Central projections from the skin of the hand in squirrel monkeys

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Abstract
Central termination patterns of afferents from the hands of squirrel monkeys were studied after subdermal injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP) or cholera toxin subunit B conjugated to HRP (BHRP). WGA‐HRP more effectively labeled axons terminating in the superficial dorsal horn of the spinal cord, while BHRP more effectively labeled axons terminating in the deeper layers. Injections of both tracers, when restricted to parts of glabrous digits, palm, or dorsal hand, revealed somatotopic patterns in the spinal cord and pars rotunda of the cuneate nucleus that were, in some respects, similar and, in other respects, quite different from those previously reported for macaque monkeys (Florence et al., J. Comp. Neurol. 286:48–70, 1989). As in macaques, injections in digits 1–5 produced a rostrocaudal sequence of foci of terminations in the cervical spinal cord. However, inputs from the palm were located medial to those from the digits, whereas the palm is represented lateral to the digits in macaque monkeys. Since inputs from the palm are also medial in the dorsal horn in cats (Nyberg and Blomqvist, J. Comp. Neurol. 242:28–39, 1985), the condition in squirrel monkeys may be similar to the generalized state. In the cuneate nucleus, single injections in the hand produced dense label in the pars rotunda, and sparse label in the rostral and caudal poles. As in macaque monkeys, inputs from specific parts of the hand related to rostrocaudal clusters of cells that are cytochrome oxidase dense. The representation of the digits differed from macaques in that the digits were represented dorsal to the palm, rather that ventral to the palm as in macaques. Again, comparisons with cats suggest that squirrel monkeys have the more generalized pattern. Finally, inputs from the hairy, dorsal surfaces of the digits terminated on the same clusters as the inputs from the glabrous, ventral surfaces, apparently overlapping somewhat. The proximity of these terminations from dorsal and ventral surfaces of the digits may be related to observations that cortical representations of the glabrous surfaces of digits become responsive to dorsal surfaces of the same digits when inputs from glabrous skin are chronically deactivated (e.g., Merzenich et al., Neuroscience 3:33–55, 1983).