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Masseter muscles from normal and dystrophic mice were investigated with the electron microscope and by the use of histochemical techniques for myosin ATPase and succinic dehydrogenase activity. The observations show that the muscle fibers are severely affected in the masseter of the dystrophic mouse.
The value and change with time of the impedance of surface EMG electrodes and the effects of their difference between the bipolar electrodes on the electromyographic activity from the anterior temporal muscle and the masseter muscle in six adult male subjects with normal occlusion were studied. The results were as follows: 1. In the anterior temporal muscle, if the impedance of the electrode was under 20 k omega it was stable from just after the electrode disc was applied to the skin. In the masseter muscle, if the impedance was under 30 k omega it became stable within two minutes after the electrode was applied. 2. The difference of impedance between the bipolar EMG electrodes did not correlate with EMG activity.
The masseter muscle of several animal species was investigated by use of a histochemical method for the demonstration of acid-stable and alkali-stable myosin adenosine triphosphatase (ATPase). The following subdivisions of fiber types were used: Type I fibers show weak ATPase activity at pH 9.4, type IM fibers react moderately, and type II fibers react strongly. Rat and mouse masseter muscles contained type II fibers only, as did some rabbit masseter muscles, whereas other rabbit masseter muscles possessed equal amounts of type I and II fibers. Cat and dog masseter muscles possessed both type II and I fibers, with type II predominating. Cow masseter muscle consisted mainly of type I fibers, although some cow masseter muscles contained a very small number of type II fibers. Pig masseter muscle had both type I, II, and IM fibers. One of the characteristics of human masseter muscle is type IM fibers, which are rarely seen in muscles other than the masticatory muscles. Therefore, pig masseter muscle might be a suitable animal model for experimental studies, such as an investigation of the distribution and diameter of fiber types in the masticatory muscles before and after orthognathic surgery.
The effect of the transection of the infraorbital nerve on d 20 after birth on the formation of the annulospiral ending of Ia fibers of muscle spindles in the masseter muscle was investigated in developing mice. On the 20th d after the operation, there were significant differences between control and treated animals in the ratio of muscle spindles with non-coiled endings to all muscle spindles, and the ratios of muscle spindles with non-coiled endings in the masseter muscle on the operated and sham-operated sides were about equal to each other. On the 30th d after the operation, the ratio for the masseter muscle on the operated side was approximately twice that for the one on the sham-operated side. The ratio for the operated side was about equal to that for 15-d-old control mice, and about twice that for 20-d-old control animals. Thus, the present results indicate that disorder of sensory inputs following the transection of the infraorbital nerve leads to degeneration and/or inhibition of the formation of annulospiral endings in muscle spindles of the masseter muscle, and suggest that disorder of the masticatory mechanism may be caused by the degeneration and/or inhibition of the formation of the endings in many muscle spindles of the masseter muscle.
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Masseter muscles were surgically removed in six young female rabbits so that we could study adaptations of the superficial temporalis muscles (ST) to increased functional requirements. Eight weeks following surgery, we used morphological measurements, histochemistry, contractile properties in situ, and occlusal force in vivo to compare the muscles in the experimental animals and six control rabbits. Analysis of the results demonstrated a decrease in fatigability of ST after masseter myectomy. Incisal occlusal force decreased by 65% during the first two weeks, and no recovery was observed during the following six weeks. At eight weeks post-surgery, the mass, twitch tensions, and tetanic tensions of ST were not significantly different from those of the controls. An increase in the percent of the cross-sectional area composed of fast fatigue-resistant fibers, a slower time-to-peak twitch tension, and a decrease in fatigability suggest an increase in oxidative metabolism. Analysis of these results suggests that muscles used for highly repetitious activities with submaximal loadings adapt to increased functional requirements by increasing fatigue-resistant properties.
The effect of lidocaine-HCl on muscle spindles in the masseter muscle of developing mice was investigated. Repeated injections of mice with anesthetic in the short term decreased the diameters of primary endings, intrafusal muscle fibers and outer capsules in the equatorial regions of muscle spindles, and caused a drop in the succinic dehydrogenase activity in intrafusal muscle fibers of the muscle spindles. In addition, the diameters did not recover to the control value even after about 10 weeks following cessation of anesthetic treatment. Thus, the present results suggest that repeated use of lidocaine-HCl in developing animals may cause dysfunction of the skeletal muscles.
In the masticatory muscles, neuromuscular spindles have a very important role in controlling the jaw movement since they act as stretch receptors in skeletal muscles. The continuous intake of fine-grained diet which is easily chewable leads to degeneration of the sensory endings of Ia fibers in many muscle spindles of the mouse masseter muscle in only 120 d after birth.
The morphogenesis of the muscle spindles of the masseter muscle was investigated by electron microscopy. The mice used in this study were those from the 16-day-old fetuses to the four-day-old young, and the three-month-old adult ICR mice. At the time of observation the formation of the masseter muscle spindle had already started in the 16-day-old fetus. In the 17-day-old fetus, the sensory nerve terminated not only in the myotube but also in the myoblast in which no myofilaments were yet formed. From the observations on the 16-day-old and 17-day-old fetuses, it is suggested that the myotubes of the intrafusal muscle fibers are formed by the fusion of the myocytes already containing the myofilaments. The gamma motor end-plate appeared two days later than the sensory nerve terminal formation in the muscle spindle. The initial inner capsule of the spindle was observed in the four-day-old young. The cell without the myofilaments appeared amidst the cluster of the intrafusal muscle fibers in the three-day-old young. It is considered that this cell may be the origin of the inner capsule cell. The periaxial space formation and the myelination of the masseteric nerve fiber occurred in the four-day-old young. The formation of the postsynaptic folds was observed in the extrafusal muscle fiber at the same stage. Thus, the muscle spindles, nerve fibers and the extrafusal muscle fibers showed a remarkable development at the same time in this early postnatal stage.
This study examines in vitro the contractures induced by halothane and succinylcholine in skeletal muscle taken as biopsy specimens from 42 patients referred to the authors' laboratory for diagnosis of malignant hyperthermia (MH) susceptibility. In addition, possible differences between the response of preparations from these same patients with and without a history of masseter muscle rigidity following succinylcholine (SCh) administration were determined to investigate the in vitro relationship of masseter muscle rigidity to MH. Halothane 3%-induced contractures in preparations from MH susceptibles were similar, whether the group had a history of masseter muscle rigidity (1.15 +/- 0.18 g; n = 10) or not (1.02 +/- 0.21 g; n = 14). Halothane did not induce significant contractures in those diagnosed as normals. Succinylcholine alone did not elicit contractures from preparations derived from MH susceptibles or nonsusceptibles. Succinylcholine induced significant contractures in all preparations preexposed to halothane. Preparations from MH-negative patients with a history of masseter muscle rigidity were rendered sensitive to halothane (contractures of 1.17 +/- 0.30 g; n = 4) when SCh was present. In contrast, halothane added in the presence of SCh did not induce contractures to the same extent in preparations from MH-negative patients without a history of masseter muscle rigidity. This is the first reported in vitro method by which to examine the clinically troublesome interaction between SCh and halothane. This approach also may prove to be important in further investigations of the relationship between masseter muscle rigidity and MH.
An antiserum against basic fibroblast growth factor (bFGF) was characterized by immunoblot analysis and used to investigate the fine structure of bFGF-containing processes in muscle spindles of the rat masseter muscle. The bFGF antiserum recognized purified bFGF and bFGF-like materials with the same molecular weight as bFGF in crude homogenate of the brainstem from which bFGF fibers in the masseter muscle spindles presumably originate. Immunoelectron micrographs demonstrated that both proprioceptive and motor nerve endings in contact with intrafusal fibers contain bFGF. These findings suggest that bFGF of central (possibly trigeminal mesencephalic and motor nucleus) origin is transported into the muscle spindles subserving mechanoreception.
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The lamination of the masseter muscle in 21 Japanese serows of different sexes and ages was studied by the method of Yoshikawa et al. who proposed a lamination theory for this muscle. The masseter muscle in the Japanese serow was found to be composed of I) the proper masseter muscle which included 1) the first superficial, 2) the second superficial, 3) the intermediate and 4) the deep masseter muscles, in which the deep masseter muscle could be subdivided into a pars anterior and pars posterior and II) the improper masseter muscle which included 5) the maxillomandibular and 6) the zygomaticomandibular muscles, in which the maxillomandibular muscle was further divided into first and second layers. These findings indicate that the lamination of the masseter muscle in the Japanese serow is the same as that in the goat and sheep.
Effects of lidocaine-HCl on extrafusal muscle fibers in the masseter muscle of developing mice were studied histologically and morphometrically. In the affected region, many extrafusal muscle fibers were circular and smaller in diameter than unaffected extrafusal muscle fibers, and the SDH activity of the affected fibers was very irregular. In lidocaine-HCl injected groups, the ratios of extrafusal muscle fibers with central nuclei to total extrafusal muscle fibers were higher than those in the saline solution-injected groups. The diameters of extrafusal muscle fibers in the masseter muscle treated with lidocaine-HCl were smaller than those in the saline solution-injected groups. These changes in the muscle with lidocaine-HCl continued for 45 days after a single injection. Five injections of lidocaine-HCl into developing mice caused long-term degeneration of the masseter muscle. Thus, the present study suggests that a local anesthetic agent caused degeneration of immature muscle fibers of the masseter muscle in developing mice and may result in long-term decrease of masticatory capacity. Therefore, lidocaine-HCl may inhibit the synchronized development of masticatory organs in developing animals.
During sustained isometric contraction of the jaw muscles, the surface electromyography (EMG) power spectrum compresses into lower frequencies. This compression has been mainly attributed to the change of the conduction velocity of the action potentials along the muscle fibers. In this study, the muscle fiber conduction velocity of the masseter muscle was measured by using the surface electrode array and the cross-correlation technique. The conduction velocity of the masseter muscle varied from 6.83 to 18.01 m/s among the eight subjects, and slightly increased with the contraction level.
The coordination of masseter muscle contraction is inseparably wrapped up in the occlusal elements. This study measured bilaterally the masseter muscle activity of 22 subjects without temporomandibular joint disease. The EMG recordings of the left and the right masseter muscles were repeated 20 times to test symmetry, reproducibility and appearance of silent periods. In 6 subjects the beginning of the muscle contraction showed laterality characteristics depending on handedness. The complex processes of recorded mandibular movements showed more biological variations than the recording patterns of occlusal indicator foils may disclose. That indicates the possibility of pseudocontact markings. The occluding surfaces therefore should allow free mandibular movements.
In order to examine the effect of an occlusal splint on the integrated electromyography (EMG) of the masticatory muscles, EMG of bilateral masseter muscles of 23 patients with temporomandibular joint disturbance syndrome (TMJDS), with and without an occlusal splint, was measured and integrated on line during maximum clenching. It was found that the integrated myoelectrical value of the masseter muscle on the involved and non-involved side was reduced with the occlusal splint. The absolute difference between integrated myoelectrical values in the left and right masseter muscles was reduced with an occlusal splint, but the relative difference remained virtually unchanged. These results indicate that the occlusal splint can decrease masseter muscle activity and thus exert a therapeutic effect.
Eighteen rats (thirty-six sides) were injected with red latex into the peripheral arteries through the left ventricle in the heart and fixed in 10% formalin to demonstrate the arterial architecture. According to the method of Yoshikawa et al. who proposed the lamination theory of this muscle, the latex injected specimens were dissected under a stereoscopic microscope. The masseter muscle in the rat was distributed by the masseteric branches of the facial, external carotid and dorsal branch of the infraorbital arteries as well as the transverse facial, masseteric and buccal arteries. This finding was essentially the same as observed on other species which included the dog, cat, crab-eating monkey, rabbit, cow and horse. However, the origin, course and distribution of the posterior deep temporal and masseteric arteries in the rat were considerably different from those of other species. Furthermore, since the way of development of the arteries and the subdivided muscles of the masseter muscle varies among species, the relationships between these arteries and the subdivided muscles seem to differ to some extent from species to species. Outline of the arterial system of the lateral aspect in the rat's head was shown in Fig. 1. The arteries and masseteric branches which were distributed to the subdivided muscles of the masseter muscle in the rat were as follows. 1) The first and second superficial and intermediate masseter muscles were distributed by the masseteric branches of the facial and external carotid arteries as well as the transverse facial, buccal and masseteric arteries. 2) The anterior portion of the deep masseter muscle was supplied by the masseteric branch of the facial, the masseteric and the buccal arteries. 3) The posterior portion of the deep masseter muscle received only the masseteric artery. 4) The maxillomandibular muscle was vascularized by the masseteric branch of the dorsal branch of the infraorbital and the buccal arteries. 5) the zygomaticomandibular muscle included only the masseteric artery.