Many studies have been conducted on the monosynaptic reflex (H-reflex) changes which occur before a voluntary movement in man. Especially, it has been shown that there were the changes of H-reflex prior to the rapid plantarflexion movement which was called the premotor inhibition.
In the present study, the changes in soleus H-reflex prior to a choice reaction time (RT) were studied, giving the special attention to the possible functional difference of motor patterns to generate the premotor inhibition during a period when the physiological mechanisms involved in the movement.
The changes of H-reflex were measured in choice RT experiment in 9 and 6 male subjects under the different two conditions : (a) selection of the response sides (left or right plantarflexion) -the movement pattern was known before the presentation of a response signal ; and (b) selection of the movement patterns (plantar or dorsiflexion) -the responce side was known before.
The premotor inhibition was observed only in case of (a), and was dependent on a prerequisite for movement patterns. Moreover, the physiological significance of this phenomenon was indeed related to a presynaptic inhibition, and was related to a modulation of alpha-motoneurone excitability by the pyramidal system. Thus, the premotor inhibition might be related to central motor preprogram controlling presynaptic inhibition on the reflex pathways.

In this study, it was attempted to clarify the neurophysiological mechanism of voluntary movement (phonation) recording evoked EMG (H-wave) which have been studied by Magladery and others basing on the Hoffman's study about monosynaptic reflex. The indicator was the amplitudes of H-wave which were evoked on M. soleus by stimulation on the afferent nerve fibers contained in N. tibialis. Variations of H-wave amplitudes were plotted in relation to the time elapsed from initiation of phonation and were investigated about the central nervous system in relation to it.
As the results of this study, following conclusions were led;
1) At the stage of pre-phonation period, average values of H-wave amplitude were higher than those of control. This result was understood as impulses which were commanded to contract the muscles for phonation raised alpha motoneurones in reflex arc of lower limbs, simultaneusly.
2) The damping phenomenon of H-wave amplitudes were observed just prior to pre-phonation. But the neurophysiological mechanism of it is unknown in detail, now.
3) The H-wave amplitudes showed the highest value in agreement with initiation of phonation. This phenomenon was called as Jendrassik's Maneuver. This effect was understood that Jendrassik's Maneuver had some functional connection to the reticular formation of brain stem.
4) After the initiation of phonation, the inhibitory phenomenon of H-wave amplitudes (this level was lower than that of control) was observed at most of subjects. The neurological mechanism of this phenomenon is unknown.

It has been shown that there were the changes of H-reflex prior to the rapid voluntary movement. The present paper is designed to examine the changes of H-reflex about the time course prior to the rapid voluntary movement (dorsiflexion and plantarflexion), and premotor silent period in detail.
H-reflexes were elicited from subjects' right foot at varying intervals after the response signal (sound stimulus) of reaction time task when the response involved the rapid dorsiflexion or plantarflexion of the foot. In plantarflexion, H-reflexes were elicited from two response situations ; one was the situation which subjects were not given the preparatory stimulus (without warning = W -), the other was given the preparatory stimulus (with warning= W+) .
The following results were obtained ;
1) Dorsiflexion and plantarflexion
In case of dorsiflexion, although there were no obvious changes of H-reflex prior to EMG onset, in case of plantarflexion the damping phenomenon of H-reflex prior to EMG onset was observed and this phenomenon was the inhibitory one. Especially, under w-situation, it was obviously and it was about 70-80msec prior to EMG onset.
2) Silent period
The damping phenomenon of H-reflex was observed in accordance with silent period but this value was much the same of control level. This value of the damping of H-reflex was smaller than that of premotor silent period (about 30-40msec) . The time when the damping of H-reflex was observed was in accordance with that of prior to EMG onset of simple plantarflexion and this damping of H-reflex was the inhibitory phenomenon.
These results suggest that the upper center sents some inhibitory motor commands to cotralateral agonist motorneurons in advance of the voluntary movement in spite of the peripheral motor set, and those from the upper center precede the voluntary movement by ahnnt 70-80msec.

Specific facilitation of the agonist motoneuron pool (pretibial muscle; TA) was investigated under the two different reaction time conditions, i, e, simple (S) and choice (C) reaction time tasks. The motor task was bilateral ankle dorsiflexion, and two different strengths of voluntary ankle dorsiflexion (25%, (S) and 50% (L) of maximum contraction force) were controlled by visually guided tracking. The excitability of the agonist motoneuron pool was investigated by the H-reflex method.
As the differences between the EMG reaction times of TA on both sides were extremely small under both the S and C tasks, it might be reasonable to use the non-stimulated side as the onset of voluntary movement. Not only the onset of H-reflex facilitation with respect to EMG onset, but also the reaction times were significantly prolonged under the conditions of task C compared with those of task S. In addition, movement times were also prolonged and peak dF/dt values were significantly decreased under the conditions of task C. These results allow us to conclude that the modulation of pre-motor facilitation of the agonist H-reflex is controlled by descending command from the brain, and would be caused by slower EMG reaction times. In addition, we suggest that the prolonged motor times and decreased dF/dt values under the conditions of task C are attributable to slower recruitment and a lower firing rate of motor unit.

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