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Traversing nerve root of spinal nerve S3

The traversing nerve root of spinal nerve S3 is one of the sacral spinal nerve roots that descends within the lumbar cistern before exiting through the sacral foramina. Like other traversing roots, it courses inferiorly from its origin at the conus medullaris within the thecal sac until it joins to form the mixed spinal nerve. It contributes significantly to pelvic organ function, perineal sensation, and sacral plexus innervation.

The S3 root has high clinical importance due to its parasympathetic contributions to bladder, bowel, and sexual function, and its role in sensory and motor innervation of the perineum.

Synonyms

  • Sacral nerve root S3

  • Third sacral traversing nerve root

  • Sacral spinal nerve root (S3 segment)

Origin, Course, and Exit

  • Origin:

    • Arises from dorsal and ventral rootlets of the S3 spinal cord segment at the conus medullaris

  • Course:

    • Rootlets combine into dorsal and ventral roots

    • Traverse the thecal sac, descending vertically with other sacral roots in the cauda equina

    • Join together at the intervertebral level to form the mixed spinal nerve root

  • Exit:

    • The S3 spinal nerve exits through the third sacral foramen

Relations

  • Anteriorly: Sacral bodies and pelvic organs

  • Posteriorly: Sacral lamina and ligamentous structures

  • Superiorly: Traversing S2 nerve root

  • Inferiorly: Traversing S4 nerve root

  • Laterally: Exiting sacral nerves and dorsal root ganglion

Function

  • Motor contributions:

    • Supplies pelvic floor muscles and parts of lower limb through sacral plexus connections

    • Contributes to pudendal nerve, controlling external anal and urethral sphincters

  • Sensory contributions:

    • Provides sensation to perineum, genitalia, and parts of the buttock region

  • Autonomic contributions:

    • Parasympathetic fibers play a major role in bladder emptying, bowel function, and sexual response

Clinical Significance

  • Radiculopathy: Compression (disc disease, stenosis, tumor) can cause perineal pain, numbness, or sphincter dysfunction

  • Cauda equina syndrome: Involvement leads to urinary retention, fecal incontinence, and saddle anesthesia

  • Pelvic surgery: At risk during procedures involving the sacrum, pelvis, or rectum

  • Imaging: Evaluation is essential in suspected plexopathy or sacral pathology

MRI Appearance

T1-weighted images:

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

  • Surrounded by bright perineural fat and epidural fat, aiding in visualization

T2-weighted images:

  • Demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF in the thecal sac, giving clear contrast

  • Pathological processes (edema, inflammation, infiltration) appear as increased signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal S3 nerve root shows minimal or no enhancement

  • Abnormal root (neuritis, tumor infiltration, metastasis, infection) shows focal or diffuse enhancement

3D T2 SPACE / CISS:

  • S3 traversing root shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by very bright CSF in the thecal sac, giving excellent contrast

  • Best sequence for evaluating compression, displacement, or entrapment of sacral roots

MRI image

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