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PubMed1987

Relation to extrafusal fibre-type composition in muscle-spindle structure and location in the human masseter muscle.

Muscle-spindle density and size, in relation to extrafusal fibre-type, were examined in different portions of the masseter of six normal males by enzyme histochemistry. A direct relationship between spindle density, intrafusal fibre density and spindle size, and proportion of type I muscle fibres was found in the deep portion. Out of 410 muscle spindles in random samplings, 74 per cent were in the deep portion which is predominantly composed of type I fibres. Spindle density was three times greater than in the other portions, and spindles were commonly clustered, sometimes sharing capsule tissue. The deep masseter spindles were distinctive; they had the largest diameter and the most intrafusal fibres, the fibre density being four times greater than in other portions. Eleven per cent of the spindles, mainly in the deep portion, contained an unusually high number of intrafusal fibres (15 or more fibres). The findings imply that individual portions of the masseter have specialized functions. An especially powerful proprioceptive reflex mechanism is suggested for the deep portion. Thus, the human masseter may be part of an intricate, evolutionarily advanced, motor system also connected with speech.

Adolescent↗
PubMed1998

Development of trigeminal mesencephalic and motor nuclei in relation to masseter muscle innervation in mice.

This study evaluated temporospatial changes in the central organization of trigeminal mesencephalic (mesV) and motor (motV) nuclei during their development. Very little is known regarding the timing of formation of these trigeminal nuclei and the role that target tissue interactions may have on their spatial organization. Cells located in motV innervate muscles of mastication while the mesV nucleus contains populations of primary afferent cells that innervate muscle spindles in jaw closing muscles and periodontal mechanoreceptors around the roots of teeth. To label mesV afferents and motV efferents during their development, lipophilic dyes, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) or 4-(4-dihexadecylamino)styryl)-N-methylpyridinium iodide (DiA), were inserted into various jaw muscles, the dorsomedial mesencephalic region or tooth buds of maxillary or mandibular teeth of embryonic or postnatal mouse pups. Parasagittal sections were evaluated under epifluorescence to determine the temporospatial organization of trigeminal nuclei and the timing of outgrowth of their processes to target tissues. Early in development, motV motoneurons were organized in columns or clusters closely associated with groups of motoneuron axon fascicles that innervated a specific muscle. Double labelling of masseter and temporalis muscles showed that the columns containing motoneurons associated with these muscles were interdigitated early in development but later condensed into respective motoneuron pools. In contrast, no spatial organization of mesV afferent cell bodies was observed throughout the developmental sequence examined. The results of this study also demonstrated that motV processes enter into jaw muscle at least 1 day prior to proprioceptive afferents. MesV afferent appearance at the tooth was further delayed by 10 days suggesting different signaling mechanisms for these two targets.

Animals↗
PubMed1985

Bite force and masseter muscle electromyographic activity during onset of an isometric clench in man.

The surface electromyogram (EMG) of the masseter muscle and interocclusal force were recorded in six human subjects during fast and slow clenches. The simple linear force/EMG relation that applies for steady bite-force levels was inadequate to explain their relationship under conditions of changing force: a second component of force, its rate of change, was necessary. EMG/force equations for fast and slow clenches were broadly similar. The rate of change factor may account for the apparent EMG/force lag observed in earlier studies.

Adult↗
PubMed1983

Proliferative myositis of the masseter muscle. A case report and a review of the literature.

A case of proliferative myositis of the masseter muscle in a 62-year-old woman is presented and pertinent literature is reviewed. This benign pseudosarcomatous proliferative lesion has not been previously reported in any of the muscles of mastication. Clinically the patient presented with pain and difficulty in opening of the mouth. On initial examination, a malignant salivary gland tumor was suspected. The proliferative myositis exhibits a disturbing microscopic appearance characterized by the presence of numerous hyperchromatic fibroblasts and oval or spindle-shaped giant cells which resemble malignant rhabdomyoblasts. These lesions have been often misdiagnosed as rhabdomyosarcoma and other types of sarcomas. Awareness of the microscopic characteristics of proliferative myositis and the possibility of its occurrence in the perioral soft tissues is important in avoiding misdiagnosis of this benign condition.

Cell Division↗
PubMed1999

Comparative data from young men and women on masseter muscle fibres, function and facial morphology.

The primary aim was to relate information about masseter muscle fibres and function to aspects of facial morphology in a group of healthy young men. The secondary aim was to investigate possible sex differences using data previously obtained from a comparable group of age-matched, healthy women. Dental status and facial morphology were recorded in 13 male students aged 20-26 years. Functional examinations included bite-force measurements and electromyographic recordings of masseter activity. A biopsy was removed from the masseter of each participant during surgical extraction of a wisdom tooth, and the tissue examined for myosin ATPase activity. Further, the cross-sectional areas of the different fibre types were measured. In spite of using age-matched healthy men and women with a full complement of teeth, statistically significant sex differences were found among measures related to muscle function and some measures of facial morphology. Thus data from men and women should not be pooled uncritically. The greater bite force in men than women corresponded with the greater diameter and cross-sectional area of type II fibres. Further, the males had more anteriorly inclined mandibles and shorter anterior facial height, suggesting a relation between the greater muscle force and the shape of the face. However, linear regression analysis failed to demonstrate any significant association between bite force and facial morphology among men and women. Thus, craniofacial morphology could be a result of far more contributing factors than previously believed.

Adult↗
PubMed1983

Changes in the pattern of SDH and PhR staining in fibres of rat masseter muscle following long-term functional stretch.

Histochemical changes in the three basic fibre types of the deep masseter muscle of the rat were studied following functional stretch due to increased vertical dimension of the occlusion. A large number of muscle fibres showed changing histochemical profiles for succinic dehydrogenase (SDH) and phosphorylase (PhR) during degeneration and subsequent regeneration of muscle fibres. On the other hand, a small number of muscle fibres with well defined sarcolemmas presented a normal pattern (i.e. low SDH and high or intermediate PhR activities) during this experiment. The ratio between the three basic fibre types remained unchanged after recovery of the muscle fibres. These results suggest that fast-twitch-oxidative-glycolytic and slow-twitch-oxidative fibres underwent degeneration and regeneration, while almost all the fast-twitch-glycolytic fibres retained normal SDH and PhR staining following functional stretch.

Animals↗
PubMed1982

Delayed onset of masseter muscle pain in experimental tooth clenching.

At 100% and about 50% of maximum voluntary isometric contraction strength of the masseter muscle, six healthy human subjects exercised tooth clenching until there was onset of muscle pain in 42 and 146 sec, respectively. Onset of pain, at maximum contraction strength, might have been due to the cumulative amount of activity in relatively large motor units. The delayed onset of pain might have been associated with a decline in the quantity and quality of contracting large motor units. The Clinical implications of this study suggest that an increase of the vertical dimension of the face will delay the onset of muscle pain in patients suffering from tooth clenching.

Adult↗
PubMed1984

Histochemical study of the masseter muscle in patients with vertical maxillary excess.

This study was designed to examine the morphologic properties of the masseter muscle in individuals with vertical maxillary excess (VME). The deep surface of the anterior aspect of the superficial masseter was biopsied in nine patients at the time of corrective surgery. Individual muscle fibers were classified as slow-twitch-oxidative (SO), fast-twitch-oxidative-glycolytic (FOG), or fast-twitch-glycolytic (FG) based on staining characteristics using a battery of histochemical procedures. Most patients had normal tissue, although there were considerable differences in size and distribution of the three fiber types. Generally, SO fibers were the largest and most prevalent type of fiber. Two individuals, however, demonstrated evidence of muscle pathology that primarily affected the FG fiber population. Histochemical evidence of muscle pathology has not previously been reported in patients with VME, and the pattern of SO dominance observed in these patients may be different from the distribution that would be seen in the masseter of a person free of craniofacial deformities.

Adenosine Triphosphatases↗
PubMed1992

Unusual presentation of common acute lymphoblastic leukemia antigen-positive extramedullary disease in childhood. Two patients with isolated masseter muscle involvement.

Two children with unusual extramedullary common acute lymphoblastic leukemia antigen (CALLA)-positive (CD10) disease are reported. Isolated masseter infiltration with CD10/CD19-positive lymphoblasts was present in both patients with no other evidence of disease. One child had relapse of common acute lymphoblastic leukemia, and the other had primary disease. Disease may have spread from lymph nodes overlying the masseter muscle. Immunophenotyping and immunogenotyping provided a rapid and accurate diagnosis for both children.

Child↗
PubMed1990

Convergence patterns of afferent information from the temporomandibular joint and masseter muscle in the trigeminal subnucleus caudalis.

Convergence patterns of afferent information from the temporomandibular joint (TMJ) and the masseter muscle (Mm) in the trigeminal subnucleus caudalis in response to natural stimulation were examined in anesthetized rats for a better understanding of masticatory pain phenomena. All neurons tested (135) could be placed in one of the following three classes according to their responsiveness to natural stimulation in the TMJ and/or the Mm: Class I neurons, excited by only innocuous stimuli; Class II neurons, excited by only noxious stimuli; Class III neurons, excited by both. Afferent inputs from the TMJ and the Mm converged on 108 (80%) of the 135 neurons. Of these convergent neurons, 79% received nociceptive information from the TMJ and/or the Mm. The results suggest that convergent caudalis neurons that receive nociceptive information from the TMJ and/or the Mm may be important to referred pain associated with dysfunction of the masticatory system.

Action Potentials↗
PubMed1975

Human masseter muscle: H- and tendon reflexes. Their paradoxical potentiation by muscle vibration.

We developed a method for direct electrical stimulation of the masseter nerve in man. Both direct M-reponses and genuine H-reflexes were recorded from the ipsilateral masseter muscle. Muscle vibration that inhibits the Achilles tendon reflex and the soleus H-reflex was found to potentiate the masseter tendon reflex and also the masseter H-reflex. This unexpected contrast may be related to peculiar brain stem circuitry of the masseter reflex mechanism.

Achilles Tendon↗
PubMed1994

Automated cytometry of fibre size and spatial distribution in the superficial masseter muscle of the rat at three ages.

An image cytometry program was applied to sections of the superficial masseter muscle of female and male 21-, 42- and 105-day-old rats. Lesser diameter and spatial distribution of more than 6000 muscle fibres were automatically measured in digital images from muscle sections stained for myofibrillar ATPase activity. In this muscle, only type 2A, 2B and 2C fibres were observed. At the three ages and in both sexes, 2A fibres were found to have the largest diameter and were the most frequent (> 54%). In the 21-day-old animals, females presented larger diameters than did males; in contrast, in the 105-day-old animals, the three fibre types were larger in males than in females. At all ages and in both sexes, type 2A occupied 32 to 80% more cross-sectional area than type 2B. Most images (98%) showed a random spatial distribution of their fibre types. All fibre types grew in diameter with age. The coefficient of variation of the diameter was age-independent and remained under 23%. The finding of an age-independent variable could have a practical application: an increase of the coefficient of variation (> 25%) can be considered as pathological, even without a perfect age-matched control.

Aging↗
PubMed1998

Exteroceptive suppression of masseter muscle: assessment of two methods for quantitating suppression periods.

OBJECTIVES: The late exteroceptive suppression period (ES2) of the masseter muscle is conventionally quantified by a combination of measures of duration, amplitude, or mean activity. The combination of measures makes it difficult to compare ES2s obtained from different set-ups. An unbiased method for quantitating ES2 is introduced allowing ranking of ES2s. The introduced method is utilized to evaluate stimulus and recording conditions under which ES2 is maximal. MATERIAL AND METHODS: Fifteen male volunteers participated in 3 experiments designed to determine the effect of level of teeth clenching, stimulus intensity, and perceived pain intensity on the ES2. RESULTS: Coefficients of variance (1 h/1 month) were: duration: 11%/43%, latency: 9%/12%, and magnitude of suppression: 11%/12%. A level of teeth clenching of 66% of maximal voluntary bite force, and a stimulus intensity below pain detection threshold was associated with maximal magnitude of suppression of ES2. CONCLUSION: The introduced method is reproducible and allows ranking and stimulus-response estimations of ES2. It is suggested that ES2 does not reflect activity along the nociceptive pathways in the trigeminal region.

Adult↗
PubMed1981

Muscular and mandibular adaptation after lengthening, detachment, and reattachment of the masseter muscle.

This investigation was undertaken to study the skeletal and muscular adaptations in the mandible after lengthening, detachment, and reattachment of the masseter. Placing the bite-opening appliance and altering the length of the masseter, with and without surgical detachment and reattachment, resulted in a number of mascular and skeletal adaptations: First, a posterior and inferior rotation of the mandible was produced. In the experimental period, forward repositioning of the gonial angle and upward repositioning of the symphyseal region occurred. The amount of skeletal repositioning was greater in those animals in which the associated musculature was not detached. Second, if the masseter muscle was lengthened and not detached from the mandible, a gradual return of the muscle to its original length was observed. After 48 weeks, a 50% return occurred. Third, if a muscle was lengthened and then surgically detached, immediate shortening of the muscle occurred in an anterior and superior direction, with ultimate reattachment often at a length shorter than the original. Fourth, if a muscle was surgically detached and then surgically reattached without placement of a bite-opening appliance, a slight shortening of the muscle occurred. Fifth, if the bite was opened and the masseter was surgically detached and then surgically reattached, the muscle returned to its approximate original length and had a more predictable postsurgical positioning. It may be concluded that, in surgical procedures where lengthening of the masticatory muscles is mandatory, it is beneficial, if possible, to surgically detach these muscles from their insertions. In doing so, one would maintain the original orientation of the muscle and at the same time presumably impose less stress on the newly placed, surgically repositioned skeletal elements.

Adaptation, Physiological↗
PubMed1986

Electromyographic power-spectrum changes during repeated fatiguing contractions of the human masseter muscle.

During 7 sessions at weekly intervals, 10 healthy males performed the same endurance test three times by isometrically contracting the masseter muscles at 20, 30, 40, 50, 60, 75 or 90 per cent of maximal electromyographic activity. The interval between the tests in any one session was 30 s. Changes in the surface electromyogram were monitored by calculating the changes in mean power frequency (MPF) of the signals. With the higher contraction levels, endurance times were shorter, the MPF shifted to lower frequencies more rapidly and the MPF value was lower at the end of the test. At these levels, the subjects reported lack of power as the sole reason for stopping the test but at low levels, pain was the important reason. The endurance time and the rate of MPF shift for the second and third test of each experiment were the same. For the first test, the endurance time was longer (p less than 0.025) and the rate of MPF shift was lower (p less than 0.05). Thus the analysis of specific electromyographic (MPF) indicators provides information about the development of muscle fatigue.

Adult↗
PubMed1992

Myosin heavy chain expression in rabbit masseter muscle during postnatal development.

The expression of isoforms of myosin heavy chain (MHC) during postnatal development was studied in the masseter muscle of the rabbit. Evidence is presented that in addition to adult fast and slow myosin, the rabbit masseter contains neonatal and 'cardiac' alpha-MHC. During postnatal growth myosin transitions take place from neonatal and fast (IIA, IIA/IIB--referring to a fibre containing both IIA and IIB MHCs) MHC to adult 'cardiac' alpha-MHC and I/alpha-MHC. Since there is a temporary population of fibres containing IIA/alpha-MHC during the first 4 wk of development with a peak in the 3rd to 4th wk, the transition from IIA-MHC to alpha-MHC may occur in these IIA/alpha-MHC-containing fibres. The appearance of 'cardiac' alpha-MHC coincides with the timing of weaning, suggesting that the changes in MHC content, that probably result in a transition to a lower speed of contraction, have functional significance related to weaning. The finding of neonatal MHC in adult rabbits indicates that the masseter develops at a rate and in a way that is distinct from most other skeletal muscles. A spatiotemporal variation in expression of myosin isozymes within the masseter was observed, with many fibres containing more than one myosin type, indicating developmentally regulated spatial differences in function.

Adenosine Triphosphatases↗
PubMedDate not supplied

[The persistence of ontogenic characteristics in the adult masseter muscle].

During embryonic and foetal development, the masseter is formed from two successive generations of muscle fibers in a manner which is very similar to that which has been previously described for other skeletal muscles. This phenotype is characterised by the persistence of ontogenic myosin isoforms (embryonic and foetal myosin heavy chains, embryonic light chain) and by the presence of two distinct populations of fibers: small diameter fibers which coexpress the embryonic, foetal and fast isoforms of the myosin heavy chains but never express the slow isoform; large diameter fibers which express the slow myosin heavy chain either exclusively or in variable associations with the other isoforms. These characteristics of the human masseter muscle probably correspond not only to its embryological origin and its special innervation, but also to the functional constraints to which it is submitted after birth.

Adult↗