CORRELATION BETWEEN PATTERNS OF HORIZONTAL CONNECTIVITY AND THE EXTENT OF SHORT-TERM REPRESENTATIONAL PLASTICITY IN RAT MOTOR CORTEX

Plasticity of representational maps in adult cerebral cortex has been documented in both sensory and motor cortex, but the anatomical basis for cortical plasticity remains poorly understood. To investigate hori zontal connectivity in primary motor cortex (M1) as a putative anatomi cal substrate for short-term, functional plasticity of adult motor cor tical representations, a combination of electrical stimulation and bio cytin labeling was used to examine pre-existing patterns of intrinsic connections in adult rat M1 in relationship to the pattern of reorgani zation of the motor movement map induced by transection of the contral ateral facial nerve. Two hours after nerve cut, small, circumscribed r egions of the forelimb representation expanded medially into territory previously devoted to the vibrissae representation. Outside of this n ovel, expanded forelimb region, no forelimb movement could be evoked f rom the former vibrissae representation at any time over the period of hours tested, thus representing silent cortex. Injections placed into vibrissae cortex representing the newly expanded forelimb representat ion gave rise to labeled axons and dense terminal fiber labeling which crossed the forelimb/vibrissae border and extended up to 1.2 mm withi n the low-threshold forelimb representation. In contrast, injections p laced into silent vibrissae cortex gave rise to labeled axons and term inal boutons which remained mostly restricted to the original vibrissa e representation, with only sparse projections that crossed into the l ow-threshold forelimb representation. Thus, these results suggest that the extent of short-term, functional reorganization of M1 induced wit hin the first several hours following peripheral nerve cut is mediated , and constrained, by an anatomical framework of pre-existing, horizon tal projections which traverse representation borders.