Dermatomes

The skin divided into dermatomes, one band per spinal nerve root on each side, from C2 at the back of the head to S5 round the anus, with the face supplied by the trigeminal nerve. Tap a band for its root, the nerves that carry it and where to test it (the ISNCSCI key sensory points); the skeleton, spinal cord and nerves lie faint beneath. Dermatome maps differ between sources, and neighbouring dermatomes overlap, so losing a single root often leaves only a small numb patch, or none. People differ too. The ISNCSCI key points, where one root is most reliably alone, are the places to test.