Biomedical subjects

W A Weijs

Publications and source records attributed to W A Weijs.

At least 19 recordsLinked to original sources

PubMed1992

Presence of cardiac alpha-myosin correlates with histochemical myosin Ca2+ ATPase activity in rabbit masseter muscle.

A combined enzyme-histochemical (ATPase reactivity) and immunohistochemical study has been performed on sections of rabbit masseter muscle. The majority of the fibres previously designated as type IIC and/or type I according to their ATPase activity were found to contain 'cardiac' alpha-myosin heavy chain in addition to other myosin heavy chains. All alpha-myosin heavy chain-containing fibres reveal ATPase activity after pre-incubation at pH 4.2-4.6 similar to that of the classical type I fibres, while, in that pH range, limb type IIC fibres show intermediate ATPase activity. One group of these fibres reveal ATPase activity after pre-incubation at pH 10.1-10.3 as well, but not at pH 10.4-10.5. These fibres contain exclusively either alpha- or alpha- and I-myosin heavy chains but do not contain the IIA-myosin heavy chain. The second part of the fibres reveals ATPase activity after treatment within the whole alkaline pre-incubation range (pH 10.1-10.5) and these fibres contain alpha-myosin and IIA-myosin but no I-myosin heavy chain. It is concluded that the classical IIC fibre type is not present in the rabbit masseter muscle. Furthermore, ATPase reactivity does not allow us to distinguish fibres on their myosin heavy chain content in rabbit masseter muscle.

Animals↗

PubMed1992

Electromyographic heterogeneity in the human masseter muscle.

The complex, pennate architecture of the human masseter muscle points to a functional division into more than the commonly distinguished deep and superficial parts. In this study, the possible existence of regional differences in activation was examined. EMG activity was registered in three deep and three superficial regions with the use of bipolar fine-wire electrodes. Recordings were made during different static bite tasks, in specified directions, and with a specified bite-force magnitude. A linear bite-force/EMG relationship was observed. Furthermore, it appeared that muscle regions showed a different pattern of change in activity as a function of bite-force direction. Heterogeneity was nearly absent in anteriorly-, anteriomedially-, and medially-directed bites, but became increasingly obvious in the other bite-force directions. The posterior deep region showed the most aberrant activation pattern, which was almost opposite that from the other regions. This part was fully active in posterolaterally-directed bites. The posterior superficial region showed the largest variability in activity as a function of bite-force direction. The results point to a functional partition of the masseter muscle in at least three parts: anterior deep, posterior deep, and superficial. A further subdivision of the superficial portion might be present, but was not as obvious as the division of the deep masseter.

Adult↗

PubMed1992

A comparison of jaw muscle cross-sections of long-face and normal adults.

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 less than 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.

Adult↗

PubMed1992

Preweaning feeding mechanisms in the rabbit.

Muscle contraction patterns and mandibular movements of infant rabbits during suckling and chewing were compared. Oral muscle activity was recorded by fine-wire electromyography, while jaw movements and milk bottle pressure were registered. Suckling and mastication have a comparable cycle duration and share a common pattern of oral muscle activity which consists of a succession of a jaw closer burst, during which the jaw closes and undergoes a power stroke (in mastication), a suprahyoid burst with a stationary or slightly opening jaw and a digastric burst with fast jaw opening (the power stroke of suckling). Compared to suckling, mastication shows decreased jaw opener activity, increased jaw closer activity, development of jaw closing activity in the lateral pterygoid, and increased asymmetry in the masseter by development of a new differentiated motor pattern on the working side. The study shows that the suckling motor pattern enables the infant rabbits to change to chewing with just a few modifications.

Animals↗

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↗

PubMed1991

Biomechanical changes in the rabbit masticatory system during postnatal development.

Using dissection, biometry, and two three-dimensional mechanical models, the postnatal changes of the rabbit masticatory muscles were studied by analyzing their three-dimensional orientation, their strength and fiber lengths, and certain functional consequences of these changes. The first mechanical model uses length-tension relationships of the muscles and predicts the maximum bite force as a function of mandibular position. It shows that young rabbits are able to generate large bite forces at a wider gape than adult animals and that the forces are directed more vertically. In spite of the postnatal changes the mechanical advantage of the system remains about equal. However, the muscles are reoriented so that they exert a larger degree of parallel action, suggesting a larger bite force magnitude but a smaller range of bite force directions. The second model-predicts this range. It shows that during postnatal development a relative gain occurs in the possibilities for the system to exert forces directed rostrodorsally. In all other directions the capability to exert force decreases. The results suggest that during development the possibility of the system to generate large bite forces is increased at the cost of a restriction in the range of jaw excursion and that a restriction takes place in the range of possible force directions that can be exerted at the molars.

Animals↗

PubMed1991

Demonstration of 'cardiac-specific' myosin heavy chain in masticatory muscles of human and rabbit.

Human and rabbit masticatory muscles were analyzed immuno- and enzyme-histochemically using antibodies specific to 'cardiac' alpha, slow and fast myosin heavy chain isoforms. In human masseter, temporalis, and lateral pterygoid muscle 'cardiac' alpha myosin heavy chain is found in fibres that contain either fast, or fast and slow myosin heavy chain. In rabbit masseter, temporalis and digastric muscles, fibres are present that express 'cardiac' alpha myosin heavy chain either exclusively, or concomitantly with slow myosin heavy chain or fast myosin heavy chain. Our results demonstrate a much broader distribution of 'cardiac' alpha myosin heavy chain than hitherto recognized and these might explain in part the specific characteristics of masticatory muscles. The 'cardiac' alpha myosin heavy chain is only found in skeletal muscles originating from the cranial part of the embryo (including the heart muscle), suggesting that its expression might be determined by the developmental history of these muscles.

Adult↗

PubMed1991

Relationships between jaw muscle cross-sections and craniofacial morphology in normal adults, studied with magnetic resonance imaging.

In 32 Caucasian adult males serial MRI scans of the jaw muscles were taken approximately perpendicular to the mean fibre direction of the jaw muscles to determine their cross-sectional areas. These areas are proportional to the maximal isometric strength of a muscle. To describe facial skeletal variation, nine angular and 21 linear cephalometric measurements were recorded, and statistically reduced by means of multiple regression and principal component analysis. Six components were extracted, rotated, and subsequently correlated with the maximal cross-sectional areas of the jaw elevators and anterior digastric muscle. Positive significant correlations were found between a linear combination of several transversal skull dimensions on the one hand, and the maximal temporalis and masseter cross-sections on the other. A negative significant correlation was found between the flexure of the cranial base and the temporalis cross-section. Surprisingly, no significant correlations were found between either anterior facial height or posterior facial height and any of the jaw muscles cross-sections. It was concluded that, in adult males with normal skull shape, relationships exist to a limited extent between craniofacial morphology and the cross-sectional areas of the jaw muscles.

Adult↗

PubMed1991

Relationships between spindle density, muscle architecture and fibre type composition in different parts of the rabbit masseter.

The occurrence of muscle spindles was studied in the masseter muscle of rabbits by light-microscopy of whole muscle sections. The distribution of the spindles appeared to be heterogeneous. Most spindles lay in the anterior/deep and mid-parts of the masseter. The lateral/superficial part contained only a few muscle spindles. The distribution of the spindles is correlated to the distribution of slow twitch (type I) extrafusal fibres. This means that spindles, like type I fibres might be involved in the control of fine movements and posture. Spindle density and type I fibre density increase with distance from the temporomandibular joint. This could mean that spindles are involved in controlling bite force.

Animals↗

PubMed1990

Elastic and collagenous fibers in the temporomandibular joint capsule of the rabbit and their functional relevance.

This study deals with the form and function of the temporomandibular joint capsule and compares the histological structure of the capsule and kinetics of condylar and disc movement in rabbit and man. The morphology of the capsule of the jaw joint of the rabbit was studied by means of histological sections of isolated capsules, celloidin sections of decalcified heads, and cryostat sections containing the joint, fixed in jaw open and closed positions. General staining methods and selective methods for elastin and collagen were used. The volume density of the elastic fibers was determined morphometrically. The capsule consists of a portion connecting disc and skull and a portion connecting disc and mandible. The former portion has a fibrous outer layer of dense connective tissue. It contains numerous bundles of specifically oriented collagen but hardly any elastic fibers. Few irregularly arranged elastic fibers are present in the lateral and medial areas of the capsule. In contrast, the portion connecting disc and condylar process contains not only oriented collagen but also large amounts of elastic fibers. Anteriorly, there is a thin elastic band that is stretched greatly at jaw opening. Posteriorly, the capsule takes the appearance of a large fibro-elastic pad that covers the posterior extension of the condyle. This part is somewhat stretched when the jaw is closed. The findings correlate well with the results of jaw movement studies, showing a remarkable capability for condylar protrusion and retrusion. In contrast to the situation in humans, this movement occurs mainly between condyle and disc. It is made possible by the rather tight connections between skull and disc and flexible ones between disc and condyle.

Animals↗

PubMed1990

A new model for calculating muscle forces from electromyograms.

A muscle model is described that uses electromyogram (EMG), muscle length and speed of contraction to predict muscle force. Physiological parameters are the Hill constants and the shape of the twitch response to a single stimulus. The model was incorporated in a jaw model of the rabbit and tested by predicting the bite force produced by the jaw muscles during mastication. The time course of the calculated force appeared to match the bite force, measured in vivo by a strain gauge, applied to the bone below the teeth. The variation in peak strain amplitude from cycle to cycle correlated with the variation predicted by the model. The peak amplitude of the integrated EMGs of individual jaw muscles showed an average correlation with peak strain of 0.41. Use of the sum of the available peak amplitudes, weighted according to their effect upon the bite force increased the correlation to 0.46; the model predicted bite forces showed a correlation of 0.57 with the strain. The increase in correlation was statistically significant. The muscle forces were calculated using a minimum number of easily obtainable constants.

Animals↗

PubMed1990

Computer-assisted estimation of lines of action of human masticatory muscles reconstructed in vivo by means of magnetic resonance imaging of parallel sections.

The orientation of these lines of action was estimated in 9 healthy subjects, by reconstructing the muscle shape from a series of parallel sections obtained by MRI. In order to gain insight into sources of error, the lines of action of the masseter and medial pterygoid were estimated from two mutually perpendicular series of sectional images. Average results were compared with anatomical data from the literature. The results indicated that the accuracy of the estimate was principally dependent on the reliability of the reconstructions; the average accuracy of the estimated orientations was about 5 degrees.

Adult↗

PubMed1990

Coactivation of jaw muscles: recruitment order and level as a function of bite force direction and magnitude.

The aim of this study was to obtain insight into the coactivation behaviour of the jaw muscles under various a priori defined static loading conditions of the mandible. As the masticatory system is mechanically redundant, an infinite number of recruitment patterns is theoretically possible to produce a certain bite force. Using a three-component force transducer and a feedback method, subjects could be instructed to produce a bite force of specific direction and magnitude under simultaneous registration of the EMG activity of anterior and posterior temporal, masseter and digastric muscles on each side. Forces were measured at the second premolars. Vertical, anterior, posterior, lateral and medial force directions were examined; in each direction force levels between 50 N and maximal voluntary force were produced. The results show that for all muscles the bite force-EMG relationship obeys a straight-line fit for forces exceeding 50 N. The relationship varies with bite force direction, except in the case of the digastric muscles. Variation is small for the anterior temporal and large for the posterior temporal and masseter muscles. The relative activation of muscles for a particular force in a particular direction in unique, despite the redundancy.

Adult↗

PubMed1990

Growth patterns of the rabbit masticatory muscles.

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 or physiological cross-section (PCS) of muscles is computed from weight and fiber length. It increases positively allometrically in the jaw closers 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 relationships are age-dependent.

Animals↗

PubMed1990

Histochemical and functional fibre typing of the rabbit masseter muscle.

The fibre-type distribution of the masseter muscle of the rabbit was studied by means of the myosin-ATPase and succinate dehydrogenase reactions. Six different fibre types were found and these were unequally distributed between and within the anatomical compartments of the muscle. Most of the masseter consists of slow- and fast-twitch oxidative fibres. The slow fibres increase in numbers in the deeper and more anterior regions of the muscle. Fast-twitch glycolytic fibres were almost exclusively found in the most posterior portions of the superficial and deep masseter. The fibre composition within the sagittally orientated anatomical compartments was found to be correlated with maximal contraction speeds during natural mastication as estimated from a mechanical model. However, the differences in fibre composition between the anatomical compartments (and hence between superficial and deep layers) appeared not to be correlated with contraction speed. The regional and compartmental specialisation within the masseter permits the muscle to perform many different functional roles in the generation and control of the jaw movements, jaw position and bite forces.

Aerobiosis↗

PubMed1989

Jaw movements and muscle activity during mastication in growing rabbits.

To investigate the biomechanical effects of juvenile growth changes in the rabbit masticatory apparatus a comparison was made of mastication in just-weaned and adult animals. Mandibular movements in two planes were registered by cineradiography. Masticatory muscle activity was recorded by fine-wire electromyography. The same pattern of unilateral mastication was present in the two ages. The most important changes in the jaw movements are 1) a decrease of jaw opening speed and chewing frequency and an increase in jaw opening time, 2) a decrease in maximum gape in soft food and an unaltered gape in small-particle hard food, and 3) an increase in lateral jaw excursion, mainly due to a more pronounced movement of the jaw to the balancing side (lingual phase). The contraction patterns were basically similar in the two ages. The higher chewing frequency in young animals was attained by a larger degree of overlap between opening and closing muscle activities. Young animals used relatively more EMG activity to chew hay, the hardest food. The changes in opening speed, gape, and chewing frequency are consistent with earlier predictions from the morphological changes, and so is the extra activity needed to chew hard food. The increase in lateral excursion was not predicted. It is suggested to be caused by cheek teeth wear, making possible smooth occlusal guidance of the jaw at the balancing side. Some of the changes in juvenile morphology can be viewed as adaptations to a changing diet.

Aging↗

PubMed1989

An iterative procedure to estimate muscle lines of action in vivo.

A method is described to estimate the line of action of muscles in the three-dimensional space from serial images of parallel muscle sections obtained in vivo by means of CT or MRI scanning. The external shape of a muscle, reconstructed from the series of parallel sections, is mathematically divided into a series of imaginary slices directed arbitrarily in the three-dimensional space. The line of action is estimated initially as a regression line through the centroids of these mathematical slices. A new series of mathematical slices is constructed perpendicular to the regression line and a new estimate of the line of action is obtained from their centroids. This procedure is repeated until the estimated line of action is perpendicular to the mathematical slices; it can then be considered as a reliable estimate of the line of action. The accuracy of the method has been tested for various reconstruction parameters and muscle shapes. The results of these tests show that the accuracy is relatively independent of the direction in which the sectional images have been made and that, except for relatively short and thick muscles, the estimated lines of action deviated less than about 2 degrees from the theoretical one. The presented method is a relatively simple mathematical technique which can be used easily for muscles reconstructed in vivo from routinely obtained sectional MRI or CT images.

Biomechanical Phenomena↗

PubMed1989

Electromyographic activity of the human masticatory muscles during submaximal clenching in the inter-cuspal position.

The activity patterns of the masseter and the anterior temporal muscles were studied in twenty-one healthy male subjects while clenching at 10, 20, 30, 40 and 50% of the maximum clenching level. At low clenching levels the temporal muscle activity tended to dominate, at high levels the masseter muscle activity was stronger (P less than 0.001). The asymmetry in muscle activity also depended upon the clenching level (P less than 0.001), while at each level the masseter muscle asymmetry was greater than the temporal muscle asymmetry (P less than 0.05-P less than 0.025). By comparing the electromyographic activities of the left and right side within each subject it was found that the masseter muscle with the higher electromyographic activity tended to have the larger cross-sectional area (P less than 0.01) and at the 50% clenching level it tended to be on the side with the greater number of post-canine tooth contacts (P less than 0.001).

Adult↗