Comparative anatomy of the buccinator muscle in cat (Felis domestica).
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Published descriptions of the buccinator muscle of the cat (Felis domestica) differ from those for the same muscle in other mammals. Only an oral component of the muscle has been described in cats, not a buccal part. The purpose of this study was to identify the buccinator muscle in the cat and report on its anatomical features in detail. Dissections of the facial muscles were carried out on 12 specimens of adult cats (6 males and 6 females) that had been fixed with 10% formalin. We then observed the facial muscles and traced their innervations, arteries, and veins under a binocular microscope. The buccinator muscle in the cat was identified underneath an orbicularis oris, arising from the lower buccal membrane and from the molar region of the alveolar border of the mandible. It was about 3 mm wide at its origin, 4 mm wide at its insertion, and about 11 mm in length from origin to insertion. This contrasts with humans, in whom the muscle arises not only from the mandible, but also from the maxilla. Apart from this difference, this muscle in cats displays the following similarities to the buccinator muscle of other mammals: 1) it is innervated by the facial nerve; 2) it supports the buccal membrane; 3) it seems to insert into the modiolus; 4) its bundles run antero-posteriorly; 5) the posterior part of the muscle is located medially to the masseter muscle; 6) the parotid duct, facial nerve, artery, and vein run lateral to the muscle; 7) it is located deeper than other facial muscles; and 8) the buccal nerve runs on its surface. These relationships are spatially similar to those of the buccinator muscle in mammals. This muscle may aid in mastication, including suckling, and in expelling air forcibly, like the buccinator in humans. (+info)
Control of oral closure in lingual stop consonant production.
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Previous work has shown that the lips are moving at a high velocity when the oral closure occurs for bilabial stop consonants, resulting in tissue compression and mechanical interactions between the lips. The present experiment recorded tongue movements in four subjects during the production of velar and alveolar stop consonants to examine kinematic events before, during, and after the stop closure. The results show that, similar to the lips, the tongue is often moving at a high velocity at the onset of closure. The tongue movements were more complex, with both horizontal and vertical components. Movement velocity at closure and release were influenced by both the preceding and the following vowel. During the period of oral closure, the tongue moved through a trajectory of usually less than 1 cm; again, the magnitude of the movement was context dependent. Overall, the tongue moved in forward-backward curved paths. The results are compatible with the idea that the tongue is free to move during the closure as long as an airtight seal is maintained. A new interpretation of the curved movement paths of the tongue in speech is also proposed. This interpretation is based on the principle of cost minimization that has been successfully applied in the study of hand movements in reaching. (+info)
Reflex control of human jaw muscles.
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The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects. (+info)
Trigeminal and polyradiculoneuritis in a dog presenting with masticatory muscle atrophy and Horner's syndrome.
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A 9-year-old, spayed female, Airedale Terrier was euthanatized and necropsied after a progressive clinical course that included Horner's syndrome of the left eye and unilateral atrophy of the masticatory muscles. Although gross lesions were limited, a polyradiculoneuritis and ganglionitis that was most severe in the trigeminal nerves and ganglia were confirmed histologically. The inflammatory infiltrate consisted predominantly of macrophages and B and T lymphocytes that were phenotypically confirmed by immunostaining. Horner's syndrome was the result of damage to postganglionic sympathetic fibers that were incorporated in segments of the inflamed trigeminal nerve and its ophthalmic branch. Histologically, the character and distribution of the inflammation was similar to previously described syndromes of suspected immune-mediated etiology in humans and animals. (+info)
'Superfast' or masticatory myosin and the evolution of jaw-closing muscles of vertebrates.
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There are four fibre types in mammalian limb muscles, each expressing a different myosin isoform that finely tunes fibre mechanics and energetics for locomotion. Functional demands on jaw-closer muscles are complex and varied, and jaw muscles show considerable phylogenetic plasticity, with a repertoire for myosin expression that includes limb, developmental, alpha-cardiac and masticatory myosins. Masticatory myosin is a phylogenetically ancient motor with distinct light chains and heavy chains. It confers high maximal muscle force and power. It is highly jaw-specific in expression and is found in several orders of eutherian and marsupial mammals including carnivores, chiropterans, primates, dasyurids and diprotodonts. In exceptional species among these orders, masticatory myosin is replaced by some other isoform. Masticatory myosin is also found in reptiles and fish. It is postulated that masticatory myosin diverged early during gnathostome evolution and is expressed in primitive mammals. During mammalian evolution, mastication of food became important, and in some taxa jaw closers replaced masticatory myosin with alpha-cardiac, developmental, slow or fast limb myosins to adapt to the variety of diets and eating habits. This occurred early in some taxa (rodents, ungulates) and later in others (macropods, lesser panda, humans). The cellular basis for the uniqueness of jaw-closing muscles lies in their developmental origin. (+info)
Vibration-induced discharge patterns of single motor units in the masseter muscle in man.
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Single motor unit potentials were recorded with small bipolar wires from intact masseter muscles in the adult man and a detailed parametric analysis of the effects of muscle vibration on motor unit discharges was carried out. 2. When the vibration amplitude was kept constant, each unit started firing at a definite threshold of vibration frequency. With higher frequencies the rate of firing rapidly reached a maximum. Units recruited at higher frequencies presented a lower maximum rate of firing. 3. When the vibration frequency was kept constant, each masseter unit discharged at a definite threshold of vibration amplitude. With higher amplitudes the unit quickly reached a maximum rate of discharge. Units with a higher frequency threshold tended to also present a higher amplitude threshold. Motor unit "excitability" curves could be plotted using the combined threshold conditions for frequency and amplitude of applied vibrations. 4. With a given parametric set of vibration, the units only started firing at a given delay after the onset of vibration. The delay was quite different for different units and it increased considerably, sometimes by several seconds, when the vibration amplitude was made smaller. 5. In all the experimental conditions tested, and even when the unit discharge did not start until several seconds after vibration onset, the unit potential presented a close and highly consistent temporal relation to the vibration cycles. The slow recruitment process is thought to involve a polysynaptic excitatory mechanism which progressively depolarizes the masseter motoneurones close to their threshold, the actual firing being triggered by monosynaptic excitatory post-synaptic potentials from I(a) afferents, hence the small latency jitter recorded. This special pattern of tonic vibration reflex in jaw-closing muscles in man may result from the lack of reciprocal inhibition from the jaw-opening muscles. (+info)
Dynamics of the human masticatory system.
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In this review, the movement characteristics of the human masticatory system are discussed from a biomechanical perspective. The discussion is based upon the three fundamental laws of mechanics applied to the various anatomical structures that are part of the masticatory system. An analysis of the forces and torques applied to the mandible by muscles, joints, articular capsules, and teeth is used to assess the determinants of jaw movement. The principle of relating the interplay of forces to the center of gravity of the lower jaw, in contrast to a hinge axis near its joints, is introduced. It is evident that the muscles are the dominant determinants of jaw movement. The contributions of the individual muscles to jaw movements can be derived from the orientation of their lines of action with respect to the center of gravity of the lower jaw. They cause the jaw to accelerate with six degrees of freedom. The ratio between linear and angular accelerations is subtly dependent on the mass and moments of inertia of the jaw, and the structures that are more or less rigidly attached to it. The effects of articular forces must be taken into account, especially if the joints are loaded asymmetrically. The muscles not only move the jaw but also maintain articular stability during midline movements. Passive structures, such as the ligaments, become dominant only when the jaw reaches its movement boundaries. These ligaments are assumed to prevent joint dislocation during non-midline movements. (+info)
Biomechanical influences of head posture on occlusion: an experimental study using finite element analysis.
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The biomechanical influences of head posture on the cervical column and craniofacial complex during masticatory simulation were quantified using three-dimensional (3D) finite element analysis (FEA). Three types of finite element model (FEM) were designed to examine relationships between the position of the head and malocclusion. Model A was constructed to have a standardized cervical column curve, model B a forward inclined posture, and model C a backward inclined posture. The results of the spinal displacements revealed that model B moved in a forward direction and model C in a backward direction during masticatory simulation. The stress distributions on the cervical column (C1-C7) for models A, B, and C showed differences; stress converged at the atlas in model A, high-level stresses were observed at the spinous processes of C6 and C7 in model C, and the stress converged at the anterior edge in the vertebral body of C4 of model B. Stress distribution on the occlusal plane and maxillofacial structure did not show absolute differences among the three models. Alteration of head posture was directly related to stress distribution on the cervical column, but may not always directly influence the occlusal state. (+info)