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Spinal nerve S1

The first sacral spinal nerve (S1) is a mixed motor and sensory nerve formed by the union of dorsal (sensory) and ventral (motor) roots from the S1 spinal segment. Rootlets emerge from the lower end of the spinal cord and descend in the cauda equina before converging at the S1 dorsal root ganglion. The nerve exits the vertebral canal via the S1–S2 intervertebral foramen of the sacrum.

S1 is a major contributor to the lumbosacral plexus and provides motor fibers to gluteal, hamstring, and calf muscles while supplying sensory innervation to the posterior thigh, leg, and lateral foot.

Synonyms

  • First sacral nerve

  • S1 nerve root

  • Sacral segment 1 nerve

Origin, Course, and Branches

  • Origin:

    • Dorsal and ventral rootlets from the S1 spinal cord segment

    • Merge at the dorsal root ganglion (S1 level) to form the spinal nerve

  • Course:

    • Travels within the cauda equina, then exits the vertebral canal via the S1–S2 sacral foramen

    • Joins the sacral plexus in the pelvic cavity

  • Branches:

    • Contributes to the sciatic nerve (tibial and common fibular divisions)

    • Contributes to the superior gluteal nerve and inferior gluteal nerve

    • Contributes to posterior femoral cutaneous nerve

    • Contributes to the pudendal nerve and other pelvic splanchnic branches

Relations

  • Anteriorly: Sacral ala, pelvic viscera, and sacral plexus

  • Posteriorly: Sacral lamina and ligaments

  • Superiorly: L5 spinal nerve

  • Inferiorly: S2 spinal nerve

Function

  • Motor: Innervates gluteus maximus, gluteus medius, hamstrings, gastrocnemius, soleus, and intrinsic foot muscles (via sciatic nerve branches)

  • Sensory: Supplies posterior thigh, posterior leg, lateral malleolus, heel, and lateral foot (S1 dermatome)

  • Reflex: Mediates the ankle (Achilles) reflex

Clinical Significance

  • Disc herniation at L5–S1 is the most common cause of S1 nerve root compression (sciatica)

  • Radiculopathy causes pain radiating down posterior thigh, calf, and lateral foot

  • Weakness: plantar flexion, hip extension, foot eversion

  • Loss of Achilles reflex is a key finding in S1 involvement

  • Important for imaging, surgery, and selective nerve blocks

MRI Appearance

T1-weighted images:

  • Root appears as low-to-intermediate signal linear structure within bright epidural fat

T2-weighted images:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Compression, neuritis, or edema increases signal intensity

STIR (Short Tau Inversion Recovery):

  • Normal root: low signal

  • Pathological root: bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal nerve: minimal or no enhancement

  • Abnormal nerve: focal or diffuse enhancement (radiculitis, tumor, infection)

3D T2 SPACE / CISS:

  • S1 root shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF in the thecal sac, allowing clear visualization

  • Excellent for evaluating foraminal stenosis, nerve compression, or root sleeve cysts

CT Appearance

Non-Contrast CT:

  • Root not directly seen; inferred at the S1–S2 sacral foramen

  • Foraminal bony margins and surrounding fat help localization

Post-Contrast CT:

  • Nerve itself does not enhance significantly

  • Pathologic conditions (nerve sheath tumor, metastatic infiltration, infection) appear as enhancing soft tissue masses in or near the foramen

MRI image