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 |
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| Insertion | Encircles the anal canal as a thickened ring, ending 1 cm above the anal verge |
| Nerve Supply | Inferior hypogastric plexus (sympathetic, from L5-S2, via hypogastric nerves); Pelvic splanchnic nerves (parasympathetic, from S2-S4) |
| Blood Supply | Superior rectal artery; Inferior rectal artery |
| Actions | Maintains involuntary anal resting tone (contributes 50-80% of resting anal pressure) |
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Internal anal sphincter integrity is essential for passive faecal continence, particularly for liquid stool and flatus. Sphincter injuries from obstetric trauma, fistula surgery, and internal sphincterotomy for chronic anal fissure can produce passive faecal incontinence. The intersphincteric groove between the internal and external sphincters is a key landmark in ano-rectal surgery. Chronic anal fissure results from internal sphincter hypertonia causing ischaemia of the posterior midline anoderm, treated by lateral internal sphincterotomy dividing the distal portion of this muscle.
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.
