1Department for Stomatognathic Dysfunction and 2Department of Physiology, Faculty of Dentistry, Tokyo Medical and Dental University, Tokyo 113-8549, Japan
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ABSTRACT |
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Takada, Yoshiyuki, Takao Miyahara, Tatsuya Tanaka, Takashi Ohyama, and Yoshio Nakamura. Modulation of H Reflex of Pretibial Muscles and Reciprocal Ia Inhibition of Soleus Muscle During Voluntary Teeth Clenching in Humans. J. Neurophysiol. 83: 2063-2070, 2000. A previous study has demonstrated that the soleus H reflex is facilitated in association with voluntary teeth clenching in proportion with biting force in humans. The present study tried to elucidate the functional significance of this facilitation of the soleus H reflex, by examining 1) whether the facilitation of the H reflex is reciprocal or nonreciprocal between the ankle extensors and flexors and 2) whether the reciprocal Ia inhibition of crural muscles is facilitated or depressed in association with voluntary teeth clenching. The H reflex of the pretibial muscles was evoked by stimulation of the common peroneal nerve in seven healthy subjects with no oral dysfunction. The pretibial H reflex was facilitated in association with voluntary teeth clenching in a force-dependent manner. The facilitation started preceding the onset of electromyographic activity of the masseter muscle. Stimulation of the common peroneal nerve at low intensities subthreshold for evoking the M wave of the pretibial muscles inhibited the soleus H reflex after a short latency corresponding with a disynaptic inhibition, indicating that the reciprocal Ia inhibition was depressed in association with voluntary teeth clenching. Thus, the present study has shown that voluntary teeth clenching evokes a nonreciprocal facilitation of ankle extensor and flexor muscles and attenuated reciprocal Ia inhibition from the pretibial muscles to the soleus muscle. It is concluded that voluntary teeth clenching contributes to improve stability of stance rather than smoothness of movements.
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INTRODUCTION |
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Clenching of the teeth is often observed in
association with voluntary movements requiring strong effortse.g.,
during weight lifting. With the development of sports medicine, an
increasing number of analyses have recently been made of the possible
correlation of teeth clenching with efficiency of motor performance as
well as muscle strength of the extremities (Miyahara et al.
1996
). A previous study in our laboratory has demonstrated that
the human soleus H reflex is facilitated in association with voluntary
teeth clenching (Miyahara et al. 1996
). The facilitation
started preceding the onset of the masseter electromyographic (EMG)
activity and increased in magnitude linearly with the increment of
biting force. However, the functional significance of this facilitation
in motor control has remained to be studied.
In association with locomotion, ankle flexors and extensors are activated alternately during stance and swing phases, respectively. During the stance phase, the ankle extensors contract and ankle flexors relax to extend the ankle joint for propulsion of the body mass. On the other hand, during the swing phase the ankle flexors contract and the extensors relax. Thus, the reciprocal inhibition plays a major role for smooth performance of locomotion. In contrast, to stabilize the posture, the ankle extensors and flexors co-contract to fix the ankle joint, as seen in the positive supporting reaction.
It has been reported in humans that in association with voluntary
extension and flexion of the ankle joint, the soleus H reflex is
facilitated and inhibited, respectively (Tanaka 1983).
It is also known that the gain of H reflexes of lower limbs is
modulated in a phase-dependent manner during stepping, walking, and
running in humans (Brooke et al. 1997
). In addition, the
reciprocal group Ia inhibition of the soleus H reflex by stimulation of
the lowest threshold fibers in the common peroneal nerve was
demonstrated in patients with bilateral athetosis in the resting
condition (Mizuno et al. 1971
) and during voluntary
contraction of pretibial muscles in healthy subjects (Tanaka
1974
).
Thus, to reveal the significance of the facilitation of the soleus H
reflex in association with voluntary teeth clenching, we studied
whether the facilitation of H reflexes of leg muscles in association
with voluntary teeth clenching is reciprocal or nonreciprocal between
ankle extensors and flexors. We also studied whether the reciprocal Ia
inhibition from the pretibial muscles to the soleus H reflex
(Tanaka 1974) is depressed or facilitated in association
with voluntary teeth clenching.
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METHODS |
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Subjects and materials
Experiments were performed on seven healthy male volunteers, aged 25-30 yr, who had all given informed consent to participate in the study. Five of them participated in both the first and second series of experiments. One of the remaining two subjects participated only in the first series, and the other in the second series only. Subjects were comfortably seated in a reclining chair. The angle of the knee and foot joints on both sides was kept constant at ~120° and 100°, respectively, by means of an immobile footplate. The head rested on a headrest, and the forearm and hand rested on an armrest. The armrest and the footplate served to keep the muscles of the arm and leg relaxed during the resting period in the experiment.
In the first series of experiments, in which we studied modulation of the H reflex of the pretibial muscles during voluntary teeth clenching, EMG activity was recorded from the pretibial and masseter muscles on the right side with bipolar surface cup electrodes (diameter: 8.0 mm) placed 3 cm apart longitudinally over the middle part of the tibialis anterior muscle and over the masseter muscle just below the zygomatic arch. The EMG activity was amplified with conventional amplifiers (time constant: 0.03 s; high cut frequency: 10 kHz).
The H reflex of the pretibial muscles was evoked on the right side with a pair of surface electrodes (diameter: 8.0 mm) taped to the skin along the common peroneal nerve: the cathode positioned at the head of the fibula and the anode 2.5 cm apart distally. A ring silver plate was placed around the skin between the stimulating and recording electrodes as the ground electrode, to reduce the artifact because of spread of the stimulating current along the surface of the skin. The stimulating pulse of 0.5 ms was used at an intensity 1.1 times the threshold (×T) for the M wave. At this stimulus intensity, a small M wave was obtained together with the H reflex; this M wave was used to monitor the stability of the stimulating condition.
In the second series of experiments, we studied modulation of the reciprocal Ia inhibition from the afferents of the pretibial muscles to the soleus muscle during voluntary teeth clenching. The same type of bipolar surface recording electrodes as used in the first series were placed 3 cm apart longitudinally over the right soleus muscle just below the gastrocnemius muscle belly to record the soleus H reflex, in addition to the stimulating and recording electrodes used for the H reflex of the pretibial muscles in the first series. A surface stimulating electrode (diameter: 8.0 mm) was positioned in the popliteal fossa over the tibial nerve as a cathode and a silver plate (35.0 × 45.0 mm) placed on the patellar region as an anode, to evoke the soleus H reflex. Test stimulating pulses of 1.0 ms in duration were applied to the tibial nerve at intensities that evoked the H reflex with an amplitude ~30% of the maximum M wave. The conditioning stimulating pulse was applied to the common peroneal nerve at an intensity of 0.95 × T for the M wave of the pretibial muscles. Experimental data were simultaneously recorded on a data recorder (Sony PC216A, flat frequency response: DC, 5.0 kHz) and on a floppy disk through an A/D converter (Canopus ADX-98E, 10 kHz per channel) with the use of a microcomputer (NEC-98 model 60). Hard copies of records were obtained with a thermal array recorder (Nihon Kohden RJA-1300, flat frequency response: DC, 10 kHz).
Experimental procedures
Experimental procedures were similar to those in our previous
study (Miyahara et al. 1996). The strength of teeth
clenching with the upper teeth against the lower teeth was monitored by the amplitude of the full-wave-rectified, integrated masseter EMG,
because it is known that there is a rectilinear relationship between
the amplitude of the masseter EMG and bite force during isometric
contraction (Moller 1966
). Two vertical cursors were shown on the computer display in front of the subject (Fig.
1A). The cursor in the
top half of the display (target cursor) consisted of a pair
of vertical lines; the center between them indicated the instructed
force level, and each line corresponded to the instructed force level
±5.0% of the maximum contraction of the masseter muscle. The level of
maximum contraction was defined as the mean amplitude of the
full-wave-rectified, integrated masseter EMG during voluntary teeth
clenching with maximum effort for 3 s. The strength was set from
25 to 100% of the maximum contraction in 25% steps. The cursor at the
bottom half of the display (force cursor) continuously
indicated the instantaneous amplitude of the full-wave-rectified,
integrated masseter EMG during the instructed muscle contraction. It
moved from the left to the right on the display with an increase in
contraction of the masseter muscle. The left and right ends of the
display corresponded to the 0 and 100% levels of the voluntary muscle
contraction, respectively. In advance of each experiment, the subject
was instructed to match the position of the force cursor between the
pair of target cursors as soon as possible by adjusting the strength of
contraction of the masseter muscle.
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The subject maintained the jaw in the rest position during the resting period when neither cursor was shown in the display. The start of the trial was indicated by a warning signal of beep (1 kHz for 300 ms), which was given at a constant interval of 10 s (Fig. 1Ba). After various intervals from the warning signal (Fig. 1Bb), both the target and force cursors were shown simultaneously on the display as the go signal. The target cursor was shown at the position representing the instructed level of contraction of the masseter muscle, and the force cursor at the left end of the display corresponding to the null biting force during the rest period. In response to the go signal, the subject quickly matched the force cursor to a position between two vertical lines of the target cursor by adjusting the strength of contraction of the masseter muscle (Fig. 1B, bottom trace) and kept this level until the test stimulus was given or for ~3 s when the test stimulus was not given. A session consisted of 20-30 trials, and a series of 6-12 sessions was performed in a day with a 5-min interval after each session.
In the first series, a trial consisted of one of the following three
tasks: 1) test stimulus to the common peroneal nerve without
teeth clenchingi.e., null biting force (control trial), 2)
test stimulus with teeth clenching (conditioned trial), and 3) no test stimulus with teeth clenching. Neither the
subject nor the experimenter could predict which task would be required in the next trial or when the next go signal would be given, because the delay of the go signal (Fig. 1Bb) as well as test
stimulation of the common peroneal nerve from the go signal (Fig.
1Bc) in each trial was ordered randomly by the computer.
Before starting a session, both 1) the number of trials in
the session and 2) the number of trials in which either
conditioning or test stimulus was not given were randomly determined.
In the trials to study the relation between the biting force and the magnitude of the pretibial H reflex, the interval between the warning signal and the go signal was set from 0.5 to 3.0 s in 0.5-s steps in random sequence (Fig. 1B, 2nd and 3rd traces). In the trials to study the time course of modulation of magnitude of the pretibial H reflex with respect to the onset of masseter EMG, the subject was instructed to clench his teeth at a strength of 75% of the maximum effort. The interval between the go signal and the stimulation of the common peroneal nerve was varied randomly from 1 s before the go signal to 3 s after it.
In the second series, trials consisted of the following four kinds: 1) test stimulation of the tibial nerve without teeth clenching, 2) test stimulation with teeth clenching, 3) conditioning stimulation of the common peroneal nerve followed by test stimulation of the tibial nerve without teeth clenching, and 4) the same as 3) with teeth clenching. As in the first series, the sequence of these four kinds of trial was randomized by a computer in advance of each session. The level of masseter contraction was set at 70% of the maximum voluntary contraction throughout this series of experiments.
Data analysis
The difference in magnitude of the H reflexes of the pretibial and soleus muscles between the control trials without teeth clenching and the conditioned trials with teeth clenching was statistically tested by one-way ANOVA followed by Scheffé's F test, Welch's t-test and a paired t-test. Except for the results of the experiments, in which the time course of modulation of amplitude of the pretibial H reflex was studied, the results in those trials were excluded from the analysis, in which 1) the masseter EMG activity was present before the go signal, 2) the test stimulus applied before the masseter EMG reached the instructed level, and 3) the level of the masseter EMG activity was outside the instructed level.
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RESULTS |
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Modulation of H reflex of pretibial muscles during voluntary teeth clenching
In the first series of experiments, we studied whether the H reflex of the pretibial muscles was modulated in association with voluntary teeth clenching. Figure 2 illustrates an example of the EMG of the pretibial muscles evoked by stimulation of the common peroneal nerve. It consisted of two successive responses: an early and a late response. At low stimulus intensities, the early response had a rather simple wave form with a latency of ~15 ms, whereas the late response consisted of a pair of component waves after a latency of ~30 ms. Each component wave of the late response had the same threshold of 0.90-0.95 × T as the early response. Both the early and late responses increased in magnitude in parallel with an increase in stimulus intensity at a range lower than ~1.2 × T for the early response. Beyond this level, the early response steadily increased in magnitude with a further increase in stimulus intensity, whereas the late response gradually diminished to finally disappear as the stimulus intensity was increased. Because these properties corresponded to those of the M wave and the H reflex in other limb muscles, we regarded the early and late responses as the M wave and the H reflex of the pretibial muscles, respectively.
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At higher stimulus intensities, the M wave showed a notable change with an increase in stimulus intensity: in addition to an increase in amplitude of the M wave that was induced at lower stimulus intensities, another wave appeared after a shorter latency and increased in amplitude with an increase in stimulus intensity. Thus at high stimulus intensities, the M wave itself consisted of a pair of distinct component waves.
We tested modulation of the pretibial H reflex evoked by stimulation at intensity of 1.1 × T for the H reflex during voluntary teeth clenching. As shown in Fig. 3, every component wave comprising the pretibial H reflex (Fig. 3A) increased in amplitude during maximum voluntary teeth clenching (Fig. 3B), which is seen in the superimposed record of both traces in A and B (Fig. 3C).
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The magnitude of the pretibial H reflex, was defined as the area lying between the EMG trace and a straight line extending from the onset of the first deflection to the peak of the last deflection (the shaded area of the inset in Fig. 3). It increased in parallel with the strength of teeth clenching. Figure 4A is a scatter diagram of the magnitude of pretibial H reflex against the force level of teeth clenching obtained from a subject. The mean H reflex magnitudes at the force levels of 25, 50, 75, and 100% of the maximum voluntary contraction were 109.5, 130.0, 154.0, and 163.3% of the control magnitude, respectively. Compared with the magnitude of the control H reflex without teeth clenching, it was larger during the teeth clenching as a whole at all strengths of teeth clenching than the control magnitude without teeth clenching (P < 0.05), and steadily increased with an increase in strength of teeth clenching with a significant difference between one level and the level after the next (i.e., between 0 and 50%, 25 and 75%, and 50 and 100%; P < 0.05). The correlation coefficient was 0.72 (P < 0.05). Figure 4B shows the correlation between the strength of teeth clenching and the mean values of magnitude of the pretibial H reflex at respective intensity level in each of six tested subjects (open circles); in each subject there was a significant positive correlation between the magnitude of the H reflex and the stimulus intensity (r = 0.49-0.74, P < 0.05). The filled circles represent the grand mean values of the magnitude of all the six subjects against the strength of teeth clenching; a significant positive correlation was found between the magnitude of the H reflex and the stimulus intensity (r = 0.94, P < 0.05).
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Time course of modulation of the pretibial H reflex in association with teeth clenching
Figure 5 shows the time course of the modulation of magnitude of the pretibial H reflex in relation to EMG activity of the masseter muscle during teeth clenching at an intensity of 75% of the maximum in a subject. The onset of the masseter EMG activity is shown as 0 on the abscissa. Each circle in Fig. 5A represents the magnitude of the reflex as a percent of the control amplitude at the time when the test stimulus was applied. As seen in Fig. 5B, which shows the plot in Fig. 5A on an expanded time base, the increase in magnitude of the pretibial H reflex apparently started at >200 ms before the onset of the masseter EMG activity, reached a peak ~60 ms before the onset, and then declined to a plateau ~400 ms after the onset. If we take the control value +2 SD as the minimum value of a significant increase in magnitude, the increase started at 124 ms before the onset of the masseter EMG activity (Fig. 5B, arrow). In the six tested subjects, the magnitude of the pretibial H reflex started to increase at 83-124 ms before the onset of the EMG activity (99.3 ± 15.7 ms, mean ± 1 SD).
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Modulation of the soleus H reflex by stimulation of the common peroneal nerve
Conditioning stimulation of the common peroneal nerve at an intensity subthreshold for the M wave of the pretibial muscles depressed the soleus H reflex. Figure 6 shows an example of the time course of the depression of the soleus H reflex induced by conditioning stimulation of the common peroneal nerve at an intensity of 0.95 × T for the M wave of the pretibial muscles in a subject. It started at a conditioning-testing interval of 1.0 ms and reached a peak at 2.0 ms, then gradually tended to return to the control level, although the depression was still present at an interval of 4.0 ms (P < 0.05).
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Figure 7 shows the results obtained from a session in a subject. The soleus H reflex was depressed by conditioning stimulation of the common peroneal nerve at an intensity subthreshold for the M wave of the pretibial muscles under the conditions with teeth clenching (right) and without (left). The control soleus H reflex obtained without teeth clenching (amplitude: 4.20 mV; top left) was depressed by conditioning stimulation of the common peroneal nerve applied 2.0 ms preceding test stimulation to the tibial nerve (amplitude: 3.53 mV; bottom left). The facilitated soleus H reflex during teeth clenching (amplitude: 5.08 mV; top right) was also depressed by stimulation of the common peroneal nerve (amplitude: 4.38 mV) at the same intensity as used during the rest control condition without teeth clenching (bottom right). Although the soleus H reflex was depressed not only in trials without teeth clenching but also in those with teeth clenching, the depression was reduced from 19.0% in trials without teeth clenching to 13.8% in those with teeth clenching.
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In this subject, the depression of the soleus H reflex was compared
among all the trials without teeth clenching and those with teeth
clenching (Fig. 8A). The mean
amplitude of the soleus H reflex was reduced to 86.2%
(n = 75) of the control (n = 79) in the
trials without teeth clenching, whereas during teeth clenching the mean
amplitude was reduced to 93.1% (n = 82) of the
facilitated amplitude (n = 84) of the soleus H reflex.
Although the depression was significant in both conditions, the soleus
H reflex was less depressed during teeth clenching (P < 0.05)i.e., the soleus H reflex was partially disinhibited in
association with the teeth clenching. Figure 8B shows the
depression of the soleus H reflex without teeth clenching (open
circles) and during teeth clenching (filled circles) in each of all the
six tested subjects, showing a significant reduction of the depression
of the soleus H reflex during teeth clenching (P < 0.05).
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To exclude the possibility that the reduction of the depression is simply due to an increased amplitude of the soleus H reflex during teeth clenching, the relation of the magnitude of depression of the soleus H reflex to its amplitude was studied. Figure 9A illustrates the relation of the amplitude of the control soleus H reflex (open circles) and the conditioned soleus H reflex (filled circles) with the intensity of test stimulation of the tibial nerve in a subject. Although the amplitude increased with stimulus intensity in both control and conditioned soleus H reflexes, it was smaller in the conditioned H reflex in the range from 10 to 70% of the maximum magnitude of M wave except for 15 and 67% (P < 0.05). However, the reduction was clearly dependent on the amplitude of the soleus H reflex. As shown in Fig. 9B, the depression was increased until the amplitude of the soleus H reflex reached ~50% of the maximum M wave (correlation coefficient: 0.95, P < 0.01). Beyond this level, however, the inhibition tended to decrease with an increase in amplitude of the soleus H reflex.
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Because the amplitude of the soleus H reflex never exceeded 45% of the maximum M wave even during teeth clenching in the present study, the reduction of depression of the soleus H reflex in association with teeth clenching showed a change in the opposite direction to that expected simply from an increase in amplitude of the soleus H reflex.
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DISCUSSION |
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The present study has demonstrated that 1) the pretibial H reflex is facilitated during voluntary teeth clenching in proportion with the strength of biting force monitored by the amplitude of the masseter EMG, 2) the facilitation started preceding the onset of masseter EMG activity, and 3) the depression of the soleus H reflex induced by conditioning stimulation of the common peroneal nerve at an intensity subthreshold for evoking the M wave in the pretibial muscles is reduced during voluntary teeth clenching.
Modulation of pretibial H reflex during voluntary teeth clenching
It was reported that the H reflex could be evoked in the pretibial
muscles in selected subjects as well as during voluntary contraction of
these muscles (Davies 1985; Deschuytere and
Rossele 1971
; Pierrot-Deseilligny et al. 1973
,
1981
; Schieppati and Crenna 1985
; Tanaka
1974
; Upton et al. 1971
). In the present study,
transcutaneous stimulation of the common peroneal nerve evoked a pair
of successive EMG responses recorded from the bipolar surface electrode
placed on the belly of the tibialis anterior muscle: an early and a
late response. Both these responses appeared after the latencies
comparable to those reported in the studies described, and the relation
between the stimulus intensity and the magnitude of the early and late responses was essentially the same as that of the M wave and H reflex
of other limb muscles. Thus, the present study confirmed that the M
wave and H reflex could be evoked by transcutaneous stimulation of the
common peroneal nerve in healthy subjects at rest.
It was noted in this study that the H reflex recorded from the surface
over the tibialis anterior muscle consisted of a pair of distinct waves
separated from each other by a rather silent period. This pattern is in
sharp contrast to the soleus H reflex, which consists of a single wave.
The difference in waveform may be due to a difference in composition of
muscle fiber types as well as spinal motoneurons between the two
muscles in humans. It has been reported that the number of type II
fibers amounts to 27% (Johnson et al. 1973) or 34%
(Henriksson-Larsén et al. 1985
) of all the fibers
in the tibialis anterior muscle, whereas it is only 12.3% in the
soleus muscle in humans (Henriksson-Larsén et al.
1985
). Thus a significant portion of the fibers consisted of
type II fibers in the tibialis anterior muscle, whereas nearly all the
fibers are type I in the soleus muscle. The group of motoneurons innervating the type I fibers have axons with slower conduction velocities than those innervating the type II fibers. The volley monosynaptically evoked in the two groups by stimulation of the common
peroneal nerve arrives at the tibialis anterior muscle after
appreciably different latencies. It has been reported that stimulation
of the sciatic nerve in cats evokes monosynaptic reflex volleys in the
tibial nerve consisting of two clearly discernible two, an early and a
late (Kubota et al. 1965
). The volley recorded from the
medial gastrocnemius nerve shows a single peak corresponding with the
early peak recorded from the tibial nerve, whereas the volley in the
soleus nerve consists of a single peak corresponding to the late peak.
It was proposed that the early and late peaks represent efferent
volleys conducting in the axons of phasic and tonic motoneurons. It
could be assumed that the two component waves of the pretibial H reflex
represent EMGs in type I and II muscle fibers evoked by efferent
volleys along axons with different conduction velocities in two
distinct groups of spinal motoneurons innervating the two groups of the
tibialis anterior muscle fibers. Similarly, the same two groups of
tibialis anterior motoneurons may be implicated in the two distinct
waves in the M wave of the pretibial EMG evoked by stimulation at high intensities.
The common peroneal nerve innervates the extensor digitorum longus and the extensor hallucis longus muscles as well as the tibialis anterior muscle. It is possible therefore that the H reflex recorded from the surface of the belly of the tibialis anterior muscle also includes the H reflexes evoked in these pretibial muscles, which are located in parallel with the tibialis anterior muscle adjacent to one another. The separate waves of the pretibial muscles may represent the EMG activities evoked in these pretibial muscles in addition to the tibialis anterior muscle. We checked whether these pretibial muscles were activated by stimulation of the common peroneal nerve. Although palpation could find no contraction of the extensor digitorum longus muscle, we could not detect whether the extensor hallucis longus muscle was activated, because it was located deep in the pretibial region. Accordingly, we cannot exclude the possibility that the H reflex recorded from the surface of the belly of the tibialis anterior muscle may include the activities evoked in the pretibial muscles other than the tibialis anterior muscle.
The facilitation of the pretibial H reflex started when test
stimulation was applied to the common peroneal nerve at 83-124 ms
(98.3 ± 15.7 ms, mean ± 1 SD) before the onset of the
masseter EMG. The conduction time for the afferent volley evoked by the test stimulation of the common peroneal nerve to reach the pretibial motoneuron pool should be shorter than the difference in latency between the H reflex and M wave (~15 ms). The difference represents the period from the time of test stimulation of the common peroneal nerve to the time of arrival of the reflexively evoked efferent volley
of pretibial motoneurons at the point of stimulation of the common
peroneal nerve. Because the facilitation of the pretibial H reflex
started when test stimulation was applied at 83-124 ms before the
onset of the masseter EMG, the excitability of the pretibial
monosynaptic reflex must have been elevated 68 ms (i.e., 83
15 ms) preceding the onset of masseter EMG.
In association with voluntary teeth clenching in monkey, the
excitability of the masseter motoneuron pool was reported to start to
increase at 25-45 ms preceding onset of masseter EMG (Blair-Thomas and Luschei 1975). With regard to
isometric bite task in monkeys, Larson et al. (1981)
reported that it was only after the onset of increase in bite force and
of the EMG of the temporalis and masseter muscles that afferents from
muscle spindles in jaw-closing muscles and periodontal mechanoreceptors
started to fire. Accordingly, it is suggested that the command for
facilitation of the pretibial H reflex be issued from the cerebral
cortex to the spinal cord in parallel with that for excitation of
jaw-closing motoneurons. During steady biting, oral-facial afferent
impulses induced by biting as well as supraspinal descending impulses
may be involved in the facilitation of the pretibial H reflex, as reported with respect to the facilitation of the soleus H reflex in
association with voluntary teeth clenching (Miyahara et al. 1996
).
Modulation of reciprocal Ia inhibition of soleus H reflex during voluntary teeth clenching
It has been reported that the volleys in the common peroneal nerve
exerted a short-latency depressive effect on the soleus H reflex in
patients with bilateral athetosis (Mizuno et al. 1971) and during voluntary dorsiflexion of the ankle joint (Tanaka
1972
) as well as in resting condition (Tanaka
1974
) in normal human subjects. This depression is regarded as
the reciprocal Ia inhibition, because the depression is evoked by
stimulation of the common peroneal nerve after a disynaptic latency at
intensities subthreshold for evoking the M wave in the pretibial
muscles (Tanaka 1974
).
In the present study the soleus H reflex was depressed by conditioning
stimulation of the common peroneal nerve at an intensity subthreshold
for evoking the M wave in the pretibial muscles, with the same time
course as the reciprocal Ia inhibition of the human soleus H reflex
(Tanaka 1983). In addition, the changes in amount of
reciprocal inhibition of soleus H reflex in relation to its size in the
present study was virtually the same as the reciprocal Ia inhibition of
the soleus H reflex reported in humans (Crone et al.
1990
). Thus, the depression of the soleus H reflex evoked by
stimulation of the common peroneal nerve can reasonably be regarded as
the reciprocal Ia inhibition of the soleus H reflex.
The present study has demonstrated that this reciprocal Ia inhibition
of the soleus H reflex is reduced during voluntary teeth clenching. It
has been reported that the amount of the reciprocal Ia inhibition
depends on the amplitude of the soleus H reflex (Crone et al.
1990). The inhibition increases with an increase in amplitude
of the test soleus H reflex until it reaches a plateau at the test
reflex size of 40-50% of the maximum M wave. In the present study the
size of the test soleus H reflex was set ~30% of the maximum M wave
and it never exceeded ~45% of the maximum M wave, even during
voluntary teeth clenching. In this range the amount of the inhibition
should have increased with an increase in amplitude of the test soleus
H reflex. As a matter of fact, the amount of inhibition decreased
during voluntary teeth clenching even though the amplitude of the test
soleus H reflex was increased by teeth clenching. The result indicates
that the amount of the reciprocal Ia inhibition of the soleus H reflex
was really reduced during voluntary teeth clenching.
Thus, combining the findings in the present study and our previous one, we conclude that in association with voluntary teeth clenching 1) the H reflexes in the ankle extensor and flexor are nonreciprocally facilitated, and 2) the reciprocal Ia inhibition from the ankle flexor on the ankle extensor is reduced.
Functional significance of the modulation of the H reflex and reciprocal Ia inhibition of leg muscles in association with teeth clenching
H reflexes in human soleus muscle have been reported to be
inhibited during walking (Capaday and Stein 1986;
Morin et al. 1982
) and running (Capaday and Stein
1987
) compared with standing, even when the contraction level
of the muscle is the same for the two conditions. The nonreciprocal
facilitation of H reflexes of leg muscles in combination with the
reduction of their reciprocal Ia inhibition is consistent with an
improved control of position required to maintain a stable posture. We
propose that voluntary teeth clenching would possibly contribute to
stabilization of postural stance rather than smoothness of movement.
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ACKNOWLEDGMENTS |
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Present addresses: Y. Nakamura, Dept. of Welfare and Information, Faculty of Informatics, Teikyo Heisei University, Ichihara, Chiba 290-0193, Japan; T. Ohyama, The First Department of Prosthodontics, Faculty of Dentistry, Tokyo Medical and Dental University, Tokyo 113-8549, Japan.
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FOOTNOTES |
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Address for reprint requests: Yoshiyuki Takada, Department of Stomatognathic Dysfunction, Faculty of Dentistry, Tokyo Medical and Dental University, 5-45 Yushima 1-chome, Bunkyo-ku, Tokyo 113-8549, Japan.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 15 March 1999; accepted in final form 4 October 1999.
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REFERENCES |
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