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Spinal nerve S5

The spinal nerve S5 is the fifth sacral nerve and one of the smallest spinal nerves. It originates from the sacral spinal cord and emerges through the sacral hiatus via the sacral foramina or sacral canal, depending on anatomic variation. It plays a minor but important role in sensory innervation of the perianal region and contributes fibers to the coccygeal plexus, which supplies the skin over the coccyx.

Although small, S5 is clinically relevant in pelvic surgery, coccygeal pain syndromes, and imaging of sacral nerve roots, especially in cases of cauda equina lesions, tethered cord, or sacral tumors.

Synonyms

  • Fifth sacral spinal nerve

  • Sacral nerve root S5

  • Coccygeal plexus contributor

Origin, Course, and Branches

  • Origin:

    • Arises from the ventral and dorsal roots of the spinal cord at the sacral level

    • Exits the sacral canal through the sacral hiatus or foramina

  • Course:

    • Short course within the sacral canal

    • Emerges with S4 and coccygeal nerves to form the coccygeal plexus

  • Branches:

    • Ventral ramus: Joins S4 and coccygeal nerve to form the coccygeal plexus

    • Dorsal ramus: Supplies the skin over the coccyx

    • Meningeal branches: Supply dura mater and periosteum around sacral canal

Relations

  • Anteriorly: Sacral canal contents (filum terminale, coccygeal plexus)

  • Posteriorly: Sacrum and sacral hiatus

  • Laterally: Surrounding sacral foramina and pelvic fascia

  • Inferiorly: Coccygeal nerve and coccyx

Function

  • Sensory: Supplies skin over the coccyx and perianal region

  • Motor: Minor contribution to pelvic floor muscles via coccygeal plexus

  • Reflex: Participates in sensory-motor pathways of anal and pelvic reflex arcs

  • Autonomic: Small contribution to visceral afferents of pelvic organs

Clinical Significance

  • Coccygodynia: Irritation or entrapment of S5 may cause coccyx pain

  • Sacral lesions: Tumors, fractures, or infections involving the sacrum may compress S5

  • Cauda equina syndrome: Rare involvement may produce perianal sensory loss or bowel/bladder dysfunction

  • Surgical relevance: Important in procedures involving sacral hiatus, caudal anesthesia, and sacral nerve stimulation

MRI Appearance

T1-weighted images:

  • S5 root appears as a thin low-to-intermediate signal linear structure

  • Surrounded by bright fat in sacral canal, aiding visualization

T2-weighted images:

  • Appears as intermediate to mildly hyperintense signal compared to muscle

  • Pathological nerve (edema, compression, neuritis) appears brighter

STIR (Short Tau Inversion Recovery):

  • Normal nerve shows low signal

  • Inflamed or infiltrated nerve appears bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Normal nerve shows minimal or no enhancement

  • Pathological nerve shows focal or diffuse enhancement (neuritis, tumor, infection)

3D T2 SPACE / CISS:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by very bright CSF or fat, giving excellent contrast

  • Allows high-resolution tracing of the small S5 nerve root in sacral canal and foramina

CT Appearance

Non-Contrast CT:

  • S5 nerve itself not directly visible

  • Seen indirectly as a soft tissue density within sacral canal or sacral hiatus, outlined by fat

  • Sacral fractures or deformities may impinge on its course

Post-Contrast CT:

  • Nerve does not enhance directly

  • Pathology (nerve sheath tumor, infection, or inflammatory infiltration) may appear as enhancing tissue around sacral canal or foramina

  • Surrounding fat planes may show stranding in inflammation

MRI image

Spinal nerve S5 mri axial anatomy image-img-00000-00000_00001

MRI image

Spinal nerve S5 mri CORONAL anatomy image-img-00000-00000