Transcutaneous Spinal Direct Current Stimulation (tsDCS) Modulates Human Corticospinal System Excitability

This study aimed to assess the effects of thoracic anodal and cathodal transcutaneous spinal direct current stimulation (tsDCS) on upper- and lower-limb corticospinal excitability. Yet, despite studies assessing thoracic tsDCS influences the spinal ascending tract and reflexes, none assessed the effects of this technique over upper- and lower-limb corticomotorneuronal connections. In 14 healthy subjects we recorded motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) from abductor hallucis (AH) and hand abductor digiti minimi (ADM) muscles before (baseline, B), and at a different time-points (0 and 30 minutes) after anodal or cathodal tsDCS (2.5 mA, 20 minutes, T9-T11 level). In 8 of the 14 subjects we also tested the soleus H-reflex, the F-waves from AH and ADM before and after tsDCS. Both anodal and cathodal tsDCS left the upper-limb MEPs and F-wave unchanged. Conversely, while leaving lower-limb H-reflex unchanged, they oppositely affected lower-limb MEPs: whereas anodal tsDCS increased resting motor threshold (mean±SEM 107.33 ± 3.3%, increase immediately after tsDCS, and 108.37 ± 3.2% increase 30 min after tsDCS compared to baseline), and had no effects on MEP area and latency, cathodal tsDCS increased MEP area (139.71 ± 12.9% increase immediately after tsDCS and 132.74 ±22.0% increase 30 min after tsDCS compared to baseline) without affecting resting motor threshold and MEP latency. Our results show that tsDCS induces polarity specific changes in corticospinal excitability that last for more than 30 min after tsDCS offset and selectively affect responses in lower-limb muscles innervated by lumbar and sacral motorneurons.