Masseter Muscle Fibers Research Articles

The contribution of the deep fibers of the masseter muscle to selected tooth-clenching and chewing tasks

Anatomically, the human masseter muscle consists of at least two portions (pars superficialis, pars profunda) with distinctly different fiber directions. The purpose of this study was to describe functional behavior in the deep fibers of the masseter muscle and to define any differences in its behavior from that of the superficial fibers. In 20 subjects, EMG activity of the superficial and the deep portions of the masseter muscle was recorded during specific parafunctional (intercuspal and eccentric tooth clenching) and functional (unilateral chewing) tests. Superficial and deep activity was measured with bipolar surface electrodes and intramuscular fine-wire electrodes. Simultaneously, displacement of a lower incisor point was recorded in three dimensions. The data were collected and stored for analysis by a disk-based computer system. The results indicated that changes in the direction of effort, in mandibular position, and in the side used for chewing all influenced activity in both parts of the muscle to different extents. The most distinct separation of activity occurred when intercuspal clenching was directed retrusively; the deep fibers of the masseter muscle response reduced to 47.5% of its maximum value while that of the superficial fibers of the masseter muscle fell to 5.5%. During chewing, activity in the deep fibers of masseter muscle was distributed evenly bilaterally, whereas that in the superficial fibers of the masseter muscle was biased significantly toward the chewing side. Differentiation of activity within the masseter muscle may be relevant to the distribution of regional tenderness in the muscle when it is involved in parafunctional activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of rabbit masseter muscle fibers

Myosins of histochemically distinguishable single fibers of rabbit masseter muscle--type 1, 2A, 2B, and slow fibers--have been characterized by gel electrophoresis under dissociating (sodium dodecyl sulfate) and nondissociating (inorganic pyrophosphate) conditions, and by analysis of peptide maps of the heavy chains following limited proteolytic degradation. Type 2B fibers contain more LC3 homodimer than type 2A fibers; peptide maps of their heavy chain are different although the two myosins comigrate on pyrophosphate gel electrophoresis. Slow fiber myosin migrates more slowly than fast myosin and has a distinct peptide map. Differences were also found among fibers of the same histochemical type but originating in different muscles. In adductor magnus 2B myosin the LC1 + LC3 heterodimer band is the strongest, while in masseter 2B myosin the heterodimer is the weakest. Statistical considerations suggest that in masseter there is a mechanism preferentially forming the homodimers. More work is needed to determine the mechanism by which phenotypical differences occur among various fiber types in the same muscle and between corresponding fiber types in different muscles.

Histochemical fibre-type profile in the human masseter muscle

A histochemical characterization of the masseter muscle was performed on biopsy samples of dentate subjects with normal occlusion. There was a continuum of ranges of oxidative and glycolytic capacities of the masseter muscle fibres. Besides the lightly-stained type I and the darkly-stained type II fibres, fibres with intermediate staining reactions for standard ATPase at pH 9.4, IM fibres, were seen in all biopsy samples. IM fibres had some staining characteristics in common with type I, i.e. the reaction for NADH-TR and for ATPase after preincubation at pH 4.6 and 4.2. Like type II fibres they showed strong reaction for menadione-linked α-glycerophosphate dehydrogenase and for phosphorylase. The ATPase reaction after preincubation at pH 4.6 did not generally reveal subtypes of type II. It is concluded that the masseter muscle in normal human subjects has a very special fibre composition, with ATPase-IM fibres being a part of the normal fibre population.

Adaptation of the masseter and temporalis muscles following alteration in length, with or without surgical detachment.

Histochemical properties, muscle fiber cross-sectional area, muscle fiber length, and the oxidative capacity of masticatory muscles of female rhesus monkeys were assessed following alteration in functional length by an intraoral appliance or by detachment of the muscle. Experimental groups received the appliance only (A); the appliance and subsequent detachment of the masseter (AD); the appliance and detached masseter, but with surgical reattachment of the masseter to the pterygomasseteric sling (ADR); no appliance, but detachment and reattachment of masseter (DR); or an appliance which was removed after 24 weeks to study posttreatment responses (PT). Animals were sacrificed and the muscles were studied at intervals from 4 to 48 weeks after initiation of the experimental period. The results of these studies led to the following conclusions: (1) Stretching the masseter and temporalis muscles within physiological limits did not significantly alter the proportion of fiber types, although oxidative capacity of the fibers was reduced. (2) Fibers with "intermediate" myofibrillar ATPase activity were no more prevalent in experimental than control muscles. (3) The cross-sectional area of Type I fibers of masseter muscles decreased following some experimental procedures, indicating that recruitment of these fibers is the most sensitive to altered jaw function. (4) Minimal alteration of muscle capillarity was induced by any of the experimental procedures. (5) The lengths of masseter muscle fibers in Group PT and of temporalis muscle fibers in groups AD and ADR were greater than in control animals.

Electrogenesis of muscle fibers of the rat masseter muscle

It was shown by intracellular recording of resting membrane potentials (RMP) and action potentials that the superficial layers of the rat masseter muscle contain chiefly fibers with a high MPP and small overshoot, whereas the deep layers contain mainly fibers with a low MPP but a high overshoot. The excitability of the cytoplasmic membrane of muscle fibers with different MPP levels was found to be similar with respect to its electrical parameters. It is suggested that the rat masseter muscle contains a high proportion of fast phasic fibers in its superficial layers and slow phasic fibers in its deep layers.