Real-Time Changes in Corticospinal Excitability during Voluntary Contraction with Concurrent Electrical Stimulation

Tomofumi Yamaguchi,Naoshin Yoshida,Satoshi Tanaka,Meigen Liu,Shigeo Tanabe,Yoshihiro Muraoka,Kenichi Sugawara,Kei Saito,Natasha M Maurits

doi:10.1371/journal.pone.0046122

Abstract

While previous studies have assessed changes in corticospinal excitability following voluntary contraction coupled with electrical stimulation (ES), we sought to examine, for the first time in the field, real-time changes in corticospinal excitability. We monitored motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation and recorded the MEPs using a mechanomyogram, which is less susceptible to electrical artifacts. We assessed the MEPs at each level of muscle contraction of wrist flexion (0%, 5%, or 20% of maximum voluntary contraction) during voluntary wrist flexion (flexor carpi radialis (FCR) voluntary contraction), either with or without simultaneous low-frequency (10 Hz) ES of the median nerve that innervates the FCR. The stimulus intensity corresponded to 1.2× perception threshold. In the FCR, voluntary contraction with median nerve stimulation significantly increased corticospinal excitability compared with FCR voluntary contraction without median nerve stimulation (p<0.01). In addition, corticospinal excitability was significantly modulated by the level of FCR voluntary contraction. In contrast, in the extensor carpi radialis (ECR), FCR voluntary contraction with median nerve stimulation significantly decreased corticospinal excitability compared with FCR voluntary contraction without median nerve stimulation (p<0.05). Thus, median nerve stimulation during FCR voluntary contraction induces reciprocal changes in cortical excitability in agonist and antagonist muscles. Finally we also showed that even mental imagery of FCR voluntary contraction with median nerve stimulation induced the same reciprocal changes in cortical excitability in agonist and antagonist muscles. Our results support the use of voluntary contraction coupled with ES in neurorehabilitation therapy for patients.

Highlights

Cortical plasticity plays a fundamental role in motor learning and neurorehabilitation
The main effect of electrical stimulation (ES) indicates that the MMG-motor evoked potentials (MEPs) was higher when flexor carpi radialis (FCR) voluntary contraction was coupled with median nerve stimulation than when it was not coupled with median nerve stimulation
These results indicate that median nerve stimulation during FCR voluntary contraction enhanced the MMG-MEP measured on the FCR, and this facilitation increased with greater voluntary contraction

Summary

Introduction

Cortical plasticity plays a fundamental role in motor learning and neurorehabilitation. Voluntary contraction coupled with electrical stimulation (ES) has been reported to induce plasticity in the sensorimotor cortex in both health and disease [1]–[9]. These studies demonstrated that electrical stimulation combined with volitional efforts or additional therapy is behaviorally more effective than ES alone in the rehabilitation of patients following stroke [4]–[6]. Voluntary contraction coupled with ES appears to be a promising adjuvant therapy for functional recovery and brain reorganization after central nerve injury. The detailed effects of voluntary contraction coupled with ES on cortical plasticity remains largely unknown

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Conclusion