Sensitivity of neurons in somatosensory cortex (S1) to cutaneous stimulation of the hindlimb immediately following a sciatic nerve crush

While a large number of studies have examined receptive field alterations in the cerebral cortex after peripheral nerve injury, descriptions of neuronal sensitivity have been largely qualitative. In the present study, quantitative changes in minimal force thresholds evoking cortical responses were examined in somatosensory cortex (S1) of the rat after peripheral nerve injury. Cutaneous receptive fields were defined by conventional multi-unit recording techniques. Thresholds for evoking cortical responses were defined using Semmes-Weinstein monofilaments to stimulate the skin over the distal and proximal hindlimb, and over the adjacent tail, lower back and abdomen. Then minimal force thresholds and representational areas were compared. Receptive fields were recorded before and within 3 h after a sciatic nerve crush injury. Group comparisons were also made with intact (control) rats in which more detailed maps were derived. The results show that the minimal force thresholds for activation of newly emerged responses were not statistically different from thresholds for evoking pre-crush responses. These quantitative data provide strong support for the notion that functional boundaries in S1 between saphenous and sciatic representations and between distal and proximal hindlimb representations are actively maintained by selective inhibition of subsets of overlapping inputs. Formerly inhibited responses can be activated by low-threshold cutaneous stimulation immediately following peripheral nerve injury.