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40.1 The spinal nerve roots are split into segments. This causes recognised patterns of innervation [nerves going into things] of skin and muscles from each segment.

40.2 A dermatome is an area of skin supplied by the dorsal [sensory] root of a spinal nerve: see Figure 19A. From the head down each segment is arranged in a segmental fashion [like an earthworm] with modifications made for the limbs. The dermatomes of the arms are supplied by the nerves from C5 to T1.

The dermatomes of the legs by the roots from L3 to S2. Knowledge of the dermatomal organisation can assist in clinical identification of the level of a cord or nerve lesion e.g. if the C6 sensory function is impaired the most likely cause is a lesion at C5/6. And also can identify non organic complaints of altered sensation.

40.3 The segmental nerve supply to the skeletal muscles are called myotomes: see Figure 20.

40.4 So for instance pressure from a prolapsed disc on the L5 nerve root may not only produce pain from irritation of the sensory fibres but also weakness in the affected muscles such as extensor hallucis longus, the great toe extensor which is predominantly supplied by L5.