Abstract:
Reaching to pick up an object is a seemingly simple everyday task, yet one that requires the central nervous system to precisely coordinate activity of multiple muscles across the upper limb. A series of experiments were conducted to investigate the role of the contralateral and ipsilateral primary motor cortex (M1) in proximal upper limb control for healthy adults and stroke patients. A literature review was conducted to inform the experimental hypotheses. Transcranial magnetic stimulation was used in each experiment to assess the excitability of contralateral and ipsilateral descending projections from M1 during upper limb motor tasks. The first experiment investigated the role of the presumed cervical propriospinal system in coordinating reaching and gripping in healthy adults. Propriospinal neurons (PNs) were disinhibited during a grip-lift task, facilitating the functional cocontraction of proximal and distal muscles. In the next three experiments, non-invasive brain stimulation (NIBS) was used to suppress M1 excitability. There was a reduction in excitability of ipsilateral projections to PNs innervating a proximal upper limb muscle, and a degradation of selective proximal muscle activation in the ipsilateral arm following suppressive NIBS of M1. Together, these experiments provide evidence that ipsilateral M1 has an important contribution to proximal upper limb motor control in healthy adults, possibly via descending projections to ipsilateral PNs in the spinal cord. The final study investigated the effect of suppressive NIBS of contralesional M1 on proximal paretic upper limb control in stroke patients. There was evidence that paretic proximal upper limb motor control was improved in patients who have little compensatory up-regulation of contralesional M1, but may be contraindicated for patients in whom contralesional M1 is playing a compensatory role. A hypothetical model to explain ipsilateral neural control of the proximal upper limb and the effects of suppressive NIBS on proximal paretic upper limb control in patients with different levels of motor impairment is presented.