Cerebellar-related long latency motor response in upper limb musculature by transcranial magnetic stimulation of the cerebellum

In this study, we aimed to identify the cerebellum-related electromyographic (EMG) response that appeared in the upper limbs musculature. Thirty times averaged transcranial magnetic stimulation (TMS) with a double-cone coil placed over the cerebellar hemisphere elicited long latency EMG responses at the bilateral extensor carpi radialis (ECR) muscles. The peak latency of this EMG response was 70.7±12.7 ms in the ipsilateral ECR and 62.9±10.2 ms in the contralateral ECR of the TMS side. These latencies were much longer than the latency of the muscle evoked potential when we stimulated pyramidal tracts at the foramen magnum level. Cerebellar hemisphere loading by the finger target pursuit test made this EMG response faster during TMS on the ipsilateral side of the cerebellum and slower during TMS on the contralateral side of the cerebellum. Furthermore, the deeper the level of drowsiness, the slower the peak latency of this EMG response became. These results suggest that this EMG potential is a specific response of the cerebellum and brainstem reticular formation, and may be conducted from the cerebellar structure to the ECR muscle through the polysynaptic transmission of the reticulospinal tract.