An Index of Topographic Normality in Rat Somatosensory Cortex: Application to a Sciatic Nerve Crush Model

Abstract

Previous studies have demonstrated that peripheral denervation of the skin is reflected in the CNS as a reorganization of somatotopic representations. In cases in which peripheral nerve regeneration occurs there is a gradual reactivation of cortex by novel receptive fields that is reversed as regenerated nerves reestablish connections with the original skin surface. Functional recovery appears to depend on the pattern in which somatotopic organization in the cortex is reestablished. The relationship between functional recovery and cortical topography is not precise, however, since the descriptions of postinjury representations in the cortex have been largely descriptive and not quantitative. The purpose of this study was to derive an index to quantify deviations from normal somatotopic organization in the somatosensory cortex. Multiunit recordings of cutaneous representations in the somatosensory cortex (S1) of the rat were defined using Semmes-Weinstein monofilaments to stimulate the skin over the distal hindlimb of the rat 2 and 4 months after a sciatic nerve crush. To derive a sensitive index of topography, the sciatic nerve crush was selected as the injury model since nerve regeneration following crush injuries has been reported to reinstate preinjury cortical topography. Group comparisons were made with an intact control group. The results show that there were subtle, but significant differences in topography between rats with a regenerated sciatic nerve and normal rats. In addition, average thresholds for evoking cortical responses were higher than normal (but within normal range) 2 and 4 months after the crush. These results demonstrate that the index of topography derived for this study can reveal deviations that may not be distinguishable from normal topography when based on qualitative descriptions.