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Journal of Neuroscience, Vol 4, 1499-1515, Copyright © 1984 by Society for Neuroscience
Cutaneous responsiveness in primary somatosensory (S-I) hindpaw cortex before and after partial hindpaw deafferentation in adult rats
JT Wall and CG Cusick
The hindpaw of the rat is normally innervated by the sciatic and saphenous
nerves. In the present studies, the hindpaws of adult rats were partially
deafferented by transection of the sciatic nerve for variable periods of
time. The organization of the hindpaw representation in primary
somatosensory (S-I) cortex was then studied with neurophysiological mapping
techniques and compared to the organization seen in normal rats. The
objective was to determine whether cutaneous responsiveness was recovered
in the cortical area which lost normal cutaneous inputs from the sciatic
nerve, and, if recovery occurred, to characterize the time course and
spatial extent of this recovery. Normal rats were found to have a
topographically organized representation of the hindpaw in S-I cortex. As
determined by nerve recording and cortical mapping, approximately 85% of
this representation is responsive to cutaneous inputs from the sciatic
nerve, while the remaining 15% is responsive to inputs from the saphenous
nerve. Following transection of the sciatic nerve, all hindpaw skin regions
normally innervated by the sciatic nerve remained denervated. In cortex,
the representation of cutaneous inputs from the saphenous nerve expanded
into parts of the hindpaw region normally representing sciatic inputs and
occupied an area about 3 times larger than the saphenous representation in
normal rats. This expansion was initially observed 1 to 2 days after
transection and was stably maintained with longer deafferentation times.
However, even after chronic deafferentation of up to 5 months, this
enlarged saphenous representation was still only half the size of the
normal hindpaw representation in normal rats. These findings suggest that
cortical representations of deafferented skin can become activated by
substitute cutaneous inputs. The rapid time course for substitution
suggests these changes are due to functional modifications in normally
existing connections. With the deafferentation conditions used in the
present study, input substitution was limited to only parts of the deprived
cortex. A hypothesis is presented which suggests these changes are due to
adjustments in the dominance of saphenous and sciatic inputs to specific
regions of cortex.
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