Combination of Static Magnetic Fields and Peripheral Nerve Stimulation Can Alter Focal Cortical Excitability

Abstract

For clinical application of transcranial static magnetic stimulation (tSMS), it is important to achieve a focal target cortical stimulation. Previous study suggested that the associative stimulation combining non-invasive stimulation of the motor cortex (M1) and the peripheral nerve stimulation (PNS) may be useful to produce cortical excitability change. To test this hypothesis, we measured the M1 excitability and intracortical circuits by using transcranial magnetic stimulation (TMS) before and after the tSMS of short duration (5 min) combined with PNS. Thirty-three normal volunteers were participated; tSMS+PNS (n = 11), sham+PNS (n = 11), and tSMS alone (n = 11). We found the transient suppression of the motor-evoked potential (MEP) of the right abductor pollicis brevis (APB) muscle, but not of the abductor digiti minimi (ADM) muscle, when combining tSMS with PNS over median nerve at the wrist. The lack of suppressive effect on APB in tSMS alone with short duration is in accord with the previous observation. In addition, the tendency of transient enhancement of the short-latency intracortical inhibition was observed immediately after intervention in the tSMS±PNS group. These findings show that the combination of tSMS and PNS can induce the cortical excitability change in target cortical motor area and potentiate the suppression effect.

Transcranial Static Magnetic Field Stimulation over the Primary Motor Cortex Induces Plastic Changes in Cortical Nociceptive Processing

“…The reasons why facilitatory HF rTMS over M1 increases SICI in patients with chronic pain are unknown, and further studies are needed. In line with these studies, we speculate that inhibitory modulation of M1 by tSMS, especially the enhancement of SICI that we demonstrated in our previous studies (Nojima et al, 2015 , 2016 ), is related to some aspects of nociceptive processing used in the generation of EPs in this study.…”

“…This could be indirectly and partly compatible with the result of our study in which tSMS over M1 exerted analgesic effects via brain responses to IES. In our previous studies, we demonstrated that 10 min of tSMS over M1 enhances SICI (Nojima et al, 2015 , 2016 ), although we did not test this in the current study. In chronic neuropathic pain studies, HF rTMS and anodal tDCS of M1 restore SICI and are correlated with the amount of induced pain relief (Lefaucheur et al, 2006 , 2012b ; Antal et al, 2010 ; Mhalla et al, 2011 ).…”

“…In line with previous cellular and animal studies, SMFs applied to the human scalp are believed to decrease cortical excitability. In recent works, tSMS over M1 was shown to not only suppress the corticospinal pathway (Oliviero et al, 2011 ; Silbert et al, 2013 ), but also to enhance short-latency intracortical inhibition (SICI; Nojima et al, 2015 , 2016 ). Further, we showed that tSMS over S1 decreases the amplitude of the N20 component of somatosensory evoked potentials (SEPs) following median nerve stimulation (Kirimoto et al, 2014 ) and alters normal somatosensory processing (Carrasco-López et al, 2017 ).…”

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

“…To obtain static magnetic fields, we utilized a cylindrical neodymium magnet (NdFeb; diameter 50 mm, length 30 mm) with a maximum magnetic flux density of 0.459 Tesla (NeoMag, Ichikawa, Japan), which is the same type used in some previous studies ( Kirimoto et al, 2014 , Nojima et al, 2015 , Kirimoto et al, 2016a , Nojima et al, 2016 ). For sham stimulation, we used a non-magnetic stainless steel cylinder of the same size and weight.…”

“…Local applications of static magnetic fields to the human cortex modulates cortical excitability ( Oliviero et al, 2011 , Kirimoto et al, 2014 , Gonzalez-Rosa et al, 2015 , Nojima et al, 2015 , Kirimoto et al, 2016a , Kirimoto et al, 2016b , Nojima et al, 2016 ). Oliviero et al (2011) were the first to report that transcranial static magnetic field stimulation (tSMS) using small, strong magnets over the human motor cortex decreased excitability of the motor cortex.…”