Functional organization of tactile inputs from the hand in the cuneate nucleus and its relationship to organization in the somatosensory cortex

Abstract

Central processing of tactile inputs from the hand begins in the main cuneate nucleus and continues in the thalamus and area 3b cortex. Little is known about cuneate functional organization in primates or about how cuneate and area 3b organization are related. In this study, neurophysiologic approaches were used to evaluate how tactile inputs from the hand and adjacent body are organized in the cuneate nucleus of squirrel monkeys. Cuneate data on the organization of hand inputs were then compared with analogous area 3b data from our earlier cortical studies that used the same approaches. Evaluations of several cuneate properties, including (1) responsiveness to tactile stimulation, (2) incidences and sizes of receptive fields, (3) somatotopic progressions, (4) properties of representations, and (5) relationships between functional inputs and cytochrome oxidase staining, suggest that tactile afferents from the hand form consistently organized cuneate representations that, in turn, relate to the parcellated organization of cuneate structural substrates. Comparisons of cuneate and area 3b organization indicate that tactile processing from the brainstem to cortex involves a preservation of tactile responsiveness and somatotopic organization but, in addition, involves transformations that produce receptive field sharpening, suppression of hairy hand inputs, amplification and refinement of glabrous inputs, and relocations of representations. Ascending lemniscal substrates are characterized by cascading excitatory convergence/divergence that increments at successively higher levels between sensory afferents and area 3b. It is suggested that the observed preservations and transformations reflect this organization but, in addition, reflect mechanisms that cause counterbalancing sharpening and suppressions of hand inputs.