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Physiological changes in the somatosensory forepaw cerebral cortex of adult raccoons following lesions of a single cortical digit representation

Experimental Neurology
Publication Date
DOI: 10.1016/0014-4886(90)90024-m
  • Biology


Abstract The purpose of this study was to determine whether restricted lesions within primary somatosensory (SmI) cortex cause changes in the functional organization of cortical areas bordering on the site of injury. Focal ablations of cortical tissue were made in the representational area for digit 3 within the SmI forepaw cortex of adult raccoons. Electrophysiological mapping experiments done 15–17 weeks later showed that significant alterations had occurred in the response properties of clusters of neurons within those representational zones adjoining the lesion—the zones for digit 2, digit 4, and the palmar pads. These three cortical areas were modified by the appearance of new, usually weaker secondary inputs and changes in some properties of the normal primary inputs from the forepaw. (i) Many neurons responded to stimulation of previously ineffective skin regions; the new inputs often originated from digit 3 but frequently involved other digits or the pads as well. (ii) Neuronal receptive fields (RFs), mapped at a standard suprathreshold stimulus intensity, were larger than normal. (iii) Skin type and submodality sensitivity typically were less specific than normal; more neurons had RFs that included both glabrous and hairy skin or claws and displayed mixtures of responsiveness to skin touch, hair deflection, or claw touch. (iv) The representation of RF location, skin type, and submodality sensitivity was more variable as a function of horizontal and vertical distance through the cortex. In general, the physiological changes were found to degrade the somatotopic order and response specificity of the intact cortical areas adjoining the lesion. The relatively mixed nature of the novel inputs following cortical damage was qualitatively similar to that produced by digital nerve transection described previously by Kelahan and Doetsch ( Somatosens. Res., 1984, 2: 49–81). The data suggest that both cortical and peripheral somatosensory injuries may produce their effects by unmasking synaptically weak neural connections. Synaptic unmasking may involve facilitation or disinhibition of neurons with convergent properties—especially those located within the heterogeneous subdivisions of SmI cortex.

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