Functional plasticity of surround inhibition in the motor cortex during single finger contraction training.

We investigated the functional changes in short intracortical inhibitory (SICI) circuits to determine whether surround inhibition is altered during a simple finger movement training. Using an electromyographic (EMG) feedback system linked to a computer monitor, participants practiced sustained index finger abduction by 40% maximum voluntary contraction of the first dorsal interosseous (FDI) while decreasing overflow EMG activity of the abductor digiti minimi (ADM) to less than 5% maximum voluntary contraction. Single transcranial magnetic stimuli (TMS) and paired-pulse TMS were applied to the left primary motor cortex to elicit motor-evoked potentials (MEPs) in the right FDI and ADM before/after training. In addition to recording MEPs from both muscles during voluntary FDI contraction, MEPs were recorded during motor imagery. MEPs from the FDI were not altered by training, indicating no functional changes in SICI circuits associated with the FDI field. In contrast, SICI circuits associated with ADM were significantly strengthened by training, as indicated by reduced baseline EMG activity during both actual FDI contraction and motor imagery and by reduced MEPs in response to post-training TMS. We propose that SICI circuits show functional plasticity during motor training and that surround circuit inhibition of nontarget muscle groups increases in proportion to the acquisition of motor skills.

Changes of excitability in M1 induced by neuromuscular electrical stimulation differ between presence and absence of voluntary drive.

The aim of this study is to investigate excitability changes in the human motor cortex induced by variable therapeutic electrical stimulations (TESs) with or without voluntary drive. We recorded motor-evoked potentials (MEPs) from extensor and flexor carpi radialis (FCR) muscles at rest and during FCR muscle contraction after the application of TES on FCR. TES application conditions were changed intensities, frequencies, and trains. In addition, to evaluate the contribution of M1 inhibitory circuits to the effects of TES application, we also recorded MEPs using paired-pulse transcranial magnetic stimulation. In resting muscle states, an increase in TES intensity resulted in an increase in MEP ratio in both the muscles. In contrast, when TES was applied to FCR during contraction, MEP ratios of both the muscles decreased with increased number of pulse trains. However, under both the states, MEP ratios decreased induced by paired-pulse transcranial magnetic stimulation in extensor carpi radialis to which TES was not applied. Excitability changes in M1 induced by TES application were reversibly modulated depending on the presence or absence of voluntary drive. This study showed that the therapy and the voluntary drive of the target muscles act together, and complement the effects of each other, which may be beneficial for optimizing the rehabilitation if the therapy accompanies voluntary drive.