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PubMed1991

Motor-unit territory in the human masseter muscle.

Motor-unit territories in human masseter are reportedly focal and related to putative subvolumes of muscle. However, in the absence of a reliable method of locating needle-electrode recording sites within the muscle in three dimensions and due to inherent weaknesses in electromyographic recording techniques, the limits of motor-unit territory in the masseter may have been underestimated. Single motor-unit responses were recorded as time-locked events from 32 paired-needle recording sites throughout the masseter muscles of three subjects. Recording sites were located stereotactically with an optical system, magnetic resonance imaging, and a common reference, then displayed graphically in three dimensions. The mean linear separation of the paired recording sites was 8.8 +/- 3.4 mm. The putative territories had a preferred orientation in the antero-posterior axis. Motor-unit territories were larger than described previously and appeared to be related to anatomical compartments. The restriction of these territories to discrete regions of the muscle provides an anatomical substrate for selective regional motor control of the human masseter muscle.

Action Potentials↗
PubMed1996

[Effects of lateral mandibular deviation on masseter muscle activity].

The investigation of the physiological response of masticatory muscle which is associated with the perception of the mandibular lateral deviation is clinically important. Therefore, the effects of various lateral mandibular deviation on the masseter muscle function were investigated on five healthy subjects. The mandibular position was deviated 0.5, 1.0, 1.5, 2.0 and 3.0 mm to the right and left from a reference position corresponding to the rest position. The mandible was passively retained in each test position by means of a bite block. Electromyographic activity (background activity) was recorded with bipolar surface electrodes applied to the right and left masseter muscles. Jaw-jerk reflex was evoked by a reflex hammer, and motoneuron activity was indirectly examined. The background activity as well as the amplitude of the jaw-jerk reflex on the mediotrusive side mainly increased in proportion to the mandibular deviation. However, most subjects showed no significant change about 3 mm from the reference position in the background activity and about 2 mm in the range of muscle activity evoked by the jaw-jerk reflex. These results suggested that the masseter jaw-jerk reflex may assist clinical examination of small mandibular deviations.

Adult↗
PubMed2002

Human single masseter muscle fibers contain unique combinations of myosin and myosin binding protein C isoforms.

Striated craniofacial and limb muscles differ in their embryological origin, regulatory program during myogenesis, and innervation. In an attempt to explore the effects of these differences on the striated muscle phenotype in humans, the expression of myosin and myosin-associated thick filament proteins were studied at the single fiber level both in the human jaw-closing masseter muscle and in two limb muscles (biceps brachii and quadriceps femoris muscles). In the masseter, unique combinations of myosin heavy chain (MyHC) and myosin binding protein C (MyBP-C) isoforms were observed at the single fiber level. Compared to the limb muscles, the MyHC isoform expression was more complex in the masseter while the opposite was observed for MyBP-C. In limb muscles, a coordinated expression of three MyHC and three MyBP-C isoforms were observed, i.e., single fibers contained one or two MyHC isoforms, and up to three MyBP-C isoforms. Also, the relative content of the different MyBP-C isoforms correlated with the MyHC isoform expression. In the masseter, on the other hand, up to five different MyHC isoforms could be observed in the same fiber, but only one MyBP-C isoform was identified irrespective MyHC isoform expression. This MyBP-C isoform had a migration rate similar to the slow MyBP-C isoform in limb muscle fibers. In conclusion, a unique myofibrillar protein isoform expression was observed in the human masseter muscle fibers, suggesting significant differences in structural and functional properties between muscle fibers from human masseter and limb muscles.

Adolescent↗
PubMed1998

Acoustic myography, electromyography and bite force in the masseter muscle.

Acoustic myography (AMG) offers some advantages over electromyography (EMG) in certain circumstances, but the use of AMG on the jaw-closing muscles has not been fully tested. The purpose of this study was to examine the relationship between AMG, EMG and force in the masseter muscles of nine healthy male subjects. The AMG was recorded using a piezoelectric crystal microphone and the EMG was recorded simultaneously with surface electrodes. Force was recorded between the anterior teeth with a strain-gauge transducer. Analysis showed that Pearson's correlation coefficient was 0.913 for force/AMG and 0.973 for force/EMG in all subjects, indicating a linear relationship between force, AMG and EMG at the four different force levels tested (25-75% of maximum). It is apparent that AMG may be used as an accurate monitor of masseter muscle force production, although some care is required in the technique.

Acoustics↗
PubMed2000

Proprioceptive afferents survive in the masseter muscle of trkC knockout mice.

Peripheral innervation patterns of proprioceptive afferents from dorsal root ganglia and the mesencephalic trigeminal nucleus were assessed in trkC-deficient mice using immunohistochemistry for protein gene product 9.5 and parvalbumin. In trkC knockout mice, spinal proprioceptive afferents were completely absent in the limb skeletal muscles, M. biceps femoris and M. gastrocnemius, as previously reported. In these same animals, however, proprioceptive afferents from mesencephalic trigeminal nucleus innervated masseter muscles and formed primary endings of muscle spindles. Three wild-type mice averaged 35.7 spindle profiles (range: 31-41), six heterozygotes averaged 32.3 spindles (range: 27-41), and four homozygotes averaged 32.8 spindles (range: 26-42). Parvalbumin and Nissl staining of the brain stem showed approximately 50% surviving mesencephalic trigeminal sensory neurons in trkC-deficient mice. TrkC-/- mice (n = 5) had 309.4 +/- 15.9 mesencephalic trigeminal sensory cells versus 616.5 +/- 26.3 the sensory cells in trkC+/+ mice (n = 4). These data indicate that while mesencephalic trigeminal sensory neurons are significantly reduced in number by trkC deletion, they are not completely absent. Furthermore, unlike their spinal counterparts, trigeminal proprioceptive afferents survive and give rise to stretch receptor complexes in masseter muscles of trkC knockout mice. This indicates that spinal and mesencephalic trigeminal proprioceptive afferents have different neurotrophin-supporting system during survival and differentiation. It is likely that one or more other neurotrophin receptors expressed in mesencephalic trigeminal proprioceptive neurons of trkC knockout mice compensate for the lack of normal neurotrophin-3 signaling through trkC.

Animals↗
PubMed1975

Evidence for a monosynaptic mechanism in the tonic vibration reflex of the human masseter muscle.

Vibration of the masseter and temporalis muscles in normal human adult subjects elicits a tonic vibration reflex with unexpected features. The electromyographic response is not asynchronous as in the limb muscles, but involves well-defined spikes with a one-to-one temporal relation to the vibration cycles. The effect of various parameters such as muscle stretch, vibration frequency or amplitude, etc, has been investigated. The small latency fluctuation of the vibration-induced spikes is compatible with a monosynaptic reflex mechanism which is considered to be assisted by a polysynaptic facilitatory background of proprioceptive origin.

Adult↗
PubMed2000

Functional lip reconstruction with a radial forearm free flap combined with a masseter muscle transfer after wide total excision of the chin.

Total lower lip reconstruction was accomplished by combining a radial forearm free flap with a masseter muscle transfer. The patient, who had T4 carcinoma, had the entire lower lip resected including the depressor anguli oris muscle. A radial forearm flap was used to reconstruct the lower lip lining and the floor of the oral cavity. The right and left masseter musculofascial flaps were elevated and transferred in the medial-superior direction, and the peripheral margins of the flaps were sutured together. The lateral margins of the flaps were then sutured to the orbicularis oris muscle of the upper lip. Good sphincter function was obtained more than 1 year after the operation, electromyography revealed almost normal mobility of the transferred masseter muscles, and no sagging of the masseter muscle sling was observed. This procedure appears to be effective for the reconstruction of sphincter function of the lower lip after wide excision of the entire chin.

Aged↗
PubMed1985

Tenomyositis of the masseter muscle: report of cases.

Inflammation occurring at the attachment of the masseter muscle to the zygomatic arch--tenomyositis--must be distinguished from the more common disorders occurring in the rest of the muscle. Anti-inflammatory treatment has been effective in the treatment of tenomyositis. The cases presented illustrate the effectiveness of this type of treatment.

Adult↗
PubMed1993

Variables related to masseter muscle function: a maximum R2 improvement analysis.

A multiple linear regression analysis, with stepwise maximum R2 improvement technique by forward selection and pair switching, was used to select the occlusal, morphologic, and histologic variables which explained most of the variation in bite force and electric masseter muscle activity. The variables comprised tooth contact and facial morphology together with thickness and fiber characteristics of the masseter muscle. The study included 13 healthy women, 21-28 yr of age, with a minimum of 24 teeth and no serious malocclusion. Significant exploratory models (R2: 0.55-0.85) were shown concerning bite force, and electromyographic amplitude during resting posture, maximal voluntary contraction (ICP), and unilateral chewing, as well as contraction time (chewing side). Muscle thickness and molar contact had a significant, positive effect on the level of forceful muscle contraction. The explorative model both demonstrated explicable relations, and offered better insight into interrelations than did univariate analysis.

Adenosine Triphosphatases↗
PubMed1979

[Electrogenesis of muscular fibers in the rat masseter muscle proper].

It was established by the method of intracellular lead of membrane rest potentials (MRP) and action potentials (AP) that fibers with high MRP and little overshot are mainly located in superficial layers of the rat masseter muscle proper whereas those with low MRP and high overshot principally in deep layers. Excitability of the cytoplasmic membrane of muscular fibers of both types proved to be related in electrical properties (critical level of depolarization, current threshold). It is suggested that the rat masseter muscle contains a great number of rapid phasic fibers in superficial layers and slow ones in deep layers.

Action Potentials↗
PubMed1978

Prenatal growth patterns of the human mandible and masseter muscle complex.

Since experimental and clinical evidence supports some role of musculature in determining the form and size of facial bones during the active periods of growth after birth, this study addresses the same basic relationships between muscle and bone during the periods of active growth before birth. The relationship between the masseter muscle and the mandible, including its ramal and body components, was chosen as the model for study in nineteen human fetuses (ages 16 to 36 weeks). Cross-sectional cephalometric data indicated that, although increases in the size of the muscle and mandible were linearly related to increasing age, the ramal portion of the mandible was more closely related to changes in the masseter muscle than to changes in the mandibular body. Moreover, it appears that reorientation of the muscle anteriorly and downward precedes a similar reorientation of the ramus, with the combination of both fetal events leading to the typical relationships of the two structures expected after birth. Although this study does not get to cause-and-effect relationships, and although the fetal specimens cannot be monitored longitudinally over time, the parallelisms between our prenatal findings and those reported for postnatal periods certainly lend further support to the observation that many aspects of morphogenesis and growth are continual processes spanning the periods on either side of birth.

Body Height↗
PubMed1985

Isometric endurance of the human masseter muscle during consecutive bouts of tooth clenching.

Three human subjects performed teeth clenching at maximum voluntary contraction strength (MVC) of the mandibular elevator muscles. At intervals of 10s, ten successive bouts of MVC clenching were exercised until total exhaustion of the contracting muscles (isometric endurance). Isometric endurance time decreased exponentially from 59 to 12s. The curve of the decrease showed a monotone power function (y = x-1). Electromyographic recordings from masseter muscles suggested that fatigable motor units, possibly fast glycolytic units, dropped out during the initial stages of maintained MVC isometric activity. The silent period of the monosynaptic jaw jerk reflex was increased by about 35% following exhaustion of the masseter muscle.

Adult↗
PubMed1998

Visual input effect on EMG activity of sternocleidomastoid and masseter muscles in healthy subjects and in patients with myogenic cranio-cervical-mandibular dysfunction.

This study was conducted in order to determine the input visual effect on electromyographic (EMG) activity of the sternocleidomastoid and masseter muscles in the supine and lateral decubitus positions. The study was performed on 22 patients with myogenic cranio-cervical-mandibular dysfunction (CMD) and 18 healthy subjects. EMG activity at rest and during swallowing of saliva and maximal voluntary clenching was recorded in the supine and lateral decubitus positions in the following conditions: 1. with eyes open; and 2. with eyes closed after 5 minutes in a dark room. A significant decrease of EMG activity at rest with closed eyes in both groups was observed in the sternocleidomastoid (lateral decubitus position) and in the masseter muscle (supine position). During swallowing of saliva a significant decrease of EMG activity with closed eyes was observed only in the sternocleidomastoid muscle (lateral decubitus position) in healthy subjects. During maximal voluntary clenching any significant differences were observed upon variation in the visual input. The significant change in EMG activity, mainly observed at rest, suggests that the visual input effect is weak. The absence of a significant change in EMG activity during maximal voluntary clenching upon variation in the visual input could be clinically relevant in patients with myogenic CMD who habitually brux.

Adolescent↗
PubMed1985

Studies of biologic parameters for denture design. Part III: Effects of occlusal adjustment, base retention, and fit on masseter muscle activity and masticatory performance.

EMG recordings of ipsilateral and contralateral masseter muscles and standardized masticatory performance tests were made in 20 denture wearers before and after modifications were made to their clinically unsatisfactory dentures. The improvement of retention and stability by correction of gross occlusal prematurities and by the use of a denture adhesive or base reline with a soft tissue-conditioning material did not significantly alter the chewing performance or muscle activity during mastication in denture wearers. The results support our previous findings that the influence of denture factors on chewing ability of denture wearers is limited. To avoid variations in EMG activity caused by changes in electrode positions, all tests for a given subject were made at the same sitting. Thus the study design did not provide time to denture wearers for their neuromuscular mechanism to adapt to the modifications prior to testing masticatory function. A replication of this study that permits sufficient adaptation time after each modification is suggested to validate the findings.

Adhesives↗
PubMed1998

Different phenotypes among slow/beta myosin heavy chain-containing fibres of rabbit masseter muscle: a novel type of diversity in adult muscle.

Difference in the phenotype of different mammalian muscle fibres are usually attributed to differences in the expression of the product of different myosin heavy chain (MyHC) genes, which are known as isoforms. We studied differences in phenotype among fibres containing a single MyHC isoform (slow/beta) of the masseter muscle of adult rabbits. Four different monoclonal antibodies to slow/beta MyHC were used to stain serial sections from muscles in males and females. All antibodies recognize a single band on immunoblots and stain the same set of fibres in rabbit postcranial muscles. However, differential staining was observed in the masseter muscles. Antibody BA-D5 reacts with the most fibres, antibody A4.951 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of these fibres, and antibody A4.840 reacts with a subset of A4.951-positive fibres. Antibody S58 reacts only with an even smaller subset of fibres. Even though differential staining using four antibodies might allow for the expression of as many as 15 different staining patterns, or patterns, or phenotypes, only four were observed on > 99% of over 30 000 fibres studied. In females, nearly 40% of the fibres stain exclusively with antibody BA-D5, while in males, fewer than 8% of the fibres express this phenotype. The proportions of fibres of the other phenotypes do not differ so strikingly with gender. We conclude that an epitope diversity exists among muscle fibres in the adult rabbit masseter and that it is not necessarily a consequence of differences in gene expression. We feel that it is a regulated process and that, at least for some phenotypes, this regulation may be hormonally influenced.

Age Factors↗
PubMed1995

Masseter muscle rigidity associated with glycine1306-to-alanine mutation in the adult muscle sodium channel alpha-subunit gene.

BACKGROUND: Succinylcholine-induced masseter muscle rigidity (MMR) is a potentially life-threatening complication of anesthesia and is closely correlated with the heterogeneous disorder malignant hyperthermia (MH) susceptibility. MMR also is identified with a variety of neuromuscular disorders, including the myotonias, that are associated with abnormal in vitro contracture test (IVCT) results. Recently, mutations in the adult skeletal muscle sodium channel alpha-subunit gene (SCN4A) have been shown to cause generalized nondystrophic myotonias, some of which are associated with mild nonspecific symptoms. The purpose of the current investigation was to begin to evaluate the molecular genetic relationship between known mutations in the SCN4A gene, MMR, and the results of the IVCT used to diagnose MH-susceptibility. METHODS: A single extended pedigree of 16 individuals was ascertained through a proband who experienced MMR and whole-body rigidity after succinylcholine administration. Subsequently, four individuals were shown to have a mild form of myotonia on clinical and laboratory examination. IVCT was carried out according to standardized protocols. Mutations in the SCN4A gene were sought in exons 22 and 24 using single-strand conformational analyses. Variability in the SCN4A gene sequence was confirmed by direct DNA sequence analyses. RESULTS: Four individuals with myotonia were shown to carry a guanine-to-cytosine mutation at nucleotide position 3917 of the reported SCN4A sequence. This DNA mutation was coinherited with MMR and an abnormal IVCT result in this family. Previous studies have demonstrated that the glycine1306-to-alanine substitution is associated with a mild clinical syndrome referred to as myotonia fluctuans. CONCLUSIONS: The current report provides direct evidence that succinylcholine-induced MMR, whole-body rigidity, and an abnormal IVCT result are associated with a mutation in the SCN4A gene.

Alanine↗
PubMed1997

Short-term effect of glucocorticoid injection into the superficial masseter muscle of patients with chronic myalgia: a comparison between fibromyalgia and localized myalgia.

The aim of this study was to investigate whether the treatment effect of intramuscular glucocorticoid injection differs between patients with fibromyalgia and those with localized myalgia of the masseter muscle concerning pain, tenderness to digital palpation, pressure pain threshold, pressure pain tolerance level, maximum voluntary occlusal force, or intramuscular temperature. Twenty-five patients with fibromyalgia and 25 patients with localized myalgia of the masseter muscle were first asked to assess their pain on a visual analogue scale; afterward, a routine clinical examination, including tenderness to digital palpation, was performed. For each patient, the pressure pain threshold, pressure pain tolerance level, and maximum voluntary occlusal force, as well as the intramuscular temperature, were recorded. Finally each patient received an injection of glucocorticoid. The examination and glucocorticoid treatment were repeated after approximately 2 weeks, and a follow-up was performed after another 5 weeks. In the fibromyalgia group, there was a reduced tenderness to digital palpation in response to the treatment. The localized myalgia group responded with a general improvement of symptoms as well as a significant reduction of pain intensity and tenderness to digital palpation. The results of this study indicate that patients with fibromyalgia and localized myalgia in many respects show a similar response to local glucocorticoid treatment.

Adult↗
PubMed1987

Effect of ibuprofen on isometric contractions and associated pain of the human masseter muscle.

A randomized, single-blind crossover trial tested the effect of 1,000 mg of ibuprofen on jaw muscle pain induced by maximum voluntary teeth clenching (MVC). Subjects exercised MVC until there was onset of pain in the masseter muscles (pain latency in seconds), and until pain and exhaustion of the masseter and anterior temporalis muscles could no longer be endured (pain tolerance in seconds). Pain intensity was quantified by visual analogue scores, and pain sensitivity by the pain sensitivity range and the pain sensitivity ratio. During MVC the mean voltage of the left masseter muscle was recorded by cumulative surface electromyography. Ibuprofen had no significant effect on the pain latency and the pain tolerance. Neither did ibuprofen significantly decrease the pain intensity nor significantly affect the pain sensitivity range and the pain sensitivity ratio. After intake of ibuprofen, the number of electromyograms with a decrease in mean voltage was significantly increased--credibly, an expression of increased central fatigue with voluntary decruitment of motor units, and possibly the result of increased contraction times because of an undisclosed effect of ibuprofen. There was no circumstantial evidence of impaired motor activity that could be attributed to biosynthesis of prostaglandins.

Adult↗