Dermatome Definition. A dermatome is the area of sensory nerves near the skin that are supplied by a specific spinal nerve root. The body can be divided into regions that are mainly supplied by a single spinal nerve. There are eight cervical (one for the head, and one for each cervical vertebra), twelve thoracic, five lumbar and five sacral spinal nerves. Dermatomes are useful for finding the site of damage to the spine. For example, leg pain from radiculopathy often indicates a problem to a specific nerve root in the lumbar spine.