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Robotic Spine Universe

Structures · Spinal cord segments

L3 spinal cord segment

The piece of spinal cord that gives rise to the L3 pair of spinal nerves. Segment names use the spinal cord level system, which does not line up with the vertebrae.

Show in the 3D spineSee what it connects to in the body

At a glance

Level system
L3 segment (spinal cord level)
Lies opposite
T12 (vertebral level)Model mapping; varies between people. Frostell et al. 2016; De Leener et al. 2018; Cadotte et al. 2014
Length along the cord
13.8 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 L3 nerve exits between the L3 vertebra and the L4 vertebra, below the L3 vertebra.Betts et al. 2022, §13.4
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

Cross-section at L3

left ventral hornright ventral hornleft intermediate zoneright intermediate zoneleft dorsal hornright dorsal hornAnteriorPosteriorLR
Regions in this section (text list)
  • left ventral horn
  • right ventral horn
  • left intermediate zone
  • right intermediate zone
  • left dorsal horn
  • right dorsal horn

Grey matter and white-matter tracts at this level. Outline from the PAM50 template and atlas, cut at the segment midpoint. De Leener et al. 2018; Lévy et al. 2015

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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

  • 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

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  • 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.

For learning only. Levels and sizes vary between people; nothing here describes a particular person or gives medical advice. Sources: Betts et al. 2022, §13.2; Frostell et al. 2016; De Leener et al. 2018; Cadotte et al. 2014; Betts et al. 2022, §13.4; Rupp et al. 2021; Kirshblum et al. 2011; Betts et al. 2022, §11.6.