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:
|Cervical||Neck||7||Supply movement and feeling to the arms, neck and upper trunk|
|Thoracic||Upper Back||12||Control the muscles and organs of the chest and abdomen|
|Lumbar||Lower Back||5||Supply the legs|
|Sacral||Sacrum||5 (Fused)||The bladder, bowel and sexual organs|
|Coccygeal||Coccyx or tailbone||4 (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 Level||Nerve Root||Motor Deficit||Sensory Deficit||Reflex Compromise|
Medial leg and foot
|L4-5||L5||Extensor hallucis longus||Lateral thigh|
|L5-S1||S1||Ankle plantar flexors||Posterior leg|
|Anterolateral shoulder and arm||Biceps|
|Lateral forearm and hand|
|Medial forearm and hand, ring and little fingers||None|
|T1-T2||T1||Hand intrinsics||Medial forearm||None|
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.