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Exiting nerve root of spinal nerve S1

The S1 spinal nerve root is one of the major lumbar-sacral nerve roots that exits the spinal canal to contribute to the lumbosacral plexus. It emerges from the S1 intervertebral foramen as the exiting nerve root and plays a vital role in motor and sensory innervation of the lower limb. It provides important fibers to the sciatic nerve, superior and inferior gluteal nerves, and contributes to muscles of the posterior thigh, leg, and foot.

Clinically, the S1 root is frequently involved in radiculopathy caused by disc herniation, foraminal stenosis, or trauma.

Synonyms

  • First sacral spinal nerve root

  • S1 exiting root

  • Sacral nerve 1

Origin, Course, and Distribution

  • Origin:

    • Formed from the ventral and dorsal rootlets arising from the spinal cord at the S1 segment

  • Course:

    • Rootlets unite to form the S1 spinal nerve within the spinal canal

    • Exits the spinal canal via the S1 intervertebral foramen

    • Lies below the pedicle of S1 vertebra, accompanied by radicular vessels

    • Contributes to the formation of the sacral plexus along with fibers from L4, L5, S2, and S3

  • Distribution:

    • Contributes fibers to the sciatic nerve, superior gluteal nerve, inferior gluteal nerve, posterior femoral cutaneous nerve, and pudendal nerve

Relations

  • Anteriorly: Vertebral body and intervertebral disc (L5–S1)

  • Posteriorly: Lamina and ligamentum flavum

  • Superiorly: L5 nerve root

  • Inferiorly: S2 nerve root

  • Laterally: Sacral foraminal margins and iliac vessels

Function

  • Motor:

    • Innervates gluteus maximus, hamstrings, gastrocnemius, soleus, and intrinsic foot muscles via plexus branches

  • Sensory:

    • Provides sensation to posterior thigh, posterior leg, heel, and lateral foot

  • Reflex:

    • Mediates the ankle (Achilles) reflex

Clinical Significance

  • Radiculopathy: Compression from L5–S1 disc herniation, foraminal stenosis, or spondylolisthesis causes pain radiating down posterior thigh and leg (sciatica)

  • Weakness: Impaired plantar flexion (gastrocnemius/soleus)

  • Sensory loss: Posterior calf, lateral ankle, heel, and sole of foot

  • Surgical relevance: Target in laminectomy, microdiscectomy, or nerve root injections

  • Imaging relevance: Frequently evaluated in MRI for disc herniation and nerve compression

MRI Appearance

T1-weighted images:

  • Nerve root shows low-to-intermediate signal intensity

  • Surrounded by bright epidural fat in the foramen, improving contrast

T2-weighted images:

  • Root shows intermediate to mildly hyperintense signal compared to muscle

  • Compression or neuritis appears as focal bright signal and thickening

STIR (Short Tau Inversion Recovery):

  • Normal root:intermediate signal

  • Pathology (edema, neuritis, compression): bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal root enhances minimally

  • Abnormal root (radiculitis, tumor, infection) shows abnormal focal or diffuse enhancement

3D T2 SPACE / CISS:

  • Root shows intermediate to mildly hyperintense signal compared to muscle

  • CSF around the root is very bright, sharply outlining the nerve

  • Excellent for detecting nerve impingement, foraminal narrowing, or subtle compressions

CT Appearance

Non-Contrast CT:

  • Nerve root itself is not directly visible

  • Course inferred within S1 foramen, surrounded by epidural and foraminal fat

  • Foraminal stenosis, bony spurs, or disc calcification may impinge on root

Post-Contrast CT:

  • Normal root does not enhance

  • Pathology may appear as enhancing soft tissue lesions (tumor, inflammation)

  • Perineural fat stranding may suggest nerve irritation or compression

MRI image

Exiting nerve root of spinal nerve S1 MRI coronal anatomy image-img-00000-00000

CT image

Exiting nerve root of spinal nerve S1 ct axial anatomy image-img-00000-00000