Spinal and muscle motor evoked potentials following magnetic stimulation in cats

Motor evoked potentials (MEP) from transcranial magnetic stimulation were successfully recorded from the spinal cord and gastrocnemius muscle in cats. The spinal MEP consisted of four negative peaks, N1 through N4, and subsequent small, multiphasic waves. The conduction velocity calculated from N1 corresponded to that of the nonpyramidal cerebellar evoked potential. After ablation of the pyramidal tract at various levels, N1 through N3 persisted, whereas N4 disappeared. The muscle MEP showed the same latency as that of MEP obtained by direct cortical electrical stimulation and disappeared after similar ablation. These findings suggest that the first three peaks of the spinal MEP are responses of the nonpyramidal cerebellospinal pathways and that the fourth peak of the spinal MEP and the muscle MEP are pyramidal tract responses. Spinal and muscle MEP from magnetic stimulation may be valuable in the qualitative evaluation of the function of the pyramidal and nonpyramidal pathways.