The surface of the skin is divided into specific areas, called dermatomes. A dermatome is an area of skin whose sensory nerves all come from a single spinal nerve root. (Sensory nerves carry information about such things as touch, pain, temperature, and vibration from the skin to the spinal cord.)
Spinal roots come in pairs—one of each pair on each side of the body. There are 31 pairs:
There are 8 pairs of sensory nerve roots for the 7 cervical vertebrae.
Each of the 12 thoracic, 5 lumbar, and 5 sacral vertebrae has one pair of spinal nerve roots.
In addition, at the end of the spinal cord, there is a pair of coccygeal nerve roots, which supply a small area of the skin around the tailbone (coccyx).
There are dermatomes for each of these nerve roots.
Sensory information from a specific dermatome is carried by sensory nerve fibers to the spinal nerve root of a specific vertebra. For example, sensory information from a strip of skin along the back of the thigh is carried by sensory nerve fibers to the 2nd sacral vertebra (S2) nerve root.