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Traversing nerve root of spinal nerve S2

The traversing nerve root of spinal nerve S2 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 travels inferiorly from its origin at the conus medullaris before joining with its dorsal and ventral roots to form the spinal nerve. It plays an essential role in motor and sensory innervation of the lower limb, pelvis, and perineum.

The S2 traversing root is clinically significant because of its contribution to the sacral plexus, sciatic nerve, and pudendal nerve. Pathology of this root may cause sensory deficits, motor weakness, or pelvic organ dysfunction.

Synonyms

  • Sacral nerve root S2

  • Second sacral traversing nerve root

  • Sacral spinal nerve root (S2 segment)

Origin, Course, and Exit

  • Origin:

    • Arises from the dorsal and ventral rootlets of the S2 spinal cord segment at the conus medullaris

  • Course:

    • Rootlets unite to form dorsal and ventral roots

    • Traverse the thecal sac within the lumbar cistern, coursing downward and medially

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

  • Exit:

    • The S2 spinal nerve exits the vertebral column through the second sacral foramen

Relations

  • Anteriorly: Vertebral bodies, sacral ala

  • Posteriorly: Lamina and ligamentous structures of sacrum

  • Superiorly: Traversing S1 nerve root

  • Inferiorly: Traversing S3 nerve root

  • Laterally: Exiting sacral spinal nerves and dorsal root ganglion

Function

  • Motor contributions:

    • Supplies muscles of the posterior thigh, leg, and foot via branches of the sacral plexus (sciatic, tibial, and common peroneal nerves)

    • Contributes to innervation of pelvic floor and perineal muscles via pudendal and other branches

  • Sensory contributions:

    • Provides sensation to posterior thigh, leg, foot, and perineum (via posterior femoral cutaneous and pudendal nerves)

  • Autonomic contributions:

    • Parasympathetic fibers contribute to control of bladder, bowel, and sexual function

Clinical Significance

  • Radiculopathy: Compression from herniated disc, stenosis, or mass may cause pain radiating to posterior thigh, calf, and foot (sciatica-like symptoms)

  • Pelvic dysfunction: Damage may lead to bladder, bowel, or sexual dysfunction due to parasympathetic fiber involvement

  • Surgical relevance: At risk during sacral and pelvic surgery

  • Imaging: Important landmark in evaluating cauda equina and sacral plexus pathology

MRI Appearance

T1-weighted images:

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

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

T2-weighted images:

  • Demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF within the thecal sac, providing high contrast

  • Pathological changes (edema, inflammation, tumor) appear as increased signal intensity

T1 Fat-Sat Post-Contrast:

  • Normal nerve root enhances minimally or not at all

  • Pathological root (neuritis, tumor, metastasis, infection) demonstrates focal or diffuse enhancement

3D T2 SPACE / CISS:

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

  • Clearly outlined by very bright CSF within the thecal sac

  • Excellent sequence for detecting nerve root compression, entrapment, or displacement in the spinal canal

MRI image

Traversing nerve root of spinal nerve S2 MRI coronal anatomy image-img-00000-00000