Trial-to-trial fluctuations in H-reflexes and motor evoked potentials in human wrist flexor

The H-reflexes and the motor potentials (MEPs) evoked by electromagnetic br ain stimulation in the human wrist flexor were recorded over many trials. T he responses from each stimulus at two steady levels of muscle activation w ere sorted into three groups, based on their amplitudes. The electromyogram (EMG) in each of these groups was rectified and averaged. The level of pre -response muscle activity was found to correlate with the amplitude of both the averaged H-reflexes and the averaged MEPs. This suggests that much of the amplitude fluctuations of both H-reflexes and MEPs can be attributed to moment-to-moment changes in the level of activity of the motoneurone pool. Overall, however, the amplitude of MEPs increased more rapidly than the am plitude of H-reflexes as the pre-stimulus EMG activity increased. This is p robably because, while the amplitude of H-reflexes depends primarily on the level of motoneurone pool excitability, the amplitude of an MEP depends no t only on this, but also on the excitability of the motor cortex, and the f ormer is to some extent also dependent on the latter. (C) 1999 Elsevier Sci ence Ireland Ltd. All rights reserved.