Repetitive magnetic stimulation of cerebral cortex in normal subjects

Changes of excitability in the neuraxis were studied in 11 healthy subjects with repetitive transcranial magnetic stimuli of the motor cortex at frequencies of 1-10 Hz. Following trains of four stimuli, compound muscle action potentials (CMAPs) were obtained from the flexor carpi radialis muscle. At an interstimulation interval (ISI) of 100-180 ms. the amplitudes of the CMAPs evoked by stimuli II and IV were inhibited as compared with the amplitude of the CMAPs evoked by the first stimulus. In contrast, the amplitude of the CMAPs evoked by stimulus III was almost unchanged. Changes in excitability depended on the stimulation intensity. Thus, it was demonstrated that the amplitude of the CMAPs evoked by stimulus III was reduced at low stimulation intensities (100 and 110% of threshold intensity) but normalized at higher stimulation intensities (120 and 130% of threshold intensity). Results of H-reflex studies indicated that excitability changes at the segmental level are not necessarily involved in the inhibition of the CMAPs evoked by stimuli II and IV at an ISI of 100 ms. It is proposed that long acting neurotransmitter modulation can explain part of the inhibition during repetitive magnetic stimulation, and that recruitment of different neurons, due to electric field differences, explains the dependence of excitability on stimulation intensity.