Static magnetic field stimulation applied over the cervical spinal cord can decrease corticospinal excitability in finger muscle

Abstract

Objective: Transcranial static magnetic field stimulation has recently been demonstrated to modulate cortical excitability. In the present study, we investigated the effect of transspinal static magnetic field stimulation (tsSMS) on excitability of the corticospinal tract.

Methods: A compact magnet for tsSMS (0.45 Tesla) or a stainless steel cylinder for sham stimulation was positioned over the neck (C8 level) of 24 able-bodied subjects for 15 min. Using 120% of the resting motor threshold transcranial magnetic stimulation intensity, motor evoked potentials (MEPs) were measured from the first digital interosseous muscle before, during, and after the tsSMS or sham intervention.

Results: Compared with baseline MEP amplitudes were decreased during tsSMS, but not during sham stimulation. Additionally, during the intervention, MEP amplitudes were lower with tsSMS than sham stimulation, although these effects did not last after the intervention ceased.

Conclusions: The results suggest that static magnetic field stimulation of the spinal cord by a compact magnet can reduce the excitability of the corticospinal tract.

Significance: Transspinal static magnetic field stimulation may be a new non-invasive neuromodulatory tool for spinal cord stimulation. Its suppressive effect may be applied to patients who have pathological hyperexcitability of the spinal neural network.

Keywords: EMG, electromyography; FDI, first dorsal interosseous; M1, primary motor cortex; MEP, motor evoked potential; Motor evoked potential; Neuromodulation; Spinal cord; Static magnetic field; TMS, transcranial magnetic stimulation; tSMS, transcranial static magnetic stimulation; tsSMS, transspinal static magnetic stimulation.

Fig. 1

Experimental setup. A cylindrical neodymium magnet for tsSMS or a stainless cylinder of the same size and appearance for sham stimulation, was positioned on the subject’s neck over C8 during the intervention step. They were fixed in place by a movable arm on a stand. MEPs from the FDI muscle were recorded by TMS applied over the left M1 cortex.

Fig. 2

Time-course of the background EMG. No significant differences were observed. Error bars represent standard deviation.

Fig. 3

Changes in normalized MEP amplitude before (pre), 5 min (during-5), and 10 min (during-10) after the start of the intervention, and immediately after (post-0) and 5 min after the end of the intervention (post-5). The asterisks (**) represent a significant difference between the pre and during-5 timepoints in the tsSMS condition (p < 0.01). The sharp symbol (#) represents significant differences between tsSMS and the sham condition at during-5 and during-10 timepoints (p < 0.05). Error bars are standard error.