Targeted stimulation of the corticospinal tract has been shown to improve voluntary motor output in humans with spinal cord injury (SCI; Bunday and Perez, 2012). Here, we used a novel protocol that targeted late synaptic inputs into corticospinal neurons in humans with and without incomplete cervical chronic SCI. We used 180 paired transcranial magnetic stimulation (TMS) pulses over the hand representation of the primary motor cortex at interstimulus intervals of 4.3 ms (targeting I3-wave circuits; iTMS protocol) and 3.5 ms (targeting no I-wave interval; control protocol) at 0.1 Hz for a total of 30 min. Motor evoked potentials (MEPs) in an intrinsic finger muscle where measured at rest before, immediately after, and up to 30 min after the stimulation with the coil oriented to induce currents in the brain in the posterior-anterior (PA) and anterior-posterior (AP) direction to preferentially activate early and late synaptic inputs to corticospinal neurons, respectively. We found that MEPs size increased in the AP but not in the PA direction after the iTMS protocol for up to 30 min after the stimulation in control (by ~175%) and SCI (by ~142%) participants. No changes in MEP size were observed after the control protocol when tested with the coil either in the PA or AP direction in both groups. Notably, EMG and force outcomes during index finger abduction increased after the iTMS protocol in control (EMG by ~135%; force by ~129%) and SCI (EMG by ~130%; force by ~127%) participants. No changes were observed after the control protocol. SCI subjects needed ~15% less time to complete the nine-hold-peg-test after the iTMS protocol compared to baseline. Thus, we propose that targeting late synaptic inputs into corticospinal neurons might represent a novel strategy for enhancing corticospinal drive and voluntary motor output after human SCI.
J. LONG, P. FEDERICO, *S. LEHMANN, M. A. PEREZ;
Dept. of Neurolog. Surgery, The Miami Project to Cure Paralysis, Univ. of Miami, Miami, FL
J. Long: None. P. Federico: None. S. Lehmann: None. M.A. Perez: None.