[The role of the masticatory and facial muscles in the etiology of dysgnatias].
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Anatomy, Comparative↗
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Influence of an occlusal splint on integrated electromyography of the masseter muscles.
In order to examine the effect of an occlusal splint on the integrated electromyography (EMG) of the masticatory muscles, EMG of bilateral masseter muscles of 23 patients with temporomandibular joint disturbance syndrome (TMJDS), with and without an occlusal splint, was measured and integrated on line during maximum clenching. It was found that the integrated myoelectrical value of the masseter muscle on the involved and non-involved side was reduced with the occlusal splint. The absolute difference between integrated myoelectrical values in the left and right masseter muscles was reduced with an occlusal splint, but the relative difference remained virtually unchanged. These results indicate that the occlusal splint can decrease masseter muscle activity and thus exert a therapeutic effect.
Adult↗
Heterogeneity among muscle precursor cells in adult skeletal muscles with differing regenerative capacities.
Skeletal muscle has a remarkable capacity to regenerate after injury, although studies of muscle regeneration have heretofore been limited almost exclusively to limb musculature. Muscle precursor cells in skeletal muscle are responsible for the repair of damaged muscle. Heterogeneity exists in the growth and differentiation properties of muscle precursor cell (myoblast) populations throughout limb development but whether the muscle precursor cells differ among adult skeletal muscles is unknown. Such heterogeneity among myoblasts in the adult may give rise to skeletal muscles with different regenerative capacities. Here we compare the regenerative response of a masticatory muscle, the masseter, to that of limb muscles. After exogenous trauma (freeze or crush injuries), masseter muscle regenerated much less effectively than limb muscle. In limb muscle, normal architecture was restored 12 days after injury, whereas in masseter muscle, minimal regeneration occurred during the same time period. Indeed, at late time points, masseter muscles exhibited increased fibrous connective tissue in the region of damage, evidence of ineffective muscle regeneration. Similarly, in response to endogenous muscle injury due to a muscular dystrophy, widespread evidence of impaired regeneration was present in masseter muscle but not in limb muscle. To explore the cellular basis of these different regenerative capacities, we analyzed the myoblast populations of limb and masseter muscles both in vivo and in vitro. From in vivo analyses, the number of myoblasts in regenerating muscle was less in masseter compared with limb muscle. Assessment of population growth in vitro indicated that masseter myoblasts grow more slowly than limb myoblasts under identical conditions. We conclude that the impaired regeneration in masseter muscles is due to differences in the intrinsic myoblast populations compared to limb muscles.
Animals↗
Masseteric repetitive nerve stimulation in the diagnosis of myasthenia gravis.
OBJECTIVE: (1) to develop a method for masseteric repetitive nerve stimulation (RNS) and to obtain normative data for amplitude and area decrement of the muscle (M) response. (2) To investigate myasthenia gravis (MG) patients with masseteric RNS. Masticatory muscles are frequently affected in MG, but no RNS test is available to investigate this district. METHODS: Fifteen healthy subjects and 17 MG patients were examined. The masseteric nerve was stimulated by a monopolar needle (cathode), inserted between the mandibular incisure and the zygomatic arch, and a surface electrode (anode), on the contralateral cheek. Masseteric M response was recorded using surface electrodes on the muscle belly and below the mandibular angle. Stimuli were delivered at 3 Hz in trains of 9, at rest and after isometric effort. RESULTS: Normal subjects: mean amplitude decrement was 0.3+/-1.2% at rest, and 1.9+/-1.3% after isometric effort. PATIENTS: 15 patients (88%) were positive on masseteric RNS; in 3 of these it was the only positive RNS test. The extent of decrement observed in masseter muscle was significantly greater than in trapezius muscle. CONCLUSIONS: Masseteric RNS is a simple and well-tolerated procedure; it offers a new possibility in testing the cranial muscles in disorders of neuromuscular transmission.
Accessory Nerve↗
Fibre composition of human intrinsic tongue muscles.
The muscle fibre composition of three human intrinsic tongue muscles, the longitudinalis, verticalis and transversus, was investigated in four anterior to posterior regions of the tongue using morphological and enzyme- and immunohistochemical techniques. All three muscles typically contained type I, IIA and IM/IIC fibres. Type I fibres expressed slow myosin heavy chain (MyHC), type II fibres fast MyHC, mainly fast A MyHC, whereas type IM/IIC coexpressed slow and fast MyHCs. Type II fibres were in the majority (60%), but regional differences in proportion and diameter of fibre types were obvious. The anterior region of the tongue contained a predominance of relatively small type II fibres (71%), in contrast to the posterior region which instead showed a majority of larger type I and type IM/IIC fibres (66%). In general, the fibre diameter was larger in the posterior region. This muscle fibre composition of the tongue differs from those of limb, orofacial and masticatory muscles, probably reflecting genotypic as well as phenotypic functional specialization in oral function. The predominance of type II fibres and the regional differences in fibre composition, together with intricate muscle structure, suggest generally fast and flexible actions in positioning and shaping the tongue, during vital tasks such as mastication, swallowing, respiration and speech.
Adenosine Triphosphatases↗
[Functional recovery with prosthetic management for segmental resection of the mandible without reconstruction].
PATIENT: The patient, a 54-year-old male, consulted the Oral Surgery Department of Iwate Medical University Hospital with a complaint of a mass in the left oral base in June 1992. In September 1992, the patient was diagnosed as having cancer in the left mandibular base, and the tumor was excised by resection of the entire cervical region on the left side. Since radiation osteonecrosis in the left mandible and mandibular fracture were detected, segmental excision of the left mandible was performed in March 1993. Although the postoperative course was good without reconstruction, the patient consulted the Second Prosthetic Department to achieve functional recovery in February 1996. This patient had no occlusal contact between the maxilla and mandible because the mandible shifted to the affected side. After fixation of a mandibular prosthetic appliance for the defective mandible, a palatal plate for the maxilla in occlusal contact with the mandibular dentition and mandibular prosthetic appliance were fixed in November 1997. After fixation of a new mandibular prosthetic appliance and dentures for the maxilla with palatal ramp in April 2001, masticatory function was observed to have improved with control of the mandible. DISCUSSION: To prevent the mandibular shift and improvement of the masticatory function, a palatal plate with a palatal ramp in the occlusal contact region was fixed, and a balance of the masticatory muscles could be maintained. An evaluation of the level of improvement in the masticatory function and the pronunciation function indicated that the mandibular prosthetic appliance and palatal plate with a palatal ramp in the occlusal contact region increased the kind of food that the patient could take. Moreover, by enlarging the narrow Donders space, the pronunciation was improved. CONCLUSIONS: Fixation of a palatal plate with a palatal ramp in the occlusal contact region without reconstruction of the mandibular bone was useful for the control of mandibular deviation to the affected side and improvement of the masticatory function.
Dental Prosthesis↗
No effect of experimental occlusal interferences on pressure pain thresholds of the masseter and temporalis muscles in healthy women.
It has been suggested that occlusal interferences may lead to pain and tenderness of the masticatory muscles. Tender jaw muscles are more sensitive to pressure pain, as assessed by means of pressure algometry. We tested the effects of occlusal interferences on the pressure pain threshold of the jaw muscles by means of a double-blind randomized crossover experiment carried out on 11 young healthy females. Golden strips were glued either to an occlusal contact area (active interference) or to the vestibular surface of the same tooth (dummy interference) and left for 8 d each. Pressure pain thresholds of the masseter and anterior temporalis muscles were assessed under interference-free, dummy-interference and active-interference conditions. The results indicated that the application of an active occlusal interference, as used in this study, did not influence significantly the pressure pain thresholds of these muscles in healthy individuals.
Adult↗
[Evaluation of the degree of restoration of function with fixed molar bridges].
The purpose of this study was to determine the degree of restoration of masticatory function with fixed molar bridges. Evaluations were made of the deep temperature of the masseter muscle, masticatory performance and occlusal contacts (duration, force distribution, area and number of contacts). Criteria were established for the parameters above mention in 30 normal subjects. Fixed bridges were placed in 10 patients with missing only the mandibular first molar. Measurements were carried out both prior to and after placement of the prosthesis. The following results were obtained. 1. The deep temperature of the masseter muscle patients recovered in 30 days after prosthesis. 2. The total number of contact points and contact area increased after placement by 30 days. 3. The duration of occlusal contacts increased up to 30 days after placement. 4. The force distribution of occlusal contacts decreased for a time but recovered as the normal subjects after 30 days. 5. The masticatory performance was restored immediately after the placement and showed no change by 45 days.
Body Temperature↗
Aberrant muscle between the temporalis and the lateral pterygoid muscles: M. pterygoideus proprius (Henle).
During examination of the positional relationships between the lateral pterygoid and the temporalis muscles and the innervating nerves, an aberrant muscle was observed in three of 66 head halves. The aberrant muscle originated from the medial surface of the anteromedial muscle bundle of the temporalis (Shimokawa et al. 1998, Surg. Radiol. Anat. 20:329-334) and inserted into the inferolateral surface of the lower head of the lateral pterygoid. Due to its location, origin and insertion this aberrant muscle slip is considered to correspond to the pterygoideus proprius described by Henle (1858, Handbuch der Anatomie des Menschen). Based on the innervation findings, the present aberrant muscle might be considered as a remnant muscle bundle between the anteromedial muscle bundle of the temporalis and the lateral pterygoid during differentiation of the lateral masticatory muscle anlage.
Adult↗
Projection of jaw-muscle spindle afferents to the caudal brainstem in rats demonstrated using intracellular biotinamide.
Intracellular staining with biotinamide was used to study the axonal projection and synaptic morphology of rat jaw-muscle spindle afferents. Intracellular recordings in the mesencephalic trigeminal nucleus (Vme) were identified as spindle afferent responses by their increased firing during stretching of the jaw-elevator muscles. Biotinamide-stained axon collaterals with boutons were found in the trigeminal motor nucleus (Vmo), Vme, the region dorsal to Vmo including the supratrigeminal region, the dorsomedial portion of the trigeminal principal sensory nucleus, and the dorsomedial part of the rostral spinal trigeminal subnucleus oralis. Additional, previously undescribed projections of jaw-muscle spindle afferents were found to the dorsomedial portion of the caudal spinal trigeminal subnucleus oralis (Vodm), the dorsomedial part of the spinal trigeminal subnucleus interpolaris (Vidm), the caudal parvicellular reticular formation, laminae IV and V of the spinal trigeminal subnucleus caudalis (Vc), and the dorsal division of the medullary reticular field. Labeled spindle boutons in Vodm formed predominately axodendritic synapses. Some of these boutons received presynaptic inputs from unlabeled P-type boutons containing clear, spherical, or flattened vesicles. In Vidm, labeled collaterals and boutons were densely clustered into glomerular-like structures. Labeled boutons in Vidm made axodendritic, axosomatic, and axoaxonic synapses and received synaptic contacts from unlabeled boutons containing clear, spherical, or flat and pleomorphic vesicles. Unlabeled presynaptic boutons in Vidm occasionally contained dense core vesicles. Labeled boutons in Vc mainly formed synaptic contacts with large diameter dendrites. This projection of jaw-muscle spindle afferents to caudal brainstem regions may play a significant role in masticatory-muscle stretch reflexes and in the integration of trigeminal proprioceptive information and its transmission to higher centers.
Afferent Pathways↗
Non-nociceptive upper limb afferents modulate masseter muscle EMG activity in man.
Recent electrophysiological data obtained in anaesthetized rats evidenced jaw muscle excitatory responses to the electrical stimulation of type II limb somatosensory afferents. In the present work, we describe an inhibitory reflex evoked in human masseter muscles by stimulation of non-nociceptive fibres travelling in the median and radial nerves (MED and RAD, respectively). Eighteen healthy volunteers participated in the study. Subjects were seated on a comfortable chair, with the complex head-mandible-neck-trunk and the limbs securely fixed to the chair. Attempts were made to minimize possible interferences due to the activation of afferents other than the stimulated ones. The subjects were instructed to contract masseter muscles at a submaximal level and to maintain a stable level of muscle contraction during all trials. EMG voluntary activity was recorded from both masseter muscles by means of coaxial needle electrodes before and after the electrical stimulation of MED and/or RAD at intensities below pain threshold. In all subjects, MED stimulation induced bilaterally a marked depression of masseter EMG activity, which occurred at a latency of 23.6 +/- 2.1 ms and lasted 27.8 +/- 6.6 ms. RAD stimulation also induced a marked reduction in masseter EMG activity, but this effect was clearly observed in 9 out of 18 subjects, and it showed latency (30.2 +/- 7.5 ms) and duration (44.9 +/- 5.4 ms) significantly longer in comparison with the MED-induced effect. All subjects exhibited the inhibitory period in masseter EMG following the simultaneous stimulation of both nerves; this one appeared at a latency not significantly different (25.3 +/- 5.9 ms) and lasted much more (37.4 +/ - 8.2 ms) than EMG depression evoked by MED stimulation. The duration of masseter muscle inhibition, induced by MED and/or RAD stimulation, was inversely related to the level of EMG activity, while latency was not related to it. Significant increases in the inhibitory period duration were also observed by increasing stimulus intensity, within a subthreshold range for the activation of nociceptive fibres. In all cases, the inhibitory period was followed by a later excitatory rebound activity, whose latency and duration depended on the duration of the preceding EMG inhibition and on the background level of masseter activation. In conclusion, results evidenced that the activation of arm somatosensory fibres modulates masseter muscle activity in normal man. This might lead to a coordination between limb and masticatory muscle activity, which is required in several complex motor acts.
Action Potentials↗
Trigeminal motoneurons with disparate dendritic geometry innervate different muscle groups in the frog.
Cobalt labelling revealed two, morphologically different types of neurons in the trigeminal motor nucleus innervating masticatory muscles and the levator bulbi muscle, respectively. As far as their neuronal morphology is concerned, levator bulbi motoneurons differed from all other motoneurons of the brainstem. They could not be found in lizards and rats; and the levator bulbi muscle was also absent in these animals. This motoneuron group is compared with the accessory abducens nucleus which innervates the retractor bulbi muscles, and is present only in animals which possess this muscle group.
Animals↗
Relationship between attachment of the superficial masseter muscle and craniofacial morphology in dentate and edentulous humans.
Previous studies have described the importance of the interaction between the masticatory muscles and the craniofacial skeleton in the control of craniofacial growth. This study describes the attachment and orientation of the superficial masseter muscle and its relationship with craniofacial morphology in dentate and edentulous subjects. Data were obtained from lateral cephalometric radiographs of a total of 31 cadavers in which the superficial masseter muscle had been defined with liquid barium. The results provide evidence that the morphology of the superficial masseter muscle in the gonion region differed significantly between dentate and edentulous subjects, with the masseter being 2.7 mm wider and the gonion-anterior muscle border distance being 4.0 mm greater in dentate subjects. The complex relationship between craniofacial morphology and the dimensions and inclination of the superficial masseter muscle were most clearly evident in dentate subjects where the position of the anterior border was related to ramus dimensions and mandibular and occlusal plane angles. The association is much less clear in edentulous subjects where normal function was disturbed. In general, age was not a significant determinant of variation in superficial masseter muscle dimensions and orientation.
Age Factors↗
Age changes in neuromuscular junctions of masseter muscle.
There is little information regarding the relationship between aging and ensuing morphological changes in the neuromuscular junction (NMJ) of the masticatory muscle. The masseter of C57BL/6J male mice at three different ages (young adult, 6 months; mature adult, 12 months; and old, 30 months) was studied. Morphological measurements were taken from zinc iodide osmium stained NMJ. Camera lucida drawings were superimposed on a computer monitor via a video camera and traced using a digitizer. The data were treated statistically using multivariate analysis of variance (MANOVA). No difference was found between 6 and 12 months of age, but a significant decrease in morphological parameters in NMJ from 30-month-old animals was found when compared with mature adult animals. Nerve terminal areas, perimeters, longitudinal extent lengths, and fiber diameters were reduced by 24%, 21%, 15.5% and 23%, respectively. Nerve terminal branches and incidence of sprouts were significantly increased in older animals. Age changes in the NMJ morphology are probably associated with altered balance between degeneration and regeneration of nerve terminals. This view is supported by increased terminal sprouting in old mice which is indicative of the plasticity or remodelling of NMJ during aging.
Aging↗
Changes in reflex responses of the masseter and digastric muscles during sleep in freely behaving rabbits.
The aim of this study was to clarify the modulation of motoneuron excitability in masticatory muscles during sleep. For this purpose, changes in the reflex responses of the masseter and digastric muscles between sleep and wakefulness were studied in freely behaving rabbits. Stimulation of the jaw closing muscle spindle afferents induced the masseteric monosynaptic reflex (MMR). During quiet sleep (QS), which gradually replaced quiet wakefulness, the mean MMR amplitude showed no change. During active sleep (AS). MMR amplitudes were markedly reduced, but they were confounded by the occasional occurrence of facilitation in the amplitude. The facilitatory reflexes were often related to rapid eye movements (REMs). However, the excitatory input probably did not originate from the same region as the REM generator, since the REM and the large MMR did not always occur simultaneously. On the other hand, jaw opening reflexes remained inhibited. The results indicated that there is mainly a difference in the excitability between the two groups of motoneurons during AS; masseter motoneuron activity was inhibited but occasionally facilitated by excitatory inputs occurring in association with REMs, however, the digastric motoneuron activity was remained inhibited. The excitatory inputs may induce dysfunctional muscle contraction of the jaw closing muscles as seen in bruxism.
Animals↗
Functions of masseter and temporalis muscles in the control of temporomandibular joint loading--a static analysis using a two-dimensional rigid-body spring model.
Masticatory forces or bite forces applied to the teeth are known to generate a compressive force on the temporomandibular joint (TMJ). This intra-articular force is referred to as TMJ loading. TMJ loading is considered to be controlled to a certain extent by the stomatognathic system, so as not to exceed a certain limit. In order to clarify the control mechanism of the TMJ during biting, the controllability of TMJ loading through coordinated activities of the masticatory muscles has been numerically analyzed using a static two-dimensional jaw model. This model consists of a rigid-body model of the jaws and a spring model of the articular disk, which includes three dominant muscle force vectors representing the masseter, temporalis and lateral pterygoid muscles. The bite force vector is assumed to be applied solely at the first molar. Static equilibrium analyses suggest that coordinated activities of the masseter muscle and anterior portion of the temporalis muscle can minimize TMJ loading and concentrate the stress within the articular disk to the intermediate zone of the disk, and that the activity of the posterior portion of the temporalis muscle has the possibility of reducing TMJ loading.
Bite Force↗
[Anatomical observations on the attachment of human temporalis and masseter muscles].
It is very important for orthodontics to investigate the bone remodeling pattern in the area of the attachment of the masticatory muscles induced by the changing muscular function. In the present study, the insertion pattern of Sharpey's fibers and the bone structure are comparatively studied on the temporalis and the masseter muscles macroscopically, light microscopically and electron microscopically, respectively. The following results are obtained: 1) The periosteum on the squamous part of the temporal bone is thinner, while the one on the lower ridge of the zygomatic arch is relatively thicker. 2) The fine and loose Sharpey's fibers are found mostly on the squamous part of the temporal bone, while the ones on the lower ridge of the zygomatic arch are relatively coarse and dense. 3) Comparing with the squamous part of the temporal bone, the fibrous bone is thinner and also few reversal lines and resting lines are observed at the lower ridge of the zygomatic arch. These findings seem to prove the fact that the bone remodeling proceeds frequently within the limited narrow area. These differences are directly due to the differences of the anatomical site and the muscular function.
Aged↗
Activity recognition in long-term electromyograms.
Long-term electromyography with portable recorders allows the study of muscle activity in the natural environment to investigate whether muscle overuse or oral habits may contribute to initiate or perpetuate a myoarthropathy of the masticatory system. At present, little is known about the behaviour of masticatory muscles over long time periods. The aim of this preliminary study was to define parameters which allow the automatic recognition of different types of oral activities from the electromyogram form. A programme with functional and parafunctional activities was performed by four volunteers (total of 333 functional and 82 parafunctional recordings). Electromyograms of the masseter and temporal muscles were recorded by means of a self-developed portable intelligent solid state recorder. Signal mean level (mw) and dynamics (dw) within a sliding window were determined. Temporal muscle recordings allowed better discrimination between function and simulated parafunction. The average of mw for clenching was 51.4 +/- 3.5% and for tooth grinding 21.2 +/- 2% of the peak electromyogram value at maximum voluntary clench. At 0.96 s sliding window duration, clenching, tooth grinding and chewing signals had maximum separation, using dw/mw as parameters (average: 0.16 +/- 0.01 for clenching, 0.39 +/- 0.01 for teeth grinding and 0.88 +/- 0.01 for chewing).
Adult↗