The spinal cord carries information between your brain and your body, specifically your skin and voluntary muscles.
Each of your 31 pairs of spinal nerves controls a set of muscles and an area of skin.
A dermatome is the area of skin controlled by one pair of spinal nerves. Derma means skin. Tome means section. Testing your sense of touch on different dermatomes can tell your doctor a lot about the severity of your Spinal Cord Injury (or SCI).
This is because the SCI disrupts signals that are normally sent between your brain and skin.