"Selective Activation of Ipsilateral Motor Pathways in Intact Humans"
The Journal of Neuroscience, in press:2014
Mouse over here for a brief summary or click to open article in a new tab.It has been proposed that ipsilateral motor pathways play a role in the control of ipsilateral movements and recovery of function after injury. However, the extent to which ipsilateral motor pathways are involved in voluntary activity in intact humans remains largely unknown. Using transcranial magnetic stimulation over the arm representation of the motor cortex we examined ipsilateral motor evoked potentials (iMEPs) in a proximal arm muscle during increasing levels of unilateral and bilateral isometric force in a sitting position. We demonstrate that iMEPs area and amplitude decreased during bilateral activation of homonymous elbow flexor muscles and increased during bilateral activation of heteronymous elbow flexor and extensor muscles compared with a unilateral contraction, regardless of the force tested. To further understand the neuronal inputs involved in the bilateral effects we examined the contribution from neck afferents projecting onto ipsilateral motor pathways. Medial (away from the muscle tested) and lateral (toward the muscle tested) rotation of the head enhanced bilateral iMEPs effects from homonymous and heteronymous muscles, respectively. In contrast, head flexion and extension exerted nonspecific bilateral effects on iMEPs. Intracortical inhibition, in the motor cortex where iMEPs originated, was modulated in the same direction as iMEPs. We conclude that ipsilateral projections to proximal arm muscles can be selectively modulated by voluntary contraction of contralateral arm muscles, likely involving circuits mediating asymmetric tonic neck reflexes acting, at least in part, at the cortical level. The pattern of bilateral actions may represent a strategy to engage ipsilateral motor pathways in a motor