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Spinal nerve L4

The spinal nerve L4 is the fourth lumbar spinal nerve, arising from the L4 spinal segment of the spinal cord. Like all spinal nerves, it is formed by the union of dorsal (sensory) and ventral (motor) roots. After traveling in the cauda equina, it exits the spinal canal through the L4–L5 intervertebral foramen.

L4 is a major contributor to the lumbar plexus and lumbosacral trunk, providing fibers to both the femoral and obturator nerves, and contributing to the sciatic nerve. It carries motor, sensory, and reflex fibers critical to lower limb function.

Synonyms

  • Fourth lumbar spinal nerve

  • L4 nerve root

  • Lumbar segment 4 nerve

Origin, Course, and Branches

  • Origin:

    • Arises from spinal cord at the L4 spinal segment (typically at vertebral level T12–L1 due to cord termination at L1–L2)

    • Formed by union of dorsal and ventral rootlets

  • Course:

    • Descends in the cauda equina, traverses lateral recess, and exits via the L4–L5 intervertebral foramen

    • Joins the lumbar plexus within the psoas major muscle

  • Branches:

    • Contributes to the femoral nerve

    • Contributes to the obturator nerve

    • Gives fibers to the lumbosacral trunk (with L5), which contributes to the sacral plexus and sciatic nerve

    • Small muscular branches to psoas major and quadratus lumborum

Relations

  • Anterior: Psoas major muscle and lumbar plexus nerves

  • Posterior: Ligamentum flavum, facet joint, laminae of L4 vertebra

  • Superior: L3 spinal nerve

  • Inferior: L5 spinal nerve

Function

  • Motor: Via femoral and obturator nerves to quadriceps femoris, hip adductors, and parts of psoas/iliacus

  • Sensory: Dermatome covers medial leg, medial malleolus, and medial foot

  • Reflex: Major contributor to the patellar reflex

Clinical Significance

  • L4 nerve root compression (often at L4–L5 disc herniation) causes:

    • Pain radiating to anterior thigh, medial leg, medial malleolus

    • Weakness in knee extension (quadriceps) and hip adduction

    • Decreased patellar reflex

  • L4 is a key target in nerve root blocks for lumbar radiculopathy

  • Involved in lumbar spinal stenosis, foraminal stenosis, and radiculitis

MRI Appearance

T1-weighted images:

  • Nerve root is seen as a linear low-to-intermediate signal structure within bright epidural fat

T2-weighted images:

  • Appears with intermediate to mildly hyperintense signal compared to muscle

  • Compressed or edematous root shows brighter signal

STIR (Short Tau Inversion Recovery):

  • Normal nerve root shows low signal

  • Inflamed or entrapped root shows bright hyperintensity

T1 Fat-Sat Post-Contrast:

  • Normally shows minimal enhancement

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

3D T2 SPACE / CISS:

  • Root shows intermediate to mildly hyperintense signal compared to muscle

  • Surrounded by bright CSF within the thecal sac at its origin, and fat within the foramen

  • Excellent for detecting compression at the L4–L5 foramen and lateral recess

CT Appearance

Non-Contrast CT:

  • Nerve root itself not directly seen

  • Course inferred in the L4–L5 foramen by surrounding fat planes and bony margins

  • Foraminal narrowing or osteophytes may suggest compression

Post-Contrast CT:

  • Nerve does not enhance appreciably

  • Pathology such as nerve sheath tumor, inflammatory infiltration, or postoperative scarring appears as enhancing soft tissue along the foramen

MRI image