Proteomics and immunohistochemistry identify the expression of α-cardiac myosin heavy chain in the jaw-closing muscles of sooty mangabeys (order Primates)

ObjectiveThe jaw-closing muscles of humans and nonprimate mammals express alpha-cardiac fibers but MyHC α-cardiac has not been identified in the jaw adductors of nonhuman primates. We determined whether MyHC α-cardiac is expressed in the superficial masseter and temporalis muscles of the sooty mangabey (Cercocebus atys), an African Old World monkey that specializes on hard seeds.DesignLC–MS/MS based proteomics was used to identify the presence of MyHC Iα.Immunohistochemistry was used to analyze the composition and distribution of fiber types in the superficial masseter and temporalis muscles of eight C. atys. Serial sections were stained against MyHC α-cardiac (MYH6), as well as MyHC-1 (NOQ7.5.4D), MyHC-2 (MY-32), and MyHC-M (2F4).ResultsProteomics analysis identified the presence of Myosin-6 (MyHC α-cardiac) in both heart atrium and superficial masseter. MyHC α-cardiac was expressed in abundance in the superficial masseter and temporalis muscles of all eight individuals and hybrid fibers were common.ConclusionsThe identification of MyHC α-cardiac in the jaw adductors of sooty mangabeys is a novel finding for nonhuman primates. The abundance of MyHC α-cardiac indicates a fatigue-resistant fiber population characterized by intermediate speed of contraction between pure MyHC-1 and MyHC-2 isoforms. We suggest that α-cardiac fibers may be advantageous to sooty mangabeys, whose feeding behavior includes frequent crushing of relatively large, hard seeds during the power stroke of ingestion. Additional studies comparing jaw-adductor fiber phenotype of hard-object feeding primates and other mammals are needed to explore this relationship further.     

MGF- and myostatin-mRNA regulation in masseter muscle after orthognathic surgery

Patients with prognathism and retrognathism show extensive morphologic changes after orthognathic surgery. The inability of muscles to adapt adequately might be one reason for observed relapses. The aim of this prospective study was to analyze the regulation of functional genes after surgery and changes in myosin heavy chain (MyHC) isoform expression, a potential molecular marker profile for inadequate muscle adaptation. 29 adult patients with prognathic and retrognathic mandibles were involved. A total of 232 muscle biopsies were taken from both masseter muscles presurgically and 6 months later. The mRNA expression of "mechano growth factor" (MGF), myostatin, and 3 MyHC isoforms were quantified by real-time polymerase chain reaction. The MGF mRNA was significantly up-regulated (P < .005), whereas myostatin mRNA showed no significant regulation. There was a shift in MyHC isoform expression. The MyHC-I mRNA was decreased (P < .005), whereas MyHC-IIa mRNA was increased (P < .005). The coherence of increased MGF expression and MyHC isoform shift 6 months after orthognatic surgery indicates an adequate muscle adaptation and higher mastication activity.     

Myosin isoform composition of the human medial and lateral pterygoid muscles

The medial and lateral pterygoid muscles are different in structure as well as in function. The medial pterygoid muscle is concentrically active during jaw closing, and the superior head of the lateral pterygoid muscle is eccentrically active during jaw closing, while its inferior head is concentrically active during jaw opening. Architecturally, the medial pterygoid can deliver higher forces than the lateral pterygoid. We investigated whether these differences are reflected in the myosin heavy-chain (MyHC) composition and the fiber cross-sectional area (f-csa) of these muscles. The pterygoid muscles from eight cadavers were investigated by means of monoclonal antibodies against different isoforms of MyHC. The proportions of pure MyHC type I fibers did not differ significantly among the muscles (32% in medial pterygoid, 34% in superior head, and 36% in the inferior head of the lateral pterygoid), nor did the total proportions of pure MyHC type IIA and IIX fibers (16% in medial pterygoid, 26% in the superior head, and 19% in the inferior head of the lateral pterygoid). The mean f-csa of type I fibers was 1315 microm2, which did not differ significantly among the muscles, and was significantly larger than the f-csa of type IIA fibers. The relative proportions of hybrid fibers, which expressed more than one MyHC isoform, were 52% in the medial pterygoid, 40% in the superior head, and 45% in the inferior head of the lateral pterygoid and did not differ significantly among the muscles. The most abundant hybrid fiber types found were fibers expressing MyHCs-cardiac alpha+IIA and MyHCs-cardiac alpha+I+IIA. Significant regional differences were found in the proportions of MyHC type I fibers in the medial pterygoid and in the inferior head of the lateral pterygoid. Although the form and function of the muscles are different, we conclude that this is not reflected in their myosin isoform composition.     

Histological changes and changes in the myosin mRNA content of the porcine masticatory muscles after masseter treatment with botulinum toxin A

Objectives

Botulinum toxin A (Botox) is increasingly used for treatment of muscle hyperfunction. For a better understanding of the possible morphologic and chewing changes in patients induced by a therapy with Botox, muscle fiber and myosin heavy chain (MyHC) mRNA alterations were examined in this animal study.

Materials and methods

The investigation was carried out on 14-week-old pigs (seven treated animals, eight controls; calculated animal size with a power of 0.5). To initialise the total immobilisation of the right masseter, the Botox injection was distributed into ten areas. After a 56-day period, muscle tissue was taken from the left and right side of the masseter (three regions), temporal (two regions), medial pterygoid and geniohyoid muscles using a standardized method. The muscle fiber cross sections were examined immunohistochemically. Fiber staining was accomplished with antibodies to specific MyHC isoforms. The MyHC mRNA changes were analysed using real-time RT-PCR.

Results

Muscles adapt to such stress by changing fiber types and MyHC mRNA content. Paralysed masseters display atrophic changes while other masticatory muscles show hypertrophic changes. The results indicated that the typical distributions of type IIa und IIb fiber types in masticatory muscles were increased in the masseter muscles due to Botox application. On the other hand, the masseters without Botox in the treated group showed a significant increase of type I MyHC.

Conclusions

Application of Botox may lead to uncontrolled structural changes in affected and unaffected muscles.

Clinical relevance

Treatment of muscle hypertrophy with Botox may cause muscle imbalance.     

Quantitative analysis of masticatory activity during unilateral mastication using muscle fMRI

Oral Diseases (2011) 17 , 407–413 Objective: Quantitative analysis of the activities of all masticatory muscles is required to elucidate the mechanism of stomatognathic dysfunction. Electromyography can be used to record the activity of masticatory muscles, but quantification of the overall activity of every masticatory muscle has not been accomplished because of methodological limitations. In this study, we used muscle functional magnetic resonance imaging for simultaneous quantification of the overall activities of the masseter, medial pterygoid and lateral pterygoid muscles during unilateral gum chewing. Methods: Seven healthy male volunteers participated in the study. We evaluated changes in the mean proton transverse relaxation time in the bilateral masseter, medial pterygoid and lateral pterygoid muscles before and after unilateral gum chewing, and to quantify the overall activity of these muscles simultaneously during unilateral gum chewing. Results: After 5 min of chewing, the activity of the ipsilateral masseter was highest among the six muscles, followed by the ipsilateral medial pterygoid, contralateral lateral pterygoid and contralateral masseter muscles. Conclusion: These results affirm the importance of the ipsilateral masseter muscle and quantitatively demonstrate the important contribution of the ipsilateral medial pterygoid and contralateral lateral pterygoid muscles during unilateral mastication.     

The association of occlusal factors with masticatory muscle tenderness in 10- to 19-year old Turkish subjects

The aims of this study were (1) to investigate the relationship between occlusal factors and masticatory muscle tenderness among 10- to 19-year-old (mean 14 years eight months) Turkish subjects and (2) to identify possible sex differences between them. The sample consisted of 716 individuals (355 male and 361 female subjects). Tenderness with palpation of masseter and temporalis muscles and functional manipulation of lateral and medial pterygoid muscles was registered. The examiners recorded the Angle classification bilaterally for molars, presence of anterior and posterior crossbites, excessive overjet, open and deep bites, functional shift, and severity of anterior crowding. Associations between the occlusal factors and muscle tenderness according to sex were evaluated with chi-square analysis. Statistically significant associations were found between masticatory muscle tenderness and all the investigated occlusal factors except posterior crossbite and functional shift. Masseter, medial, and lateral pterygoid muscle tenderness was higher in female subjects. Medial and lateral pterygoid muscle tenderness in Class I cases and masseter and medial pterygoid muscle tenderness in Class II, division 1 malocclusion cases were higher in female subjects (P < .05). In open-bite cases, medial pterygoid muscle tenderness (P < .05), in deep-bite cases, masseter (P < .01) and medial pterygoid (P < .05) muscle tenderness, and in excessive overjet cases, masseter muscle tenderness (P < .05) were also higher in female subjects. These results suggest that greater masticatory muscle tenderness in female subjects may contribute to the greater prevalence of temporomandibular disorders in them.     

Increased vertical dimension effects on masseter muscle fiber phenotype during maturation

Objective:To determine changes in mouse myosin heavy chain (MyHC) protein expression that may occur with a clinically relevant vertical dimension of occlusion (VDO) increase.Materials and Methods:Six CD-1 male mice (age: 6 weeks) underwent a 10% bite opening to replicate the clinical condition using composite on the maxillary molars and were compared to six age-matched controls. Mice were sacrificed at day 7 and 14 after bite opening. A representative masseter transverse cryosection from each animal was examined in selected sampling regions (anterior, posterior, posterior-deep, and posterior-intermediate) to assay fiber phenotype proportions and fiber size.Results:In control masseter muscles, the proportion of muscle fibers containing MyHC IIb increased in the posterior-intermediate and posterior-deep regions between 7 and 14 days (ANOVA, P < .05). The increase in the proportion of MyHC IIb fibers in the bite opening group did not occur when compared to the control group (P < .05). In addition, after 14 days of bite opening, the proportion of fibers positive for MyHC IIa was decreased in the anterior region compared to control masseter muscles. Muscle fiber diameter remained unchanged in both groups (experimental and control) and over time (P > .10).Conclusion:These data are consistent with a selective plasticity of the expression of MyHC IIb protein in the deep regions of the male masseter muscle in response to a clinically relevant VDO increase.     

Differential expression of myosin heavy chain isoforms in the masticatory muscles of dystrophin-deficient mice

The dystrophin-deficient mouse (mdx) is a homologue animal model of Duchenne muscular dystrophy (DMD) and is characterized by slowly progressive muscle weakness accompanied by changes in myosin heavy chain (MyHC) composition. It is likely that the masticatory muscles undergo similar changes. The aim of this study was to examine the masticatory muscles (masseter, temporal, tongue, and soleus) of 100-day-old mdx and control mice (n = 8-10), and the fibre type distribution (by immunohistochemistry) as well as the expression of the corresponding MyHC messenger RNA (mRNA) (protein and mRNA expression, using Western blot or quantitative real-time polymerase chain reaction (RT-PCR)). Immunohistochemistry and western blot analysis revealed that the masticatory muscles in the control and mdx mice consisted mainly of type 2 fibres, whereas soleus muscle consisted of both type 1 and 2 fibres. In the masseter muscle, the mRNA in mdx mice was not different from that found in the controls. However, the mRNA content of the MyHC-2b isoform in mdx mice was lower in comparison with the controls in the temporal muscle [11.9 versus 36.9 per cent; P < 0.01; mean ± standard error of the mean (SEM), Student's unpaired t-test], as well as in the tongue muscle (65.7 versus 73.8 per cent; P < 0.05). Similarly, the content of MyHC-2x isoforms in mdx tongue muscle was lower than in the controls (25.9 versus 30.8 per cent; P < 0.05). The observed down-regulation of the MyHC-2x and MyHC-2b mRNA in the masticatory muscles of mdx mice may lead to changed fibre type composition. The different MyHC gene expression in mdx mice masticatory muscles may be seen as an adaptive mechanism to muscular dystrophy.     

Wearing of complete dentures reduces slow fibre and enhances hybrid fibre fraction in masseter muscle

Edentulous conditions and use of complete dentures alter the function of jaw muscles, which is presumably reflected in the myosin heavy chain (MyHC) isoform composition. This study is the first dealing with MyHC isoforms expression in edentulous persons with the aim to clarify to which extent the decreased functional load following teeth loss contributes to the changed muscle phenotype during ageing. We analysed MyHC expression in old masseter muscle at decreased and full functional load by comparing age-matched edentulous and dentate subjects. Edentulous subjects had upper and lower complete dentures. Dentate subjects had at least 24 natural teeth in continuous dental arches with two molars present in each quadrant and normal intermaxillary relationship. The adaptive response to the reduced masticatory load was lower numerical and area proportion of MyHC-1 expressing fibres and higher numerical proportion of hybrid fibres in edentulous compared with dentate subjects with no significant difference in the proportion of MyHC-neo-expressing fibres between both groups. We conclude that the observed differences in the proportion of fibre types between denture wearers and dentate subjects cannot be ascribed to degenerative changes intrinsic to the ageing muscle, but to functional differences in muscle activity and to morphological alterations of stomatognathic system accompanying the complete teeth loss.     

Three-dimensional attachment morphometry and volumetric changes of masticatory muscles after free fibular flap reconstruction of the mandibular condyle

This retrospective case-series study aimed to elucidate the three-dimensional attachment morphometric features and to quantify the volumetric changes of the masticatory muscles following free fibular flap reconstruction of the mandibular condyle. Navigation software (iPlan, version 3.0; Brainlab) was used to perform delineation and volumetric measurement of the masticatory muscles using DICOM data.In total, 30 patients were included in this retrospective case series. In 25 cases (83.33%), the lateral pterygoid muscle achieved reattachment within 6 months postoperatively. The medial pterygoid muscles on the affected side achieved ectopic attachment in all cases. However, masseter reattachment on the affected side was achieved in only three cases. On the normal side, the volumes of lateral pterygoid muscle, medial pterygoid muscle, and masseter had recovered to almost preoperative levels at 1 year postoperatively. On the affected side, the volume of medial pterygoid muscle had decreased significantly (p = 2.4e-04) at 3 months postoperatively. The volumes of lateral pterygoid muscle and masseter showed mild decreases at 3 months postoperatively, but these were not significant (p = 0.52 and p = 0.05 for the pterygoid muscle and masseter, respectively). At 6 months after surgery, with the exception of the volume of the lateral pterygoid muscle (p = 0.06), the total volume of the masticatory muscles decreased significantly on the affected side. The volumes of lateral pterygoid muscle, medial pterygoid muscle, and masseter showed significant decreases at 1 year postoperatively (p = 0.03, p = 4.7e-08, and p = 1.1e-05, respectively) on the affected side. The postoperative volumes of the masseter, medial pterygoid, and lateral pterygoid muscles showed significant decreases due to the loss of reattachment.The results of this study may not help to ascertain whether reattachment of masticatory muscles will lead to better function. As a consequence, clinical trials of higher quality are needed.     

不同性别单侧咀嚼运动后咀嚼肌mfMRI分析

目的：利用肌肉功能磁共振成像（Muscle Functional MRI,mfMRI）定量记录受试者单侧咀嚼口香糖后双侧咬肌、翼内肌、翼外肌的活动,对比不同性别受试者咀嚼肌活动后mfMRI图像T2值变化有无统计学意义。方法：选取16名（男、女各8名）健康志愿者按性别分组,分别在咀嚼口香糖前、单侧持续咀嚼5min后行咀嚼肌区轴位mfMRI扫描,观察双侧咬肌、翼内肌和翼外肌平均质子横向弛豫时间（T2）的变化,利用Image-ProPlus6．0软件测量各咀嚼肌及相应脑脊液平均灰度值,求得咀嚼肌及相应脑脊液平均灰度值的比值,用比值进行两组间及肌肉咀嚼前后t检验,α=0．05。结果：男性组与女性组各组咀嚼肌在咀嚼口香糖前后平均灰度值比值t检验P〉0．05,差异无统计学意义。咀嚼后同侧咬肌的活动最强,其次是同侧翼内肌、对侧翼外肌和对侧咬肌。结论：根据本实验结果证实了同侧咬肌、同侧翼内肌、对侧翼外肌在单侧咀嚼过程中的重要作用,而性别对单侧咀嚼后咀嚼肌T2值变化的影响没有统计学意义。     

Tongue exercise and ageing effects on morphological and biochemical properties of the posterior digastric and temporalis muscles in a Fischer 344 Brown Norway rat model

ObjectiveThis study sought to examin effects of age and tongue exercise on the posterior digastric (opener) and the temporalis (closer). We hypothesized 1) age would result in differing morphological (cross sectional area) and biochemical (myosin heavy chain isoform) components of these muscles; 2) tongue exercise would result in coactivation of these muscles inducing a decrease in age-related differences between age groups.DesignYoung adult (9 months) and old (32 months) Fischer 344 Brown Norway rats were randomized into a tongue exercise or control group. Post-training, posterior digastric and temporalis muscles were harvested and analyzed using: 1) Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE) to assess percent myosin heavy chain (MyHC) content; 2) Immunohistochemical staining to determine cross sectional area (CSA).ResultsA larger proportion of slowly contracting MyHC isoforms in the posterior digastric and temporalis muscles were found in old. No significant main effects for age or exercise in fiber size were found in posterior digastric muscle. An interaction between age and exercise for temporalis cross sectional area indicated the old exercise group had smaller average cross sectional area than all other groups.Conclusions findingssuggest that: 1) Increasing age induces biochemical changes in muscles of the jaw, specifically showing an increase the proportion of slower contracting MyHC isoforms; 2) Increasing age and tongue exercise induce a reduction in muscle fiber cross sectional area in the temporalis muscle only. However, continued study of these cranial muscle systems is warranted to better understand these changes that occur with age and exercise.     

Histologic changes in rat masticatory muscles subsequent to experimental increase of the occlusal vertical dimension

Histologic changes in various masticatory muscles were studied in Wistar rats from 12 hours to 84 days after an experimental increase of the occlusal vertical dimension. The increases applied (1 to 2 mm) were based on the amount of interocclusal space (1 mm) in the rat. A sequence of tissue reaction from early acute inflammatory destruction of myofibers to healing with myofiber regeneration was most pronounced in the deep masseter muscle of the 2 mm group (12 hours to 21 days). A similar sequence was also observed to a lesser degree in the superficial masseter and anterior temporal muscles, while no changes were detectable in the posterior temporal, medial pterygoid, and anterior digastric muscles of this group. Variable severity of tissue response in different masticatory muscles might be attributable to the amount of stretching of myofibers as a result of an increased occlusal vertical dimension. On the other hand, degenerative atrophy of myofibers in the lateral pterygoid muscle occurred within 2 days with no sign of inflammation, and these condensed myofibers persisted as late as 84 days. Based on these findings, it was suggested that displacement of the mandibular condyle would exert an important influence on the prognosis after an increase in the occlusal vertical dimension. Only a transient acute inflammation occurred in the deep and superficial masseter muscles in the 1 mm group. Except for a small number of condensed myofibers in the lateral pterygoid muscle, degenerative changes of myofibers were not revealed. Therefore, increasing the occlusal vertical dimension within the extent of the interocclusal space might be regarded as appropriate for prosthodontic practices.     

Histochemical enzyme profile of the masseter, temporal and lateral pterygoid muscles of the European hedgehog (Erinaceus europeaus)

In man, there are large differences in histochemical fibre-type composition, distribution and size between jaw and trunk muscles, probably related to the special functions of the human stomatognathic system. In the hedgehog, the influence of alkaline and acid pre-incubations on the reaction for myofibrillar ATPase was different from that in man, suggesting a different myosin structure; the fibre composition was different also. The masseter, the superficial portion of the temporal and the lateral pterygoid muscles all showed a homogeneous fibre type profile with almost 100 per cent alkali-stable fibres. In two animals, the deep temporal muscles showed an apparent heterogeneous fibre pattern with 81 per cent alkali-stable fibres, 4 per cent alkali-labile fibres and 15 per cent ATPase-intermediate fibres; in one animal 87 per cent alkali-stable fibres and 13 per cent ATPase-intermediate fibres. There was no difference in cross-sectional area between the three fibre types within each muscle, but the fibres of the lateral pterygoid were smaller than the alkali-stable and the alkali-labile fibres of the masseter and temporal muscles. The limb and trunk muscles showed reactions for myofibrillar ATPase similar to the jaw muscles, but had a heterogeneous fibre-type profile. There was no significant difference in cross-sectional fibre area between the jaw and the limb muscles. Thus the jaw and limb muscles of the hedgehog have similar fibre types and about equal fibre size.     

Localization of masticatory motoneurons in the trigeminal motor nucleus of shrew and pig,with emphasis on the innervation ratio in the shrew

Objectives:We investigated the topographical representation of the masticatory muscles, primarily in the trigeminal motor nucleus (TMN) of the house musk shrew (Suncus murinus) and miniature pig, in a series of mammalian comparative anatomical studies. Additionally, correlations between motoneurons and muscles were investigated in order to examine the functionality of each muscle in the trigeminal motor system.Methods:Motoneurons were labeled using horseradish peroxidase (HRP) injection into each muscle innervated by the trigeminal nerve. In the shrew, the sizes and numbers of HRP-labeled neurons, muscle weight, and numbers of muscle fibers were measured in the innervated muscles, and correlation coefficients for the relationships among these parameters were calculated.Results:The motoneuron cluster of each muscle was arranged in the TMN in a manner similar to that observed in other previously reported animals, but the distribution of the lateral pterygoid motoneurons varied between species. In addition, considerably higher overlap was observed in each pig jaw-closer motoneuron cluster compared with other animals. The approximate innervation ratios were as follows: masseter, 337; temporal, 322; anterior digastric, 137; medial pterygoid, 110; lateral pterygoid, 79; mylohyoid, 42; tensor veli palatini, 42; transverse mandibular, 16; and tensor tympani muscles, 5.ConclusionsThe distribution pattern of the masticatory motoneurons was clearly observed in the shrew and pig TMN, and various correlations between the motoneuron and innervated muscles were determined.     

The theoretical substantiation of myofunctional correction of sagittal occlusion abnormalities and temporomandibular joint dysfunction

Simulation in 3D-model of skeletal forms of sagittal malocclusion revealed tendency in tonus' modification of muscles of mastication in formation of distal and mesial occlusion. It's shown that distal occlusion is characterized by hypotonic condition of muscles of mastication, except posterior fibers of temporal muscle. Mesial occlusion is characterized by complex combination of muscle tone with prevalence of hypotonic condition of anterior fibers of temporal muscle, superficial portion of masseter muscle and medial pterygoid muscle. We have detected that using of myofunctional devices in treatment of sagittal malocclusion, temporomandibular joint dysfunction promotes of tone increasing of muscles of mastication.     

Comparison of phosphocreatine concentration in the human masseter and medial pterygoid muscles by 31P-CSI

The aim of this study is to compare phosphocreatine (PCr) concentrations of human masseter and medial pterygoid muscles by a recently developed localized magnetic resonance spectroscopy (MRS) method, chemical shift imaging (CSI). The characteristic spectra of phosphorous metabolites including PCr and beta-ATP from the superficial part of the masseter (SM) and the deep part of the masseter (DM) and the medial pterygoid muscles (MPt) from 11 volunteers, 20-27-year-old were obtained. The study clearly demonstrated higher PCr/beta-ATP in the SM and MPt than in the DM both in mean values (P < 0.01) and in individual subjects. The results indicate that SM and MPt are power producers. There were no significant differences in the mean values of the PCr/beta-ATP ratios in SM and MPt, however, the PCr/beta-ATP ratios varied individually and the subjects could be divided into three distinct groups: values of MPt higher than SM (group A, 4 subjects); values of MPt almost equal to SM (group B, 3 subjects); and values of MPt lower than SM (group C, 4 subjects). There appears to be a close relationship between the PCr content as determined in the groups here and occlusal guidance.     

人咬肌和颞肌的肌构筑及生物学特性

目的：查明人咬肌和颞肌的肌构筑特征，分析两肌各部的生物力学属性，探讨其生理功能，方法：对人10侧咬肌和颞肌的构筑指数作测算和量化分析，结果：咬肌和颞肌均属多羽肌范畴，尤其是咬肌浅部内存在有多层腱板，肌纤维以浅-深两面附着于腱板上，对咬肌浅，深两部，颞肌前，后两部的肌构筑指数分别作两两比较，其构筑指数在各部间均有明显不同，其中，颞肌后部肌质量值最大，咬肌浅部次之，咬肌深部最轻，而生理横切面积则是咬肌浅部最大，依次为颞肌后部，颞肌前部，咬肌浅部和颞肌前部相近，最低者为颞肌后部，颞肌后部肌纤维最长，咬肌深部肌纤维最短，以上比较均有显著性差异。结论：咬肌深部属力量型肌，咬肌浅部和颞肌前部兼备力量和速度，而颞肌后部相对倾向于速度型肌，咬肌深部和颞肌前部在维持下颌位置和平衡中起重要作用，咬肌浅部和颞肌后部是产生咬合力和下颌快速移位的主体。