Definition
- Defecation is a reflex phenomenon under voluntary control, initiated when mass peristalsis of the descending and sigmoid colon pushes colonic content into the rectum.
- It is normally inhibited by a resting anorectal angle of about 90° and tonic contraction of the external anal sphincter and puborectalis muscle.
Reflex Arc of Defecation
- Receptors are stretch receptors in the wall of the rectum, stimulated by rectal distension.
Reflex arc of defecation
Component
Receptor
- Pathway
- Stretch receptors in wall of rectum
Afferent
- Pathway
- Pelvic nerve to sacral segment S3
Efferent to rectum and internal anal sphincter
- Pathway
- Pelvic nerve
Efferent to external anal sphincter
- Pathway
- Somatic nerve
Higher control
- Pathway
- Cortex, via corticospinal pathway