[Excitability of motor cortex with transcranial magnetic double stimulation in the intact man].

To evaluate the excitability of central motor tract, we studied a transcranial magnetic double stimulation with short conditioning-test (C-T) interval of 1-10ms in eight normal volunteers. In addition, H-reflex of the forearm muscle was used to study the effect of the magnetic cortical conditioning stimulus on alpha-motoneuron, and the test response evoked by electrical cortical stimulation was also used to examine the effect of the magnetic cortical conditioning stimulus. The subthreshold conditioning and suprathreshold test stimuli were applied, and compound muscle responses were recorded in the relaxed abductor pollicis brevis muscle. There was a decrease of the test response size by the conditioning stimulus at C-T interval of 1-5ms. This attenuation was probably caused by intracortical inhibition. Because the identical magnetic cortical conditioning stimulus produced increase in H-reflex size. Moreover, the test response evoked by electrical cortical stimulus was not suppressed by the magnetic cortical conditioning stimulus; whereas, response evoked by the magnetic cortical test stimulus was suppressed at C-T intervals of 2ms. With the technique of transcranial magnetic double stimulation, therefore, it is possible to evaluate the inhibitory function in the motor cortex. The technique may be of use for pathophysiology, diagnosis and estimation of treatment in the diseases.