Correlation between patterns of horizontal connectivity and the extend of short-term representational plasticity in rat motor cortex

Plasticity of representational maps in adult cerebral cortex has beendocumented in both sensory and motor cortex, but the anatomical basis forcortical plasticity remains poorly understood. To investigate horizontalconnectivity in primary motor cortex (M1) as a putative anatomicalsubstrate for short-term, functional plasticity of adult motor corticalrepresentations, a combination of electrical stimulation and biocytinlabeling was used to examine pre-existing patterns of intrinsic connectionsin adult rat M1 in relationship to the pattern of reorganization of themotor movement may induced by transection of the contralateral facialnerve. Two hours after nerve cut, small, circumscribed regions of theforelimb representation expanded medially into territory previously devotedto the vibrissae representation. Outside of this novel, expanded forelimbregion, no forelimb movement could be evoked from the former vibrissaerepresentation at any time over the period of hours tested, thusrepresenting silent cortex. Injections placed into vibrissae cortexrepresenting the newly expanded forelimb representation gave rise tolabeled axons and dense terminal fiber labeling which crossed theforelimb/vibrissae border and extended up to 1.2 mm within thelow-threshold forelimb representation. In contrast, injections placed intosilent vibrissae cortex gave rise to labeled axons and terminal boutonswhich remained mostly restricted to the original vibrissae representation,with only sparse projections that crossed into the low-threshold forelimbrepresentation. Thus, these results suggest that the extent of short-term,functional reorganization of M1 induced within the first several hoursfollowing peripheral nerve cut is mediated, and constrained, by ananatomical framework of pre-existing, horizontal projections which traverserepresentation borders.