The internal anal sphincter is a 2-3 cm long thickening of the circular smooth muscle layer of the rectum and anal canal. It operates involuntarily under autonomic control and is tonically contracted at rest, providing 50-80% of resting anal pressure. Sympathetic stimulation maintains contraction (closure), while parasympathetic stimulation and nitric oxide mediate relaxation during defaecation (rectoanal inhibitory reflex).
| Origin | Circular smooth muscle layer of the rectum, continuous distally into the anal canal |
|---|---|
| Insertion | Encircles the anal canal as a thickened ring, ending 1 cm above the anal verge |
| Nerve Supply | Sympathetic fibres (L1, L2) via the hypogastric nerves and inferior hypogastric plexus, maintaining tone; Parasympathetic fibres from the pelvic splanchnic nerves (S2-S4), promoting relaxation |
| Blood Supply | Superior rectal artery; Middle rectal artery; Inferior rectal artery |
| Actions |
|
|---|
It maintains resting closure of the anal canal involuntarily, providing most of the resting anal pressure that keeps a person continent at rest and during sleep.
Internal anal sphincter integrity is essential for passive continence, particularly for liquid stool and flatus, and its resting tone is measured by anorectal manometry, which also tests the rectoanal inhibitory reflex (reflex relaxation when the rectum distends; it is absent in Hirschsprung disease). Injury from obstetric trauma, fistula surgery or sphincterotomy causes passive leakage without urgency. Chronic anal fissure is linked to internal sphincter hypertonia and poor blood flow to the posterior midline anoderm; topical nitrates, calcium channel blockers or botulinum toxin injection relax the muscle temporarily, and lateral internal sphincterotomy divides a limited distal portion of it. The intersphincteric groove between the internal and external sphincters is a key landmark in anorectal surgery.
Palpated on digital rectal examination as the smooth firm ring at the anorectal junction, lying immediately deep to the lower border of the puborectalis sling, distinct from the striated external sphincter below and lateral.
Third and fourth degree perineal lacerations during childbirth disrupt the internal and external anal sphincters, producing faecal urgency and passive incontinence; primary repair restores continence in most cases, with secondary overlap sphincteroplasty for persistent deficits.
Lateral internal sphincterotomy for chronic anal fissure divides a portion of the internal sphincter, risking passive faecal incontinence particularly for liquid stool and flatus; risk is proportional to the length of muscle divided, favouring limited sphincterotomy over full-length division.
