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Spinal nerve L5

The spinal nerve L5 is the fifth lumbar spinal nerve and one of the largest roots of the lumbar plexus. It is formed by the union of a dorsal (sensory) root and a ventral (motor) root from the L5 spinal cord segment. It emerges from the dural sac within the cauda equina and exits the vertebral canal via the L5–S1 intervertebral foramen.

The L5 nerve contributes to the lumbosacral trunk, which joins the sacral plexus, and plays a crucial role in motor and sensory innervation of the lower limb. It is commonly involved in radiculopathy from L4–L5 or L5–S1 disc herniations.

Synonyms

  • Fifth lumbar spinal nerve

  • L5 nerve root

  • Lumbar segment 5 nerve

Origin, Course, and Branches

  • Origin:

    • Arises from the L5 spinal segment of the spinal cord

    • Formed by union of dorsal root (sensory) and ventral root (motor)

  • Course:

    • Descends as part of the cauda equina

    • Traverses the lateral recess of the spinal canal

    • Exits via the L5–S1 intervertebral foramen beneath the L5 pedicle

    • Contributes to the lumbosacral trunk, joining S1 to form the sacral plexus

  • Branches:

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

    • Fibers enter superior and inferior gluteal nerves

    • Contributes to nerves of the lumbosacral plexus supplying lower limb muscles

Relations

  • Anterior: L5 vertebral body, intervertebral disc, psoas major (laterally)

  • Posterior: Ligamentum flavum, lamina, facet joint at L5–S1

  • Superior: L4 spinal nerve

  • Inferior: S1 spinal nerve

Function

  • Motor:

    • Major supply to muscles involved in ankle dorsiflexion, toe extension, hip abduction, and inversion/eversion of foot

    • Key muscles: tibialis anterior, extensor hallucis longus, extensor digitorum longus, gluteus medius/minimus

  • Sensory:

    • Supplies sensation to the lateral leg, dorsum of the foot, and great toe

  • Reflex:

    • No single deep tendon reflex, but involved in medial hamstring reflex

Clinical Significance

  • Most commonly affected spinal nerve in lumbar disc herniation (L4–L5, L5–S1 levels)

  • Compression causes L5 radiculopathy:

    • Pain radiating down lateral leg to dorsum of foot and great toe

    • Motor deficit: foot drop (loss of dorsiflexion, toe extension)

    • Sensory deficit: dorsum of foot, especially great toe

  • Important target in nerve root blocks for pain management

MRI Appearance

T1-weighted images:

  • Nerve root appears as low-to-intermediate signal intensity structure within bright epidural fat

T2-weighted images:

  • Nerve shows intermediate to mildly hyperintense signal compared to muscle

  • Compressed or edematous root appears brighter

STIR (Short Tau Inversion Recovery):

  • Normal nerve: low signal

  • Pathology (radiculitis, compression): bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normal root: minimal or no enhancement

  • Radiculitis, tumor, or infection: focal or diffuse enhancement

3D T2 SPACE / CISS:

  • L5 root appears as intermediate to mildly hyperintense relative to muscle, surrounded by bright CSF within the thecal sac

  • Provides clear delineation of root course through lateral recess and L5–S1 foramen

  • Excellent for identifying subtle compressive lesions (disc, osteophyte, cyst)

CT Appearance

Non-Contrast CT:

  • Root not directly visible

  • Location inferred in L5–S1 foramen, bordered by pedicle, vertebral body, and disc

  • Foraminal narrowing or bony compression may be identified

Post-Contrast CT:

  • Root itself does not significantly enhance

  • Nerve sheath tumors, inflammatory masses, or infiltrations show contrast uptake

  • Perineural fat stranding may indicate inflammatory or compressive pathology

MRI image