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PubMed1996

Development of the motor endplates in the masseter muscle in the human fetus.

The development of the motor endplate (MEP) and the structure of the masseter muscle in the human fetus were examined by light and electron microscopy. At 12 weeks of gestation, the masseter muscle was composed mostly of irregularly-arranged myotubes. The number of muscle fibers increased while that of the myotubes decreases during development. After 28 weeks of gestation, the masseter muscle was composed only of muscle fibers, and these fibers rapidly increased in size in comparison with those prior to 28 weeks of gestation. The MEP appeared during the first 12 weeks of gestation, and were of an undeveloped simple type, with only one axon branch. After 20 weeks of gestation, MEPs were classified as complex or simple types in terms of the branching of the axons. The complex type of MEP was found in muscle fibers of large diameter, while the simple type was found in muscle fibers of small diameter. Schwann-like cells appeared at the surface of the MEP. After 28 weeks of gestation, only the complex type was found in the masseter muscle. These results suggest that, the development of the MEP is closely related to the development of muscle, and that the timing of the development of the masticatory muscles differs from that of other skeletal muscles, such as those in the trunk and limb.

Embryonic and Fetal Development↗
PubMed2006

Classification of muscle spindle afferents innervating the masseter muscle in rats.

Taylor et al. [Taylor, A., Durbaba, R., Rodgers, J.F., 1992a. The classification of afferents from muscle spindles of the jaw-closing muscles of the cat. J Physiol 456, 609-628] developed a method to classify muscle spindle afferents using succinylcholine (Sch) and ramp and hold stretches. They demonstrated that cat jaw muscle spindle afferents show high proportion of intermediate responses to ramp and hold jaw stretch. Together with observations on the responses to Sch their data suggests that the majority of jaw muscle spindle afferents are influenced by a combination of nuclear bag(2) and nuclear chain fibres. Relatively few are influenced solely by nuclear bag(1) fibres. The purpose of this study was to categorize jaw muscle spindle afferent in rodents in response to ramp and hold stretches. Several measures were used to classify spindle afferents including (1) conduction velocity, (2) coefficient of variation (C.V.) of the interspike interval during jaw opening, and (3) the dynamic sensitivity and the initial discharge of spindle afferents before and after succinylcholine infusion (Sch, 100mg/kg, i.v.). Consistent with observations in the cat jaw muscles, the distribution of the conduction velocity and the C.V. of Vmes masseter afferents were unimodal. Therefore, these parameters were of little value in functional classification of spindle innervation. Succinylcholine injection either markedly increased the dynamic sensitivity or produced no change in Vmes afferents. Unlike cat jaw muscle spindle afferents, the effect of Sch on the initial discharge was not clearly separable from those responding or not responding to Sch. These results suggest that rat jaw muscle spindle afferents, have physiological properties that are primarily intermediate in nature and are likely to reflect a predominance of influence from nuclear bag(2) and chain fibres. However, the distinction between bag(2) and chain fibres influences is not as clearly defined in the rat compared to the cat.

Action Potentials↗
PubMed2001

Task-dependent control of human masseter muscles from ipsilateral and contralateral motor cortex.

Corticotrigeminal projections to human masseter motoneuron pools were investigated with focal transcranial magnetic stimulation (TMS). Responses in left and right masseter muscles were quantified from the surface electromyogram (EMG) during different biting tasks. During bilateral biting, TMS elicited motor evoked potentials (MEPs) in both masseter muscles. On average, the MEP area in the masseter contralateral to the stimulus was 39% larger than in the ipsilateral muscle, despite comparable pre-stimulus EMG in both muscles. MEPs elicited while subjects attempted unilateral activation of one masseter muscle were compared with those obtained in the same muscle during a bilateral bite at an equivalent EMG level. MEPs in the masseter contralateral to the stimulated hemisphere were significantly smaller during unilateral compared with bilateral biting. There was no significant difference in the size of ipsilateral MEPs during ipsilateral and bilateral biting. We conclude that the corticotrigeminal projections to masseter are bilateral, with a stronger contralateral projection. The command for unilateral biting is associated with a reduced excitability of corticotrigeminal neurons in the contralateral, but not the ipsilateral motor cortex. We suggest that this may be accomplished by reduced activity of a population of corticotrigeminal neurons which branch to innervate both masseter motoneuron pools.

Adult↗
PubMed2005

Differential activity patterns in the masseter muscle under simulated clenching and grinding forces.

The aim of this study was to investigate (i) whether the masseter muscle shows differential activation under experimental conditions which simulate force generation during clenching and grinding activities; and (ii) whether there are (a) preferentially active muscle regions or (b) force directions which show enhanced muscle activation. To answer these questions, the electromyographic (EMG) activity of the right masseter muscle was recorded with five intramuscular electrodes placed in two deep muscle areas and in three surface regions. Intraoral force transfer and force measurement were achieved by a central bearing pin device equipped with three strain gauges (SG). The activity distribution in the muscle was recorded in four different mandibular positions (central, left, right, anterior). In each position, maximum voluntary contraction (MVC) was exerted in vertical, posterior, anterior, medial and lateral directions. The investigated muscle regions showed different amount of EMG activity. The relative intensity of the activation, with respect to other regions, changed depending on the task. In other words, the muscle regions demonstrated heterogeneous changes of the EMG pattern for the various motor tasks. The resultant force vectors demonstrated similar amounts in all horizontal bite directions. Protrusive force directions revealed the highest relative activation of the masseter muscle. The posterior deep muscle region seemed to be the most active compartment during the different motor tasks. The results indicate a heterogeneous activation of the masseter muscle under test conditions simulating force generation during clenching and grinding. Protrusively directed bite forces were accompanied by the highest activation in the muscle, with the posterior deep region as the most active area.

Adult↗
PubMed2006

The expressions of insulin-like growth factors, their receptors, and binding proteins are related to the mechanism regulating masseter muscle mass in the rat.

OBJECTIVE: The mechanism regulating skeletal muscle mass is unclear. The purpose of the present study was to investigate the extent to which insulin-like growth factors (IGFs), their receptors (IGFRs), and binding proteins (IGFBPs) are involved in the regulation of skeletal muscle mass. DESIGN: We measured the mRNA expression levels for IGFs, IGFRs, and IGFBPs in the rat masseter muscle hypertrophied by oral administration of clenbuterol for 3 weeks and determined the correlations between the weight of masseter muscle and the mRNA expression levels. RESULTS: The mRNA expression levels for IGF-I and II, IGFR1 and 2, and IGFBP4 and 6 showed clenbuterol-induced elevations and positive correlations with the weight of masseter muscle. That for IGFBP3 only exhibited a clenbuterol-induced decrease and a strong negative correlation with the weight of masseter muscle. The mRNA expression levels for IGFBP2 and 5 showed no significant changes between the control and clenbuterol groups, and no significant correlations. IGFBP1 mRNA was not detectable. CONCLUSION: These results suggest that IGF-I, II, IGFR1 and 2, and IGFBP3, 4 and 6 are related to the mechanism regulating masseter muscle mass in the rat.

Adrenergic beta-Agonists↗
PubMed1985

[Changes in the fine structure of the masseter muscle as affected by unilateral occlusal disorders].

Biopsy specimens from the masseter muscle of the miniature pig "Mini-LEWE" were analysed histochemically before and after unilateral occlusal disturbances. The percentages of different fibre types and the cross-sectional area of fibres were determined with the myosin-ATPase reaction after acid preincubation. The capillary membranes were visualized by means of modified PAS-reaction. There was demonstrated a significant increase from the percentage portion of ST-fibre types, especially of the side of occlusal disturbance, under experimental conditions. This indicates an increased isometric contraction of the masseter muscle. The size of FT-fibre types and the capillarity increase significantly on the side without occlusal disturbances, due to the dislocation of the masticatory efforts to this side. The masseter muscle fine structure is shown to be adapted good to the disturbed functional conditions. No significant side differences were found in the dry mass of the masticatory muscles, at the end of experiments.

Animals↗
PubMed2002

Glutamate-induced sensitization of rat masseter muscle fibers.

In rats, intradermal or intraarticular injection of glutamate or selective excitatory amino acid receptor agonists acting at peripheral excitatory amino acid receptors can decrease the intensity of mechanical stimulation required to evoke nocifensive behaviors, an indication of hyperalgesia. Since excitatory amino acid receptors have been found on the terminal ends of cutaneous primary afferent fibers, it has been suggested that increased tissue glutamate levels may have a direct sensitizing effect on primary afferent fibers, in particular skin nociceptors. However, less is known about the effects of glutamate on deep tissue afferent fibers. In the present study, a series of experiments were undertaken to investigate the effect of intramuscular injection of glutamate on the excitability and mechanical threshold of masseter muscle afferent fibers in anesthetized rats of both sexes. Injection of 1.0 M, but not 0.1 M glutamate evoked masseter muscle afferent activity that was significantly greater than that evoked by isotonic saline. The mechanical threshold of masseter muscle afferent fibers, which was assessed with a Von Frey hair, was reduced by approximately 50% for a period of 30 min after injection of 1.0 M glutamate, but was unaffected by injections of 0.1 M glutamate or isotonic saline. Injection of 25% dextrose, which has the same osmotic strength as 1.0 M glutamate, did not evoke significant activity in or decrease the mechanical threshold of masseter muscle afferent fibers. Magnetic resonance imaging experiments confirmed that injection of 25% dextrose and 1.0 M glutamate produced similar edema volumes in the masseter muscle tissue. Co-injection of 0.1 M kynurenate, an excitatory amino acid receptor antagonist, and 1.0 M glutamate attenuated glutamate-evoked afferent activity and prevented glutamate-induced mechanical sensitization. When male and female rats were compared, no difference in the baseline mechanical threshold or in the magnitude of glutamate-induced mechanical sensitization of masseter muscle afferent fibers was observed; however, the afferent fiber activity evoked by injection of 1.0 M glutamate into the masseter muscle was greater in female rats. The results of the present experiments show that intramuscular injection of 1.0 M glutamate excites and sensitizes rat masseter muscle afferent fibers through activation of peripheral excitatory amino acid receptors and that glutamate-evoked afferent fiber activity, but not sensitization, is greater in female than male rats.

Action Potentials↗
PubMed2004

Changes in masseter muscle activity associated with swallowing.

The purpose of this study was to investigate masseter muscle activity during swallowing in seven healthy humans. In order to observe both the increase and the decrease of masseter muscle activity, the subjects performed voluntary saliva swallowing while low-intensity tonic clenching was maintained. Electromyograms (EMG) were recorded from the right masseter muscle and the suprahyoid muscle complex. For six of seven subjects, the masseter EMG amplitude increased above the pre-swallow baseline level at the initial phase of swallowing. This increase was initiated 0.1 +/- 0.3 s before the suprahyoid EMG onset and was terminated 0.6 +/- 0.4 s after the suprahyoid EMG onset. For six of seven subjects, the masseter EMG amplitude decreased below the pre-swallow baseline level during swallowing. This decrease was initiated 0.6 +/- 0.3 s after the suprahyoid EMG onset and was terminated 1.3 +/- 0.4 s after the suprahyoid EMG onset. These findings indicate that biphasic modulation of masseter motoneuron excitability occurs during swallowing in humans. Further investigations are needed to determine whether these findings are extensively applicable to swallowing as it occurs under natural conditions.

Adult↗
PubMed1998

Biochemical properties of the superficial masseter muscle in the aging rat.

In young (15 weeks) and senescent (2 years) rats, the metabolic enzyme activities and the composition of myosin heavy chain (MHC) isoforms of the superficial masseter muscle were assessed by biochemical analysis. The succinate dehydrogenase activity and phosphofructokinase activity of the masseter muscle were significantly lower in senescent rats than in young rats. A large portion of the masseter muscle was composed of MHC IIb and IId isoforms in both young and senescent rats. As compared with young rats, no significant change in the composition of MHC isoforms was found in the masseter muscle of the senescent rats. These findings indicate that aging has a significant effect on the energy supply of the superficial masseter muscle.

Aging↗
PubMed1988

Adult human masseter muscle fibers express myosin isozymes characteristic of development.

Masseter muscle biopsies were obtained from nine patients undergoing orthognatic surgery or surgery for parotid tumors. A detailed enzyme-histochemical and immunocytochemical study of these muscles was performed using antibodies specific to the various isozymes of the myosin heavy chain (MHC) in order to identify the MHC isozymes that were present in the different fiber types. The contractile proteins in these same biopsies were analyzed by two-dimensional gel electrophoresis, native pyrophosphate gel electrophoresis, and by an immunopolypeptide mapping approach. These studies have shown that there is a very heterogeneous distribution of the myosin isozymes, with many fibers containing more than one myosin type. We also present evidence that in addition to adult fast and slow myosin, the human masseter muscle contains two proteins, neonatal MHC and embryonic myosin light chain, that are characteristic of developing muscle.

Electrophoresis↗
PubMed1990

Noninvasive estimation of the location of the end plate in the human masseter muscle using surface electromyograms with an electrode array.

The purpose of this study was to estimate the location of the end plate in the masseter muscle, and to decide the appropriate position of surface electrodes for recording electromyograms (EMG) in humans. The subjects were 16 males who had no signs or symptoms of muscular disease. Identical electrode arrays were placed on the masseter muscles on each side. Each subject was asked to clench in the intercuspal position at various levels of maximum EMG amplitude. Eleven amplified EMGs were monitored simultaneously using a linear electrode array consisting of 12 stainless steel contacts. Various values were observed in different regions of the masseter muscle for the root mean square rectified EMG during brief isometric contraction. The superior region of the muscle had lower values than the inferior. The end-plate zone, which is in the center of the lower half of the masseter muscle, showed a lower amplitude than other regions. The propagation of motor unit action potentials was also observed. It was concluded that, aside from the end-plate zone, a position within the lower half of the muscle was most suitable for recording the surface EMG of the masseter muscle.

Adult↗
PubMed1996

Masseter muscle activity in bottle feeding with the chewing type bottle teat: evidence from electromyographs.

It was reported that the activity of the masticatory muscles is reduced in bottle fed babies. The effects of chewing type bottle teats distributed in Japan on masticatory muscle activity were evaluated. Myoelectric activities of the masseter muscles of 12 babies bottle fed with chewing type bottle teats were recorded and analyzed by the same method as in the previous study. The electromyographic records of 12 breast fed babies and 12 babies bottle fed with regular sucking type bottle teats, which were reported in a previous study, were used as controls. The results show that the masseter muscle activity of babies bottle fed with chewing type bottle teats was similar to that of breast fed babies qualitatively as well as quantitatively. However, activity was visibly different from that of babies bottle fed with sucking type bottle teats, showing significant difference by ANOVA. These data suggest that babies who are ingesting milk from chewing type bottle teats do so with a chewing action similar to that of breast feeding, but very different from that of bottle feeding with sucking type bottle teats. The conclusion was that the chewing type bottle teat is useful for establishing masseter muscle activity.

Bottle Feeding↗
PubMed2003

Masseter muscle thickness and maxillary dental arch width.

The purpose of the present investigation was to study the relationship between the ultrasonographic thickness of the masseter muscle and the width of the maxillary dental arch. The sample comprised 60 consecutive orthodontic patients (37 females, 23 males), 7-18 years of age with a Class I relationship and minor malocclusion. The thickness of the masseter muscle was measured ultrasonographically. Recordings were performed bilaterally with the muscles both in relaxation and under contraction. Maxillary intermolar width was measured with an electronic calliper as the distance between the palatal surfaces of the first permanent molars. Intermolar width showed no association with age and gender. However, masseter muscle thickness showed a direct, significant (P < 0.0001) association with these two factors together, i.e. the masseter muscle was thicker in older individuals and in males. In the female group, maxillary intermolar width showed a direct, significant association with masseter thickness both during contraction (P < 0.006) and relaxation (P < 0.013), i.e. females with thicker masseter muscles had a wider maxillary dental arch. In the male group, however, no significant relationship was found between maxillary intermolar width and masseter thickness. The findings of this study indicate that the functional capacity of the masticatory muscles may be considered as one of the factors influencing the width of the maxillary dental arch.

Adolescent↗
PubMed1992

Electromyographic heterogeneity in the human masseter muscle.

The complex, pennate architecture of the human masseter muscle points to a functional division into more than the commonly distinguished deep and superficial parts. In this study, the possible existence of regional differences in activation was examined. EMG activity was registered in three deep and three superficial regions with the use of bipolar fine-wire electrodes. Recordings were made during different static bite tasks, in specified directions, and with a specified bite-force magnitude. A linear bite-force/EMG relationship was observed. Furthermore, it appeared that muscle regions showed a different pattern of change in activity as a function of bite-force direction. Heterogeneity was nearly absent in anteriorly-, anteriomedially-, and medially-directed bites, but became increasingly obvious in the other bite-force directions. The posterior deep region showed the most aberrant activation pattern, which was almost opposite that from the other regions. This part was fully active in posterolaterally-directed bites. The posterior superficial region showed the largest variability in activity as a function of bite-force direction. The results point to a functional partition of the masseter muscle in at least three parts: anterior deep, posterior deep, and superficial. A further subdivision of the superficial portion might be present, but was not as obvious as the division of the deep masseter.

Adult↗
PubMed2005

Evidence for parasympathetic vasodilator fibres in the rat masseter muscle.

The present study was designed to examine (1) whether there are vasodilator fibres in the masseter muscle, and (2) if there are, to establish the neural pathways mediating these responses in urethane-anaesthetized rats. Electrical stimulation of the central cut end of the lingual nerve (LN) elicited intensity- and frequency-dependent increases of the blood flow in the masseter muscle (MBF) and lower lip (LBF). Increases in both the MBF and LBF evoked by the LN stimulation were reduced by hexamethonium in a dose-dependent manner (1-10 mg kg(-1)). Pretreatment with phentolamine or propranolol at a dose of 100 microg kg(-1) had no effect on the increases in either MBF or LBF evoked by LN stimulation. Pretreatment with atropine (100 microg kg(-1)) significantly reduced the MBF increase induced by LN stimulation, but not that in the LBF. The sectioning of the superior cervical sympathetic trunk did not affect the responses. MBF increases occurred with electrical stimulation of the trigeminal ganglion, and these increases were significantly reduced by the administration of hexamethonium and atropine. Lidocaine microinjection into the trigeminal spinal nucleus or salivatory nuclei caused a significant attenuation of the LN-induced MBF increases. When wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected into the masseter muscle, labelled neurones were abundantly observed in the otic ganglion. The present study indicates that there are parasympathetic cholinergic and noncholinergic vasodilator fibres originating from cell bodies in the otic ganglion in the rat masseter muscle. The MBF increase evoked by activation of the parasympathetic fibres occurred via the trigeminal mediated reflex, suggesting that the novel parasympathetic vasodilator response may play an important role in the regulation of the haemodynamics of jaw muscles.

Animals↗
PubMed2005

Ketamine attenuates glutamate-induced mechanical sensitization of the masseter muscle in human males.

The purpose of the present study was to determine whether glutamate-induced mechanical sensitization of the masseter muscle in human volunteers involves activation of peripheral N-methyl-D-aspartate (NMDA) receptors. Healthy male volunteers (n=18) participated in this randomized, two-session study. During each session, the volunteers received two injections into the right masseter muscle. An initial injection of glutamate (1 M, 0.2 ml) alone was followed 30 min later by a second injection of glutamate alone or glutamate combined with ketamine (10 mM). Pressure pain threshold (PPT) was assessed over the right masseter muscle at and 2 cm above the injection site, as well as over the right temporalis muscle and left masseter muscle prior to the first injection. The PPT was reassessed at all four sites every 5 min from 10 to 30 min after the second injection and once again 60 min after the second injection. Glutamate-evoked muscle pain, pain area and the sensory pain response index of the McGill pain questionnaire were all significantly reduced by co-injection of ketamine. The mean PPT values were significantly decreased by approximately 10%, 10, 15 and 25 min after injection of glutamate, but only over the site of injection. Co-injection of ketamine with glutamate also completely blocked the glutamate-induced mechanical sensitization 15 min post-injection as compared with glutamate alone. The lack of spread of mechanical sensitization outside the area of glutamate injection is consistent with the view that glutamate-induced mechanical sensitization results from a peripheral mechanism. The attenuation of glutamate-induced mechanical sensitization by ketamine suggests that this effect is mediated, in part, through activation of peripheral NMDA receptors.

Adult↗
PubMed1993

Acoustic myography in the assessment of human masseter muscle.

The feasibility of examining electro-mechanical activity of the human masseter muscles using non-invasive recording techniques was examined in six healthy dentate adults (aged 34-57 years). Electrical activity of the muscle was examined by surface electromyography (EMG) and the mechanical activity, in the form of muscle sounds, was examined by acoustic myography (AMG). Bilateral recordings of EMG and AMG were made simultaneously using composite probes which were placed on the skin over the masseter muscles. A standardized pressure was applied to the probes via adjustable rods attached to a safety helmet. Pressures were monitored by strain gauges placed between the ends of the rods and the probes. With the subject seated, recordings of AMG and EMG were obtained during maximal jaw clenching for 4 s and the raw signals were stored on a computer. Of three maximal contractions performed, the last two were used in the analysis. The raw amplified signals underwent frequency analysis by fast Fourier Transform. Total activity was also assessed after amplification, full-wave rectification and integration, and repeatability of the results was assessed. The AMG frequency range was 6-15 Hz and was similar to values for other human skeletal muscles. The integrated values for EMG and AMG were repeatable on both sides of the face (IEMG, right r = 0.99, left r = 0.99; IAMG right r = 0.70, left r = 0.71). Simultaneous recordings of AMG and EMG from the masseter muscles may be useful for assessing electro-mechanical muscle function but further validation studies are required before the technique can be used clinically.

Acoustics↗
PubMed1988

Skin surface temperature over the masseter muscle in individuals with rheumatoid arthritis.

Temperature measurements were made on the skin surface over the masseter muscle in 71 individuals with rheumatoid arthritis (RA group) and in 52 individuals without general joint disease or symptoms (C group). The temperature recordings were performed with thermistors in contact with the skin. Symptoms in the stomatognathic system and general joint symptoms were investigated by means of a questionnaire. A clinical examination was made of the stomatognathic system. In addition, a medical examination including clinical articular indices and laboratory tests was made. The skin surface temperature over the masseter muscle was generally decreased for the individuals with RA compared with the C group but increased with duration of temporomandibular joint (TMJ) symptoms, approaching normal values. Duration was also the most important variable among those investigated in determining the skin surface temperature over the masseter muscle. Hypothermia in the RA group was correlated with craniomandibular disorders such as lateral deviation of the mandible on mouth opening and TMJ clicking, whereas individuals with a history of swelling in the TMJ region had a higher temperature than average in this group. The results of this study show that there is a correlation between craniomandibular disorders and decreased skin surface temperature over the masseter muscle in individuals with RA.

Arthritis, Rheumatoid↗