Modulation of interhemispheric inhibition during passive movement of the upper limb reflects changes in motor cortical excitability.

We investigated modulation of inhibition of motor evoked potentials in the left forearm musculature attributable to changes in corticomotor excitability induced by passive rhythmic movement of the right limb. In the first experiment, eight healthy volunteers pre-activated their left flexor carpi radialis (FCR) in a simple isometric contraction (2.5-7.5% MVC) while their right hand underwent passive wrist flexion-extension. Transcranial magnetic (TMS) or electrical (TES) stimulation was applied to the right motor cortex and responses recorded from the test (left) limb in eight phases of the wrist flexion-extension cycle of the passively driven right limb. In half of the trials TMS conditioning was applied to the left motor cortex. The conditioning stimulus significantly inhibited TMS-evoked responses in the test FCR muscle, whereas TES-evoked responses did not appear to be inhibited. For TMS-evoked responses only, inhibition in the static pre-activated left FCR was modulated such that inhibition was greater when the right wrist was passively flexing than when it was extending. In the second experiment TMS was applied to the right motor cortex, contralateral to the test (left) limb, with the right hand either passively extending or flexing through the neutral position. Conditioning was applied to the left motor cortex at a range of intensities adjusted to threshold for flexion and extension movements. No difference was evident in the maximum magnitude of inhibition between the extension and flexion conditions. We propose there is an increased absolute threshold for recruitment and a decreased gain of inhibitory callosal pathways during extension phases of the wrist flexion-extension cycle.