Multiple descending corticospinal volleys demonstrated by changes of the wrist flexor H-reflex to magnetic motor cortex stimulation in intact human subjects.

To study noninvasively multiple descending volleys on alpha motoneurons, we used a 30-50% maximum H-reflex in the wrist flexor muscle conditioned by a subthreshold magnetic stimulus of the contralateral motor cortex as a parameter of motoneuronal excitability in seven neurologically healthy subjects. The time interval of the median nerve (test) stimulus and the magnetic (conditioning) stimulus was varied. In all subjects the conditioned H-reflex significantly (P < 0.05) enhanced at conditioning-test intervals between -1 ms and +10 ms. Superimposed on the overall facilitation there were at least two short duration periods of enhanced facilitation in all but one subject. These periods were separated by 2-4 ms and suggest increased excitability at the alpha motoneuronal pool brought on by the descending volleys. We conclude that with subthreshold magnetic cortical stimuli it is possible to produce multiple descending corticospinal volleys, which probably represent descending I waves. This should be taken into account when analyzing motor evoked potentials (MEPs) in clinical studies.