Motor cortex bilateral motor representation depends on subcortical and interhemispheric interactions.

The corticospinal tract is a predominantly crossed pathway. Nevertheless, the primary motor cortex (M1) is activated bilaterally during unilateral movements and several animal studies showed that M1 has a bilateral motor representation. A better understanding of the uncrossed corticospinal system is especially important for elucidating its role in recovery of limb control after unilateral injury. We used intracortical microstimulation (ICMS) to determine the representation of contralateral and ipsilateral forelimb joints at single M1 sites in the rat. Most sites representing an ipsilateral joint corepresented the same joint contralaterally. We next determined whether ipsilateral responses evoked in one hemisphere depended on the function of M1 in the opposite hemisphere using reversible inactivation and pyramidal tract lesion. Ipsilateral responses were eliminated when the homotopic forelimb area of M1 in the opposite hemisphere was inactivated or when the pyramidal tract on the nonstimulated side was sectioned. To determine the role of transfer between M1 in each hemisphere we sectioned the corpus callosum, which produced a 33% increase in ipsilateral ICMS thresholds. Neither M1 inactivation nor callosal section changed contralateral response thresholds, indicating the absence of tonic excitatory or inhibitory drive to the opposite M1. Finally, ipsilateral responses following M1 inactivation and pyramidal tract lesion could be evoked after systemic administration of the K(+) channel blocker 4-aminopyridine, suggesting the presence of latent connections. Our findings show important interactions between the corticospinal systems from each side, especially at the spinal level. This has important implications for recruiting the ipsilateral corticospinal system after injury.