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Robotic Spine Universe

Structures · Spinal cord segments

S4 spinal cord segment

The piece of spinal cord that gives rise to the S4 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
S4 segment (spinal cord level)
Lies opposite
L1 (vertebral level)Model mapping; varies between people. Frostell et al. 2016; De Leener et al. 2018; Cadotte et al. 2014
Length along the cord
6.7 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 S4 nerve leaves through the S4 sacral foramina.Betts et al. 2022, §13.4
ISNCSCI key sensory point
Perianal area, less than 1 cm lateral to the mucocutaneous junctionRupp et al. 2021; Kirshblum et al. 2011

Cross-section at S4

AnteriorPosteriorLR
Regions in this section (text list)

    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 lateral horn , which is only found in the thoracic, upper lumbar, and sacral regions, contains cell bodies of motor neurons of the autonomic nervous system.” Betts et al. 2022, §13.2
    “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

      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

    • Spinal nerves and plexuses

      Pudendal nerve: voluntary control of the urethral sphincter

      Holding urine in is voluntary: the pudendal nerve, from S2 to S4, controls the external urethral sphincter.

      Betts et al. 2022, §25.2 · 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

    • Parasympathetic outflow

      Sacral parasympathetic: S2–S4 to the pelvic organs

      Parasympathetic neurons in sacral segments S2 to S4 leave in the pelvic splanchnic nerves to the lower large intestine, the bladder and the reproductive organs. They empty the bladder.

      Wulf and Tom 2023; Betts et al. 2022, §25.2 · Show on the body

    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, §25.2; Wulf and Tom 2023.