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)