1Department of Oral Physiology and 2Department of Orthodontics, Osaka University Faculty of Dentistry, Suita, Osaka, 565-0871 Japan
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ABSTRACT |
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Hidaka, O., T. Morimoto, T. Kato, Y. Masuda, T. Inoue, and K. Takada. Behavior of Jaw Muscle Spindle Afferents During Cortically Induced Rhythmic Jaw Movements in the Anesthetized Rabbit. J. Neurophysiol. 82: 2633-2640, 1999. The regulation by muscle spindles of jaw-closing muscle activity during mastication was evaluated in anesthetized rabbits. Simultaneous records were made of the discharges of muscle spindle units in the mesencephalic trigeminal nucleus, masseter and digastric muscle activity (electromyogram [EMG]), and jaw-movement parameters during cortically induced rhythmic jaw movements. One of three test strips of polyurethane foam, each of a different hardness, was inserted between the opposing molars during the jaw movements. The induced rhythmic jaw movements were crescent shaped and were divided into three phases: jaw-opening, jaw-closing, and power. The firing rate of muscle spindle units during each phase increased after strip application, with a tendency for the spindle discharge to be continuous throughout the entire chewing cycle. However, although the firing rate did not change during the jaw-opening and jaw-closing phases when the strip hardness was altered, the firing rate during the power phase increased in a hardness-dependent manner. In addition, the integrated EMG activity, the duration of the masseteric bursts, and the minimum gape increased with strip hardness. Spindle discharge during the power phase correlated with jaw-closing muscle activity, implying that the change in jaw-closing muscle activity associated with strip hardness was caused by increased spindle discharge produced through insertion of a test strip. The increased firing rate during the other two phases may be involved in a long-latency spindle feedback. This could contribute to matching the spatiotemporal pattern of the central pattern generator to that of the moving jaw.
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INTRODUCTION |
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Masticatory movements are regulated by the
texture of food, and greater activities of the jaw-closing muscles are
produced when the hardness of the food increases (Lund
1991). This suggests that the control of mastication is highly
dependent on sensory feedback. Experiments using cortically induced
rhythmic jaw movements (CRJMs) in rabbits (Lavigne et al.
1987
; Morimoto et al. 1989
) have demonstrated
that the integrated electromyogram (EMG) of the masseter muscle
(Ma-IEMG) increases during CRJMs when a test material is chewed between
the opposing cheek teeth. The effect is dramatically reduced when the
periodontal and muscle spindle afferents are cut, indicating that the
two sensory inputs are essential for the facilitatory effect. On the
other hand, this facilitatory effect is only partially decreased when
periodontal sensation alone is eliminated, and Ma-IEMG and masticatory
force are still regulated according to strip hardness (Hidaka et
al. 1997b
). During mastication, therefore, there is the
potential for muscle spindle afferents to regulate the jaw-closing
muscle activity in a hardness-dependent manner. It seems that a simple reflex mechanism alone cannot explain the whole facilitatory effect (Hidaka et al. 1997b
). This is because an early part of
the facilitation of the masseter muscle activity sometimes occurs
before the minimum latency for the masseter response to strip
application (i.e., before the minimum latency [6 ms] of the
monosynaptic stretch reflex). Sometimes the masseter facilitation
occurs even before the onset of a load on the teeth. The early activity
persists after cutting the periodontal afferents, suggesting that
muscle spindle afferents play a crucial role in the production of the early facilitatory effect. In support of this proposal, a lesion of the
mesencephalic trigeminal nucleus (MesV) almost abolishes the early
facilitatory response (Komuro et al. 1997
).
The behavior of spindle afferents during fictive mastication induced by
cortical stimulation has been reported by other investigators (Kolta et al. 1990). But the activity of muscle spindle
afferents during CRJMs has only been noted in two brief reports
(Morimoto et al. 1995a
,b
). How muscle spindles are
involved in the regulation of jaw-closing muscle activity therefore
remains an unanswered question.
In the present study we sought to provide an insight into the neural
mechanism for mastication by analyzing how jaw muscle spindle afferents
behave, according to the hardness of a chewed material, during CRJMs.
The use of CRJMs in studies of mastication enables measurement of the
effect of the chewed material alone on parameters relating to
mastication. The texture of the material can be controlled, and CRJMs
obtained during chewing can be compared with those obtained when not
chewing. Some data from this study have been reported briefly
(Hidaka et al. 1997a).
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METHODS |
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All surgical procedures were approved by the Osaka
University Faculty of Dentistry Intramural Animal Care and Use
Committee and were almost the same as those described previously
(Liu et al. 1993). In this investigation, 24 male
rabbits weighing 2.5-3.5 kg were used. Each animal was anesthetized
with ketamine (16 mg/kg, Ketalar 10, Sankyo) and thiamylal sodium (20 mg/kg) injected in an auricular vein. The trachea was cannulated, and
anesthesia was maintained during surgery by a mixture of halothane and
oxygen so that a corneal reflex and spontaneous eye movements were not present. A photo diode was attached to the mentum, and jaw movements were monitored with an optoelectronic recording apparatus (C2399, Hamamatsu Photonics, Hamamatsu City, Japan). The animal's head was
fixed in a stereotaxic apparatus so that the lambda was 1.5 mm below
the bregma (Sawyer et al. 1954
). The cortical surface was exposed (1-7 mm anterior and 3-8 mm lateral of the bregma) for
electrical stimulation of the cerebral masticatory area (CMA). The
rectal temperature was maintained between 36 and 38°C with a heating
pad. The EMGs were recorded with pairs of Teflon-coated stainless steel
wires (with bared tips 1.0 mm in length) inserted into the left
masseter and digastric muscles on the side contralateral to the
cortical stimulation. On the basis of the results of our previous study
(Morimoto et al. 1989
), masseteric activity was recorded
from the deep anterior part of the muscle.
Cortical stimulation
Intracortical microstimulation was achieved with square pulses
(0.2 ms, <80 µA, 30 Hz) delivered through glass-coated metal electrodes with an impedance of 1-2 M at 1 kHz, and crescent-shaped jaw movements were induced. The anesthesia during the experiment was
controlled at a level slightly lighter than that during surgery. The
heart rate was ~230 beats/min. The lowest arterial pressure of the
femoral artery was ~75 mmHg, and the highest pressure ranged between
110 and 130 mmHg. Pupillary constriction appeared when a light was
directed into the animal's eye. No obvious changes in the heart rate,
respiration, pupillary size, or arterial pressure were induced by CMA stimulation.
Intraoral stimulation
During CRJMs, a thin test strip of polyurethane foam was held with forceps by an experimenter and placed between the animal's opposing left molar teeth. Three test strips, each 2-mm thick, 5-mm wide, and 15-mm long were used, and each strip was designated as soft, medium, and hard, respectively (hardness: 27, 67, and 91 respectively; Japanese Industrial Standard, K 6301).
Recording from muscle spindle afferents
A craniotomy was performed to allow access to the left MesV
(11-16 mm posterior and 0-4 mm lateral of the bregma), and a small cylinder was fixed to the skull at the defect and filled with liquid
paraffin. Passive movements were applied to the lower jaw with a
servo-controlled stretcher (EMIC 513A) attached to screws at the
symphysis menti. During sinusoidal and ramp-and-hold stretches, spikes
were recorded from the MesV neurons with a glass-coated tungsten
microelectrode (15-µm-diam tip, 1-3 M impedance at 1 kHz). The
spindle responses were recorded at least 6 hours after the
administration of thiamylal sodium. The following criteria (see also
Masuda et al. 1997
) were used for identification of MesV
spindle units: histological identification of the recording sites in
the MesV, a low threshold for unit discharge to passive jaw opening;
neuronal firing rates >100 Hz during mastication, absence of response
to occlusal contact of the upper and lower teeth, and absence of
response to pressure applied to the teeth, gingiva, or vibrissae. In
addition, we identified masseteric units by the following features: a
response to gentle palpation of the masseter surface (Fig.
1A) and a decrease or
elimination of any preexisting discharges in response to weak
electrical stimulation of the muscle belly (Fig. 1B). For
the nonmasseteric units, we confirmed a response to gentle palpation of
a small superficial portion of the temporal muscle and a response to a
brief pressure applied to the eyeball.
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At the finish of the experiments, electrolytic lesions were made at the recording sites to permit identification of their locations. Each rabbit was given a fatal dose of thiamylal sodium and perfused intracardially with saline and then with buffered formalin. The brains were extracted, sectioned at the thickness of 50 µm in the frontal plane, and stained with cresyl violet.
Data analysis
An eight-channel digital audio tape data recorder (PC-208 M, Sony-Magnescale, Tokyo, Japan) was used to record the jaw movements, EMGs, and spindle discharges. The data were transferred to a computer (Power Macintosh 9500/200), digitized at 500 Hz for the jaw movements, 2 kHz for the EMGs, and 20 kHz for the spindle discharges. Spindle units were discriminated and identified as single units on the basis of shape (including amplitude) of the units by software (Spike2, Cambridge Electronic Design, Cambridge, UK). The EMG data were software rectified and smoothed by a 9-point moving average, and the area under the smoothed EMG record was denoted as integrated EMG (IEMG).
The CRJMs were recorded before and during the strip application, and these periods were designated as control cycles and experimental cycles, respectively. The interval between two consecutive maximum jaw-openings was defined as a chewing cycle, and its length was designated as a total cycle length (TCL). A chewing cycle was composed of three phases: The jaw-opening phase was defined as the time from the most closed position to the most opened position, the jaw-closing phase was the most opened position to the lateralmost position, and the power phase was the phase between the first two phases.
The mean values were calculated for each of the following variables from five successive chewing cycles, and the mean value was used as a representative value for the chewing cycles: the minimum gape, the mean frequency of spindle discharge, IEMG, and the EMG burst duration.
The EMG burst duration was determined in the following way. The
mean ± SD of the digitized EMG during one-tenth of the TCL immediately before the maximum jaw opening was first calculated for the
masseteric EMG. Each burst was then identified by when the muscle
activity exceeded the mean level by 3 SD for 30 ms.
Statistical analysis
In each group, each variable was tested for a normal
distribution with a 2 test, and the
homogeneity of all variances was tested with a Bartlett test. The
results of these tests determined which statistical analysis was
performed next. A one-way ANOVA or the Kruskal-Wallis test was used to
examine for differences among the three groups classified by strip
hardness. A Wilcoxon signed-rank test was used to determine significant
differences between control cycles and experimental cycles.
Scheffé's F test was used for multiple comparisons when a
statistically significant difference was found among the three groups.
P < 0.05 was assumed to be statistically significant.
The data were presented as the mean ± SE.
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RESULTS |
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We examined data, including 43 spindle afferents, from 13 rabbits. Twenty-one of the spindle afferents were from the masseter muscle, 3 were from the temporalis, and the muscle of origins for the remaining 19 spindle afferents were not clearly identified. In the masseter, the spindles were located in the anterior or central region. Forty-three stable units were recorded during chewing of the soft test strip and during chewing of the medium strip. However, only 36 stable units were recorded when the hard test strip was applied. The weakness of the rabbit skull (in comparison with other animals, e.g., cats) sometimes prevented a rigid fixation to the stereotaxic frame and hampered stable recording conditions. Consequently, fewer stable units were obtained during chewing of the hard test strip.
Behavior of muscle spindle afferents during CRJMs
Records of the discharges of a spindle neuron are shown in Fig. 2A. The receptive field for this neuron was in the ventral anterior part of the masseter muscle. When rhythmic jaw movements were induced by rubbing the hard palate (a chewing strip was not applied), spindle discharges were absent at the end of the jaw-closing phase (Fig. 2A, Palatal stimulation). This period of silence was also apparent in records obtained during CRJMs when a chewing-strip was absent (Fig. 2A, No strip) although the regularity of the spindle discharge in a jaw movement cycle was different. Spindle discharges continued throughout the jaw closure, however, when a chewing strip was applied. Changes in the number of discharges of the unit, according to strip hardness, are shown in Fig. 2B. The activity of the masseter muscle increased with strip hardness.
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We compared the discharge frequency of this unit during CRJMs with and
without chewing strips after normalizing the latter so that the time
course matched (Fig. 3). When a test
strip was applied, the minimum gape increased, and the muscles were at
a longer length during the power phase with the test strip in place than during this phase in control cycles (i.e., the muscles were not
shortened to the same degree). The discharge pattern in control cycles
(Fig. 3, +) was similar to the pattern in experimental cycles (Fig. 3,
) during the jaw-opening and jaw-closing phases. The discharge
patterns were very different, however, during the power phase, when
discharges were observed only when a chewing-strip was applied.
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Discharges of a different unit are illustrated in Fig. 4A. The muscle of origin for this afferent discharge was not the masseter muscle and could not be clearly identified. This unit is likely to be a muscle spindle afferent (see DISCUSSION). When a test strip was absent, neuronal activity was observed during the jaw-opening phase; during the remaining part of the chewing cycle the unit did not discharge. A different spindle discharge pattern was observed, however, after application of a soft chewing strip. Discharges were then observed mainly in the latter half of the jaw-opening phase and the period around the minimum jaw opening. A similar pattern of discharge activity was observed during the chewing of the medium strip, but the firing rate in the power phase was higher. The increased rate of spindle discharge was further pronounced when the hard strip was chewed, and the discharge became continuous throughout the chewing cycles. The discharge pattern displaying an increased frequency around the time of minimum gape (i.e., around maximum jaw closure) was observed in 7 (including 3 masseteric) of the 43 units.
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The behavior of another masseteric unit is shown in Fig. 4B. During the power phase of control cycles, a clear cessation of firing was not observed. In experimental cycles, however, spindle discharge showed behavior similar to that shown in Fig. 4A, according to strip hardness.
A clear cessation of spindle discharge during the power phase of control cycles (e.g., as shown in Figs. 2 and 4A) was observed in 26 (60%) of the 43 units. These 26 units included 14 masseteric units and 12 nonmasseteric. No distinct relation was found between the receptive field and the presence or absence of a clear cessation of firing. The mean spindle discharge rate increased significantly in a hardness-dependent manner during the power phase (one-way ANOVA, P < 0.001; Fig. 5A). In addition, the mean firing rate during each phase was significantly greater in experimental cycles than in control cycles (Wilcoxon signed-rank test; P < 0.01; Fig. 5, A, B, and C). However, the mean firing rates during the jaw-opening and jaw-closing phases did not significantly change, according to strip hardness (Fig. 5, B and C).
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Changes in masseter activity and the minimum intermaxillary distance related to strip hardness
The masseteric IEMG activity and the mean duration of the masseteric EMG bursts increased significantly in a hardness-dependent manner (Kruskal-Wallis test, P < 0.001, Fig. 5D; one-way ANOVA, P < 0.001, Fig. 5E). The minimum gape was increased by insertion of a test strip and became wider significantly with increasing strip hardness (one-way ANOVA, P < 0.001; Fig. 5F). On the other hand, the horizontal component of chewing in experimental cycles did not change significantly with strip hardness.
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DISCUSSION |
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Twenty-one (49%) of the spindle afferents examined were from the
masseter muscle. Within the MesV of the cat, no difference has been
found in the relative distribution of spindle afferent somata from the
masseter, temporalis, and medial pterygoid muscles (Cody et al.
1972; Jerge 1963
). During fictive mastication in the rabbit, central modulation (a phasic inhibition in most units) of
afferent discharge has been recognized, and neurons showing modulated
and unmodulated characteristics have been encountered at all levels of
the MesV (Kolta et al. 1990
). These findings suggest
that there was little bias in the sampling of the units that were
recorded from the MesV. The recording sites of the units were confirmed
histologically. These units responded to the jaw opening, but did not
respond to pressure applied to the teeth, gingiva, or vibrissae.
Furthermore, the first-order neurons of Golgi tendon organs are located
in gasserian ganglia rather than in the MesV (Lund et al.
1978
). Accordingly, all units examined in the current study
were thought to be muscle spindle units.
The receptive fields of the masseteric units of the present
investigation were located in the anterior or middle part of the muscle, which accords well with anatomic findings (Bredman et al. 1991). The temporal muscle of a rabbit is divided into a
small superficial portion and a much larger part originating from the posterior wall of the orbit (Weijs and Dantuma 1981
). In
the present study, we restricted palpation of the temporal muscle to
the small superficial portion because of the difficulty in reaching the larger part. Three units were observed in the small portion. In the
rabbit, the number of the spindles in the lateral pterygoid muscle, if
any are present, is extremely small (Smith and Marcarian 1967
). We believe therefore that the spindles of the remaining 19 units are in the deep portion of the temporal muscle or in the
medial pterygoid muscle.
Behavior of muscle spindle afferents during CRJMs
The activity patterns of muscle spindle afferents were divided
into two types, depending on the presence or absence of a clear cessation of discharge during the power phase before strip application. We cannot clarify what caused the different spindle discharge patterns,
but possible explanations include the type of ending of the spindle
afferent and the location and direction of the spindle. Because jaw
muscle spindles cannot be classified reliably by their conduction
velocity (Inoue et al. 1981), a distinction between the
two types of afferents, primary and secondary, was not made in the
present investigation. Moreover, it seemed inappropriate in the present
study to use the more reliable way of classifying spindle afferents by
using ramp-and-hold stretches and a depolarizing neuromuscular blocking
agent (succinylcholine). This is because the agent may cause such an
unfavorable condition that the output from motoneurons is not
transferred to muscles in a physiological manner, thereby causing an
unfavorable effect on the behavior of EMG activity according to strip
hardness. Furthermore, it is uncertain whether the test with
succinylcholine is valid for the rabbit. The test has been used
exclusively on the cat. Consequently, which type of afferents
contribute to the regulation of jaw-closing muscle activity remains
uncertain at present. However, we believe that both primary and
secondary afferents are involved in the regulation, because both types
are activated by static fusimotor drive during the jaw-closing and
power phases, and because both afferents project to trigeminal
motoneurons monosynaptically (Appenteng et al. 1978
;
Dessem et al. 1997
) and may also be involved in
polysynaptic jaw muscle stretch reflexes (Dessem et al.
1997
). The topographic distributions of the two types of
afferents, however, are different (rat: Dessem et al.
1997
; cat: Kishimoto et al. 1998
), implying that
the afferents play different functional roles.
The spindle afferent firing showed less regularity in CRJMs than in
rhythmic jaw movements induced by palatal stimulation. Although the
possibility remains that these different discharge patterns were caused
by the dissimilar jaw movements induced, the firing patterns could also
result from a difference in fusimotor activity. In support of this
proposal, a very irregular discharge of the spindle afferents was noted
during active movements, whereas spindle firing was more regular when
the same movements were reproduced passively (Taylor et al.
1997). It seems therefore that the fusimotor drive is more
active in CRJMs than in reflexively evoked rhythmic jaw movements.
A clear cessation of spindle discharge during the power phase was
observed before strip application but was absent in most units after
strip application. The discharge during shortening of the jaw-closing
muscles could result from the static fusimotor drive that would prevent
unloading of sensory endings (Taylor and Appenteng
1981). It has been proposed that the static fusimotor fibers
are activated principally during muscle shortening to help keep both
primary and secondary endings active and take up slack in the
intrafusal fiber. In contrast, during the whole cycle of chewing, the
dynamic fusimotor system acts tonically to set the incremental
sensitivity of primary endings to stretch (Appenteng et al.
1980
). In the present study the discharge increased
coincidentally with the transition from fast closing (jaw-closing
phase) to slow closing (power phase), which is in agreement with
previous reports (Cody et al. 1975
; Goodwin and
Luschei 1975
; Taylor and Cody 1974
; Taylor and Davey 1968
; Taylor et al.
1981
). The discharge often persisted throughout the power
phase, especially when a tougher strip was applied. During the power
phase the spindle discharge increased with strip hardness, which seemed
to parallel jaw-closing muscle activity. The increased spindle
discharge during the power phase could effectively raise the
excitability of trigeminal motoneurons (Chase and McGinty
1970
) and thus is most likely involved in the facilitatory
effects on jaw-closing muscle activity. The increased firing rate for a
tougher strip is probably caused by a greater intermaxillary distance
throughout the power phase (Morimoto et al. 1995a
) and
by an increased slowing of muscle shortening. Any loading that slowed
the expected shortening would cause an increase in spindle afferent
firing, which would reflexively compensate for the loading. In this
manner, the patterned discharge of the static fusimotor drive may
represent a temporal template of the intended movement (Taylor
and Appenteng 1981
). In this concept, the secondary spindle
afferents would tend to display more firing, because the greater
dynamic sensitivity of the
b1b2c afferents (afferents
with the predominant influences of the bagl,
bag2 and chain intrafusal fibers; primary) would
make it more difficult for the static fusimotor burst to prevent
unloading than for the b2c afferents (afferents
with the predominant influences of the bag2 and
chain intrafusal fibers; probably secondary).
During the jaw-opening phase, excitatory inputs from spindle afferents
are inhibited in trigeminal motoneurons by hyperpolarizing inputs from
the pattern generator (Goldberg and Chandler 1981; Nakamura and Kubo 1978
). However, the spindle firing
during the jaw-opening phase could be concerned with the regulation of
jaw-closing muscle activity. It has been noted by other investigators
(Dessem et al. 1997
; Donga and Dessem
1993
) that jaw-muscle spindle afferents project directly to the
cerebellum. These direct projections could provide reliable signals for
the comparison of peripheral movements with centrally generated motor
commands and thereby produce a long-latency spindle feedback that might
contribute to matching the central commands with peripheral movements.
For this function secondary spindle afferents may be particularly
important (Perreault et al. 1995
). The early
facilitatory EMG response to strip chewing (Hidaka et al.
1997b
), which was almost abolished by deafferentation of muscle
spindle afferents (Komuro et al. 1997
), may be produced by the increased firing rate during the jaw-opening phase.
During the jaw-closing phase, the discharge increased when any strip
was applied. A perturbation in a strip-chewing cycle starts
approximately at the end of the jaw-closing phase (Hidaka et al.
1997b). Gellman et al. (1985)
have proposed that
it is the climbing fiber system that signals unexpected events to a normal movement pattern. According to their study, a climbing fiber
response is consistently elicited if an unexpected displacement or
contact occurs. We propose that for the trigeminal system, a
trigeminally evoked climbing fiber response may signal a perturbation (i.e., an event outside the programmed chewing pattern) during mastication, thereby triggering a corrective movement or modulating the
cerebellar output to alter movement commands. The cerebellum may
modulate masticatory rhythmic jaw movements by exerting its effects
on the masticatory central rhythm generator as well as trigeminal motoneurons (Nakamura and Katakura 1995
).
Hardness-dependent changes in EMGs of the masticatory muscles
Both periodontal sensory receptors and jaw muscle spindles are
primarily responsible for the facilitatory effects on masseteric EMGs
during mastication (Hidaka et al. 1997b; Morimoto
et al. 1989
). However, these sensory receptors have different
physiological properties (Appenteng et al. 1982
;
Inoue et al. 1981
) and different regulatory actions on
masticatory force (Hidaka et al. 1997b
). The
facilitatory masseteric response, including the early EMG response
before a loading on the teeth, almost remains even after deprivation of
periodontal sensation by cutting the maxillary and inferior alveolar
nerves to the teeth (Hidaka et al. 1997b
), implying that
spindle afferents contribute to the remaining facilitation of the
masseteric EMG. Chase and McGinty (1970)
electrically
stimulated the MesV and recorded the resultant H-reflex of the masseter
muscle. They noted that during CRJMs in the freely moving cat, the
H-reflex was facilitated during jaw closure and inhibited during jaw
opening. Nakamura et al. (1976)
reported that in
immobilized cats, tonic jaw depression increased the amplitude of
cortically induced rhythmical masseteric nerve activity. It could be
stated from these results that the increased spindle discharges, at
least during the jaw-closing and power phases, could raise the
excitability of motoneurons and lead to a facilitatory EMG response
during CRJMs, according to the hardness of the material that was chewed.
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ACKNOWLEDGMENTS |
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This study was partly supported by Grants 10307045 and 10771010 from the Japanese Ministry of Education.
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FOOTNOTES |
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Address for reprint requests: O. Hidaka, Department of Oral Physiology, Osaka University, Faculty of Dentistry, 1-8 Yamadaoka, Suita, Osaka 565-0871 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 21 December 1998; accepted in final form 2 August 1999.
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REFERENCES |
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