L3 spinal nerve

At a glance

Level system

L3 nerve (named for its spinal cord segment)

Where it leaves the canal

The L3 nerve exits between the L3 vertebra and the L4 vertebra, below the L3 vertebra.Betts et al. 2022, §13.4

Distance down the canal before it exits

128 mmMeasured on one model: the Visible Human male, imaged as a supine cadaver. Not a normal range; drawn root, schematic

ISNCSCI key sensory point

Medial femoral condyle above the kneeRupp et al. 2021; Kirshblum et al. 2011

ISNCSCI key muscle

Knee extensors: tested by asking the person to extend the kneeRupp et al. 2021; Kirshblum et al. 2011

Roots, ganglia and nerves are drawn schematically from each segment to its exit; their paths are not traced from imaging.

What the sources say

“For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column.” Betts et al. 2022, §13.4

“All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots.” Betts et al. 2022, §13.4

“The sensory axons enter the spinal cord as the dorsal nerve root.” Betts et al. 2022, §13.4

“The motor fibers, both somatic and autonomic, emerge as the ventral nerve root.” Betts et al. 2022, §13.4

“The lumbar plexus arises from all the lumbar spinal nerves and gives rise to nerves innervating the pelvic region and the anterior leg.” Betts et al. 2022, §13.4

Connections to the rest of the body

• Spinal nerves and plexuses

  Lumbar plexus: the front of the thigh

  The lumbar nerves form the lumbar plexus, which supplies the pelvic region and the front of the thigh.

  Betts et al. 2022, §13.4 · Show on the body

• Key muscles (ISNCSCI)

  L3: knee extensors

  In the standard neurological examination after spinal cord injury (ISNCSCI), the strength of the knee extensors is the key test of segment L3. The muscle also receives other segments; the key muscle is a convention for testing, not the whole supply.

  Rupp et al. 2021; Betts et al. 2022, §11.6 · Show on the body

• Key sensory points (ISNCSCI)

  Key sensory points: one patch of skin per segment

  The examination tests light touch and pinprick at one key point per segment on each side, from C2 behind the ear to S4–5 beside the anus. The points here were placed on this body by following the standard’s written descriptions from bony landmarks, so they are approximate.

  Rupp et al. 2021; Kirshblum et al. 2011 · Show on the body

Read more

• The stretch reflex

  The knee jerk explained. A stretched muscle's spindle makes the same muscle contract directly through the cord and relaxes its antagonist, keeping the muscle at a constant length.