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Spinal nerve L3

The spinal nerve L3 is the third lumbar spinal nerve, formed by the union of dorsal (sensory) and ventral (motor) roots of the L3 spinal segment. Its rootlets arise from the conus medullaris region, descend within the cauda equina, and the named L3 nerve exits the vertebral canal through the L3–L4 intervertebral foramen. It is a mixed nerve, contributing heavily to the lumbar plexus and providing motor and sensory supply to the anterior thigh and medial leg.

Synonyms

  • Third lumbar spinal nerve

  • L3 nerve root

  • Lumbar segment 3 nerve

Origin, Course, and Branches

  • Origin:

    • Arises from dorsal (sensory) and ventral (motor) rootlets of the L3 spinal segment

    • Roots unite in the intervertebral foramen to form the spinal nerve

  • Course:

    • Exits through the L3–L4 intervertebral foramen

    • Courses laterally into the psoas major muscle

    • Contributes significantly to the lumbar plexus within psoas major

  • Branches:

    • Contributes to the femoral nerve

    • Contributes to the obturator nerve

    • Contributes to the lateral femoral cutaneous nerve

    • Muscular branches to psoas and quadratus lumborum

    • Communicating branches with sympathetic chain

Relations

  • Anterior: Psoas major muscle

  • Posterior: Laminae, ligamentum flavum, and facet joint at L3–L4

  • Superior: L2 spinal nerve

  • Inferior: L4 spinal nerve

Function

  • Motor: Via femoral and obturator nerves, supplies quadriceps femoris, adductors, and part of iliacus

  • Sensory: L3 dermatome—anterior thigh, medial thigh, and medial knee

  • Reflex: Contributes to the patellar reflex (quadriceps reflex)

Clinical Significance

  • L3 root compression (e.g., L3–L4 disc herniation or foraminal stenosis) may cause:

    • Pain radiating to anterior thigh and medial knee

    • Weakness of quadriceps, difficulty with knee extension

    • Reduced patellar reflex

  • Important in lumbar plexus blocks and anterior thigh surgical approaches

  • May be affected in trauma, tumor infiltration, or diabetic lumbosacral plexopathy

MRI Appearance

T1-weighted images:

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

T2-weighted images:

  • Root shows intermediate to mildly hyperintense signal compared to muscle

  • Edema or compression leads to increased signal

STIR (Short Tau Inversion Recovery):

  • Normal nerve root is low signal

  • Inflamed or compressed root becomes bright hyperintense

T1 Fat-Sat Post-Contrast:

  • Minimal enhancement in normal root

  • Focal or diffuse enhancement in radiculitis, tumor, or infection

3D T2 SPACE / CISS:

  • Root appears intermediate to mildly hyperintense compared to muscle

  • Surrounded by bright CSF in the thecal sac, allowing precise visualization of root exit at the L3–L4 foramen

  • Excellent for detecting foraminal stenosis, disc compression, or subtle nerve sheath lesions

CT Appearance

Non-Contrast CT:

  • Root not directly visible; inferred in the L3–L4 intervertebral foramen by fat planes and bony margins

  • Foraminal narrowing may indicate possible root compression

Post-Contrast CT:

  • Nerve itself does not enhance normally

  • Pathological processes (nerve sheath tumor, inflammatory infiltration) appear as enhancing soft tissue in or near the foramen

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