The representation of peripheral nerve inputs in the S-I hindpaw cortex of rats raised with incompletely innervated hindpaws

Abstract

The hindpaws of 1-d-old rats were partially denervated by transection and ligation of the sciatic nerve. Following growth to adulthood, the topographical organization of the hindpaw representation in primary somatosensory (S-I) cortex was studied with neurophysiological mapping techniques and compared to the organization of previously studied normal adult rats and adult rats that had sciatic transection. The goals were (1) to determine how the topographical organization of the hindpaw representation is affected when development occurs with an incomplete set of peripheral inputs, and (2) to identify possible differences in the capacity of neonatal and adult CNSs to adjust to loss of inputs. The rat hindpaw is relatively immature on the day of birth. Neonatal transection and ligation of the sciatic nerve stunted gross development of the hindpaw and resulted in a permanent loss of low-threshold mechanoreceptor inputs from hindpaw zones normally innervated by the sciatic nerve. A comparison of the cortical representations of neonatally denervated and normal rats indicated that early loss of sciatic inputs caused several changes in the topographical organization of the hindpaw cortex, including (1) a loss of the representation of hindpaw skin areas innervated by the sciatic nerve, (2) a limited infringement of cutaneous inputs from the hindquarter into the cortical zone deprived of sciatic hindpaw inputs, (3) increased variability in the topographical relationships of the hindpaw and hindquarter representations, and (4) a decrease in the size of the cortical area responsive to cutaneous inputs. A comparison of the cortical representations of neonatal and adult denervates indicated that the general cortical reaction to sciatic injury at both ages was similar: Neurons in some parts of the deprived hindpaw cortex were activated by cutaneous inputs from uninjured nerves, whereas neurons in other parts of this cortex were unresponsive to cutaneous stimulation. The topographical organization and size of projection zones of uninjured peripheral inputs were different, however, after denervation in neonatal and adult rats. From these findings we suggest that (1) development of a normal, topographically organized hindpaw representation requires integration of hindpaw inputs in a spatially specific manner, (2) more than one pattern of cortical adjustment occurs after sciatic injury, and age is an important determinant of the pattern that is established.(ABSTRACT TRUNCATED AT 400 WORDS)