Each dorsal root ganglion and associated spinal nerve arises from an iterated series of embryonic tissue masses called somites. This explains the overall segmental arrangement of somatic nerves in adults. The skin region innervated by the sensory axons of a single dorsal root and its related spinal nerve is called a dermatome. In humans, the area of each dermatome has been defined via studies of patients in whom specific dorsal roots are affected (e.g., herpes zoster, or "shingles") or surgically interrupted (for pain relief). These studies show that dermatomal maps vary among individuals. Dermatomes also overlap substantially, so that injury to an individual dorsal root does not lead to complete loss of sensation in the relevant skin region. Dermatomal overlap is more extensive for touch, pressure, and vibration than for pain and temperature.

Neuroscience, Sinaur Associates (QP355.2.N487 1997)