Repeated cathodal transspinal pulse and direct current stimulation modulate cortical and corticospinal excitability differently in healthy humans.

Noninvasive transspinal stimulation of the thoracolumbar region, where leg motor circuits reside, produces prominent plasticity of brain and spinal cord circuits. However, reorganization of cortical and corticospinal excitability after multiple sessions (i.e. repeated) remains elusive. In this study, we investigated changes in intracortical inhibition, intracortical facilitation, and corticospinal excitability after 10 sessions of cathodal transcutaneous delivery of pulse or direct current stimulation, termed here transspinal (tsPCS, tsDCS), in resting healthy humans. tsPCS was delivered at sub- and supra-threshold intensities, while intensity for tsDCS ranged from 2.24 to 2.34 mA within a session. Intracortical inhibition and facilitation were assessed based on the tibialis anterior (TA) motor evoked potential (MEP) amplitude following subthreshold transcranial magnetic stimulation (TMS) at the conditioning-test (C-T) intervals of 1, 2, 3, 10, 15, 20, 25, and 30 ms. The TA MEP recruitment input-output curves were also assembled to establish changes in corticospinal excitability. For both transspinal stimulation protocols, the active cathodal electrode was placed over the T10 spinal process. Results indicated that repeated tsPCS did not alter intracortical inhibition or intracortical facilitation but decreased corticospinal excitability for the right M1 and increased corticospinal excitability for the left M1. tsDCS decreased intracortical inhibition, increased intracortical facilitation, did not affect the maximal MEP amplitude but increased the slope of the right TA MEP input-output curve. Neurophysiological changes may be attributed to neural mechanisms involved in learning and memory. These results support that noninvasive transspinal stimulation alters both cortical and corticospinal neural excitability in resting healthy humans.