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Traversing nerve root of spinal nerve S1

The traversing nerve root of spinal nerve S1 refers to the intradural and extradural rootlets and root fibers of the S1 spinal nerve as they descend within the lumbar thecal sac and traverse the spinal canal before exiting. Unlike the exiting nerve root, which leaves at its corresponding intervertebral foramen, the traversing nerve root continues caudally within the canal until it exits at its designated level.

For S1, the traversing root travels within the lumbar canal (commonly affected at L5–S1 disc level) and exits through the S1 intervertebral foramen. It is highly clinically relevant as it is commonly compressed by disc herniations, lateral recess stenosis, or degenerative changes, leading to classic S1 radiculopathy (pain radiating down the posterior thigh, calf, and lateral foot).

Synonyms

  • Descending root of S1

  • Cauda equina root of S1

  • S1 traversing root fiber

Origin, Course, and Exit

  • Origin:

    • Arises from the dorsal (sensory) and ventral (motor) rootlets emerging from the spinal cord at the conus medullaris (L1 level)

  • Course:

    • Descends within the thecal sac as part of the cauda equina

    • Joins into a single S1 nerve root fascicle within the lumbar canal

    • Traverses the lateral recess at L5–S1

  • Exit:

    • Leaves the spinal canal through the S1 intervertebral foramen, uniting with the dorsal root ganglion and ventral ramus to form the mixed spinal nerve

Relations

  • Anteriorly: Posterior longitudinal ligament and intervertebral disc (L5–S1)

  • Posteriorly: Ligamentum flavum and lamina of L5/S1

  • Laterally: Pedicles of L5 and S1, facet joint

  • Medially: Other cauda equina nerve roots within the thecal sac

Function

  • Motor: Supplies gastrocnemius, soleus, hamstrings (part), gluteus maximus (via inferior gluteal nerve), and intrinsic foot muscles

  • Sensory: Provides sensation to posterior thigh, calf, heel, lateral foot, and little toe

  • Reflex: Contributes to the Achilles (ankle) reflex

Clinical Significance

  • Disc herniation (L5–S1): Most common site of S1 traversing root compression → sciatica, foot weakness, absent ankle reflex

  • Spinal stenosis: Narrowing of lateral recess compresses traversing root

  • Tumors / cysts: Can mimic or compress root (schwannomas, synovial cysts)

  • Trauma: Fractures of sacrum or lumbosacral junction may injure S1 root

  • Imaging relevance: Differentiating traversing from exiting root is crucial in pre-surgical planning

MRI Appearance

T1-weighted images:

  • Nerve root appears as linear/oval low-to-intermediate signal structure within bright epidural and perineural fat

T2-weighted images:

  • Root appears as intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by very bright CSF in the thecal sac, improving visibility

  • Pathology (edema, compression) may show increased signal

T1 Fat-Sat Post-Contrast:

  • Normal traversing root shows minimal or no enhancement

  • Pathological root (radiculitis, tumor infiltration) enhances focally or diffusely

3D T2 SPACE / CISS:

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

  • Surrounded by very bright CSF, providing high contrast

  • Excellent for mapping root course, visualizing lateral recess narrowing, and differentiating from vessels

MRI image

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