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Exiting nerve root of spinal nerve S5

The exiting nerve root of spinal nerve S5 is the terminal sacral spinal nerve root. It is much smaller than other sacral roots and emerges at the level of the sacral hiatus through the fifth sacral foramen or sacral canal. It combines with S4 and the coccygeal nerve to form the coccygeal plexus, which provides sensory and motor fibers to the skin around the anus, coccyx, and small pelvic floor muscles.

Although small, the S5 root is clinically important because of its contribution to perineal sensation, anal sphincter function, and pelvic floor innervation.

Synonyms

  • Fifth sacral nerve root

  • Terminal sacral spinal nerve

  • Coccygeal plexus root

Origin, Course, and Exit

  • Origin:

    • Arises from the dorsal and ventral rootlets of the S5 spinal cord segment at the terminal part of the conus medullaris/cauda equina

  • Course:

    • Rootlets converge within the thecal sac

    • The nerve root courses a short distance downward in the sacral canal

  • Exit:

    • Leaves the spinal canal through the fifth sacral foramen or via the sacral hiatus, joining with S4 and coccygeal nerves

Relations

  • Anteriorly: Pelvic viscera (rectum, sigmoid colon)

  • Posteriorly: Sacral lamina and sacrococcygeal ligament

  • Superiorly: S4 nerve root

  • Inferiorly: Coccygeal nerve root

  • Laterally: Sacral ala and foramina

Function

  • Motor: Contributes to pelvic floor muscles and external anal sphincter via coccygeal plexus

  • Sensory: Supplies skin around anus, coccyx, and perianal region

  • Autonomic: Works with S2–S4 parasympathetic fibers for pelvic organ function

Clinical Significance

  • Injury: Sacral fractures or sacral surgery can damage the root, leading to perineal sensory loss or sphincter dysfunction

  • Cauda equina syndrome: Involvement of S5 root causes saddle anesthesia and bowel/bladder dysfunction

  • Oncology: May be infiltrated by pelvic tumors, sacral metastases, or chordomas

  • Target for blocks: Used in caudal epidural anesthesia and nerve block techniques

MRI Appearance

T1-weighted images:

  • Nerve root appears as a thin low-to-intermediate signal linear structure

  • Surrounded by high signal fat in the sacral canal and foramina

T2-weighted images:

  • Appears as intermediate to mildly hyperintense signal compared to muscle

  • Bright CSF within the sacral canal outlines the nerve root

  • Pathology (edema, infiltration, compression) shows increased signal

T1 Fat-Sat Post-Contrast:

  • Normal root shows minimal to no enhancement

  • Pathologic involvement (inflammation, tumor, infection) shows focal or diffuse enhancement

3D T2 SPACE / CISS:

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

  • Surrounded by very bright CSF in the sacral canal, giving excellent contrast

  • Allows detailed visualization of small roots like S5 and coccygeal root, useful for detecting entrapment or compression

MRI image

Exiting nerve root of spinal nerve S5 MRI coronal anatomy image-img-00000-00000