Defecation and continence !
- Regulation
- For completeness
When stool fills the rectum, a reflex opens the internal sphincter; the voluntary external sphincter decides whether it is time to go.
Updated 2026-11-01
Also called: Rectoanal inhibitory reflex, Fecal continence
See it in 3D: Sigmoid colon · Rectum · Anal canal · External anal sphincter · Puborectalis
What it is
Continence depends on two sphincters and an angle. The internal anal sphincter (smooth muscle, involuntary) provides most of the resting pressure; the external sphincter and the puborectalis (striated, voluntary, pudendal nerve) close the canal and hold the anorectal angle. Rectal distension starts reflexes that relax the internal sphincter, and the cortex decides whether the external one relaxes too.
How it works, step by step
- A mass movement carries stool from the sigmoid into the rectum, which distends.
- Stretch receptors activate the myenteric plexus: the distal colon contracts and the internal sphincter relaxes.
- The signal travels up the cord to the cortex: conscious urge appears.
- If it is not the time, the external sphincter and puborectalis contract; the rectum accommodates and the urge fades.
- If it is the time, the external sphincter and puborectalis relax, the anorectal angle opens and a Valsalva manoeuvre is made.
- The sacral parasympathetic reflex intensifies contractions and stool is expelled.
Clinical notes
- Obstetric anal sphincter injury: a common cause of fecal incontinence in women.
- Pelvic floor dyssynergia: the puborectalis contracts instead of relaxing on straining → outlet-obstruction constipation.
- Cauda equina syndrome: incontinence, saddle anesthesia and loss of the anal reflex; a surgical emergency.
- Opioids and dehydration harden stool and favour constipation; heavy straining favours hemorrhoids and rectal prolapse.
Test yourself
Think of the answer, then tap the question to see it.
What role does the rectoanal inhibitory reflex play in continence?
Rectal distension briefly relaxes the internal sphincter and lets contents touch the sensitive anal canal, so gas, liquid or solid can be told apart while the external sphincter keeps it closed.
Why does a cervical spinal cord injury allow reflex but not voluntary defecation?
The sacral reflex arc (S2–S4) and the enteric plexus are intact, but the link to the cortex, which voluntarily controls the external sphincter and puborectalis, is lost.
What does the puborectalis muscle do for continence?
It slings around the anorectal junction and pulls it forward, holding an angle of ≈90° that acts as a valve; it must relax for defecation.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Small bowel and colon motility
- The enteric nervous system, slow waves and GI control
- Micturition and the bladder reflex
- The autonomic nervous system: organization
- Spinal cord reflexes
Related conditions
- Haemorrhoids
- Rectal prolapse
- Obstetric perineal tears and anal sphincter injury
- Spinal cord injury
- Cauda equina syndrome
- Irritable bowel syndrome
In Pharmacology
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 23.5 The Small and Large Intestines (OpenStax (Rice University), 2022); Anatomy, Abdomen and Pelvis: Anal Canal (StatPearls (NCBI Bookshelf), 2024); Irritable Bowel Syndrome (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 64). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.