Long lasting structural changes in primary motor cortex after motor skill learning: a behavioural and stereological study

Many motor skills, once acquired, are stored over a long time period, probably sustained by permanent neuronal changes. Thus, in this paper we have investigated with quantitative stereology the generation and persistence of neuronal density changes in primary motor cortex (MI) following motor skill learning (skilled reaching task). Rats were trained a lateralised reaching task during an "early" (22-31 days old) or "late" (362-371 days old) postnatal period. The trained and corresponding control rats were sacrificed at day 372, immediately after the behavioural testing. The "early" trained group preserved the learned skilled reaching task when tested at day 372, without requiring any additional training. The "late" trained group showed a similar capacity to that of the "early" trained group for learning the skilled reaching task. All trained animals ("early" and "late" trained groups) showed a significant Inter hemispheric decrease of neuronal density in the corresponding motor forelimb representation area of MI (cortical layers II-III). It is concluded that learning a skilled reaching task implies long lasting structural changes in restricted cortical regions of the motor cortex. The generation and persistence of these changes probably reflect a plastic reorganization for storing and retrieving motor skills. The plastic changes were also observed in the older rats, suggesting that motor cortex maintains its plastic capacity throughout the lifespan.