Interhemispheric interactions between trunk muscle representations of the primary motor cortex

Unilateral arm movements require trunk stabilization through bilateral contraction of axial muscles. Interhemispheric interactions between primary motor cortices (M1) could enable such coordinated contractions but these mechanisms are largely unknown. Using transcranial magnetic stimulation (TMS), we characterized interhemispheric interactions between M1 representations of the trunk stabilizing muscle erector spinae at 1^st lumbar vertebra (ES L1) during a right isometric shoulder flexion. These interactions were compared to those of Anterior Deltoid (AD), the main agonist in this task, and the first dorsal interosseous (FDI). TMS over the right M1 elicited ipsilateral silent periods (iSP) in all three muscles on the right side. In ES L1 - but not AD or FDI-, ipsilateral motor evoked potential (iMEP) could precede the iSP or replaced it. iMEP amplitude was not significantly different whether ES L1 was used to stabilize the trunk or was voluntarily contracted. TMS at the cervicomedullary junction showed that the size of CMEP was unchanged during the iSP but increased during iMEP, suggesting that the iSP is due to intracortical mechanisms, but not the iMEP. Using a dual coil paradigm with two coils over the left and right M1, interhemispheric inhibition could be evoked at interstimulus interval of 6ms in ES L1 and 8ms in AD and FDI. Together these results suggest that interhemispheric inhibition is dominant when axial muscles are involved in a stabilizing task. The ipsilateral facilitation could be evoked by ipsilateral or subcortical pathways and could be used depending on the role axial muscles play in the task.

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