The internal anal sphincter is a smooth muscle thickening providing the majority of resting anal tone. It relaxes reflexively with rectal distension (recto-anal inhibitory reflex — RAIR), which is absent in Hirschsprung's disease.
| Origin | Circular smooth muscle of the rectum — a distal thickening of the inner circular coat |
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| Insertion | Not a separate muscle — a continuation of the rectal wall circular smooth muscle |
| Nerve Supply | Pelvic splanchnic nerves (S2-S4) — parasympathetic for relaxation; Sympathetic fibres (L1-L2) — for contraction |
| Blood Supply | Superior and inferior rectal arteries |
| Actions | Maintains resting anal tone — contributes 55-85% of resting anal pressure; Involuntary faecal continence at rest |
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Absent RAIR on anorectal manometry is the diagnostic criterion for Hirschsprung's disease — the aganglionic bowel cannot relax the IAS in response to rectal distension. Internal sphincter tears from obstetric trauma produce reduced resting tone. Botulinum injection into the IAS treats chronic anal fissure by reducing resting sphincter pressure.
Not palpable separately from external sphincter on digital rectal examination.
Internal anal sphincter botulinum injection reducing resting sphincter pressure to allow healing of chronic posterior anal fissures, the non-surgical alternative to lateral internal sphincterotomy.
