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PubMed1991

Direct entry of rabies virus into the central nervous system without prior local replication.

Rabies virus pathogenesis was studied in a mouse model by inoculation of the masseter muscle. At different intervals, the masseter muscle, trigeminal ganglia, and brain were analyzed for virus-specific RNA with a polymerase chain reaction assay, which revealed that as early as 18 h postinfection (p.i.), virus-specific RNA was present in the trigeminal ganglia, and at 24 h p.i., viral RNA was identified in the brain stem. Analysis of the masseter muscle demonstrated virus at 1 h p.i. but no virus-specific RNA between 6 and 30 h p.i., indicating that virus invaded the nerve ending directly, without prior replication in the muscle. At 36 h p.i., viral RNA was detected again in the masseter muscle. Selective amplification of plus- and minus-strand RNA isolated from the masseter muscle at 96 h p.i. revealed that the majority of the rabies virus-specific RNA was in the positive sense, suggesting virus replication in muscle tissue during late stages of infection.

Animals↗
PubMed1997

[A patient with unilateral atrophy of trigeminal nerve-innervated muscle associated with contralateral glossopharyngeal and vagal neuropathy].

A 23-year-old woman was admitted to our hospital because of the left cheek swelling. On neurological examination, marked muscle atrophy of the right masseter muscle was recognized on chewing. The jaw was not deviated when she opened her mouth. Corneal reflex and the sensation on the face were intact. Curtain phenomenon was observed on the right side. The hypesthesia in the left palatal mucosa was observed. Routine laboratory data including cerebrospinal fluid analysis were normal. EMG from the right masseter muscle was not elicited, but normal in the left masseter muscle. The CT scan showed atrophy and low density areas in the right masseter, temporalis, lateral and medial pterygoid muscles, and hypertrophy in left masseter muscle. The MRI on T1 and T2 images confirmed the above muscle involvement. The etiology of trigeminal motor neuropathy, glossopharyngeal and vagal neuropathy were unknown. Masseter muscle hypertrophy was thought to be work hypertrophy compensated for the contralateral muscular atrophy.

Adult↗
PubMed1986

Electromyography and bite force studies of muscular function and dysfunction in masticatory muscles.

Electromyographic (EMG) activity versus bite force was studied during a gradually increased isometric contraction up to maximal effort for patients with painful masseter muscles and referents. The masseter muscle, the anterior temporal muscle and the descending part of the trapezius muscle were chosen for the recordings. Bite force was registered with a bite force sensor placed between the first molars. The effects of double blind intramuscular injections of lidocaine and saline in the patients' masseter muscle were evaluated by EMG versus bite force and by assessment of discomfort. EMG activity during unilateral chewing was compared in terms of relative masticatory force between referents and patients by amplitude probability distribution analysis. Regression analyses showed intra-individually steeper slopes for high force levels than for low force levels for the masseter muscle. This was not observed for the anterior temporal muscle. These differences in slopes of the EMG versus force regressions for the masseter muscle and the anterior temporal muscle could be due to differences in recruitment pattern. The same intra-individual relationship between low and high force levels was found for referents and patients. An increased activity, especially among the patients, was found for the descending part of the trapezius muscle during stronger activity of the mandibular elevators. The EMG versus force relationship for low force levels of the masseter muscle was less steep after an intramuscular injection of lidocaine but not after saline. Both solutions for injection had a positive effect on the patients' assessments of discomfort one week after the injection. Three days after injection the patients who received lidocaine experienced a reduction in muscular discomfort. This reduction was not found among patients receiving saline. The amplitude probability distribution analysis revealed that the patients used greater relative masticatory forces than the referents during the chewing of an almond for all probability levels analysed below the peak load of the masseter muscles. Rough estimates of the peak masticatory forces in Newton (N) were for chewing an almond 364 N (referents); 373 N (patients) and for gum-chewing 239 N (referents); 238 N (patients) as regards the masseter muscle. The values were similar for the anterior temporal muscle.

Bite Force↗
PubMed1984

Relationship between occlusal contacts and jaw-closing muscle activity during tooth clenching: Part I.

Electromyographic recordings from the anterior temporal muscle fibers bilaterally, the posterior temporal muscle fibers bilaterally, the superficial masseter muscle bilaterally, and the left medial pterygoid muscle were used to study the effects of changing the location, size, and direction of effort on specific contact points during maximal clenching tasks in human subjects. Vertical clenching efforts in the natural or simulated intercuspal position generally showed the highest muscle activities for all the muscles recorded. When the contact point moved posteriorly along the arch from incisors to molars, the activity in the ipsilateral temporal muscles was seen to increase, while the activity in the ipsilateral medial pterygoid and the masseter muscles bilaterally was seen to decrease during vertical clenching tasks. Eccentric efforts on specific contact points generally resulted in lower activity than the corresponding vertical effort. This was usually seen in all muscles, but not all values were significant. The ipsilateral temporal and contralateral pterygoid muscles showed the most activity during maximal clenches in lateral direction with little contribution from the other muscles. The temporal muscles showed the most activity in retrusive clenching, with activity in the other muscles nearly nonexistent. The medial pterygoid and masseter muscles were found to be the most active muscles during protrusive and incisal clenching, while the temporal muscle activity was low. When the size and number of contacts were increased anteriorly, a generalized increase in muscle activity was seen. The same trend occurred posteriorly but was not as consistent or significant. Cross-arch contacts were associated with a slight but significant bilateral increase in masseter muscle activity and an increase in temporal muscle activity ipsilateral to the cross-arch contact when maximum vertical clenches were performed. However, no significant increases were observed when the effort was directed laterally. The findings of this electromyographic study on change of the contact point, size of contact point, and the direction of effort applied on a contact point confirm their specific associations with the activity of muscle groups. Significant data have also been made available for a biomechanic approach of the investigation of degenerative joint changes.

Adult↗
PubMed2006

Relationship between head position and the muscle hardness of the masseter and trapezius muscles: a pilot study.

The purpose of this study was to examine the hardness of the masseter and trapezius muscles at various head positions and to explore the relationship of these two muscles to each other in terms of their respective levels of muscle hardness at different head positions. Twenty-two asymptomatic male subjects participated in this study. Using a hand-held hardness meter, muscle hardness was first measured in a relaxed position as a baseline. The subjects were then asked to assume five deviated head positions, and the muscle hardness was measured again. The data obtained at each deviated head position were compared to those at baseline. In addition, the subjects were asked to maintain a five minute sustained anterior flexion of the head, and muscle hardness was also measured and compared to the baseline. As a result, there was a significant increase in muscle hardness at the point of the whole trapezius with 30-degree anterior flexion, while there was a significant decrease at the point of the right masseter. A significant increase in hardness was seen in the upper trapezius muscle in conjunction with right side bending and in the right upper trapezius muscle on left side bending. Also, a significant decrease in hardness was observed in the right masseter with right side bending. With reference to the axial rotation, there was a significant increase in hardness in the upper right trapezius muscle upon right axial rotation. Moreover, there was a significant increase in the muscle hardness in the whole trapezius after the five minute anteriorly sustained head task. This study provides evidence that deviated head positions lead to an increase in hardness of the trapezius muscle. The data also revealed the simultaneous occurrence of the elevation of muscle hardness in the upper trapezius muscle and the decrease in muscle hardness in the masseter muscle associated with right side bending and anterior flexion.

Adult↗
PubMed2003

Measurement of muscle hardness using a hardness meter: application to the masseter and temporal muscles and reproducibility of measurement.

This study was designed to evaluate the intra-examiner and inter-examiner reliability of measurements of masticatory muscle hardness and to confirm that the muscle hardness increases with contraction using a commercially available muscle meter. Twenty healthy asymptomatic female subjects participated in this study. Hardness was expressed as numerical relative values (0-100). First, muscle hardness was measured at a standardized point located in the masseter muscle and temporal muscle in a randomized order by two examiners, and again by one of the same examiners after ten minutes for the reproducibility study. Then the muscle hardness was measured at each point for 0 kgf, 3 kgf, 6 kgf, and 9 kgf levels of bite force. As a result, intraclass correlation coefficient (ICC) analysis revealed good intra-examiner reliability in the masseter muscle (ICC = 0.711), good intra-examiner reliability in the temporal muscle (ICC = 0.643), good inter-examiner reliability in the masseter muscle (ICC = 0.631), and unacceptable inter-examiner reliability in the temporal muscle (ICC = 0.008). Also, our results showed that muscle hardness increased with contraction, and relationships with a slope of 1.229, a y-intercept of 62.513, and a correlation coefficient of 0.448 were observed in the masseter muscle. However, no correlation was found between muscle hardness and bite force in the temporal muscle. The findings indicate that measurement of hardness provides reliable physiological information about the masseter muscle in this setting.

Adolescent↗
PubMed1998

A comparison of jaw-opener and jaw-closer muscle activity in humans to overcome an external force counteracting jaw movement.

In contrast to the jaw-closer muscles, no or very few spindles are present in the jaw-opening digastric muscle. Therefore sensory feedback to the digastric muscle may be different from feedback to the jaw-closer muscles, resulting in a different reaction when jaw movement is perturbed. This possible difference was investigated by comparing the reaction of the digastric muscle when jaw opening is perturbed, with the reaction of the masseter muscle when jaw closing is perturbed. Subjects made rhythmic, 1-Hz open-close movements of the jaw under control of a metronome. During jaw opening (digastric muscle) or, in the other experiments, during jaw closing (masseter muscle), an external force counteracting jaw movement could appear. Series of movements without the force were unexpectedly alternated by series with the force. In both muscles sensory induced activity started approximately 25 ms after the onset of the force and consisted of two phases. In the masseter muscle the maximum of the first increase was reached significantly sooner (37 +/- 2 ms SEM) than in the digastric muscle (54 +/- 3 ms). The second increase appeared much sooner in the masseter muscle (73 +/- 4 ms) than in the digastric muscle (159 +/- 10 ms). When the force was expected, in both muscles an increase in preprogrammed muscle activity was observed. Also an increase in reflex activity, generated before 120 ms after the onset of the force, was observed, compared with when the force appeared unexpectedly. The relative increase in reflex activity was approximately 2 times larger than the relative increase in preprogrammed activity. Therefore, the increase in reflex activity when the force was expected may have been caused not only by an increase in recruitment, but also by an increase in the gain of the reflex loops. Reflex activity relative to preprogrammed activity was on average 4 times larger in the masseter muscle than in the digastric muscle. This indicates that the masseter muscle can react more adequately to disturbances of jaw movement than the digastric muscle.

Adult↗
PubMed1986

The distribution of muscle primary afferents from the masseter nerve to the trigeminal sensory nuclei.

Transganglionic transport of horseradish peroxidase--wheat germ agglutinin conjugate was used to study the pattern of termination of somatic afferent fibers innervating the masseter muscle within the trigeminal sensory nuclear complex (TSNC) of the cat. The central processes of the masseteric nerve terminated in the caudal third of the pars interpolaris, and laminae I/V through the caudal two-thirds of caudalis and rostral parts of the C1 spinal cord segment. The functional significance of the masseteric afferent projections to the TSNC with a preferential pattern was discussed, particularly with respect to muscle pain.

Animals↗
PubMed1992

Temporomandibular-related pressure thresholds: a model for establishing baselines.

The use of pressure threshold measurement to quantify tender spots and trigger points in muscles is well established. Its application to the field of temporomandibular disorders, however, has been more recent and less widely published. Pressure threshold measurement may be useful to define values for dysfunctional muscles or structures so that changes in muscle condition through a course of treatment can be quantified. Normal values for many muscles in the body have been established, but studies of this type in the masticatory system are limited and have often used a fixed, pre-determined pressure cutoff, rather than measuring thresholds across a number of patients. This pilot study seeks to determine whether normal values exist for the anterior temporalis muscles, the masseter muscles, and the lateral capsules of the temporomandibular joints. Intrarater and interrater reliability in measuring thresholds was also studied with the aim of contributing toward a methodological model for further study.

Adolescent↗
PubMed1997

Masticatory muscle function after unilateral condylar fractures: a prospective and quantitative electromyographic study.

Electromyographic (EMG) recording of masticatory muscle activity was performed in 9 adult men with unilateral condylar fracture immediately after conservative treatment of the condylar fracture (T0) and 4 (T1) and 8 (T2) months after the trauma. From T0 to T2, maximal voluntary contraction of the anterior and posterior temporal muscles and of the masseter muscle opposite the fracture side (contralateral) increased significantly. Activity in the anterior temporal muscles also rose significantly during natural chewing, whereas the masseter muscles remained at the same level, but activity consistently tended to be strongest contralaterally to the condylar fracture. In addition, there was a tendency from T1 to T2 for natural chewing to take place predominantly on the contralateral side. The most obvious overall changes during the follow-up were shorter and stronger contractions in all muscles during chewing, on the side of the impaired joint. The increase of activity in the anterior temporal muscles during maximal bite and natural chewing, and the occurrence of stronger and shorter contractions during ipsilateral chewing, were interpreted as signs of improved function due to healing and relief of pain from the impaired joint. On the other hand, there was a specific rise of maximal activity only in the contralateral masseter, and during natural chewing, activity was constantly stronger in the same muscle in combination with the tendency of increasing predominance of contralateral strokes. These traits specifically related to the masseter muscles were considered an indication of a permanent functional distortion due to reflex suppression on the fracture side as an after-effect of the injury.

Adult↗
PubMed1993

Morphological classification of the masticatory muscles in dogs based on their innervation.

The masticatory muscles and their innervations in dogs are classified, from deep to superficial layers, as follows: 1) medial pterygoid muscle and medial pterygoid nerve, 2) lateral pterygoid muscle and lateral pterygoid nerve, 3) anterior deep part of temporal muscle and anterior deep temporal nerve, 4) posterior deep part of temporal muscle and posterior deep temporal nerve, 5) superficial part of temporal muscle and superficial temporal nerve, the only branch of the temporal nerve which passes out from the infratemporal fossa, 6) zygomaticomandibular part of masseter muscle, which is thought to belong to the masseter muscle, and masseter nerve, 7) deep layer of masseter muscle and masseter nerve, 8) superficial layer of masseter muscle and masseter nerve.

Animals↗
PubMed1997

A fully automated system for the evaluation of masseter silent periods.

Exteroceptive suppression of masseter muscle activity, 'masseter inhibitory reflex', comprises one or 2 silent periods (SP1 and SP2) interrupting the voluntary activation. The main problem when evaluating exteroceptive suppression is the lack of an objective and precise measure for the onset and end of the silent period which so far has not been overcome by various automated systems. We describe a new fully automated system for determining the onset and end of the masseter silent period. The decision approach is essentially based upon deterministic properties of median filters which are used to partition the local variances of the EMG traces into constant segments and edges between them. The system was tested in 13 healthy volunteers with 2 subjects tested serially 10 times each to get estimates of the inter- and intra-individual variability. The performance of the system compared favourably to that of a simpler approach and to earlier results from our laboratory. The inter-individual variability of the SP1 onset was 17 times smaller than when based on a subjective decision process.

Adolescent↗
PubMed2005

Analysis of masticatory muscle condition using the 4-dimensional muscle model for a patient with square mandible.

The present study was conducted to ascertain characteristics of mandibular movements in patients with SQM, observe the kinetics of masticatory muscles using a four-dimensional (4D) muscle model, and kinetically investigate the etiology of Square Mandible (SQM). As results, 1, In the maximum opening position, location of the condyle was beyond the articular tubercle for volunteer, but within the mandibular fossa for SQM patient. 2, While the temporal muscle of volunteer was markedly expanded, that of SQM patient was not. 3, In both volunteer and SQM during left lateral excursion, the right mandibular condyle moved to a position slightly before the lowest point of the articular tubercle. The 4D muscle model showed that the cause of limited mouth opening in SQM patient was insufficient expansion of the temporal muscle, and not dysfunction of the opening muscles. Insufficient expansion of the temporal muscle stresses the masseter muscle and leads to hypertrophy of the masseter muscle and hyperplasia of the mandibular angle, resulting in the unique facial configuration.

Adult↗
PubMed1995

[Experimental investigation on mouse masseter by means of denervation].

Histochemical and electron microscopic observations were made on normal and denervated masseter muscles of mice to elucidate differences of characteristics between the masticatory muscles and other skeletal muscles. The three types of muscle fibers were classified by means of Succinic dehydrogensae (SDH) reaction. In masseter muscles of normal adult mice, the three types of fibers, that is, high SDH activity fibers, intermediate SDH activity fibers and low SDH activity fibers, were recognized. More oxidative fibers in the deep portion of masseter muscles were found than that in the superior portion, which suggest that each portion has its own functional role. It is generally believed that denervation gradually causes atrophy in skeletal muscles. On the other hand, denervated masseter muscles of mice clearly showed different reactions which had two phases. In the first phase right after denervation, many masseter muscle fibers were degenerated, necrosed and regenerated. A few weeks later, in the second phase, muscle fibers remained immature, denervation atrophy was occurred, and dense connective tissue was proliferated. The other side of nondenervated masseter muscle was not influenced by denervated ones.

Animals↗
PubMed1987

Dental stability and maximal masticatory muscle activity.

The electromyographical (EMG) activity of masticatory muscles during full clenching in the retruded contact position (RCP) and intercuspal position (IP) with and without a posterior stabilizing splint was studied. The linear envelope EMG signal from three bilateral muscles was recorded (masseter, anterior temporal and posterior temporal). Thirty-seven subjects were evaluated. Clenching in the RCP without the splint and with the presence of an unstable occlusal contact inhibited the masseter muscle activity and reduced anterior temporal and posterior temporal muscle activity. The masticatory muscle activity returned to normal when clenching in the RCP with a splint that permitted stability in the dentition. EMG activity was the same in the IP with and without the splint. The results indicate that the determinant of maximal masseter isometric muscle contraction is the amount of stability in the dentition rather than the jaw position. If the dentition takes the major role of stabilizing the mandible, i.e., there is good intercuspation, the masseter muscle can exert maximal isometric contraction. If the stability is not provided by the dentition, i.e., there is a premature contact, the jaw muscles must contribute to the stabilization and reduce the magnitude of the maximal contraction to avoid damage to the structures involved in the compensatory stabilization.

Dental Occlusion↗
PubMed1990

Variation in histochemical enzyme profile and diameter along human masseter intrafusal muscle fibers.

The histochemical enzyme profile of human masseter intrafusal muscle fibers was analyzed in consecutive serial cross sections along the individual fibers. Two hundred intrafusal fibers in 21 muscle spindles were classified. On the basis of equatorial nucleation, myosin ATPase-staining reactions after alkaline and acid preincubations and diameter, four different populations or types of intrafusal fiber were identified: large-diameter alkaline-stable and acid-stable fibers, bag2; two types of fiber with intermediate-diameter, alkaline-labile and acid-labile fibers corresponding to bag1 and alkaline-labile and acid-stable fibers designated as AS-bag1; and small-diameter alkaline-stable and acid-stable (pH 4.6)-acid-labile (pH 4.3) fibers called chain fibers. Regional variability in staining and diameter along the individual fibers was noted. In general, intrafusal fibers showed stronger oxidative reactions than did extrafusal fibers. The enzyme profile of the human masseter intrafusal fibers differed from that of extrafusal fibers in jaw, limb, and trunk muscles and also from that reported for spindles in limb and trunk muscles in man. The result suggests unique properties of human jaw muscle spindles and the jaw motor system.

Adult↗
PubMed1995

Effect of chronic and experimental jaw muscle pain on pain-pressure thresholds and stimulus-response curves.

Pain-pressure thresholds (PPTs) and stimulus-response (S-R) curves in masseter muscles and index fingers of 11 female patients with chronic jaw-muscle pain were compared with that of 11 matched control subjects. Experimental hyperalgesic and hypoalgesic conditions in the masseter muscles of control subjects were induced by intramuscular injection of 5% saline and of local anesthetic, respectively. The PPTs were found to be significantly lower in the masseter muscles of pain patients than in those of control subjects. The mean slopes of the S-R curves were significantly steeper for the masseter muscles of pain patients (0.481 +/- 0.213) than of control subjects (0.274 +/- 0.201, P < .0256). There were no statistically significant differences in PPTs or S-R curves for the index finger. The PPTs in masseter muscles of control subjects were not significantly affected by injection of 5% saline; however, the slopes of the S-R curves for the masseter muscles were significantly steeper for saline-injection values compared to baseline values (21.7% +/- 29.6%, P < .037). Injection of local anesthetic into masseter muscles of control subjects increased the PPTs significantly and reduced the slopes of the S-R curves significantly as compared to baseline values (-22.9% +/- 34.6%, P < .0155). The present results suggest that PPTs and S-R curves are valuable tools for quantitative description of chronic and experimental jaw muscle pain.

Adult↗