Excitability changes in the ipsilateral primary motor cortex during rhythmic contraction of finger muscles

Abstract

The aim of this study was to determine whether the excitatory ipsilateral primary motor cortex (ipsi-M1) is affected by changes in the frequency of rhythmic voluntary contraction of the left first dorsal interosseous (FDI) induced by repetitive abduction of the left index-finger. Transcranial magnetic stimulations were delivered to the left M1 during repetitive left index-finger abduction at 1, 2, and 3Hz, and motor evoked potentials (MEPs) were simultaneously evoked in the resting right (Rt)-FDI, Rt-abductor pollicis brevis, and Rt-abductor digiti minimi. The stimulus-response (S-R) curve of the MEP at each frequency was recorded. In addition, F-waves were recorded from the Rt-FDI during these rhythmic contraction tasks in order to examine the changes in spinal motoneuron excitability. MEPs were markedly increased under the 3Hz conditions compared with the other conditions. However, F-waves were hardly changed under these conditions. The S-R curve of the MEP induced under the 3Hz conditions was significantly steeper than the curves produced under other conditions. Our results indicate that the excitability of ipsi-M1 is affected by the frequency of rhythmic voluntary contraction of unilateral finger movement, which may be caused by neural inputs delivered via a transcallosal pathway.