Rectum does not fill the anus with fecal matter constantly because of the tonic contractions in the following areas

Voluntary and subconscious control is exerted on the external anal sphincter while the internal sphincter responds to reflexes.

The process of defecation is controlled in the anus by reflexes called defecation reflexes. It works as follows. Feces which enters the
rectum distends it and also distends its myenteric plexuses deep in its muscularis externa. The plexuses excites the formation of peristaltic wave which develops retrograde and prograde in the descending colon, sigmoid colon and rectum. The effect of these peristaltic waves is to force feces from these locations towards the anus.  As this peristaltic wave reaches the anus it automatically releases the internal anal sphincter while the external anal sphincter is consciously relaxed; feces comes out in the process known as defecation.

Parasympathetic defecation reflex: Parasympathetic sacral outflow is responsible for this reflex. Due to stretching of the rectum, the rectum is stimulated and transmits its signals via its myenteric plexuses to the spinal cord and back to the descending colon, sigmoid colon, rectum and anus by way of parasympathetic nerve fibers called the pelvic splanchnic nerves. These parasympathetic signals augment the peristaltic waves  thereby converting the myenteric intrinsic reflexes into a  powerful means of defecation which will assist the emptying of the gut from the splenic flexure of the colon to the anus.













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