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Spinal nerve S3

The spinal nerve S3 is the third sacral spinal nerve, formed by the union of dorsal (sensory) and ventral (motor) roots of the S3 spinal segment. It emerges from the spinal canal via the third anterior and posterior sacral foramina of the sacrum. Like other sacral nerves, it contributes both motor and sensory fibers, playing an essential role in pelvic organ innervation, perineal sensation, and sphincter control.

Synonyms

  • Third sacral spinal nerve

  • S3 nerve root

  • Sacral segment 3 nerve

Origin, Course, and Branches

  • Origin:

    • Dorsal and ventral rootlets from the S3 spinal cord segment (within conus medullaris)

    • Roots descend in the cauda equina and join to form the S3 spinal nerve

  • Course:

    • Passes through the third anterior and posterior sacral foramina

    • Joins adjacent sacral nerves to form the sacral plexus on the anterior surface of the piriformis muscle

  • Branches:

    • Contributes fibers to the pudendal nerve (major branch to perineum and sphincters)

    • Fibers to the pelvic splanchnic nerves (nervi erigentes), providing parasympathetic supply to pelvic viscera

    • Contributions to inferior gluteal nerve, posterior femoral cutaneous nerve, and sciatic nerve (minor)

Relations

  • Anteriorly: Pelvic organs and pelvic fascia

  • Posteriorly: Sacral bone and posterior sacral foramina

  • Superiorly: S2 spinal nerve

  • Inferiorly: S4 spinal nerve

  • Laterally: Piriformis muscle and sacral plexus

Function

  • Motor: Contributes to control of pelvic floor muscles, external anal sphincter, external urethral sphincter, and parts of the levator ani

  • Sensory: Provides sensation to perineum, anal region, and external genitalia via pudendal and other branches

  • Autonomic (parasympathetic): Via pelvic splanchnic nerves, supplies bladder, rectum, and sexual organs (erection, micturition, defecation)

Clinical Significance

  • Lesions of S3 may cause urinary and fecal incontinence, erectile dysfunction, and loss of perineal sensation

  • Important in cauda equina syndrome evaluation (saddle anesthesia, bladder dysfunction)

  • S3 roots are a key target in sacral neuromodulation for urinary/fecal incontinence therapy

  • May be affected by sacral fractures, tumors, or pelvic surgeries

MRI Appearance

T1-weighted images:

  • Nerve roots show low-to-intermediate signal intensity within bright epidural fat in sacral foramina

T2-weighted images:

  • Nerve roots show intermediate to mildly hyperintense signal compared to muscle

  • Edematous or compressed nerves appear brighter

STIR (Short Tau Inversion Recovery):

  • Normal roots appear low signal

  • Pathological roots (neuritis, inflammation, compression) show bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal roots show minimal or no enhancement

  • Pathological conditions (radiculitis, neoplasm, infection) show focal or diffuse enhancement

3D T2 SPACE / CISS:

  • S3 roots show intermediate to mildly hyperintense signal relative to muscle, surrounded by bright CSF in the sacral canal

  • Allows excellent delineation of their exit through the S3 sacral foramina and detection of subtle compressive lesions

CT Appearance

Non-Contrast CT:

  • Nerve roots not directly visualized

  • S3 foramina visible as bony canals transmitting the roots

  • Fat within foramina outlines possible nerve location

Post-Contrast CT:

  • Nerves themselves do not enhance significantly

  • Pathology (nerve sheath tumor, metastasis, inflammation) appears as enhancing soft tissue mass within/around foramina

MRI image