Spinal Excitability Changes after Transspinal and Transcortical Paired Associative Stimulation in Humans

Figure 1

Paired associative stimulation (PAS) protocol. (a) Simplified diagram of transcranial magnetic stimulation (TMS volley) and transspinal stimulation-mediated volleys during paired stimulation. TMS motor volleys are descending, whilst transspinal stimulation produces both ascending and descending volleys. The ascending volleys are expected to reach both brain hemispheres since transspinal stimulation delivered alone evokes transspinal-evoked potentials (TEPs) in muscles of both legs. (b) Timing of PAS between transcortical and transspinal stimulation. PAS was delivered at customized interstimulus intervals for each subject during which corticospinal neurons activated via TMS arrived at the corticospinal neuron before spinal motoneurons were activated transsynaptically by the transspinal stimulation (transcortical-transspinal PAS), and during which transspinal-mediated ascending volleys arrived at the motor cortex before TMS was delivered over the left primary motor cortex leg area (transspinal-transcortical PAS).

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