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Spinal Nerves

04 September 2023

Anatomy of the Spinal Cord

To discuss the spinal nerves and how they work we first have to understand the anatomy of the spinal cord and column.  The spinal column supports the head and protects the spinal cord, which is attached to the brain and extends down the centre of the back.  It is approximately the width of a human finger.  Along with the brain it forms the central nervous system.  It is made up of millions of nerve fibres which transmit electrical signals to and from the limbs, trunk and organs.  The spinal column is made up of 33 vertebrae which are divided up as follows:

CervicalNeck7Supply movement and feeling to the arms, neck and upper trunk
ThoracicUpper Back12Control the muscles and organs of the chest and abdomen
LumbarLower Back5Supply the legs
SacralSacrum5 (Fused)The bladder, bowel and sexual organs
CoccygealCoccyx or tailbone4 (fused)

Spinal Cord Numbering

Spinal nerves carry information to and from different segments along the spinal cord, and are numbered according to their location, similar to the numbering of the vertebrae.  There are 31 pairs of spinal nerves, along the spinal cord, they exit the spinal column through holes in the vertebrae called foramen.

Dermatomes and Myotomes

A dermatome is an area of skin supplied by a single spinal nerve whilst a myotome is a group of muscles supplied by a single spinal nerve.

Irritation or injury to the spinal nerve root through conditions such as spinal stenosis and disc herniation can lead to a sensory deficit in the distribution of the dermatome, or a motor deficit in the distribution of the myotome.  


Radicular symptoms occur because of irritation of a nerve root.  These symptoms can include pain, numbness and tingling.

The table below outlines common radicular syndromes.

Disc LevelNerve RootMotor DeficitSensory DeficitReflex Compromise
L3-4L4QuadricepsAnterolateral thigh
Anterior knee
Medial leg and foot
L4-5L5Extensor hallucis longusLateral thigh
Anterolateral leg
Middorsal foot
Medial hamstrings
L5-S1S1Ankle plantar flexorsPosterior leg
Lateral foot
Anterolateral shoulder and armBiceps
C5-6C6Wrist extensors
Lateral forearm and hand
Pronator teres
C6-7C7Wrist flexors
Finger extensors
Middle fingerTriceps
C7-T1C8Finger flexors
Hand intrinsics
Medial forearm and hand, ring and little fingersNone
T1-T2T1Hand intrinsicsMedial forearmNone

Neurologists and neurosurgeons are not the only experts in this field. Orthopaedic surgeons have a deep level of understanding of spinal nerves necessary for them to assess spinal injuries.

Spinal Nerves