Magnetically evoked EMGs in rats

Magnetic stimulation of the brain and spinal cord was carried out in rats to record electromyogram (EMGs) from the gastrocnemius. A figure-eight coil was set over the middle of the dorsum, and shifted from the cervical vertebrae to the sacrum. The motor evoked potentials (MEPs) with 4.8 msec latency by transcranial magnetic stimulation and the descending wave with 4.7 msec latency by C3-C4 stimulation were recorded. In evoked EMGs by magnetic stimulation over T9-T10, L4-L5, S2-S3 and Ca2-Ca3 spinal cord levels, the causes of these two evoked components with short (1.5 msec) and long (4.1 msec) latencies were estimated to be the eddy current generated from the rostral to the caudal portion of the spinal cord. With the increase in magnetic stimuli, the relative sizes and disappearance of H- and M-like responses were comparable with the ordinary M- and H-responses in electrically evoked EMGs. The magnetic stimulation of the spinal cord activated the sciatic nerve at their vertebral exit, because the latencies of the H- and M-responses were constant despite the changing stimulus sites. Although magnetic stimulation with the figure-eight coil can be focused on the target, it is necessary to take into consideration the influence of the eddy current flowing in the body.