Modulations of motor system excitability following high-frequency electrical stimulation in normal subjects

Introduction. Short- and long-term modulations in the motor system have been observed following peripheral stimulation, movement learning, repetition of movements and immobilization. In this study, we studied the effects of high-frequency electrical stimulation on motor system excitability. In our electrophysiological examination, we monitored the motor evoked potentials (MEP) using trancranial magnetic stimulation (TMS) and F-wave. Materials and methods. We investigated 10 normal subjects, aged 20-35 years. We performed a stimulation session using high frequency electrical stimulation. The duration of the stimulation session was 20 min. In 5 subjects we stimulated the fifth finger and in 5 the mixed nerve. We performed the electrophysiological examination (TMS and F-wave) before the stimulation session and then 1,5,10,15,20 and 30 min after the session. With TMS we analyzed the rest motor threshold, the amplitude and the latency of MEP. To monitor the spinal excitability we analyzed the F-wave amplitude and latency. Results. Digital stimulation of the fifth finger produced a significant decrease in the amplitude of MEP lasting at least 20 min. Stimulation of the mixed nerve produced a mild but significant increase in the amplitude of MEP lasting at least 15 min. No statistically relevant differences in MEP latency and in F-wave amplitude and latency have been observed. Discussion. The effects of high frequency stimulation has been used in cellular models to investigate the long-term potentiation and depression. The peripheral stimulation modulates the motor system in different ways (inhibitory for the cutaneous afferences, excitatory for the propioceptive ones). The excitatory and inhibitory mechanisms in the motor system appear to be only partially located at the spinal level. Our study could explain some effects of the peripheral high frequency stimulation in the rehabilitation field.