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Spinal nerve S2

The spinal nerve S2 is a mixed spinal nerve formed by the union of dorsal (sensory) and ventral (motor) roots from the S2 spinal cord segment. The rootlets arise from the conus medullaris region, descend within the cauda equina, and the nerve exits the sacral canal via the second anterior and posterior sacral foramina. S2 contributes heavily to the sacral plexus and is involved in motor, sensory, and autonomic innervation of the pelvis, perineum, and lower limb.

Synonyms

  • Second sacral spinal nerve

  • S2 nerve root

  • Sacral segment 2 nerve

Origin, Course, and Branches

  • Origin:

    • Arises from the S2 spinal cord segment, formed by dorsal and ventral roots

  • Course:

    • Rootlets descend in the cauda equina within the thecal sac

    • Exits the sacral canal via the anterior and posterior sacral foramina of S2

    • Joins adjacent sacral nerves to contribute to the sacral plexus

  • Branches:

    • Contributes to sciatic nerve

    • Contributes to pudendal nerve

    • Contributes to posterior femoral cutaneous nerve

    • Contributes to pelvic splanchnic nerves (parasympathetic fibers)

    • Muscular branches to pelvic floor and gluteal region

Relations

  • Anteriorly: Piriformis muscle and pelvic viscera

  • Posteriorly: Sacrum and sacrospinous ligaments

  • Superiorly: S1 spinal nerve

  • Inferiorly: S3 spinal nerve

Function

  • Motor: Contributes to sciatic nerve (hamstrings, lower limb), pudendal nerve (perineal muscles), pelvic floor muscles

  • Sensory: S2 dermatome covers posterior thigh, parts of perineum, and genitalia

  • Autonomic: Pelvic splanchnic nerves from S2–S4 provide parasympathetic innervation to pelvic organs (bladder, rectum, genitalia)

  • Reflexes: Plays a role in anal reflex, bulbocavernosus reflex, and micturition control

Clinical Significance

  • Injury leads to posterior thigh sensory loss, perineal anesthesia, or bladder/bowel/sexual dysfunction

  • Can be affected by sacral fractures, tumors, disc disease, tethered cord, or surgical trauma

  • Important in pudendal nerve blocks and neuromodulation for pelvic floor dysfunction

  • Critical in assessing cauda equina syndrome when multiple sacral roots are involved

MRI Appearance

T1-weighted images:

  • Nerve root appears as a thin low-to-intermediate signal structure within bright epidural or foraminal fat

T2-weighted images:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Pathology (edema, neuritis, compression) makes nerve appear brighter

STIR (Short Tau Inversion Recovery):

  • Normal nerve shows low signal

  • Inflamed or compressed nerve shows bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal nerve root enhances minimally

  • Radiculitis, tumor infiltration, or infection show focal or diffuse enhancement

3D T2 SPACE / CISS:

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

  • Surrounded by bright CSF in the sacral canal, making its origin and foraminal exit at S2 clearly visible

  • Excellent for detecting root compression, tethering, or subtle lesions

CT Appearance

Non-Contrast CT:

  • Root not directly seen; position inferred at S2 anterior and posterior sacral foramina

  • Fat planes and foraminal bony margins help localize

Post-Contrast CT:

  • Nerve itself does not enhance significantly

  • Pathologic processes (nerve sheath tumor, inflammation, metastasis) appear as enhancing soft tissue masses or thickening in or around the sacral foramen

MRI image