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The temporomandibular joint (TMJ) is a specialized synovial joint essential for the function of the mammalian jaw. The main components of the TMJ are the mandibular condyle, the glenoid fossa of the temporal bone, and a fibrocartilagenous disc interposed between them. The genetic program for the development of the TMJ remains poorly understood. Here we show the crucial role of sprouty (Spry) genes in TMJ development. Sprouty genes encode intracellular inhibitors of receptor tyrosine kinase (RTK) signaling pathways, including those triggered by fibroblast growth factors (Fgfs). Using in situ hybridization, we show that Spry1 and Spry2 are highly expressed in muscles attached to the TMJ, including the lateral pterygoid and temporalis muscles. The combined inactivation of Spry1 and Spry2 results in overgrowth of these muscles, which disrupts normal development of the glenoid fossa. Remarkably, condyle and disc formation are not affected in these mutants, demonstrating that the glenoid fossa is not required for development of these structures. Our findings demonstrate the importance of regulated RTK signaling during TMJ development and suggest multiple skeletal origins for the fossa. Notably, our work provides the evidence that the TMJ condyle and disc develop independently of the mandibular fossa. [ABSTRACT FROM PUBLISHER]

Size measurements of jaw muscles reflect their force capabilities and correlate with facial morphology. Using MRI, we examined the size and orientation of jaw muscles in patients with mandibular laterognathism in comparison with a control group. We hypothesized that the muscles of the deviated side would be smaller than those of the non-deviated side, and that the muscles of both sides would be smaller than in controls. In patients, a comparison of deviated and non-deviated sides showed, in orientation, differences for masseter and medial pterygoid muscles, but, in size, differences only for the masseter muscle. Nevertheless, muscle sizes in patients were much smaller than in controls. Lateral displacement of the mandible can explain the orientation differences, but not the smaller muscle size, in patients. It is possible that the laterodeviation initiates an adaptive process in the entire jaw system, resulting in extensive atrophy of the jaw muscles. [ABSTRACT FROM AUTHOR]

Quantification of the overall activity of every masticatory muscle is requisite for the analysis of stomatognathic function, which has not been accomplished by conventional electromyography. We used positron emission tomography and 18F-fluoro- deoxy-glucose to quantify the overall activity of every masticatory muscle during lateral excursion, and to evaluate the relative contribution of each masticatory muscle to lateral excursion. The present study suggested that lateral and medial pterygoid muscles are more responsible for lateral excursion than are masseter and temporal muscles. In particular, the contralateral lateral pterygoid muscle plays a major role, followed by the contralateral medial pterygoid muscle. [ABSTRACT FROM AUTHOR]

Muscle cross-sectional area (CSA) is used as a measure for maximum muscle force. This CSA is commonly determined at one location within the muscle and for one jaw position. The purpose of this study was to establish a method to standardize the analysis of the CSA of the masticatory muscles in vivo, and to compare the CSAs along their entire length for two different jaw positions (opened and closed). The CSAs in the planes perpendicular to the long axes of the masseter, medial, and lateral pterygoid muscles were measured in ten normal young adult subjects by magnetic resonance imaging. Our results showed large differences among the muscles and a nonuniform change in CSA after jaw-opening. The method enables the CSA measurement to be standardized in vivo, and allows for a correct comparison of CSAs in different skull morphologies. [ABSTRACT FROM AUTHOR]

There is controversy as to the jaw tasks for which the superior head of the human lateral pterygoid muscle (SHLP) becomes active. The aim was to describe the functional activities of SHLP single motor units (SMUs) during horizontal isometric force tasks. In 11 subjects, 48 SMUs were recorded from computer-tomography-verified SHLP sites during generation of horizontal isometric force in the contralateral (CL), protrusive (P), and ipsilateral (IL) directions and intermediate directions (CL-P, IL-P). In eight subjects, SHLP SMUs were active in CL, CL-P, and P. Qualitatively, SHLP EMG activity increased with increased isometric force. Forty-two SMUs were active in directions other than IL; 6 exhibited activity at IL and other directions. The similarity of these data to previous human lateral pterygoid (IHLP) data supports the notion that SHLP and IHLP should be regarded as a single muscle, with activities shaded according to the biomechanical demands of the task. [ABSTRACT FROM AUTHOR]

Most jaw muscles are complex, multipennate with multiple components. The morphologic heterogeneity of masticatory muscles reflects their functions. We hypothesized that the volume of masticatory muscles changes between jaw closing and opening, and that there is a difference in the volume change among the muscles. Magnetic resonance images of the entire head were obtained in ten normal young adult subjects before and after maximum jaw opening. The volume changes of the masseter, medial, and lateral pterygoid muscles were measured. Only slight changes were seen in the masseter and medial pterygoid muscles. The lateral pterygoid muscle, however, significantly decreased its volume during jaw opening. The results provide normative values of muscle volume in living subjects, and suggest that the volume changes differ among jaw muscles. [ABSTRACT FROM AUTHOR]

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 µm², 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 α+IIA and MyHCs-cardiac α+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. [ABSTRACT FROM AUTHOR]

Masticatory muscles contain a large variety of motor units with different physiological and morphological properties. In this study, we tested the hypothesis that a relationship exists between the mechanical and myo-electric properties of single motor units in the masseter muscle of the rabbit. It was expected that faster-contracting motor units, which usually have a relatively large number of fibers with large diameters, should have faster action potentials with larger amplitudes than slower motor units. Single motor units were stimulated. A two-dimensional force transducer registered mechanical parameters of the units. EMG electrodes were used to determine amplitude and frequency parameters of the action potentials of the same units. The results showed that faster-contracting motor units indeed produced action potentials with higher conduction velocities. However, faster motor units had no significant larger amplitude of the action potential. Small but significant positive correlations were found between the tetanic peak force and the amplitude of the action potentials. Little difference was found among the various frequency and amplitude parameters, respectively, making them equally suitable to describe the action potential. Surprisingly, a negative correlation between the amplitude and frequency parameters of the action potential was found, which may result from variability in arrival times of action potentials at the electrode site. Regional differences in the frequency parameters were found between the anterior and posterior parts of the superficial masseter. [ABSTRACT FROM AUTHOR]

To determine the times of the appearance of myoblasts, early myotubes, late myotubes, and myofibers, we studied a region between two aponeuroses of the medial pterygoid masticatory muscle in embryos of two strains of rabbits, without disturbing the normal innervation. The objectives of this study were to define the quantitative relations among these cells and to determine their kinetics statistically. We used Fauve de Bourgogne and New Zealand rabbit embryos on day 17, day 17 plus 12 hours, day 18, day 18 plus 12 hours, and days 20, 22, and 28 of gestation. Cell proliferation was studied with a light microscope, by means of counting methods. Similar development was observed in the two strains of rabbits. The numbers of myoblasts decreased as follows: (i) a marked decrease; (ii) a sudden cessation of the decrease, marked by a rebound at 18 days, and lasting less than 24 hours; and (iii) a plateau between embryonic days 22 and 28. The onset of reduction in the number of early myotubes coincided with the rebound of myoblasts. The number of late myotubes increased at the time of maximal early myotube density and during rebound of the myoblasts. Myofiber densities were similar to late myotube densities on day 22. We suggest that early myotubes are formed very gradually by fusion of myoblasts, and that the significant increase in the numbers of myoblasts corresponds to the second generation of myoblasts necessary for differentiation of late myotubes. [ABSTRACT FROM AUTHOR]

Muscle force is proportional to the physiological cross-sectional area (PCSA), and muscle velocity and excursion are proportional to the fiber length. The length of the sarcomeres is a major determinant of both force and velocity. The goal of this study was to characterize the architecture of the human pterygoid muscles and to evaluate possible functional consequences for muscle force and muscle velocity. For the heads of the lateral and medial pterygoid, the length of sarcomeres and of fiber bundles, the PCSA, and the three-dimensional coordinates of origin and insertion points were determined. Measurements were taken from eight cadavers, and the data were used as input for a model predicting sarcomere length and active muscle force as a function of mandibular position. At the closed-jaw position, sarcomeres in the lateral pterygoid (inferior head, 2.83 ± 0.1 µm; superior head, 2.72 ± 0.11 µm) were significantly longer than those in the medial pterygoid (anterior head, 2.48 ± 0.36 µm; posterior head, 2.54 ± 0.38 µm). With these initial lengths, the jaw angle at which the muscles were capable of producing maximum active force was estimated to be between 5° and 10°. The lateral pterygoid was characterized by relatively long fibers (inferior, 23 ± 2.7 mm; superior, 21.4 ± 2.2 mm) and a small PCSA (inferior, 2.82 ± 0.66 cm² superior, 0.95 ± 0.35 cm²), whereas the medial pterygoid had relatively short fibers (anterior, 13.5 ± 1.9 mm; posterior, 12.4 ± 1.5 mm) and a large PCSA (anterior, 2.47 ± 0.57 cm² posterior, 3.53 ± 0.97 cm²). The mechanical consequence is that the lateral pterygoid is capable of producing 1.7 times larger displacements and velocities than the medial pterygoid, whereas the medial pterygoid is capable of producing 1.6 times higher forces. The model showed that jaw movement had a different effect on active force production in the muscles. [ABSTRACT FROM AUTHOR]

Long-face subjects have smaller maximum molar bite forces than do normal individuals. This has been attributed both to differences in moment arms and size of the jaw muscles. In this study, a comparison was made between the mid-belly cross-sectional areas of the jaw muscles of 13 long-face and 35 normal adults by means of serial MRI scans. The subjects were selected on the basis of anterior lower face height as a percentage of anterior total face height. These and other cephalometric variables were measured from lateral radiographs. In the long-face group, the cross-sectional areas of the masseter, medial pterygoid, and anterior temporal muscles were, respectively, 30%, 22%, and 15% smaller than in the control group. By a discriminant analysis and a multivariate analysis of variance, these differences were found to be significant (p < 0.001). The findings of this study hint that differences in the sizes of the jaw muscles of long-face and normal subjects might explain, in part, the observed differences in maximum molar bite force. [ABSTRACT FROM AUTHOR]

The post-natal growth of the masticatory muscles in the rabbit was examined. By means of anatomical dissection and measurement, total muscle length, muscle fiber length, and muscle weight were determined in animals varying in age between one week and 36 months and exhibiting a 50-fold weight increase. Growth data were fitted by linear regression models with facial skull length used as the independent variable. Many deviations occur from size-dependent isometric growth. The muscles can be divided into three groups, according to their pattern of weight increase: The jaw openers grow negatively allometrically, and their contribution to total muscle weight decreases with time; the temporal muscle grows negatively allometrically, but its relative weight proportion remains about the same; the masseter and medial pterygoid muscles have positively allometric growth, and their contribution to total muscle weight increases strongly. Generally, the length of the muscles and of their fibers increases at lower rates than does the length of the facial skull. After weaning, the rate of longitudinal growth drops steeply in some muscles. Total fiber area orphysiological cross-section (PCS) of muscles is computed from weight and fiber length. It increases positively allometrically in the jaw losers and negatively allometrically in the jaw openers. In the lateral pterygoid muscle, the increase of PCS changes from negatively- to positively-allometric growth after weaning. The study demonstrates that individual oral muscles follow different patterns of longitudinal and cross-sectional growth, so that their functional capacities (force, range of contraction) and mutual functional relation- ships are age-dependent. [ABSTRACT FROM AUTHOR]

Cross-sectional areas of the jaw muscles were determined by means of magnetic resonance imaging (MRI) in 12 healthy adult male subjects. These findings were compared with the cross-sectional areas of the jaw muscles of the same subjects, obtained by means of computer tomography (CT) in a previous study (Weijs and Hillen, 1985). Significant correlations (r > 0.7) were found between the CT and MRI cross-sections of the masseter, medial pterygoid, and temporalis muscles. The low correlation between the CT and MRM cross-sections of the lateral pterygoid muscle could be explained by the different imaging techniques (slice thickness) of MRP and CT scanning. CT and MRM cross-sectional areas of the masseter and medial pterygoid muscle (but not the temporalis muscle) showed highly positive and significant correlations with the maximal voluntary bite force. In living subjects, the cross-sections of the masseter and medial pterygoid muscles can be visualized with CT and MR. Compared with CT, MR1 has some advantages, such as the absence of adverse effects (no radiation) and the excellent soft-tissue imaging. Furthermore, a series of frontal, horizontal, sagittal, and angulated MRI scans can be made without modification of the patient's position, facilitating reconstruction of the jaw muscles. [ABSTRACT FROM AUTHOR]

To study the relationship between craniofacial form and jaw muscle function, we evaluated 25 adult male subjects with Obstructive Sleep Apnea (age, 30-61 years; weight, 58-122 kg) on the basis of CT scans obtained for routine diagnostic purposes. All scans were obtained with the Frankfort horizontal plane at right angles to the floor; each CT slice was 8 mm thick. Masseter and medial pterygoid muscle outlines were traced, digitized, and stored, and three-dimensional reconstructions were made for calculation of muscle volume. Lateral cephalometric radiographs were analyzed for quantification of selected craniofacial variables. Significant correlations could not be identified between physiological apnea variables and jaw muscle volume. An intersubject variability in masseter muscle volume was identified (range, 22.4 - 38.1 cm³). Medial pterygoid muscle volume revealed more variability (range, 7.4 - 15.2 cm³). Masseter muscle volume had a negative correlation with mandibular plane and gonial angle, and a positive correlation with posterior face height, rams height, posterior face length, condylar center to first molar point length, gonion to pterygomaxillary fissure length, and the ramus height/anterior face height ratio. Medial pterygoid muscle volume showed a positive correlation with posterior face height, rams height, posterior face length, and the lengths between condylar center to first molar contact point, gonion to pterygomaxillary fissure, and antegonion to key ridge. Subjects with large masseter and medial pterygoid muscle volumes had flat mandibular and occlusal planes, and small gonial angles. [ABSTRACT FROM AUTHOR]

EMG recordings of the left and right masseter and lateral pterygoid muscles were repeated three times in 15 young adults to test the reproducibility of the signals. Two tests were made on the same day (morning and afternoon) and one test three days later (morning). Needle and hook electrodes were used for the masseter muscle and needle electrodes for the lateral pterygoid muscle. The superficial areas of recording were tattooed on the skin. An intra-oral approach was selected for the lateral pterygoid muscle. The subjects were asked to perform three definite tasks for the masseter muscle (intercuspid occlusion, maximum protrusion, and centric relation with a bite opening of 3mm) and two for the pterygoid muscle (maximum opening and maximum protrusion). The EMG signals were directly integrated, and root mean squares of the AC components were computed. The selection of the signals to discard possible artefacts was performed by displaying EMG and RMS outputs on a six-channel UV oscillograph. The selected RMS values were submitted to different analyses of variance to define additive and non-additive models of effects. The results may be summarized as follows: The between-day sessions showed more variation than the within-day sessions. The variations related to the method accounted for a very high percentage of the total variance (48.9%). The recordings with needle electrodes displayed a large percentage of the individual variation (masseter, 31.2%; lateral pterygoid muscle, 65.9%) and significantly lower values for the variations related to the session or the side of recording. The one-side recordings were more suitable for distinguishing the biological variations than were the bilateral ones. The right side was less sensitive to variations than the left side. The study of the lateral pterygoid muscle was less dependent on "session", "side", or "task" effects than was that of the masseter muscle. The lack of significant variation resulting from the performed tasks suggested that only the lower bundle was recorded with a similar activity in protrusion and opening. The reproducibility of the superficial recordings of the masseter muscle may be improved if non-significant interactions between "task" and "area" or "side" factors, as well as between "task" and "session", are taken into consideration. A direct comparison of the different tasks performed in one "session" will help reduce the undesirable effects due to the random selection of the recording "side" or "site". [ABSTRACT FROM AUTHOR]

Cross-sectional areas of the masseter, temporalis and medial, and lateral pterygoid muscles were determined in 16 subjects by means of computer tomography. In each subject three scans were made, intersecting the thickest part of the muscles at right angles to the fiber direction. The masseter and medial pterygoid muscles are large in persons with brachycephalic skulls, short faces, and a small jaw angle. The cross-sectional areas of the temporalis and lateral pterygoid muscles showed no correlation with facial dimensions. [ABSTRACT FROM AUTHOR]

Several abstracts of articles related to dentistry are presented including "Epidermal growth factor and proliferation of odontogenic cells in culture," by N. E. Steidler, "Local Immunity to Bacteroides melaninogenicus in Periodontal Disease," by J. Kagan, and "Medial Pterygoid Activity During Unilateral Chewing in Man," by W. W. Wood and A. G. Hannam.

Histological study of the human jaw -muscle revealed that the temporal muscle displayed 342 muscle spindles, 208 in the horizontal and 134 in the vertical portion; the masseter muscle contained 114 spindles, 91 in the superficial and 23 in the profound portion; the medial pterygoid muscle had 59; and the lateral pterygoid muscle contained 6, four in the upper head and two in the lower head. These data suggest that the extensive mobility of the temporomandibular joint, and the maintenance of mandibular posture during mastication and speech are strongly influenced by proprioceptive mechanisms. [ABSTRACT FROM AUTHOR]

This study of the masticatory muscles of the primate showed that the temporal muscle contained 107 muscle spindles, 45 in the horizontal portion and 62 in the vertical portion; the masseter muscle contained 70, 58 in the profundus portion and 12 in the superficial portion; the medial pterygoid muscle contained 15; the lateral pterygoid muscle contained 6; and the zygomaticomandibular muscle contained 9. The muscle spindles were located around the coronoid process and mandibular ramus. [ABSTRACT FROM AUTHOR]

Histological study of the masticatory muscles of the tree shrew showed that 140 muscle spindles are concentrated in the restricted areas of the inner lowermost layer of the horizontal and vertical portions of the temporal muscle (48 and 47 spindles, respectively), the profundus portion of the masseter muscle (42 spindles), and the zygomaticomandibular muscle (3 spindles). The medial and lateral pterygoid muscles are devoid of spindles. [ABSTRACT FROM AUTHOR]

The article presents research concerning forces exerted on the human mandible by the individual muscles of occlusion and their lines of action. Past research methods and finding on the topic are assessed. The scheme used to measure the direction of force of different muscles and approximate absolute force of each is explained. Details of the relative forces exerted by the masseter, internal pterygoid, and temporal muscles are presented.

This investigation was designed to obtain anatomical information about the neuromuscular mechanism of human jaw movement by a comparative study of insectivores. Study of the masticatory muscles of the Japanese shrew-mole showed that muscle spindles are concentrated in restricted areas of the inner layer of the horizontal and vertical portions of the temporal muscle, the medial portion of the medial pterygoid muscle, and the deep portion of the masseter muscle. The lateral pterygoid muscle contains no spindles. [ABSTRACT FROM AUTHOR]

Electromyography using wire microelectrodes and multichannel recording of the masseter and medial pterygoid muscles demonstrated that in the rabbit, the ipsilateral medial pterygoid muscle actively opposes lateral mandibular movement. In the rat, the ipsilateral medial pterygoid muscle and contralateral masseter muscle oppose the movement. The results correlate with the musculoskeletal arrangements of the two animals. [ABSTRACT FROM AUTHOR]

This article presents abstracts of research studies offered at the Sixth Annual Meeting of the Australian Division of the International Association for Dental Research held at the University of Adelaide in Adelaide, Australia from August 20-22, 1968, including "Third-Molar Development in Australian Aborigines and Whites," "Muscle Spindles in the Lateral Pterygoid Muscle," and "Sympathetic Nerves in Oral Tissue."

The muscles of mastication of the rabbit, cat, dog, and monkey were characterized as mixed red and white muscle types on the basis of succinic dehydrogenase and phosphorylase content. The chacterization was related to the known function of the masticatory apparatus peculiar to each animal. [ABSTRACT FROM AUTHOR]

Morphologic observation and EMG recordings of the lateral pterygoid muscle were performed on the cat. During repetitive mandibular movements, EMG activities of the lateral pterygoid muscle were recorded at the opening stage of the mandible. This muscle was shown to keep the articular disc in forward position during mandibular opening movement. [ABSTRACT FROM AUTHOR]

The article focuses on the dimensional and positional variations of the ramus of the mandible. The authors noted that the dimension of the attachment surface of the internal pterygoid muscle was subject to significantly large variations. This is important because this muscle limits one side of the pterygomandibular space wherein the solution is deposited.