Physical and physiological specification of magnetic pulse stimuli that produce cortical damage in rats

The effects of transcranial magnetic pulse stimuli on the brain tissue of rats were examined. In Experiment I, 52 male albino rats received pulsed magnetic stimulation of the head. Stimulus intensity, number of stimulations, stimulated sites, and interval between last stimulation and sacrifice for neuropathological examination were varied. High stimulus intensity (2.8 T) and 100 or more stimulations produced clearly defined microvacuolar changes in the neuropil portion of cortical layers 2-6 (especially layers 3 and 4) in 12 of 24 animals. Fewer stimulations and lower intensities produced no such effects in 28 rats given that stimulation. Midline stimulation and stimulation over the left hemisphere produced similar results. No other brain, ocular, or spinal structures manifested such changes. Lesions were present in animals that had intervals up to 30 days between the last stimulation and perfusion. In Experiment II with 18 animals, compound motor action potentials (CMAPs) evoked by magnetic stimulation of the cortical motor area and recorded from the right lower extremity were examined. The electromyographic threshold was 0.83 T. Further increases in stimulus intensity produced increases in CMAP amplitude, up to approximately 1.9 T. It was noted that the lesion-producing intensity (2.8 T) was 0.9 T greater than the intensity needed for near-asymptotic reactions and was 3.4 times the CMAP threshold value.