Expansion
Rectal distension triggers internal sphincter relaxation, with voluntary external sphincter control
Expansion
- A mass movement propels stool into the rectum
- Rectal distension is sensed by stretch receptors
- Rectoanal inhibitory reflex: the internal anal sphincter relaxes reflexly, allowing sampling of the contents by the sensitive anal canal mucosa
- The external sphincter and puborectalis contract voluntarily to defer, or relax to proceed
- If proceeding: squatting straightens the anorectal angle, the Valsalva manoeuvre raises intra-abdominal pressure, and the pelvic floor descends
The reflex arc is mediated by the pelvic splanchnic nerves (S2 to S4) and modulated by the enteric nervous system; voluntary control travels in the pudendal nerve.
Clinical uses of the anatomy:
- Absent rectoanal inhibitory reflex confirms Hirschsprung disease
- Failure of the puborectalis to relax gives obstructed defecation
- Spinal cord injury above the sacral segments leaves a reflex bowel that can be triggered by stimulation, while a cauda equina lesion gives a flaccid, areflexic bowel