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 (purple) innervated by enteric neurons found in the myenteric plexus. The external anal sphincter is skeletal muscle (red) that can be consciously controlled, and is innervated by somatic efferent neurons. The figure is a highly 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.
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The events that occur in defecation are outlined in the flow chart below. The first step occurs when a mass movement triggers a defecation reflex. A mass movement is a maintained, high-intensity contraction of the circular muscle in the colon, which will propel feces toward the rectum. If sufficient volume is deposited in the rectum, the distension 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 the defecation reflex triggers the urge to defecate by causing increased pressure in the rectum.
Next, depending on the circumstances, either delay or defecation will occur.