Myotomes and Dermatomes
Myotome and Dermatome Overview
Spinal nerves have motor fibres and sensory fibres. The motor fibres innervate certain muscles, while the sensory fibres innervate certain areas of skin. A skin area innervated by the sensory fibres of a single nerve root is known as a dermatome. A group of muscles primarily innervated by the motor fibres of a single nerve root is known as a myotome. Although slight variations do exist, dermatome and myotome patterns of distribution are relatively consistent from person to person.
Spinal Cord Segmental Myotomes
Each muscle in the body is supplied by a particular level or segment of the spinal cord and by its corresponding spinal nerve. The muscle, and its nerve make up a myotome. This is approximately the same for every person and are as follows:
- C3,4 and 5 supply the diaphragm (the large muscle between the chest and the belly that we use to breath).
- C5 also supplies the shoulder muscles and the muscle that we use to bend our elbow.
- C6 is for bending the wrist back.
- C7 is for straightening the elbow.
- C8 bends the fingers.
- T1 spreads the fingers.
- T1 - T12 supplies the chest wall & abdominal muscles.
- L2 bends the hip.
- L3 straightens the knee.
- L4 pulls the foot up.
- L5 wiggles the toes.
- S1 pulls the foot down.
- S3,4 and 5 supply the bladder. bowel and sex organs and the anal and other pelvic muscles.
Spinal Cord Segmental Dermatomes
Dermatome is a Greek word which literally means "skin cutting". A dermatome is an area of the skin supplied by nerve fibres originating from a single dorsal nerve root. The dermatomes are named according to the spinal nerve which supplies them. The dermatomes form into bands around the trunk but in the limbs their organisation is more complex as a result of the dermatomes being "pulled out" as the limb buds form and develop into the limbs during embryological development.
In diagrams or maps the boundaries of dermatomes are usually sharply defined. However in life there maybe considerable overlap of innervation between adjacent dermatomes. Thus, if there is a loss of afferent nerve function by one spinal nerve sensation from the region of skin which it supplies is not usually completely lost as overlap from adjacent spinal nerves occurs, however, there maybe be a reduction or change in sensation.