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Traversing nerve root of spinal nerve S4

The traversing nerve root of spinal nerve S4 is one of the lower sacral nerve roots that descends within the thecal sac before exiting via the sacral foramina. Like other traversing roots, it originates from the conus medullaris, courses downward in the cauda equina, and contributes to the sacral plexus after exiting the sacrum.

The S4 traversing root is smaller in caliber compared to upper sacral roots but has a critical role in parasympathetic and somatic innervation of the pelvic organs, pelvic floor, and perineum.

Synonyms

  • Sacral nerve root S4

  • Fourth sacral traversing nerve root

  • Sacral spinal nerve root (S4 segment)

Origin, Course, and Exit

  • Origin:

    • Arises from the dorsal and ventral rootlets of the S4 spinal cord segment at the conus medullaris

  • Course:

    • Rootlets converge to form dorsal and ventral roots

    • Travel downward within the cauda equina in the thecal sac

    • Join together to form the mixed spinal nerve root

  • Exit:

    • The S4 nerve root exits the vertebral column via the fourth sacral foramen

Relations

  • Anteriorly: Pelvic viscera (rectum, bladder, uterus/prostate) separated by sacral wall

  • Posteriorly: Sacral lamina and ligamentous structures

  • Superiorly: Traversing S3 nerve root

  • Inferiorly: Traversing S5 nerve root and coccygeal root

  • Laterally: Sacral canal and foramina through which it exits

Function

  • Motor contributions:

    • Supplies muscles of the pelvic floor (levator ani, coccygeus)

    • Contributes to external anal sphincter and external urethral sphincter innervation via pudendal nerve

  • Sensory contributions:

    • Provides sensation to perianal skin and parts of external genitalia (via pudendal and perineal branches)

  • Autonomic contributions:

    • Parasympathetic fibers regulate bladder, rectal, and sexual function

  • Reflex contributions:

    • Participates in anal wink reflex and pelvic visceral reflex arcs

Clinical Significance

  • Radiculopathy: Compression (tumor, fracture, stenosis) may cause pelvic pain, perianal numbness, sphincter dysfunction

  • Pelvic dysfunction: Lesions may lead to urinary incontinence, fecal incontinence, or sexual dysfunction

  • Surgical relevance: Important during sacral surgeries and pelvic oncologic resections

  • Imaging: Key root to evaluate in cases of cauda equina syndrome or pelvic plexopathies

MRI Appearance

T1-weighted images:

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

  • Surrounded by bright epidural or perineural fat, aiding visibility near the foramen

T2-weighted images:

  • Demonstrates intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF within the thecal sac for excellent contrast

  • Pathology (edema, neuritis, infiltration) appears brighter

T1 Fat-Sat Post-Contrast:

  • Normal S4 root shows minimal to no enhancement

  • Pathological root (inflammation, tumor, infection) shows 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 in the thecal sac

  • Excellent for detecting compression, displacement, or small lesions involving sacral roots

MRI image

Traversing nerve root of spinal nerve S4 MRI coronal anatomy image-img-00000-00000