Histoenzymology and morphometry of the masticatory muscles of tufted capuchin monkey (Cebus apella Linnaeus, 1758).
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Samples of the anterior and posterior regions of the masseter and temporal muscles and of the anterior belly of the digastric muscle of 4 adult male tufted capuchin monkeys (Cebus apella) were removed and stained with HE and submitted to the m-ATPase reaction (with alkaline and acid preincubation) and to the NADH-TR and SDH reactions. The results of the histoenzymologic reactions were similar, except for acid reversal which did not occur in fibers of the fast glycolytic (FG) type in the mandibular locomotor muscles. FG fibers had a larger area and were more frequent in all regions studied. No significant differences in frequency or area of each fiber type were detected, considering the anterior and posterior regions of the masseter and temporal muscles. The frequency of fibers of the fast oxidative glycolytic (FOG) and slow oxidative (SO) types and of FOG area differed significantly between the anterior belly of the digastric muscle and the mandibular locomotor muscle. The predominance of fast twitch (FG and FOG) fibers and the multipenniform and bipenniform internal architecture of the masseter and temporal muscles, respectively, are characteristics that permit the powerful bite typical of tufted capuchin monkeys. (+info)
Functional morphology of the mastication muscles in the lesser and greater mouse deer.
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The mastication muscles were examined in the lesser (Tragulus javanicus) and greater mouse deer (Tragulus napu) to clarify the form of the mastication muscles in these primitive artiodactyls. The M. masseter was well-developed in both species, however the attachment area of its origin was not confirmed in the rostral facial part. The masseter bundles were not observed on the lateral side of the maxilla bone, and their origin was restricted to the zygomatic arch area. This suggests that the M. masseter may not act as a motor raising the mandible rostro-dorsally, but pull the insertion vertically unlike the highly derived grazer of Bovidae. The Crista temporalis was weak and the M. temporalis was thin in the mouse deer, and this indicates that the M. temporalis may not be important in the mastication in the primitive artiodactyls. These findings suggest that the browser such as mouse deer has been adapted for the feeding on soft leaves, and functional-morphologically different in mastication strategy from the grazer such as developed Bovidae species. The architecture of the mastication muscles was not different between the two species. However, in the muscle weight ratios per body weight, the M. temporalis and the M. digastricus were significantly smaller in greater mouse deer than in lesser mouse deer. (+info)
Modelling the masticatory biomechanics of a pig.
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The relationships between muscle tensions, jaw motions, bite and joint forces, and craniofacial morphology are not fully understood. Three-dimensional (3-D) computer models are able to combine anatomical and functional data to examine these complex relationships. In this paper we describe the construction of a 3-D dynamic model using the anatomical (skeletal and muscle form) and the functional (muscle activation patterns) features of an individual pig. It is hypothesized that the model would produce functional jaw movements similar to those recordable in vivo. Anatomical data were obtained by CT scanning (skeletal elements) and MR imaging (muscles). Functional data (muscle activities) of the same animal were obtained during chewing by bipolar intramuscular electrodes in six masticatory muscles and combined with previously published EMG data. The model was driven by the functional data to predict the jaw motions and forces within the masticatory system. The study showed that it is feasible to reconstruct the complex 3-D gross anatomy of an individual's masticatory system in vivo. Anatomical data derived from the 3-D reconstructions were in agreement with published standards. The model produced jaw motions, alternating in chewing side, typical for the pig. The amplitude of the jaw excursions and the timing of the different phases within the chewing cycle were also in agreement with previously published data. Condylar motions and forces were within expected ranges. The study indicates that key parameters of the pig's chewing cycle can be simulated by combining general biomechanical principles, individual-specific data and a dynamic modelling approach frequently used in mechanical engineering. (+info)
Control of facial muscle development by MyoR and capsulin.
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Members of the MyoD family of basic helix-loop-helix (bHLH) transcription factors control the formation of all skeletal muscles in vertebrates, but little is known of the molecules or mechanisms that confer unique identities to different types of skeletal muscles. MyoR and capsulin are related bHLH transcription factors expressed in specific facial muscle precursors. We show that specific facial muscles are missing in mice lacking both MyoR and capsulin, reflecting the absence of MyoD family gene expression and ablation of the corresponding myogenic lineages. These findings identify MyoR and capsulin as unique transcription factors for the development of specific head muscles. (+info)
Sagittal occlusal relationships and asymmetry in prematurely born children.
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The aim of this investigation was to examine the effect of pre-term birth on sagittal occlusal relationships. The subjects were 328 prematurely born white and black children and 1804 control children who participated in the cross-sectional study of the Collaborative Perinatal Project (USA) in the early 1960s and 1970s. Dental examinations, including dental casts and photographs, were performed at the age of 6-12 years. The sagittal occlusion of the permanent molars and the canine relationship was recorded by examining and measuring the hard stone casts. The pre-term and comparison groups were divided by sex and race. A significantly greater prevalence of pre-normal canine relationships was found in the pre-term group than in the controls (P < 0.001). The incidence of a bilateral symmetrical canine relationship was 60.3 per cent in both the pre-term and control groups, but in the pre-term group the girls had better symmetry than the boys. Asymmetry occurred significantly more often on the left side (P < 0.001), especially in the control boys, but this was not so clear in the pre-term group. The prevalence of mesial molar occlusion was greater in the pre-term group. These results suggest that premature birth and the consequent exceptional adaptation from intra- to extra-uterine nutrition may influence dental occlusal development. This emphasizes the importance of early functional activity and differences in masticatory muscle activity and the largely unknown phenomenon of early catch-up growth. Individual differences in neonatal factors, in the need for intubation and other medical care are also of importance. Pre-term birth may also interfere with the development of symmetry and lateralization. (+info)
Ontogeny of feeding function in the gray short-tailed opossum Monodelphis domestica: empirical support for the constrained model of jaw biomechanics.
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The constrained model of masticatory function enables specific predictions of bite force potentials in skulls of differing craniodental configurations. In this study, empirical support for the constrained model is provided using maximum voluntary bite force data along Region I and II of the jaws of gray short-tailed opossums Monodelphis domestica. Then, growth series of M. domestica are used to assess how bite force potential changes with growth by evaluating craniodental changes using longitudinal sets of dorsoventral radiographs and by assessing maximal bite force potential at the Region I-II boundary of the jaw in juveniles (aged 70-80 days) and adults. Our findings show that, while juveniles and adults alike enclose at least three molariform teeth within Region II (the area of highest bite force potential along the jaw), age-dependent elongation of the masticatory muscle resultant lever arm and narrowing of the palate relative to jaw length especially enhance the mechanical advantage of the adductor muscle resultant in adults. While maximal bite forces at the Region I-II boundary are absolutely greater in adults, these bite forces scale isometrically with body mass, which suggests that mass-specific forces exerted by jaw adductor muscles of larger (adult) opossums are disproportionately smaller than those exerted by smaller (juvenile) opossums. (+info)
Jaw reflexes and masseter electromyograms in mesencephalic and pontine lesions: an electrodiagnostic study.
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Jaw reflexes and masseter electromyograms were studied in five patients with mesencephalic and 11 patients with pontine lesions, vascular or tumorous in nature. In the former group jaw reflexes were abnormal, being delayed or absent, whereas masseter electromyograms were normal. In the latter group, both jaw reflexes and masseter EMG, showing denervation, were abnormal in six and both normal in five cases. It is suggested that the afferent limb of the human jaw reflex passes through the mesencephalic trigeminal nucleus. The methods proved to be valuable in the diagnosis of mesencephalic and pontine lesions involving the fifth cranial nerve. (+info)
Pulsatile control of the human masticatory muscles.
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Spectral analysis of jaw acceleration confirmed that the human mandible 'trembles' at a peak frequency around 6 Hz when held in its rest position and at other stationary jaw openings. The 6 Hz tremor increased during very slow movements of the mandible, but other lower-frequency peaks became prominent during more rapid jaw movements. These lower-frequency peaks are likely to be the result of asymmetries in the underlying, voluntarily produced, 'saw-tooth' movements. In comparison, finger tremor at rest and during slow voluntary movements had a mean peak frequency of about 8 Hz: this frequency did not change during rhythmical finger flexion and extension movements, but the power of the tremor increased non-linearly with the speed of the movement. The resting jaw tremor was weakly coherent with the activity of the masseter and digastric muscles at the tremor frequency in about half the subjects, but was more strongly coherent during voluntary movements in all subjects. The masseter activity was at least 150 deg out of phase with the digastric activity at the tremor frequency (and at all frequencies from 2.5-15 Hz). The alternating pattern of activity in antagonistic muscles at rest and during slow voluntary movements supports the idea that the masticatory system is subject to pulsatile control in a manner analogous to that seen in the finger. (+info)