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PubMed1990

[Magnetic stimulation of the scalp: the responses of the masseter muscles. II].

Magnetic stimulation of nervous elements is a new technique of investigation in intact alert man. Electromyographic responses of the masseter muscles were recorded in normal volunteers and in two patients with hemispheric or capsular lesions, both by surface and needle electrodes. In the patients "cortical" responses were absent when stimulating the affected hemisphere and present when stimulating the unaffected one. These findings suggest that the direct facilitating cortico-nuclear projections for the masseter muscle are mainly crossed. Nevertheless the presence of clear ipsilateral responses after stimulation of the unaffected hemisphere demonstrates the existence of uncrossed projections; in fact only the ipsilateral motor cortex can be the site of origin of the responses in these patients.

Adult↗
PubMed1986

Length-related changes in activation threshold and wave form of motor units in human masseter muscle.

Action potentials of single motor units in the human masseter muscle were recorded during isometric contractions at different muscle lengths over most of the working range of the temporomandibular articulation. The shape of the unit potential wave form changed progressively as the muscle length changed. This was considered to be due principally to a change in the geometric relationship of the electrode to the muscle fibres at various muscle lengths. The recruitment thresholds for all units increased sharply when the jaw opening approached the maximal gape. The pattern of change of unit thresholds with muscle length closely paralleled the curve of length versus passive tension for the combined jaw-closing muscles, temporomandibular articulation and associated soft tissues. It was concluded that the change in unit recruitment thresholds with length is likely to be a consequence of the length-passive tension relationship of the muscles, joint and soft tissues. The results of this study indicate that attempts to assign masseter motor units to one or other of the accepted functional categories on the basis of recruitment force or wave form only are unlikely to give reliable results.

Action Potentials↗
PubMed1985

Characterization of the effect of pH on the excitation-contraction coupling system of canine masseter muscle.

The effect of pH on the excitation-contraction coupling system of canine masseter muscle was studied by evaluating the functional integrity of the sarcoplasmic reticulum (SR) and myofibrils. Increasing proton concentration (pH 7.0-5.8) significantly reduced oxalate supported SR calcium uptake velocity, while Ca2+-stimulated, Mg2+-dependent ATPase activity was unaffected by pH. The efficiency ratio of calcium transport, or the coupling ratio (mumoles Ca2+ transported/mumoles ATP hydrolyzed), decreased from 1.094 +/- 0.042 at pH 7.0 to 0.946 +/- 0.036 at pH 6.0 (P less than 0.05) and to 0.780 +/- 0.024 at pH 5.8 (P less than 0.01). Myofibrillar pCa (-log [free Ca2+] )-ATPase activity was unaffected between pH 7.0 and pH 6.5. At pH 6.0, increasing Ca2+ concentration inhibited myofibrillar ATPase activity, and this inhibitory phenomenon was accentuated at pH 5.8. Kinetic analysis of the myofibrillar pCa-ATPase data, utilizing double-reciprocal plots, demonstrated an increase in Km at low pH. It is concluded that acidosis significantly uncouples calcium transport from ATP hydrolysis in the SR of masseter muscle and significantly alters myofibrillar ATPase activity. It is hypothesized that these defects may explain an observed depression in skeletal muscle cell function during ischemia.

Adenosine Triphosphatases↗
PubMed1990

[Properties of N-acetyl-beta-glucosaminidase in the masseter muscle of the mouse].

The present study was undertaken to clarify various characteristics of N-acetyl-beta-glucosaminidase (NAG) in the masseter muscle of the mouse. The pH optimum of NAG in tissue homogenate was 4.3, and Km value for p-nitrophenyl-N-acetyl-beta-glucosaminide was 0.98 mM. By means of isoelectric focusing. NAG in the masseter muscle was separated into four enzymes (NAG I-IV). The isoelectric points were: I, 4.9; II, 6.4; III, 7.2 and IV, 8.7. The pH optima of NAG I-IV ranged from 4.2 to 4.4, and Km values from 0.61 to 0.83 mM. The molecular weights of I, II, III and IV were 135,000, 120,000, 135,000 and 110,000, respectively. Studies on heat stability showed that NAG I and IV were slightly labile as compared with II and III. The activities of NAG I-IV were dose dependently inhibited by both N-acetyl-glucosamine and N-acetyl-galactosamine. Both Ag and Hg2+ ions caused a marked inhibition on activities of NAG I-IV.

Acetylglucosaminidase↗
PubMed1995

Ultrasound imaging of the masseter muscle in myotonic dystrophy patients.

Ultrasound technique was applied to measure the thickness and examine the internal structure of the masseter muscle in a group of 16 adult patients (nine women and seven men) with myotonic dystrophy (MyD) and 16 healthy individuals matched in age, sex and number of occluding teeth. The masseter thickness was measured bilaterally under both relaxed conditions and during maximal clenching. The error of measurement was found to be small, not exceeding 0.45 mm. The imaging characteristics of the masseter in most of the MyD patients was an obvious atrophy of the muscle with increased echointensity of the intramuscular tissue and loss of the internal structure concerning tendons and fasciae. The mean masseter thickness (+/- SD) in the MyD group was 10.4 (+/- 2.2) mm under relaxed conditions and 11.1 (+/- 2.4) mm during maximal clenching, compared with 13.3 (+/- 2.2) mm and 14.1 (+/- 2.4) mm, respectively, in the healthy group (P < 0.001). In conclusion, our results indicate that, in most of the myotonic dystrophy patients, the masseter muscle is atrophic with obvious signs of degeneration. Ultrasound is a useful method for both qualitative and quantitative evaluation of the condition of the masseter muscle.

Adult↗
PubMed1992

[Mapping spectral EMG parameters of the masseter muscle in normal probands under defined loads].

In 20 healthy volunteers 16-channel surface electromyograms were monopolarly recorded from the right and left masseter muscle during defined functional conditions (force-constant bite, right and left hand side, 196, 333, 530 N chewing force, during a compensation of a load directed from a frontal or a lateral position to the mandible (20, 49, 69 N), during rest position of the mandible). The EMG curves were quantified by the calculation of spectral EMG-parameters (FFT). With respect to the muscle a topographic oriented representation of the myoelectrical activity was realised by a mapping of spectral EMG-parameters. The monopolarly recorded EMG activities significantly differed between the 16 electrode positions (topographic EMG distribution pattern). During the several examination conditions of the masseter muscle different topographic distribution pattern of the myoelectrical activity were found. Moreover, the total EMG power between the muscle functions differed too. The level of the load influenced the spectral EMG power as well as, partly, the topographic EMG distribution pattern.

Adult↗
PubMed2006

Electromyographical analysis of the masseter muscle in dentulous and partially toothless patients with temporomandibular joint disorders.

This study had as objective to analyze with computerized electromyography the masseter muscles bilaterally in twenty individuals with temporomandibular joint dysfunction (DTM), being: the group I consisting of ten individuals with complete dentition and group II constituted by ten individuals with posterior dental absences; comparing nine clinical activities: rest before and after exercises, maximum habitual intercuspation (MIH), right and left laterality, forced centric occlusion, protrusion, bilateral molar bite and chewing. It was utilized the Electromyography K6I-Myotronics, with eight canals and surface silver electrodes. By means of the results analysis, we verified significant differences between the groups (p < 0.01), being group I, presented greater electromyographic activity (32.98 microvolts), than the group II (22.31 microvolts), suggesting that this individuals presented low muscular activity. The interaction between the groups and the clinical activities was significant (p < 0.01), as well as between the groups, clinical activities and muscles (p < 0.05). To clarify which amongst the relative averages the clinical activities were different, calculated the critical value of Tukey, being that rest before and after exercises, MIH, right and left laterality, presented averages with similar distributions, with values below of the values of Tukey, as well as forced centric occlusion, protrusion, bilateral molar bite and chewing presented higher values than Tukey. Based on this research's data, we concluded that the electromyographical analysis of the masseter muscles in Individuals with TMD, dentulous and with dental absence showed that individuals with TMD, dentulous or not, presented elevated muscular activity in rest position and individuals with TMD, dentulous, presented higher electromyographical activity than the individuals with TMD and lacking posterior teeth.

Adult↗
PubMed2003

[Expression of collagens in reattached masseter muscles to mandibles following a surgical detachment].

OBJECTIVE: The aim of this study was to investigate the expression of collagen in the process of masseter muscle reattachment to the cortical and cancellous bones of mandible. METHODS: A total of nine adult goats were used in the study. One was the control. The other eight were treated with bilateral detachment of the masseter muscles. The biopsies of bone and muscle were taken at 2, 4, 8 and 12 weeks after the operation. The characteristics of the healing muscle-bone interfaces were examined using immunohistochemical techniques. RESULTS: Immunohistochemical analysis illustrated that the locations of collagen type I, II and III were different during the healing process, but similar in the cortical and cancellous bones. CONCLUSION: This study demonstrates that the distribution of the three types of collagens at the muscle-bone interfaces is associated with time, but not related with their locations.

Animals↗
PubMed1980

Activity of the temporal and masseter muscles in class II, division 1 malocclusions. An electromyographic investigation.

A quantitative analysis of EMG activity in the masticatory muscles was performed in homogeneous groups of boys with Class II, Division 1 malocclusion and normal occlusion. Integrated EMG recordings from the temporal and masseter muscles were analyzed during maximal biting in the intercuspal position (centric occlusion) and during the chewing of peanuts. The results of the investigation revealed the following: (1) During maximal biting in intercuspal position the boys with Class II malocclusion exhibited less EMG activity in the masseter and temporal muscles than the boys with normal occlusion. In the Class II boys the reduction in EMG activity was most apparent for the masseter muscle. (2) During chewing the Class II subjects showed less EMG activity in the masseter muscle than the normal occlusion subjects. For the temporal muscle, no differences were found between the two occlusion groups. (3) High positive correlations were found between the EMG activity during maximal biting and chewing for both muscles in the two occlusion groups. The impaired muscle activity found in the Class II cases may be attributed to a diverging dentofacial morphology and unstable occlusal contact conditions.

Child↗
PubMed1995

Alterations to mandibular form following motor denervation of the masseter muscle. An experimental study in the rat.

The form of the mandible in young rats was studied following denervation of the masseter muscle. Wistar-derived male rats aged 27 d were randomly assigned either to experimental, sham-operated or control groups. The main trunk and initial branches of the masseteric nerve were resected on one side in each experimental animal. The nerve was exposed, but not resected, in the sham-operated group. No surgery was performed on the control group. All animals were killed at 69 d of age, examined and standardised contact radiographs taken of the cleaned and dried mandibles. Radiographic measurements describing the form and size of the mandibles were compared. On the operated side in the experimental animals the masseter muscle was smaller and the temporalis muscle larger as compared with the unoperated side. The angular process and the subcondylar incisure were also smaller and the coronoid process was positioned more rostrally. The most striking feature was that relatively little change accompanied denervation of masseter in the growing experimental rats. It is concluded that some of the changes previously attributed to muscle denervation may be due to other causes.

Analysis of Variance↗
PubMed2001

Brief exposure to testosterone is sufficient to induce sex differences in the rabbit masseter muscle.

In order to evaluate whether testosterone is responsible for inducing long-lasting sex differences in myosin heavy chain gene expression in the rabbit masseter muscle, we castrated young adult animals and administered supra-physiologic doses of testosterone for 3 or 6 weeks duration. Biopsies were taken from the masseter muscles of these animals at the time of castration and 3, 6 and 9 weeks later. Both immunoblot and immunohistochemical analyses of the myosin heavy chain (MyHC) isoform content of these muscle samples were performed. Exposure to testosterone for either duration resulted in a dramatic decrease in the content of the cardiac alpha MyHC isoform and a comparable increase in the content of the IIa MyHC isoform. The decrease in the cardiac alpha MyHC isoform content persisted for as long as 6 weeks after the end of treatment, but the increase in the content of the IIa MyHC isoform declined to normal during this time. Significant numbers of fibers were found containing both the cardiac alpha (and slow) and the IIa isoforms. Several fibers were encountered that contained both IIa and, presumably, the IIx isoform. Thus, a brief exposure to testosterone during postnatal maturation is able to produce a long lasting myosin heavy chain isoform switch that is similar in magnitude to that found during normal development.

Animals↗
PubMed2001

Sex differences in the hemoglobin oxygenation state of the resting healthy human masseter muscle.

AIMS: To determine whether sex differences exist in tissue oxygen saturation (StO2) and the hemoglobin (Hb) oxygenation state of the resting human masseter muscle. METHODS: Near-infrared spectroscopy (NIRS) was used to measure StO2 and Hb oxygenation state in 20 healthy adult volunteers (10 women and 10 men). To determine the measurement range and reliability of the NIRS recording probe, the probe was set up on 12 layers of white acrylic resin plate, each 3 mm thick. Total hemoglobin levels were measured while a red vinyl resin plate, 1 mm thick, was inserted in turn between each of the 12 layers. Distances from the skin surface to the lateral surface (S-L) and to the medial surface (S-M) of the right masseter at the middle portion of the masseter were measured on T1-weighted magnetic resonance images (repetition time 500 ms, echo time 23 ms). Thickness of the masseter was calculated by subtraction [(S-M)--(S-L)]. For the study of Hb oxygenation state, the probe was positioned at the same position on the skin surface at the mandibular postural (rest) position. RESULTS: The measurement range of the NIRS probe was from 9 to 21 mm under the skin, and the reliability of the probe was judged by intra- and inter-class correlation coefficients. There was no sex difference in S-L and the thickness of the masseter; the means of S-L and masseter thickness were 9.3 mm and 15.5 mm in men and 9.8 mm and 14.3 mm in women, respectively. Except for StO2 values, there were significant sex differences in the Hb oxygenation parameters, with the mean values in the men being approximately twice those in the women. CONCLUSION: These results provide evidence that a sex difference in the Hb oxygenation state may exist in the masseter muscle of normal healthy subjects.

Adult↗
PubMed1995

Hydroxyl radical-mediated reduction of Ca(2+)-ATPase activity of masseter muscle sarcoplasmic reticulum.

To understand the effect of oxygen free radicals on Ca(2+)-ATPase, we used sarcoplasmic reticulum (SR) microsomes of canine masseter muscle as a model system in which to explore the effects of oxidation on a biological membrane, and we investigated the effect of hydroxyl radicals (.OH) generated from Fenton's reagent (H2O2/FeSO4). H2O2 (10 mM) alone had no effect on Ca(2+)-ATPase activity; in the presence of FeSO4 (0.2 mM), H2O2 inhibited the enzyme activity. Oxygen free radical species generated from H2O2/FeSO4 under the conditions employed in the Ca(2+)-ATPase assay were verified by highly sensitive electron spin resonance spectroscopy and the spin-trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in the absence of SR vesicles; the 1:2:2:1 quartet (AN = A beta H = 1.49 mT), characteristic of the DMPO-OH spin adduct, was observed. The Ca(2+)-ATPase activity was inversely correlated with the calculated signal intensity of DMPO-OH, which is indicative of the amount of .OH radical generated. The effect of Fenton's reagent was effectively inhibited by catalase, dimethylsulfoxide, and dimethylthiourea; the effect was also inhibited by sulfhydryl (SH) group reducing agents, cysteine and dithiothreitol. The SH group modifying agents, p-chloromercuric benzoate and 5,5'-dithiobis(2-nitrobenzoic acid) depressed Ca(2+)-ATPase activity; the effects of the SH group modifying agents used were potentiated in the presence of Fenton's reagent. It is suggested that .OH radical-induced oxidant injury may be caused primarily by modification of the key SH group(s) on the ATPase molecule of masseter muscle SR vesicles.

Adenosine Triphosphatases↗
PubMed1999

Adaptation of guinea-pig superficial masseter muscle to an increase in occlusal vertical dimension.

To study the effect of increased occlusal vertical dimension on the fibre phenotypes of the superficial masseter muscle, the composition of myosin heavy-chains (MHC), myosin light-chains (MLC) and tropomyosin was investigated by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and two-dimensional gel electrophoresis in conjunction with densitometric analysis in normal (control) and bite-opened (5.7 mm increase in the vertical dimension for 1 week) guinea-pigs. The superficial masseter contained two fast-type MHC isoforms, II-1 and II-2, in both the bite-opened and control groups; their relative content (mean+/-SD, n = 7) was 47.8+/-2.9% and 52.2+/-2.9%, in the bite-opened and 44.4+/-3.0% and 55.6+3.0% in control preparations, indicating no significant (p>0.05) changes in MHC composition in association with the bite opening. On the other hand, significant differences in MLC and tropomyosin composition were found between the two preparations. Although the MLC consisted of three components, LC1f, LC2f and LC3f, in both preparations, their relative content (mean+/-SD, n = 7) was 37.1+/-2.4%, 49.6+/-1.6% and 13.2+/-3.2%, respectively, in the bite-opened and 28.1+/-3.1%, 50.9+/-1.6% and 21.0+/-3.5% in the control preparations, indicating that the bite opening induced a significant (p < 0.0001) increase in the relative content of LC1f at the expense of that of LC3f. Although the tropomyosin consisted of two components, TM-alpha and TM-beta, in both preparations, their relative content (mean+/-SD, n = 7) was 91.8%+/-1.9% and 8.2+/-1.9%, respectively, in the bite-opened and 95.9+/-0.7% and 4.1+/-0.7% in the control preparations, showing a significant (p < 0.001) increase in the relative content of TM-beta in relation to the bite opening. These results indicate that in guinea-pigs an increase in occlusal vertical dimension for 1 week changes the composition of MLC and tropomyosin, with no significant change in MHC, in the masseter muscle. These changes might be required to meet altered functional demands.

Adaptation, Physiological↗
PubMed1999

Intramuscular hemangioma of the masseter muscle.

Intramuscular hemangiomas (IMH) are uncommon tumors of the head and neck, but often occur in the trunk and extremities. When present in the head, the masseter muscle is the most frequently involved site, although constituting only 0.8% of all hemangiomas. Accurate preoperative diagnosis is uncommon without cytology or biopsy. A case of IMH of the masseter muscle in a 24-year-old Turkish woman is presented. Clinical, radiologic and histologic findings and treatment modalities are reviewed.

Adult↗
PubMed2004

[Masseter muscle rigidity after suxamethonium during induction and postoperative abortive malignant hyperthermia in a patient with esophageal achalasia].

A 38-year-old man diagnosed as esophageal achalasia developed masseter muscle rigidity after intravenous suxamethonium during anesthetic induction. Anesthesia was maintained with intravenous agents and epidural blockade, while the masseter muscle rigidity continued. After the surgery, his body temperature increased to 38.8 degrees C concomitantly with the appearance of myoglobinuria suggesting the occurrence of abortive malignant hyperthermia. These symptoms were dissolved by dantrolene administration. He was later proved to be negative with CICR test.

Adult↗
PubMed1996

Functional somatotopic organisation of motoneurons supplying the rabbit masseter muscle.

The localisation within the trigeminal motor nucleus of motoneurons supplying different regions of the rabbit masseter muscle was investigated to test the hypothesis that muscle regions with different motor tasks are controlled from different subregions of the motor nucleus. Motoneurons were labeled retrogradely with horseradish peroxidase, applied surgically to small sections of the masseter in 22 animals, and also by applying this tracer to the cut masseteric nerve. After sacrifice, the labeled muscle sections were mapped. The distribution of labeled motoneurons within the nucleus was described and compared for the muscle regions. The motoneurons for the masseter muscle are confined to the dorsal and lateral sections of the motor nucleus, along its full rostrocaudal extent. Within this subnucleus, the motoneurons for the superficial masseter occupy the dorsolateral portion, the motoneurons for the deep masseter the dorsomedial portion. The anatomical and functional subdivision of the deep masseter into an anterior and posterior portion appeared to be matched by a separation of the motoneurons for these portions in the rostrocaudal direction along the nucleus. The separation of the motoneurons for the anterior and posterior deep masseter is not complete; the territories in the motor nucleus overlap each other for about 50%. The well-established differentiation in motor tasks between the masseter portions during feeding is thus clearly reflected in a separation of motoneurons, making possible differentiation of descending or afferent input to the separate regions in the nucleus.

Animals↗
PubMed1999

Relation between perceived stimulus intensity and exteroceptive reflex responses in the human masseter muscles.

OBJECTIVES: Two periods of exteroceptive suppression are elicited in human masseter muscle following perioral electrical stimulation: ES 1 and ES2. Furthermore, in the masseter EMG, two periods of apparent excitation can be distinguished from the background activity: intersuppression period (ISP) and post-suppression period (PSP). In the present study, the relationship between nociception and ES1, ISP and PSP was investigated. METHODS: The reflex responses were electrically elicited in 15 volunteers. A novel computer algorithm was used to detect and quantitate ES1, ES2, ISP and PSP. RESULTS: (a) ES1 and ES2 were more susceptible to changes in magnitude of suppression and excitation, respectively, than ISP and PSP; (b) the on-set latencies were almost unaffected; (c) both ES1 and ES2 occurred at stimulus intensities well below pain threshold; (d) magnitude of ES1 and ES2 suppression increased with increasing stimulus intensity, but supra pain threshold stimulation did not result in further suppression; and (e) no correlation was found between perceived stimulus intensity and ES1, ISP or PSP. CONCLUSIONS: The increase in ES1 and ES2 suppression seen in our study at increasing stimulus intensity is suggested to be mediated by activity in both nociceptive and non-nociceptive fibres converging onto central interneurones in the reflex circuits. We suggest that neither ISP nor PSP are reflex responses. It is unlikely that ES1, ES2, ISP and PSP evoked by electrical stimuli are directly linked to noxious activity.

Adult↗