Central mechanisms underlying postural and volitional control of trunk muscles

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The aim of this research program is to better understand the role of different cortical motor areas in the control of trunk muscles. In the planned experiments, the function of cortical motor areas will be tested using cervicomedullary and transcranial magnetic stimulation (CMS/TMS). Over the next 5 years, three short-term objectives will be pursued: Objective 1: Determine the contribution of the corticospinal tract to the motor evoked potential (MEP) of the trunk muscles. Multiple pathways and levels of the central nervous system (e.g. spinal, cortical) contribute to the control of trunk muscles. CMS in combination with TMS might be used to assess the contribution of the corticospinal tract to the MEP using collision paradigm. Our hypothesis is that CMS will occlude the TMS evoked responses when timed adequately suggesting a contribution of the corticospinal tract to the MEP of trunk muscles. However, smaller occlusion of what was shown for distal muscles is possible considering that other descending pathways might contribute. Objective 2: Determine the corticospinal contribution of the supplementary motor area (SMA) and primary motor cortex (M1) in preparation of a postural and volitional task. M1 and SMA both contribute to motor control but their role might be different. SMA has been suggested to be involved in the control of postural adjustment and to project to the spinal cord through the corticospinal tract. Since trunk muscles massively contribute to postural control, SMA might influence trunk control through the corticospinal tract. Our hypothesis is that TMS over SMA will elicit larger MEP during postural task whereas TMS over M1 will produce larger MEP during volitional task. Cortical networks (rather than spinal) will drive the MEP modulation. Objective 3: Determine the contribution of SMA and M1 in the performance of a postural and volitional task. Several brain regions are involved in motor control. Repetitive TMS (rTMS) may be used to `virtually' lesion a specific area to disrupt transiently its excitability / normal functioning and assess its contribution to a specific function. This technique has been used to provide the first evidence of SMA contribution to postural adjustment. Indeed, a `virtual' lesion of the SMA using low-frequency rTMS reduced the duration of the postural adjustment during gait initiation. However, it is not known whether altering the function of M1 or SMA will influence differently the performance of a volitional or postural task. Our hypothesis is that SMA `lesion' will impact on the performance of the postural task whereas M1 `lesion' will impact on the performance of the volitional task. The effects will be mainly mediated through a change in cortical networks. Impact: Thanks to an integrated and systematic methodology, this research program will make important discoveries on the neural control of human trunk muscles in postural and volitional conditions.