L4 spinal cord segment

At a glance

Level system

L4 segment (spinal cord level)

Lies opposite

T12 to T12-L1 disc (vertebral levels)Model mapping; varies between people. Frostell et al. 2016; De Leener et al. 2018; Cadotte et al. 2014

Length along the cord

13.3 mmMeasured on one model: the Visible Human male, imaged as a supine cadaver. Not a normal range. De Leener et al. 2018; Frostell et al. 2016

Its nerve

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

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

What the sources say

“The posterior horn is responsible for sensory processing.” Betts et al. 2022, §13.2

“The anterior horn sends out motor signals to the skeletal muscles.” Betts et al. 2022, §13.2

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.

• 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.

• The withdrawal reflex

  Pulling away from a hot stove or a tack. Pain excites the motor neurons of the withdrawing muscle and, through an inhibitory interneuron, quiets the opposing muscle so the movement is faster.