a straigther passage through the anal canal. Look at Figure 1 in this paper for an illustration of the role of the puborectalis muscle.

The events that occur in defecation are outlined in the flow chart below. The first step is the defecation reflex. A mass movement is a maintained, high-intensity contraction of the circular muscle, which will propel feces toward the sigmoid colon and rectum. If sufficient volume is deposited in the rectum, the increased pressure will stimulate a reflexive contraction in the sigmoid colon and rectum, relaxation of the internal anal sphincter and contraction of the external anal sphincter. Defecation does not occur with the defecation reflex, rather it triggers the urge to defecate. Next, depending on the circumstances, either delay or defecation will occur. With delay, continued contraction of the external anal sphincter and high pressure eventually trigger reverse peristalsis in the rectum. Defecation involves the relaxation of the external anal sphincter (as well as relaxation of the puborectalis muscle, to create a broader anorectal angle). Expulsion of feces is aided by the Valsalva maneuver (closure of the glottis along with contraction of abdominal muscles), which increases intra-abdominal pressure.

Answer 3

The defecation reflex is one element of healthy bowel function. When it is working properly,

the defecation reflex produces muscular contractions that signal the body that stools are ready and it is time to permit a bowel movement.

When the reflex is weak or unresponsive,

the signal to move the bowel can become unreliable. Stools that are withheld (not moved when they are ready) can dry out, loose volume, harden, and become difficult to pass. This is one way that constipation can develop.

Moving the bowel whenever the reflex is activated, maintains the responsiveness of this reflex and helps to avoid the development of constipation. The text below explores the role this reflex plays in healthy bowel function.

Answer 4

The presence of feces in the rectum triggers the defecation reflex. As the rectum stretches due to the accumulation of feces, sensory receptors in the rectal walls send signals to the spinal cord, which then initiates the reflex to relax the internal anal sphincter and contract the colon muscles to push out the feces.