Feces forced into the rectum by mass movements stretch the rectal wall and initiate the defecation reflex. This spinal cord–mediated parasympathetic reflex causes the sigmoid colon and the rectum to contract, and the internal anal sphincter to relax. As feces are forced into the anal canal, messages reach the brain allowing us to decide whether the external (voluntary) anal sphincter should be opened or remain constricted to stop feces passage temporarily.
If defecation is delayed reflex contractions end within a few seconds, and the rectal walls relax.With the next mass movement, the defecation reflex is initiated again—and so on, until the person chooses to defecate or the urge to defecate becomes unavoidable.
