CorpusHuman Anatomy
DigestiveMuscle

Internal anal sphincter

Musculus sphincter ani internus

Also known as: IAS

The internal anal sphincter is the involuntary smooth-muscle thickening of the inner circular layer around the upper two-thirds of the anal canal. Its constant tone provides most of the resting anal pressure.

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Overview

The sphincter is a thickened continuation of the circular muscle of the rectum, ending a little above the lower border of the external sphincter. It stays tonically contracted. This tone comes from intrinsic myogenic activity together with sympathetic (α₁) input from the hypogastric plexuses. Parasympathetic and enteric inhibitory neurons, releasing NO and VIP, relax it.

When the rectum distends, the rectoanal inhibitory reflex (RAIR), mediated by the myenteric plexus, briefly relaxes the internal sphincter. This lets a small sample of rectal contents reach the sensitive transition zone, so a person can tell gas from liquid or solid ("sampling"). At the same moment the external sphincter contracts reflexly to keep continence.

Location

Upper two-thirds of the anal canal, deep to the external anal sphincter and separated from it by the intersphincteric space.

Anatomy

Innervation

Sympathetic (L1–L2, via the hypogastric plexuses) contracts it. Parasympathetic (S2–S4, pelvic splanchnic nerves) and enteric inhibitory neurons relax it.

Histology

Smooth muscle, a thickened inner circular layer of the muscularis externa.

Function

  • Maintains resting continence (about 70–85% of resting pressure).
  • Relaxes for sampling (RAIR) and during defecation.

Clinical significance

Absent RAIR in Hirschsprung disease

With no enteric ganglion cells, rectal distension fails to relax the internal sphincter. Absence of the rectoanal inhibitory reflex on anorectal manometry is a key diagnostic clue.

Anal fissure and sphincter injury

Hypertonia of the internal sphincter maintains fissures; treatments aim to relax it (GTN, diltiazem, botulinum toxin, sphincterotomy). Obstetric or surgical damage to the sphincter causes passive soiling.

Compiled from standard anatomy and physiology references — see the Digestive System sources. For education only; not medical advice.