L4 spinal nerve

At a glance

Level system

L4 nerve (named for its spinal cord segment)

Where it leaves the canal

The L4 nerve exits between the L4 vertebra and the L5 vertebra, below the L4 vertebra.Betts et al. 2022, §13.4

Distance down the canal before it exits

152 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 malleolusRupp et al. 2021; Kirshblum et al. 2011

ISNCSCI key muscle

Ankle dorsiflexors: tested by asking the person to dorsiflex the ankleRupp 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

“The sacral plexus comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4.” 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

• Spinal nerves and plexuses

  Sacral plexus: the back of the thigh and the calf

  L4 to S4 form the sacral plexus, the source of the nerves to the back of the leg.

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

• Key muscles (ISNCSCI)

  L4: ankle dorsiflexors

  In the standard neurological examination after spinal cord injury (ISNCSCI), the strength of the ankle dorsiflexors is the key test of segment L4. 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

• Dermatomes and myotomes

  The map from spinal segments to skin and muscle. How dermatomes and key muscles are used to find the level of a spinal cord injury in the standard examination.

• Spinal nerves and roots

  The 31 pairs of mixed nerves that connect the cord to the body. How each forms from a sensory and a motor root, where it leaves the column, and how the nerves regroup into plexuses.

• The dorsal root ganglion

  The swelling on each dorsal root that holds the cell bodies of the body's sensory neurons. Why those neurons have a single split process, and what surrounds them.

• The L4–L5 motion segment

  Two lumbar vertebrae, the disc between them, the joints and ligaments that tie them, and the nerves that pass by. One of the two most common places for a disc to herniate, and a good place to see how bone, disc and nerve fit together.