Spinal nerve distributions in the upper limb: the organization of the dermatome and afferent myotome.

Single fibres were dissected from the dorsal spinal roots of the nerves serving the brachial plexus in African green monkeys. The dermatomal organization of these spinal nerves was deduced from data concerning the receptive fields of 2834 single afferent fibres. These data were collected in an attempt to reconcile some of the discrepancies that exist in published descriptions of the dermatomes in primates; our results and the literature reviewed suggest that the cutaneous region served by one spinal nerve is actually much wider and much more variable in location than is generally recognized. This makes any summary diagram a misleading indicator of the true complexity of the spinal innervation of the upper limb. In spite of this variability among individuals, within any specific individual there is a regular and orderly progression of innervation which allows prediction of the region served by a particular spinal nerve when information concerning the site of innervation of adjacent nerves is available. The territory of each myotome tended to be larger than the dermatome of the same spinal nerve. Most muscles of the limb received afferent innervation from three to four different spinal nerves. Further, the territory of the myotome did not of necessity coincide with the dermatome of the same spinal nerve. Even those nerves innervating the hand still innervated axial muscles. These observations have important implications for the diagnosis of spinal nerve injuries.