Biphasic plasticity of dendritic fields in layer V motor neurons in response to motor learning

•Motor learning induces an enlargement of dendritic branches in M1 layer V motor neurons.•This increment in dendritic length reflects an expansion of the distal dendritic compartment.•The increase in dendritic length occurs faster in basal than in apical dendrites.•After four weeks of growth, a pruning occurs even though animals retain task performance.•Dendritic plasticity has a biphasic course with initial expansion and subsequent shrinkage. Motor learning is associated with plastic reorganization of neural networks in primary motor cortex (M1) that advances through stages. An initial increment in spine formation is followed by pruning and maturation one week after training ended. A similar biphasic course was described for the size of the forelimb representation in M1. This study investigates the evolution of the dendritic architecture in response to motor skill training using Golgy–Cox silver impregnation in rat M1. After learning of a unilateral forelimb-reaching task to plateau performance, an increase in dendritic length of layer V pyramidal neurons (i.e. motor neurons) was observed that peaked one month after training ended. This increment in dendritic length reflected an expansion of the distal dendritic compartment. After one month dendritic arborization shrinks even though animals retain task performance. This pattern of evolution was observed for apical and basal dendrites alike – although the increase in dendritic length occurs faster in basal than in apical dendrites. Dendritic plasticity in response to motor training follows a biphasic course with initial expansion and subsequent shrinkage. This evolution takes fourth as long as the biphasic reorganization of spines or motor representations.