Effects of H-reflex conditioning upon the contralateral alpha motoneuron pool.

H-reflex recovery curves have been elicited by subliminal conditioning stimuli applied to the ipsilateral and contralateral posterior tibial nerves in 10 healthy female subjects. In both types of recovery curve there was clear evidence of a period of facilitation in the ipsilateral soleus motoneuron pool 75-250 msec after the conditioning stimulus. These results indicate the bilateral nature of the facilitation and show it to be most probably produced by stimulus-evoked inputs as opposed to twitch-evoked inputs. If the facilitation is produced by descending long-loop reflex influences, then complementary evidence is provided for previous electromyographic data showing the bilaterality of long-loop reflexes evoked by percutaneous electrical stimulation. It is impossible, however, at the present time, to rule out the possible involvement of cutaneous afferent discharges or other stimulus evoked inputs in the late facilitation.