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Inferior rectal nerve

The inferior rectal nerve is a sensory and motor branch of the pudendal nerve. It arises in the ischioanal fossa and supplies the external anal sphincter and the skin around the anus and perianal region. This nerve plays an essential role in anal continence and perineal sensation. Due to its superficial position in the ischioanal fossa, it is clinically relevant in perianal surgery, nerve entrapment, and perineal pain syndromes.

Synonyms

  • Inferior anal nerve

  • Hemorrhoidal nerve

  • Branch of pudendal nerve (S2–S4)

Origin, Course, and Branches

  • Origin:

    • Arises from the pudendal nerve (ventral rami S2–S4) after it enters the ischioanal fossa

  • Course:

    • Passes medially across the ischioanal fossa through fat and connective tissue

    • Runs towards the anal canal

    • Lies inferior to the levator ani muscle and medial to the ischial tuberosity

  • Branches:

    • Motor fibers to the external anal sphincter

    • Sensory fibers to the skin of the anus and perianal region

Relations

  • Anteriorly: Ischioanal fat and perineal skin

  • Posteriorly: Sacrotuberous ligament

  • Superiorly: Levator ani muscle

  • Inferiorly: Perianal skin

  • Laterally: Ischial tuberosity

Function

  • Provides motor innervation to the external anal sphincter (voluntary control of defecation)

  • Provides sensory innervation to the skin of the anus and perianal region

  • Contributes to the anal reflex and continence mechanism

Clinical Significance

  • Perianal pain syndromes: May be injured or entrapped in pelvic floor dysfunction

  • Hemorrhoid surgery: Nerve is at risk during hemorrhoidectomy and other perianal procedures

  • Fistula surgery: Must be preserved to prevent sensory and continence deficits

  • Pudendal neuralgia: May involve inferior rectal branch, leading to localized perianal pain

MRI Appearance

T1-weighted images:

  • Nerve appears as a thin low-to-intermediate signal intensity structure within the ischioanal fat, which is bright

T2-weighted images:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Pathology (edema, inflammation, entrapment) shows increased brightness

STIR (Short Tau Inversion Recovery):

  • Normal nerve appears dark/low signal

  • Abnormal nerve (edema, inflammation) appears bright hyperintense

  • Surrounding fat suppressed, improving contrast

T1 Fat-Sat Post-Contrast:

  • Normal nerve enhances minimally

  • Pathological nerve may show diffuse or focal enhancement (neuritis, tumor, infection)

3D T2 SPACE / CISS:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by very bright ischioanal fat, providing excellent contrast

  • Useful for tracing small nerves in the perineum and identifying entrapment or lesions

CT Appearance

Non-Contrast CT:

  • Nerve itself not directly visible

  • Appears as a fine soft tissue strand in ischioanal fat planes, inferred by location

  • Surrounding fat shows low attenuation, aiding localization

Post-Contrast CT:

  • Nerve does not enhance significantly

  • Pathological conditions (inflammation, tumor, infection) may present as enhancing soft tissue masses or stranding in ischioanal fat

  • Useful in differentiating abscesses or tumor infiltration in the perianal region

MRI image

Inferior rectal nerve mri coronal image

MRI image

Inferior rectal nerve mri axial anatomy image-img-00000-00000