Question

What is the defecation reflex and what stimulates it?

Answer 1

Defecation is the process by which fecal matter is excreted out by anal canal.

Defecation reflex is the reflex activity in which fecal matter is evacuated in response to stimulus.

Defecation reflex is of two types - intrinsic defecation reflex and parasympathetic defecation reflex.

Intrinsic defecation reflex -

Feces enters into the rectum (which acts as a stimulus) and distends the rectal wall which acts on the stretch receptors present in the rectal wall. This stimulus is carries by afferent sensory neurons and reaches to the myentric plexus which is the center where this information is integrated and processed and it sends motor impulses via efferent neurons to the smooth muscles of colon and rectum. The response is relaxation of the internal anal sphincter and peristaltic activity along the colon which forces the feces out of the anal canal.

Parasympathetic defecation reflex -

Feces acts into the rectum and distends the rectal wall which is sensed by the stretch receptors present in the rectal wall, this information is carried by sensory fibres which terminates at the level of S2 and S4 segments of the spinal cord, where this information is processed and integrated. This center then sends motor impulses via pelvic parasympathetic nerves which innervated smooth muscles of the sigmoid colon and rectum , thus increasing peristaltic activity and relaxing internal anal sphincter. It results into evacuation of the fecal matter.