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Internal anal sphincter

The internal anal sphincter is a thickened continuation of the circular smooth muscle layer of the rectum. It is involuntary and under autonomic control, playing a vital role in maintaining resting anal tone and ensuring continence. It lies deep to the external anal sphincter, separated by the intersphincteric space filled with connective tissue and vascular structures.

Synonyms

  • IAS (internal anal sphincter)

  • Involuntary anal sphincter

  • Internal sphincter of the anal canal

Origin, Course, and Insertion

  • Origin: Continuation of the inner circular smooth muscle of the rectum at the anorectal junction

  • Course: Extends inferiorly along the anal canal as a cylindrical muscular sheath, blending with surrounding fibrous tissue in the intersphincteric plane

  • Insertion: Attaches inferiorly at the anal verge, fusing with connective tissue and partially with the external anal sphincter

Nerve Supply

  • Sympathetic fibers (hypogastric plexus): maintain contraction (tonic activity)

  • Parasympathetic fibers (pelvic splanchnic nerves, S2–S4): mediate relaxation during defecation

Arterial Supply

  • Superior rectal artery (branch of inferior mesenteric artery)

  • Middle rectal artery (branch of internal iliac artery)

  • Inferior rectal artery (branch of internal pudendal artery)

Venous Drainage

  • Superior rectal vein → portal venous system

  • Middle and inferior rectal veins → systemic venous system

  • Provides communication for the porto-systemic venous anastomosis in the anal canal

Function

  • Maintains resting anal tone (responsible for ~70% of resting anal canal pressure)

  • Provides involuntary continence at rest

  • Relaxes reflexively during defecation under parasympathetic control

MRI Appearance

T1-weighted images:

  • Internal anal sphincter appears as a hypointense circular muscular ring

  • Surrounding fat planes appear bright, outlining the sphincter clearly

T2-weighted images:

  • Internal sphincter remains low signal intensity relative to high-signal ischioanal fat and anal canal lumen

  • Thickening, edema, or fibrosis appears as signal change within the ring

STIR (Short Tau Inversion Recovery):

  • Suppresses fat, making the sphincter visible as a dark band

  • Edema, inflammation, or pathology shows bright signal intensity

Proton Density Fat-Sat (PD FS):

  • Internal sphincter: low signal intensity

  • Pathological processes (abscess, inflammatory extension) show bright foci or tracks

T1 Fat-Sat Post-Contrast:

  • Normal sphincter enhances mildly and uniformly

  • Fistulas, abscesses, or tumors show enhancing tracts, rims, or masses adjacent to the sphincter

CT Appearance

Non-Contrast CT:

  • Internal sphincter is not distinctly visualized as separate from the external sphincter

  • Anal canal appears as soft tissue density; sphincter complex seen as concentric rings

Post-Contrast CT:

  • Enhancement is mild and uniform

  • Inflammation, abscesses, or neoplasms show irregular or rim enhancement within the anal canal region

  • Useful for staging anorectal tumors or evaluating perianal sepsis

MRI image

Internal anal sphincter muscle MRI axial anatomy image-img-00000-00000

CT image

Internal anal sphincter ct axial image

MRI image

Internal anal sphincter