Subthreshold corticospinal control of anticipatory actions in humans

Previous findings suggest that, by influencing the subthreshold state of motoneurons, the corticospinal pathways can set and reset the threshold position at which wrist muscle recruitment begins. Here we assumed that the corticospinal system can change the threshold position in a similar way before anticipated perturbation to pre-determine an appropriate emerging response to it. We first analyzed motor-evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) applied to the wrist area of motor cortex before unloading of preloaded wrist flexors, i.e. before the subsequent involuntary wrist motion to another position (natural unloading). Subjects then learned to diminish the post-unloading movement extent without activating antagonist (extensor) muscles before unloading or making intentional movement corrections after unloading (adjusted unloading). Although activity levels of wrist muscles before unloading were similar, MEPs of extensor but not pre-loaded flexor muscles were higher before adjusted unloading. We also applied TMS in combination with a torque pulse that shortened extensor muscles such that the MEP occurred when the motoneuronal excitability was minimized. Although diminished following muscle shortening, MEPs before adjusted unloading were still higher than before natural unloading. Results suggest that the corticospinal system, possibly together with other descending systems participated in the tonic subthreshold facilitation of antagonist motoneurons before adjusted unloading, which appears sufficient in modifying motor commands and motion leading to adjusted unloading. This study reinforces previous findings that descending systems, in particular, the corticospinal system can employ threshold position control during and after learning a novel action.