Responses in Extraocular Muscles from Magnetic Stimulation

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Purpose. This experiments were performed to investigate the origin of evoked action potentials of cat extraocular muscles by a pulsed magnetic stimulus through a circular coil or a pair of oppositely connected coils, which is placed on the scalp.Methods. Pulsed magnetic stimuli were applied to the scalp in twelve normal cats while recording electromyographical responses (EMGs) from the extraocular muscles (superior rectus, lateral rectus and inferior oblique muscles) with concentric needle electrodes. To investigate origin of magnetic evoked action potentials in extraocular muscles, alteration of EMG from lateral rectus muscle was compared before and after dissecting tectopontine pathway in five cats.Results. In all normal cats, action potentials of single neuro-muscular unit were easily recorded in all muscles examined with latency of 3.2-37.5ms (mean 15.6ms). These responses disappeared either after intravenous administration of pancuronium bromide, or by amputation of the peripheral corresponding nerve. By dissecting tecto-pontine pathway, magnetic evoked action potential with latency of approximately 15-25ms were disapeared.Conclusions. In ophthalmological field, there is no investigation of the magnetic evoked electromyography in extraocular muscles due to difficulties in both stimulation and recording. In this study magnetic evoked action potentials in extraocular muscles can be classified into two groups according to their latency, i.e.those with short latency (about 5ms), those with long latency (>15ms). Ablation experiment suggested that magnetic response with long latency was origineted in midbrain and/or cerebral cortex, and those with shory latency, therefore, in pons.