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PubMed2000

Mandibular deformities in parathyroid hormone-related protein (PTHrP) deficient mice: possible involvement of masseter muscle.

Previous studies using parathyroid hormone-related protein (PTHrP) null mutant mice have indicated severe abnormalities in the endochondral ossification, suggesting that PTHrP affects chondrocyte differentiation. In this study, we found in newborn PTHrP-deficient mice some deformities in the mandible that is formed via intramembranous ossification. The mandibular ramus was bent downwards and a prominent bone crest to which the deep layer of masseter muscle was tendinously attached was observed in the mandibular body. Transmission electron microscopic studies showed that active bone formation was progressing along the tendon fibers of the masseter muscle. The examination of 3-D reconstruction models indicated that the mandibular ramus was bent at the site of muscle attachment, which was shifted in the direction of the muscle fibers. Muscle fiber type analysis using myosin ATPase staining showed that the masseter muscle in the newborn PTHrP-deficient mice contained numerous type 2B fibers, demonstrating premature maturation of this muscle. Based on these findings, we speculated that premature maturation of the masseter muscle leads, probably due to increased tensile forces, to accelerated bone crest formation and subsequent bending of the mandibular ramus. These results further suggest that PTHrP is involved in the regulation of muscle development in normal animals.

Animals↗
PubMed2003

The effects of masseter muscle pain on biting performance.

The present study applied a standardized test food of known hardness to evaluate the biting performance of 20 female patients who had pain mainly in the masseter muscle during palpation. Another 20 women of a similar age group who were pain-free during examination served as controls. Electromyograms (EMG) of the masseter and sternocleidomastoid (SCM) muscles and the jaw position were recorded and measured when the subjects were biting through two types of test foods with known hardness (hard type, 20 kg hardness and extra-hard type, 60 kg hardness). Pressure-pain-threshold (PPT) values of both the patients and the normal subjects were obtained with an algometer. It was found that the PPT of the patients with pain was significantly lower and that the extra-hard food took more masseter muscle activity and more working side jaw movement in both the pain and the normal groups. During both hard and extra-hard food biting, a significantly longer duration of masseter muscle activity was found in pain patients while the total muscle activity was not significantly stronger. Strong correlation existed between SCM and masseter muscle activity during both hard and extra-hard food biting in the patient group, while such correlation was very weak in the normal group. In conclusion, painful masseter muscles required longer masseter and SCM muscle contraction time for breaking through a hard food of 20 kg and more, and co-activation of SCM and masseter muscles existed and was more evident when the food was harder or the pain was more severe.

Adult↗
PubMed1980

Nocturnal masseter muscle activity and urinary catecholamine levels in bruxers.

Nocturnal electromyographic recordings of masseter muscle activity were performed on 20 bruxist and ten control subjects. Each subject collected two 24-hour urine samples. An analysis of urinary catecholamine content was performed. A positive relationship was found between increased epinephrine content and high levels of nocturnal masseter muscle activity.

Bruxism↗
PubMed1978

Electron-microscopic study of the rat masseter muscle following injection of lidocaine.

The ultrastructure of rat masseter muscle was examined at 15 min, 1 and 6 h, and 1 and 2 days following a single injection of 2% lidocaine. Lesions developed within 15 min. The plasma membrane was disrupted and invaginated. The nuclei were pyknotic and the mitochondria appeared swollen. The myofibrils separated and became disoriented. By 1 and 6 h, these changes were severe. By 1 day, the macrophages appeared in damaged myofibers. The presence of a few presumptive myoblasts signaled the onset of regeneration. By 2 days, presumptive myoblasts formed within the basement membrane. The basal lamina proved most resistant to injury. Regeneration of masseter muscle following the damage produced by lidocaine appeared discontinuous in nature. The singly nucleated presumptive myoblasts seemed to arise within the lesions.

Animals↗
PubMed1979

Utilization of cadaver tissue for a scanning electron microscopic study of the insertion of the masseter muscle.

1. The generally poor state of soft tissue preservation that complicates observations by gross dissection and light microscopy posed no hindrance to scanning electron microscopic (SEM) studies of bone surface, because all of the soft tissues are chemically removed. 2. Specific and identifiable surface patterns representing locations of periosteal and fibrous muscle attachments can be readily observed in cadaver tissue with the SEM. 3. SEM observations confirmed earlier literature concerning the extent of periosteal and intraosseus attachments of the masseter muscle. The masseter muscle attached periosteally on the lateral surface of the mandible and along the inferior border. Just above the inferior border, a ridge of intraosseous attachment was noted. 4. The correlation of SEM observation of bone surface patterns with gross dissection and light microscopy demonstrated that SEM can be useful in determining the extent of muscle attachments in cadaver tissue even when the soft tissue is poorly preserved.

Adult↗
PubMed1975

Load compensation in human masseter muscles.

The reaction of masseter and digastric muscles to changes in the load applied during a movement was studied in human volunteers. 2. Rapid stretch of the jaw-closing muscles evoked a monosynaptic jaw-jerk reflex response in the masseteric electromyogram (e.m.g.) after a delay of 6-8 msec. 3. The averaged integrated e.m.g. activity of the masseter was increased 5-10 msec after the rapid application of a load during voluntary closure of the jaw. It was not necessary to stretch the muscle to obtain this apparently monosynaptic response. 4. A compensatory increase in the velocity of movement followed 6-10 msec after the e.m.g. peak. 5. Unloading the masseter muscle during contraction was followed by a fall in e.m.g. activity, beginning after a latency of 6-5--1 msec, with later phases of depression beginning 27--36 and 60--70 msec after unloading. 6. The spindle-poor digastric muscles were not monosynaptically activated during loading, but responded with a latency of 24--34 msec. 7. It is concluded that monosynaptic e.m.g. changes are a prominent feature of the load compensation system in human jaw-closing muscles but not in jaw-opening muscles.

Adolescent↗
PubMed1995

A comparison of the effects of group function and canine guidance interocclusal device on masseter muscle electromyographic activity in normal subjects.

This study investigated the role of occlusal balance and canine guidance or group function guidance in masseter muscle function of normal subjects. Two types of interocclusal devices were constructed for each of the 10 subjects. Two ME 1020 EMG analyzers and bipolar Ag/AgCI electrodes were used to record the electromyographic activity of masseter muscles during clenching in centric occlusion, during left and right laterotrusive movements, and during clenching at the extremities of the jaw both with the natural dentition only and with adjusted and intentionally unadjusted interocclusal devices in place. This study demonstrated that there was no difference in masseter muscle electromyographic activity between the use of interocclusal devices designed for canine guidance or for group function guidance in normal subjects. It showed also that altering the occlusal balance significantly reduced muscle activity.

Adult↗
PubMed1993

Masseter muscle responses to forelimb nerve stimulation in the guinea pig.

These experiments were aimed at clarifying the spinal-masseteric connections by observing the effect of electrical stimulation of radial nerve afferents on masseter muscle activity. Spontaneous and evoked electrical responses of the masseter motor units were recorded with tungsten microelectrodes and analysed on computer. Results show that electrical stimulation of both radial nerves induces responses in single masseter motor units. The response patterns were characterized most frequently by excitation, but also by a sequence of excitation inhibition. The latency in masseter muscle responses were found to be lower in contralateral recordings: 18.26 +/- 4.04 msec (x +/- SD) compared with a latency of 24.3 +/- 5.25 msec in ipsilateral recordings. These observations confirm the hypothesis that somatosensory impulses eminating from the radial nerve participate in the postural control of the masseter muscle aimed at maintaining correct jaw position during forelimb displacement.

Animals↗
PubMed2003

Activation of peripheral NMDA receptors contributes to human pain and rat afferent discharges evoked by injection of glutamate into the masseter muscle.

Peripheral N-methyl-d-aspartate (NMDA) receptors are found in deep tissues and may play a role in deep tissue pain. Injection of the endogenous NMDA receptor agonist glutamate into the masseter muscle excites deep craniofacial afferent fibers in rats and evokes pain in human subjects. It is not clear whether peripheral NMDA receptors play a role in these effects of glutamate. Accordingly, the effect of NMDA on afferent activity as well as the effect of locally administered NMDA receptor antagonists on glutamate-evoked afferent discharges in acutely anesthetized rats and muscle pain in human subjects was examined. Injection of NMDA into the masseter muscle evoked afferent discharges in a concentration-related manner. It was found that the NMDA receptor antagonists 2-amino-5-phosphonvalerate (APV, 10 mM), ketamine (10 mM), and dextromethorphan (40 mM) significantly decreased glutamate-evoked afferent discharges. The effects of APV and ketamine, but not dextromethorphan, were selective for glutamate-evoked afferent discharges and did not affect hypertonic saline-evoked afferent discharges. In human experiments, it was found that 10 mM ketamine decreased glutamate-evoked muscle pain but had no effect on hypertonic saline-evoked muscle pain. These results indicate that injection of glutamate into the masseter muscle evokes afferent discharges in rats and muscle pain in humans in part through activation of peripheral NMDA receptors. It is conceivable that activation of peripheral NMDA receptors may contribute to masticatory muscle pain and that peripherally acting NMDA receptor antagonists could prove to be effective analgesics for this type of pain.

Adult↗
PubMed2006

Effects of local serotonin administration on pain and microcirculation in the human masseter muscle.

AIMS: To investigate whether exogenously administered 5-hydroxytryptamine (5-HT) at high or low concentration influences pain and microcirculation in the human masseter muscle. METHODS: In 12 healthy female subjects, 5-HT in 2 concentrations (0.1 micromol/L and 1,000 micromol/L) and isotonic saline were injected into the masseter muscles in a randomized and balanced double-blind manner. The pain intensity after injections was recorded with Borg's rating scale, and intramuscular blood flow was monitored continuously during the experiment with a laser-Doppler technique. Nonparametric statistics were used for analyses. RESULTS: Administration of 5-HT at 1,000 micromol/L induced significantly more pain than saline (Wilcoxon: P < .05), while there was no difference between 5-HT at 0.1 micromol/L and saline. The blood flow did not change significantly after injection of 5-HT at either concentration compared to saline. However, changes in pain intensity and blood flow were positively correlated after injection of 5-HT at 1,000 micromol/L (Spearman: P < .05). CONCLUSION: Intramuscular administration of 5-HT at 1,000 micromol/L into the human masseter muscle induced pain, but 5-HT did not have any effect on local blood flow at either concentration.

Analysis of Variance↗
PubMed1991

On the laminar formation of the masseter muscle in the lion (Panthera leo).

An investigation was made of the laminar structure of the masseter muscle in 3 lions (Panthera leo s. Felis leo), and the findings obtained were evaluated in comparison with those in some other carnivora. Although the general aspect of the masseter of the lion resembled that of the cat, there was no close similarity or analogy between them. The construction of the masseter in the lion was as follows. The superficial layer consisted of primary and secondary sublayers, the intermediate layer was composed of anterior and posterior portions, and the deep layer also had anterior and posterior portions. Among these three layers (the masseter proper muscle), the superficial layer was extremely well developed as a characteristic feature of this species. The maxillomandibularis muscle was developed in a muscular element of its origin and had its tendinous insertion on the anteroinferior margin of the masseteric fossa. The zygomaticomandibularis muscle was also fairly well developed in the form of two muscular bundles which originated from the temporal crest, a shelf forming a lateral protrusion on the basis of the zygomatic process, and its posterior surface. Both muscles were also well developed as the masseter improper. Such a huge and complicated laminar pattern of the masseter muscle in the lion should be sufficient to exert a strong force as a predatory animal.

Animals↗
PubMed2000

Masseter muscle activity during vestibular stimulation in man.

Experimental data report that vestibular afferents affect trigeminal system activity. The aim of this work was to evaluate whether static vestibular stimulation affects the excitability of trigeminal motoneurons in man. In order to assess this, voluntary EMG activity of masseter muscles as well as duration and latency of the early and late components of EMG exteroceptive silent period were evaluated while keeping the subject in vertical position and during 20 degrees static tilt. The experiments were performed on ten adult subjects with no orofacial, neurologic and otologic disorders. Each subject sat on a chair, which kept the complex head-jaw-neck-trunk and the limbs securely fixed, in order to minimize any interference due to the activation of somatosensory and proprioceptive afferents from these districts. The subjects were instructed to contract masseter muscles at 25% of their maximum bite force and the isometric force monitoring was used as visual feedback. Exteroceptive silent period (ESP) of masseter EMG was elicited by electrically stimulating the inferior inter-incisal gum. Results showed that static vestibular stimulation induced asymmetrical responses on voluntary masseter muscle activity, which was reduced to 70.3 +/- 16.1% (mean +/- S.D.) of the control value during ipsilateral tilt and increased to 128.8 +/- 13.0% during contralateral tilt. The duration of the early (ESP1) and late (ESP2) silent periods was also affected: during ipsilateral tilt ESP1 and ESP2 duration increased to 130.0 +/- 3.5% and to 122.1 +/- 2.1% of control, respectively; during contralateral tilt it was reduced to 76.8 +/- 1.2% and to 83.0 +/- 1.7% of control, respectively. On the contrary, changes in latencies were not significant. These data evidenced an asymmetrical effect exerted on trigeminal motor activity by static tilt. Since the influence of all receptors which could be activated by static tilt, except that arising from the macular ones, was minimized in this study, it is likely that the observed effects, induced by static tilt on masseter muscle activity, were of macular origin.

Adult↗
PubMed1992

Immuno-electron-microscopic localization of basic fibroblast growth factor in the dystrophic mdx mouse masseter muscle.

The localization of basic fibroblast growth factor (bFGF)-like immunoreactivity in the masseter muscle of dystrophic mdx mice on postnatal day 28 was investigated by immunoblot analysis and electron microscopy. Crude homogenate of the masseter muscle, when subjected to immunoblotting with a bFGF antiserum, exhibited a main band with the same molecular weight (18 kDa) as bovine bFGF. By electron microscopy, bFGF immunoreactivity was detected in small regenerating myocytes; the smaller cells were the premature myocytes, the most intense staining was the immunoreactivity within the cytoplasm. Putative precursors of the muscle cells with a few myofilaments, which were most intensely labeled with anti-bFGF, contacted each other and possibly developed into multinucleated myocytes through cell fusion. Mature myocytes with densely packed myofilaments and peripherally located nuclei did not exhibit bFGF immunoreactivity; they formed myoneural junctions with motor nerve endings immunoreactive for bFGF. Early differentiating myocytes with intense bFGF-like immunoreactivity did not make contact with immunoreactive nerve terminals. Degenerating large myocytes with a limited number of distorted and/or disrupted myofilaments exhibited electron-dense deposits in the cristae of mitochondria; these deposits were not abolished by immunoadsorption control experiments. Thus, the cell-size-dependent decrease in bFGF immunoreactivity in regenerating but not in degenerating myocytes provides a morphological basis for an autoregulatory role of bFGF in muscle regeneration. This study suggests that neuronal bFGF is not involved in initial muscle regeneration in the dystrophic mdx mouse.

Animals↗
PubMed1996

In vitro human masseter muscle hypersensitivity: a possible explanation for increase in masseter tone.

To determine whether a difference in fiber-type caffeine and Ca2+ sensitivities exists between human masseter and vastus lateralis skeletal muscle, we compared the fiber-type caffeine sensitivities in chemically skinned muscle fibers from 13 masseter and 18 vastus lateralis muscles. Caffeine sensitivity was defined as the threshold concentration inducing > 10% of the maximal tension obtained after the fiber was loaded with a 1.6 x 10(-2) mM Ca2+ solution for 30 s. Significant difference in the mean caffeine sensitivity was found between type I masseter fibers [2.57 +/- 1.32 (SD) mM] vs. type I (6.02 +/- 1.74 mM) and type II vastus lateralis fibers (11.25 +/- 3.13 mM). Maximal Ca(2+)-activated force per cross-sectional area was significantly different between masseter and vastus lateralis fibers. However, the Ca2+ concentration corresponding to half-maximal tension (pCa50) was not significantly different between type I masseter (pCa50 5.9 +/- 0.02) and type I vastus lateralis muscle (pCa50 6.01 +/- 0.08). These results suggest that the increase in caffeine sensitivity of masseter muscle reflects the presence of a low reactivity threshold of the sarcoplasmic reticulum.

Caffeine↗
PubMed1999

Mandibular movements in response to electrical stimulation of superficial and deep parts of the human masseter muscle at different jaw positions.

Anatomical and electromyographical evidence suggests a compartmentalized function of the human jaw-closing muscles during both static and dynamic motor tasks. However, the voluntary nature of these tasks hampers unequivocal interpretation of this evidence, because it is impossible to activate voluntarily a single part of a muscle exclusively. Activation of discrete, localized regions can be accomplished with electrical stimulation. A previous study confirmed a functional subdivision of the temporalis muscle into at least three parts. Here, differences in the direction of the lower incisal-point (IP) movement in response to electrical stimulation of four different parts of the masseter muscle were examined in five healthy men. The deep masseter muscle and the anterior, middle, and posterior parts of the superficial masseter muscle were stimulated with monopolar wire electrodes in four different jaw positions (resting position; 50% maximum mouth opening; and 10-mm right and left lateral excursions, both with respect to resting position). Electrode-insertion depth was measured from magnetic resonance images. Movement responses to stimulation were recorded with the OKAS-3D jaw-movement analysis system. The variation in the direction of the IP movement in response to stimulation of parts of the masseter was partly explained by the effects of stimulus location and jaw position. The response to stimulation of the deep masseter was mainly laterovertically directed, whereas the response to stimulation of each of the superficial parts had a mainly anterovertical direction, the responses being most pronounced with the mandible in its resting position. These results provide further evidence for a functional subdivision of the masseter into a superficial part and a deep part, but not for a further subdivision of the superficial part into an anterior, middle, and posterior part.

Adult↗
PubMedDate not supplied

[Experimental validation of the use of the masseter muscle in treating paralysis of the mimetic muscles].

In 15 rabbits the distal end of masseter muscle was surgically transposed to perioral region. After 90, 120 and 180 days the altered muscle position and reduction of functional load did not cause its degeneration. Rather, these conditions elicited a functional reordering evidenced by reduction of muscular fibers' cross-sectional area, the number of surrounding capillaries, decrease in the succinate dehydrogenase activity and increase in connective tissue layer.

Animals↗
PubMed2005

Quantification of oxidative metabolism in masseter muscle of denture wearers.

This study aimed to quantify oxidative metabolism in masseter muscle using near-infrared spectroscopy, in particular for denture wearers. Fourteen normal dentate subjects without malocclusion (ND group, 25-50 years) participated in the quantification of oxidative metabolism. Eleven partially edentulous patients without occlusal stops (PD group, 64-80 years) and ten edentulous patients (CD group, 57-84 years) also participated after prosthodontic treatment. Oxidative metabolism was recorded during gum chewing, maximum clenching and regulated clenching at 5 kgf. The oxygenated hemoglobin at 5 kgf clenching level was normalized to the oxygenated hemoglobin at the lowest blood flow and expressed as oxygen consumption rate (OCR). The relationship of the OCR to the maximum clenching force was analyzed using Pearson's correlation coefficient, and differences between the PD and CD groups were tested by unpaired Student's t-test. The OCR showed a significant negative correlation with maximum clenching force in the ND group. The OCR of the PD group was significantly greater than that of the CD group, although the difference in maximum clenching force was not significant between both groups. These results suggest that the aerobic ability of masseter muscle in complete denture wearers is relatively greater than in partial denture wearers with same age level.

Aged↗
PubMed1990

The estimation of motor unit twitch tensions in the human masseter muscle by spike-triggered averaging.

Spike-triggered averaging (STA) has been used to extract twitch profiles of single motor units (SMU) within the human masseter muscle. However, the reported twitch tensions may have been biased by the voluntary firing frequency of the SMUs, the complex architecture of the muscle, and by the biomechanical linkage of the jaw. In this study, a rigid STA paradigm was used to record spike-triggered "measured tensions" (STMTs) for 32 SMUs in the masseter muscles of four subjects. STMTs were recorded at two different orientations of a force transducer placed between the incisor teeth. The STMTs produced by each unit were used to calculate jaw torque. STMTs were also recorded in 11 units with differing degrees of muscle coactivation. STMTs for each unit varied according to the orientation of the force transducer. However, no systematic changes in STMTs occurred with reciprocal changes in the jaw moment arm. STMTs could be altered significantly by different degrees of muscle co-activation. The use of STA as a method for determining SMU tension in the human masseter muscle appears to be highly task-dependent and in the presence of co-activation may be inappropriate.

Action Potentials↗