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Exiting nerve root of spinal nerve S3

The exiting nerve root of spinal nerve S3 is a sacral spinal nerve root that passes laterally from the thecal sac and exits through the third sacral foramen. After emerging, it contributes to the sacral plexus and provides both sensory and motor innervation to pelvic organs, perineum, and portions of the lower limb.

The S3 exiting nerve root is clinically significant because of its involvement in pelvic floor control, bladder and bowel function, sexual function, and posterior thigh sensation. It is a critical landmark in pelvic surgery and imaging.

Synonyms

  • Sacral nerve root S3

  • Third sacral exiting nerve root

  • Sacral spinal nerve S3

Origin, Course, and Exit

  • Origin:

    • Formed by the union of dorsal and ventral rootlets from the S3 segment of the spinal cord

  • Course:

    • Rootlets join to form dorsal and ventral roots

    • Exit laterally from the thecal sac in the sacral canal

    • Pass through the third sacral foramen to emerge into the pelvis

  • Exit:

    • Emerges via the third sacral anterior sacral foramen, joining the sacral plexus

Relations

  • Anteriorly: Pelvic viscera (rectum, bladder, uterus/prostate) and pelvic peritoneum

  • Posteriorly: Sacral lamina and ligamentous structures

  • Superiorly: Exiting S2 nerve root

  • Inferiorly: Exiting S4 nerve root

  • Laterally: Internal iliac vessels and pelvic plexus

Function

  • Motor contributions:

    • Pelvic floor muscles including levator ani and coccygeus

    • External anal sphincter and external urethral sphincter (via pudendal nerve)

  • Sensory contributions:

    • Skin over perineum, anus, and parts of external genitalia

    • Contributes to posterior thigh sensation (via posterior femoral cutaneous nerve)

  • Autonomic contributions:

    • Parasympathetic innervation to bladder, rectum, and genital organs

    • Crucial for urinary, bowel, and sexual function

Clinical Significance

  • Radiculopathy: Compression may cause pelvic pain, incontinence, erectile dysfunction, or perineal sensory loss

  • Surgical relevance: Important in pelvic, rectal, and gynecologic surgery

  • Neuromodulation: Target for sacral nerve stimulation in refractory bladder or bowel dysfunction

  • Oncology: May be infiltrated by pelvic tumors or metastases

  • Imaging: Important landmark in MRI for pelvic neuropathy or sacral plexopathy

MRI Appearance

T1-weighted images:

  • Appears as a thin low-to-intermediate signal linear structure

  • Surrounded by bright fat, which improves visualization at the sacral foramen

T2-weighted images:

  • Exiting root shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF in the sacral canal, aiding contrast

  • Pathological root (edema, tumor infiltration) shows increased signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal nerve root shows minimal to no enhancement

  • Pathological involvement (neuritis, metastasis, infection) shows focal or diffuse enhancement

3D T2 SPACE / CISS:

  • Root demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Clearly outlined by very bright CSF at the sacral canal

  • Excellent for detecting root compression, displacement, or entrapment by tumors, cysts, or vascular anomalies

MRI image

Exiting nerve root of spinal nerve S3 MRI coronal anatomy image-img-00000-00000