Abstract

Our aim of this study was to investigate how transcranial magnetic stimulations (TMS) affect the spinal motoneuron excitability as measured by the amplitude of F-waves in the first dorsal interosseous muscle. Seven healthy subjects took part in the study. The effects of TMS were examined using a conditioning-test paradigm. Conditioning pulses of magnetic stimulation were delivered over motor cortex, prefrontal cortex, parietal cortex or cervical root, at various inter-stimulus intervals (ISIs, 5–300 ms) with the subjects at rest; Test electrical stimuli were delivered to the ulnar nerve to evoke an F-wave. The conditioning stimuli over motor cortex significantly increased the peak amplitude of F-wave with either 5–20 or 165–220 ms ISI. The conditioning stimuli over prefrontal cortex significantly increased the F-wave amplitude with either 85–120 or 225–240 ms ISI. The conditioning stimuli over parietal cortex produced little effect on the F-wave. The conditioning stimuli over cervical root increased the F-wave amplitude with 125–300 ms ISI. The facilitation of F-wave by the motor cortex stimulation might result from the activation of corticospinal output or the afferent feedback due to muscle twitch. The facilitation of F-wave by the prefrontal stimulation might have a non-corticospinal origin.

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