Comparison of rhythmic jaw muscle activities induced by electrical stimulations of the corticobulbar tract during rapid eye movement sleep with those during wakefulness and non-rapid eye movement sleep in freely moving guinea pigs.

OBJECTIVE: Rhythmic jaw muscle activities (RJMAs) occur during rapid eye movement (REM) sleep in humans and animals even though motoneurons are inhibited. The present study compared the characteristics of jaw muscle activities induced by electrical microstimulations of the corticobulbar tract (CT) during REM sleep with those during wakefulness and non-REM sleep. METHODS: Eleven guinea pigs were surgically prepared for polygraphic recordings with the implantation of a stimulating electrode. Long- and short-train repetitive electrical microstimulations were applied to the CT under freely moving conditions. The response rate, latency, burst amplitude, and cycle length in the digastric muscle were calculated and cortical and cardiac activities were quantified. RESULTS: Long-train microstimulations induced RJMAs in the digastric muscle followed by masseter muscle activity during wakefulness and non-REM sleep and only induced rhythmic digastric muscle activity during REM sleep. The response rate of RJMAs and the burst amplitude of digastric muscles were significantly lower during REM sleep than during wakefulness and non-REM sleep. However, response latency did not significantly differ between REM sleep and wakefulness. Transient cortical and cardiac changes were associated with RJMAs induced during non-REM sleep, but not during REM sleep. Short-train microstimulations induced a short-latency digastric response, the amplitude of which was significantly lower during REM sleep than during non-REM sleep and wakefulness. CONCLUSIONS: These results suggest that the masticatory CPG was activated by electrical CT stimulations independently of the motoneuron inhibitory system during REM sleep.

Occurrence, presence and severity of bruxism and its association with altered state of consciousness in individuals with severe acquired brain injury.

BACKGROUND: Excessive jaw muscle activity is a frequent complication after acquired brain injury (ABI). OBJECTIVE: The study aimed to identify the occurrence and severity of jaw muscle activity and its association with altered state of consciousness in patients with ABI. METHODS: A total of 14 severe ABI patients with varied altered state of consciousness were recruited. A single-channel electromyographic (EMG) device was used to assess the jaw muscle activity for three consecutive nights during Week 1 and Week 4 following admission. Differences in number of EMG episodes/h between Week 1 and 4 were analysed using non-parametric tests and association between the EMG activity and altered state of consciousness were analysed using Spearman's correlation test. RESULTS: Nine of fourteen (64%) patients showed indications of bruxism (cutoff: >15 EMG episodes/h). The average EMG episodes/h at admission were 44.5 ± 13.6 with no significant changes after Week 4 of admission (43 ± 12.9; p = .917). The EMG episodes/h ranged from 2 to 184 during Week 1 and 4-154 during Week 4. There were no significant correlations between the number of EMG episodes/h during the three nights and the individuals altered state of consciousness during Week 1 and Week 4. CONCLUSION: Patients with ABI had a conspicuously high but variable level of jaw muscle activity at admission and it tend to remain high after 4 week of hospitalisation which could potentially lead to adverse effects such as excessive tooth wear, headaches and pain in jaw muscles. The lack of associations between individuals altered level of consciousness and EMG activity could be due to low sample size and further studies are clearly warranted in this patient group with special needs. Single-channel EMG devices can record jaw muscle activity early in the hospitalisation period and might be a helpful tools for early detection of bruxism in ABI patients.

The feeding apparatus of Rhea americana (Aves, Palaeognathae): Jaw myology and ontogenetic allometry.

In birds, the jaw musculature is a crucial adaptive feature involved in feeding. The morphological traits and postnatal growth patterns of jaw muscles constitute a useful proxy to interpret feeding function and ecology. This study aims to describe the jaw muscles of Rhea americana and explore their postnatal growth pattern. A total of 20 specimens of R. americana representing four ontogenetic stages were studied. Jaw muscles were described, weighed and their proportions with respect to body mass were calculated. Linear regression analysis was used to characterize ontogenetic scaling patterns. The morphological patterns of jaw muscles were characterized by their simplicity: bellies with few or no subdivisions and similar to those described for other flightless paleognathous birds. In all stages, the muscles pterygoideus lateralis, depressor mandibulae, and pseudotemporalis had the greatest mass values. The proportion of total jaw muscle mass decreased with age from 0.22% in 1-month-old chicks to 0.05% in adults. Linear regression analysis showed that all muscles scaled with negative allometry with respect to body mass. The progressive decrease of jaw muscle mass relative to body mass in adults could be related to the generation of less force, which is in accordance with the herbivorous diet of adults. In contrast, the diet of rhea chicks includes a large proportion of insects thus, this greater muscle proportion could be associated with the ability to generate more force, thus providing better abilities to grasp and hold more mobile prey.

Sex-specific aging in bite force in a wild vertebrate.

The manifold differences between the sexes provide wide scope for sex differences in senescence. However, detecting physiological decline in old age and evaluating any sex difference in aging in a wild population can be challenging. This applies especially to long-lived species that require marking around birth in order to recognize elderly individuals, perhaps decades later. Here, we used bite force of known-age, long-lived Nazca boobies (Sula granti, a seabird) as a functional measure of muscle strength; surprisingly, only a single study has evaluated the possibility of senescent decline in muscle strength in a wild vertebrate. The male-biased adult sex ratio of this population constrains breeding opportunities across the lifespan for males, so we predicted that slower accumulation of reproductive costs would delay senescent decline in bite force in males compared to females, matching observed patterns in some non-muscle traits in this species. Data were collected from 349 adults using a force transducer at the start of the breeding season in November 2017 on Isla Española, Galápagos. Both sexes achieved less bite force in late life. The decline began at a later age in males, providing evidence of sex-specific schedules of decline in muscle function in a wild vertebrate.

Masticatory system integration in a commensal canid: interrelationships between bones, muscles and bite force in the red fox.

The jaw system in canids is essential for defence and prey acquisition. However, how it varies in wild species in comparison with domestic species remains poorly understood, yet is of interest in terms of understanding the impact of artificial selection. Here, we explored the variability and interrelationships between the upper and lower jaws, muscle architecture and bite force in the red fox (Vulpes vulpes). We performed dissections and used 3D geometric morphometric approaches to quantify jaw shape in 68 foxes. We used a static lever model and bite force estimates were compared with in vivo measurements of 10 silver foxes. Our results show strong relationships exist between cranial and mandible shape, and between cranial or mandible shape on the one hand and muscles or estimated bite force on the other hand, confirming the strong integration of the bony and muscular components of the jaw system. These strong relationships are strongly driven by size. The functional links between shape and estimated bite force are stronger for the mandible, which probably reflects its greater specialisation towards biting. We then compared our results with data previously obtained for dogs (Canis lupus familiaris) to investigate the effect of domestication. Foxes and dogs differ in skull shape and muscle physiological cross-sectional area (PCSA). They show a similar amount of morphological variation in muscle PCSA, but foxes show lower variation in cranial and mandible shape. Interestingly, the patterns of covariation are not stronger in foxes than in dogs, suggesting that domestication did not lead to a disruption of the functional links of the jaw system.

Effect of food hardness on chewing behavior in children.

OBJECTIVE: To investigate the effects of food hardness on chewing behavior in children compared with adults. MATERIALS AND METHODS: Healthy children (3-17 years) were equally divided into five groups based on their dental eruption stages. Each participant ate soft and hard viscoelastic test food models (3 each), while the three-dimensional jaw movements and electromyographic (EMG) activity of the bilateral masseter muscles were recorded. The data from the children were compared with a control group of healthy adults (18-35 years). The data were analyzed with nonparametric tests. RESULTS: There was no significant difference in the number of chewing cycles and the duration of the chewing sequence between children groups and adults. Children with primary dentition (3-5 years) showed shorter lateral jaw movement and higher muscle activity at the end of the chewing sequence, compared with adults. Further, children's age-groups (3-14 years) failed to adapt their jaw muscle activity to food hardness. However, at the late-permanent dentition stage (15-17 years), children were capable of performing adult-like chewing behavior. CONCLUSIONS: Overall, it seems that children as young as 3-year-old are quite competent in performing basic chewing function similar to adults. Yet, there are differences in the anticipation or adaption of jaw muscle activity and jaw kinematics to food hardness. CLINICAL RELEVANCE: The study may have clinical implication in the diagnosis and management of children with chewing impairment associated with dental malocclusions and other orofacial dysfunctions.

Hierarchical Representations of Aggression in a Hypothalamic-Midbrain Circuit.

Although the ventromedial hypothalamus ventrolateral area (VMHvl) is now well established as a critical locus for the generation of conspecific aggression, its role is complex, with neurons responding during multiple phases of social interactions with both males and females. It has been previously unclear how the brain uses this complex multidimensional signal and coordinates a discrete action: the attack. Here, we find a hypothalamic-midbrain circuit that represents hierarchically organized social signals during aggression. Optogenetic-assisted circuit mapping reveals a preferential projection from VMHvlvGlut2 to lPAGvGlut2 cells, and inactivation of downstream lPAGvGlut2 populations results in aggression-specific deficits. lPAG neurons are selective for attack action and exhibit short-latency, time-locked spiking relative to the activity of jaw muscles during biting. Last, we find that this projection conveys male-biased signals from the VMHvl to downstream lPAGvGlut2 neurons that are sensitive to features of ongoing activity, suggesting that action selectivity is generated by a combination of pre- and postsynaptic mechanisms.

Involvement of the basal nucleus of Meynert on regional cerebral cortical vasodilation associated with masticatory muscle activity in rats.

We examined the neural mechanisms for increases in regional cerebral blood flow (rCBF) in the neocortex associated with mastication, focusing on the cortical vasodilative system derived from the nucleus basalis of Meynert (NBM). In pentobarbital-anesthetized rats, parietal cortical rCBF was recorded simultaneously with electromyogram (EMG) of jaw muscles, local field potentials of frontal cortex, multi-unit activity of NBM neurons, and systemic mean arterial pressure (MAP). When spontaneous rhythmic EMG activity was observed with cortical desynchronization, an increase in NBM activity and a marked rCBF increase independent of MAP changes were observed. A similar rCBF increase was elicited by repetitive electrical stimulation of unilateral cortical masticatory areas. The magnitude of rCBF increase was partially attenuated by administration of the GABAergic agonist muscimol into the NBM. The rCBF increase persisted after immobilization with systemic muscle relaxant (vecuronium). rCBF did not change when jaw muscle activity was induced by electrical stimulation of the pyramidal tract. The results suggest that activation of NBM vasodilator neurons contributes at least in part to the rCBF increase associated with masticatory muscle activity, and that the NBM activation is induced by central commands from the motor cortex, independently of feedback from brainstem central pattern generator or contracting muscles.

Clinical jaw-muscle symptoms in a group of probable sleep bruxers.

OBJECTIVES: To investigate, in a sample of probable sleep bruxers with and without temporomandibular disorder (TMD) pain, the presence and relationships between clinical jaw-muscle symptoms, and test their associations with jaw-muscle electromyographic (EMG) activity during sleep. METHODS: Pain, unpleasantness, tiredness, tension, soreness, and stiffness were scored on a 0-10 numerical rating scale (NRS) in 50 probable sleep bruxers. The sample was subdivided into two groups, i.e., with and without TMD pain. Multiple-night, single-channel EMG recordings were performed. Descriptive data, correlations between the six symptoms, and correlations between symptoms and EMG measures, i.e. EMG events/recording, EMG events/hour, and night-to-night variability in EMG events, were calculated. RESULTS: In the total sample, 90% of the participants reported at least one symptom. Tiredness and tension were the most prevalent symptoms (both 78%), and pain the least (30%). In the TMD pain group, pain remained the least reported symptom (57%). Intensity of symptoms was low to moderate, with tension presenting the highest median in the total sample (NRS 4), the TMD pain group (NRS 5), and non-TMD group (NRS 3). Significant correlations between all symptoms were found in the total sample, but not in the two subgroups. No significant associations between EMG measures and muscle symptoms emerged. CONCLUSION: Jaw-muscle symptoms other than pain were highly prevalent in a sample of probable sleep bruxers. There were no associations between these symptoms and EMG measures of jaw-muscle activity during sleep. These findings challenge the concept of simple relationships between jaw-muscle activity during sleep and clinical muscle symptoms.

To roll the eyes and snap a bite - function, development and evolution of craniofacial muscles.

Craniofacial muscles, muscles that move the eyes, control facial expression and allow food uptake and speech, have long been regarded as a variation on the general body muscle scheme. However, evidence has accumulated that the function of head muscles, their developmental anatomy and the underlying regulatory cascades are distinct. This article reviews the key aspects of craniofacial muscle and muscle stem cell formation and discusses how this differs from the trunk programme of myogenesis; we show novel RNAseq data to support this notion. We also trace the origin of head muscle in the chordate ancestors of vertebrates and discuss links with smooth-type muscle in the primitive chordate pharynx. We look out as to how the special properties of head muscle precursor and stem cells, in particular their competence to contribute to the heart, could be exploited in regenerative medicine.

Effect of Sudden Deprivation of Sensory Inputs From Periodontium on Mastication.

OBJECTIVE: To investigate the effect of sudden deprivation of sensory inputs from the periodontium on jaw kinematics and time-varying activation profile of the masseter muscle. METHODS: Fourteen (age range: 22-26 years; four men) healthy and natural dentate volunteers participated in a single experimental session. During the experiment, the participants were asked to eat six hard visco-elastic test food models, three each before and after an anesthetic intervention. The movements of the jaw in three dimensions and electromyographic (EMG) activity of the masseter muscle on the chewing side were recorded. RESULTS: The results of the study showed no significant differences in the number of chewing cycles (P = 0.233) and the duration of chewing sequence (P = 0.198) due to sudden deprivation of sensory inputs from the periodontium. However, there was a significant increase in the jaw opening velocity (P = 0.030) and a significant increase in the duration of occlusal phase (P = 0.004) during the anesthetized condition. The EMG activity of the jaw closing phase was significantly higher during the control condition [116.5 arbitrary units (AU)] than anesthetized condition (93.9 AU). The temporal profile of the masseter muscle showed a biphasic increase in the excitatory muscle drive in the control condition but this increase was virtually absent during the anesthetized condition. CONCLUSION: Sudden deprivation of sensory inputs from the periodontium affects the jaw kinematics and jaw muscle activity, with a clear difference in the time-varying activation profile of the masseter muscle. The activation profile of the masseter muscle shows that periodontal mechanoreceptors contribute to approximately 20% of the EMG activity during the jaw closing phase.

The evolution of bite force in horned lizards: the influence of dietary specialization.

Dietary specialization is an important driver of the morphology and performance of the feeding system in many organisms, yet the evolution of phenotypic specialization has only rarely been examined within a species complex. Horned lizards are considered primarily myrmecophagous (ant eating), but variation in diet among the 17 species of horned lizards (Phrynosoma) makes them an ideal group to examine the relationship between dietary specialization and the resultant morphological and functional changes of the feeding system. In this study, we perform a detailed analysis of the jaw adductor musculature and use a biomechanical model validated with in vivo bite force data to examine the evolution of bite force in Phrynosoma. Our model simulations demonstrate that bite force varies predictably with respect to the gape angle and bite position along the tooth row, with maximal bite forces being attained at lower gape angles and at the posterior tooth positions. Maximal bite forces vary considerably among horned lizards, with highly myrmecophagous species exhibiting very low bite forces. In contrast, members of the short-horned lizard clade are able to bite considerably harder than even closely related dietary generalists. This group appears to be built for performing crushing bites and may represent a divergent morphology adapted for eating hard prey items. The evolutionary loss of processing morphology (teeth, jaw and muscle reduction) and bite force in ant specialists may be a response to the lack of prey processing rather than a functional adaptation per se.

Effect of a brief episode of experimental muscle pain on jaw movement and jaw-muscle activity during chewing.

The aims of this study were to determine whether: (i) the jaw motor system develops a new pattern of jaw movement and/or jaw-muscle activity after resolution of an acute episode of jaw-muscle pain; and (ii) if jaw-muscle activity and jaw-movement features change progressively with repetition of a chewing sequence. Jaw movement and jaw muscle (masseter, anterior temporalis, and digastric) activity were recorded during free and rate-standardized chewing in eight asymptomatic participants (pain infusion group), before and at three time blocks up to 45 min after a single 0.2-ml bolus infusion of 5% hypertonic saline into the right masseter muscle. The same procedure, without infusion, was performed in another eight participants (control group). There were no significant main effects of group on jaw movement and muscle activity, suggesting that there were no persistent post-pain effects on chewing. Across groups, repetitions of free and unstandardized chewing movements were associated with progressive increases in velocity and amplitude of jaw movement and masseter and temporalis electromyographic (EMG) activity. These findings suggest that factors unrelated to pain, such as practice effects, may be playing a role in the changes in jaw movement and jaw-muscle activity observed after resolution of an acute episode of jaw-muscle pain.

Experimentally induced masseter-pain changes masseter but not sternocleidomastoid muscle-related activity during mastication.

The aim of this study was to verify the effects of induced masseter-muscle pain on the amplitude of muscle activation, symmetry and coactivation of jaw- and neck-muscles during mastication. Twenty-eight male volunteers, mean age±SD 20.6±2.0years, participated in this study. Surface electromyography of the masseter and sternocleidomastoid (SCM) muscles was performed bilaterally during mastication of a gummy candy before and after injections of monosodium glutamate solution and isotonic saline solution. As a result, we observed a decrease in the amplitude of activation of the masseter muscle on the working side (p=0.009; d=0.34) and a reduction in the asymmetry between the working and the balancing side during mastication (p=0.007; d=0.38). No changes were observed either on the craniocervical electromyographic variables. In conclusion, experimentally induced pain reduced the masseter muscle activation on the working side, thereby reducing the physiological masseters' recruitment asymmetry between the two sides during mastication. No effects on SCM activity were detected. These results may partly explain the initial maladaptative changes underlying TMD conditions.

Evaluation of jaw and neck muscle activities while chewing using EMG-EMG transfer function and EMG-EMG coherence function analyses in healthy subjects.

This study aims to quantitatively clarify the physiological features in rhythmically coordinated jaw and neck muscle EMG activities while chewing gum using EMG-EMG transfer function and EMG-EMG coherence function analyses in 20 healthy subjects. The chewing side masseter muscle EMG signal was used as the reference signal, while the other jaw (non-chewing side masseter muscle, bilateral anterior temporal muscles, and bilateral anterior digastric muscles) and neck muscle (bilateral sternocleidomastoid muscles) EMG signals were used as the examined signals in EMG-EMG transfer function and EMG-EMG coherence function analyses. Chewing-related jaw and neck muscle activities were aggregated in the first peak of the power spectrum in rhythmic chewing. The gain in the peak frequency represented the power relationships between jaw and neck muscle activities during rhythmic chewing. The phase in the peak frequency represented the temporal relationships between the jaw and neck muscle activities, while the non-chewing side neck muscle presented a broad range of distributions across jaw closing and opening phases. Coherence in the peak frequency represented the synergistic features in bilateral jaw closing muscles and chewing side neck muscle activities. The coherence and phase in non-chewing side neck muscle activities exhibited a significant negative correlation. From above, the bilateral coordination between the jaw and neck muscle activities is estimated while chewing when the non-chewing side neck muscle is synchronously activated with the jaw closing muscles, while the unilateral coordination is estimated when the non-chewing side neck muscle is irregularly activated in the jaw opening phase. Thus, the occurrence of bilateral or unilateral coordinated features in the jaw and neck muscle activities may correspond to the phase characteristics in the non-chewing side neck muscle activities during rhythmical chewing. Considering these novel findings in healthy subjects, EMG-EMG transfer function and EMG-EMG coherence function analyses may also be useful to diagnose the pathologically in-coordinated features in jaw and neck muscle activities in temporomandibular disorders and whiplash-associated disorders during critical chewing performance.

Does the habitual mastication side impact jaw muscle activity?

OBJECTIVE: To compare electrical activity in the anterior temporal and masseter muscles on the habitual (HMS) and non-habitual mastication side (NHMS), during mastication and in the mandibular postural position. In addition, the increase in electrical activity during mastication was assessed for the HMS and NHMS, analysing both working (WSM) and non-working side during mastication (NWSM). METHODS: A total of 28 healthy women (18-32 years) participated in the study. They were submitted to Kazazoglu's test to identify the HMS. Bioresearch 'Bio EMG' software and bipolar surface electrodes were used in the exams. The exams were conducted in the postural position and during the unilateral mastication of raisins, on both the HMS and NHMS. The working and non-working side on HMS and NHMS were assessed separately. The obtained data were then statistically analysed with SPSS 20.0, using the Paired Samples Test at a significance level of 95%. RESULTS: The differences in the average EMG values between HMS and NHMS were not statistically significant in the postural position (Temporal p=0.2; Masseter p=0.4) or during mastication (Temporal WSM p=0.8; Temporal NWSM p=0.8; Masseter WSM p=0.6; Masseter NWSM p=0.2). Differences in the increase in electrical activity between the masseter and temporal muscles occurred on the working side, on the HMS and NHMS (p=0.0), but not on the non-working side: HMS (p=0.9) and NHMS (p=0.3). The increase in electrical activity was about 35% higher in the masseter than in the temporal muscle. CONCLUSIONS: Mastication side preference does not significantly impact electrical activity of the anterior temporal and masseter muscles during mastication or in postural position.

Diagnostic validity of self-reported measures of sleep bruxism using an ambulatory single-channel EMG device.

PURPOSE: Self-reported measures have been widely used to indicate the presence of possible and probable sleep bruxism (SB) in both research and clinical situations. However, few studies have attempted to assess the diagnostic validity of this approach. The aim of this study was to estimate the diagnostic validity of self-reported measures of SB using an ambulatory single-channel electromyographic (EMG) device. METHODS: A total of 115 participants were enrolled and examined by standardized Research Diagnostic Criteria for Temporomandibular Disorders (RDC/TMD) including two questions related to SB: self-reported SB and morning-jaw symptoms. An ambulatory single-channel EMG device (GrindCare3™, Medotech A/S) was used for measuring jaw-muscle EMG activity during sleep for seven consecutive nights. Cut-off values for different measures of EMG activity (average, maximum and minimum) and the coefficient of variation (CV) were selected to divide participants into two groups, with higher or lower EMG activity or CV values. The sensitivity and specificity for each question and combination of them were calculated. RESULTS: Self-reported SB had the highest sensitivity (compared with morning-jaw symptoms) for all measures of EMG activity and CV, although the values were low to modest (average: 76.0%, maximum: 76.9%, minimum: 77.3%, CV: 61.0%). The specificity was low for both the questions related to the different measures of EMG activity and CV (35.1-52.4%). CONCLUSIONS: This study indicated that the diagnostic validity of self-reported measures of SB was low to modest using an ambulatory EMG device assessment as a reference. Using only self-reported measures for the assessment of SB may not have a high validity, which should be taken into consideration in the clinical evaluation of patients.

Sleep bruxism: an updated review of an old problem.

Objective To provide an update on what is known about bruxism and some of the major clinical highlights derived from new insights into this old problem in dentistry. Materials and methods A selective, non-systematic but critical review of the available scientific literature was performed. Results There are two main different types of bruxism, which are related to different circadian periods (sleep and awake bruxism) that may differ in terms of pathophysiology, but they share some common signs and symptoms. Approximately one out of 10 adult individuals may suffer from bruxism, but not all bruxers may need treatment. Bruxism is complicated to diagnose in the clinic and self-report of bruxism may not necessarily reflect the true presence of jaw muscle activity. Better understanding has been acquired of bruxism relationships with sleep stages, arousal responses and autonomic function with the help of polysomnography and controlled sleep studies. Meanwhile, there is still much more to learn about awake bruxism. With the available scientific knowledge it is possible to systematically assess the effects of bruxism and its potential risk factors for oral and general health. Moreover, we can be aware of the realistic possibilities to manage/treat the patient suffering from bruxism. Conclusion Bruxism is a parafunctional activity involving the masticatory muscles and probably it is as old as human mankind. Different ways have been proposed to define, diagnose, assess the impact and consequences, understand the pathophysiology and treat or manage bruxism. Despite the vast research efforts made in this field, there are still significant gaps in our knowledge.

Effects of the masticatory demand on the rat mandibular development.

The influence of masticatory loading stimulus on mandibular development is not fully clear. In this paper, experimental alterations in the daily muscle use, caused by a changed diet consistency, were continuously monitored, while adaptations in bone and cartilage were examined. It is hypothesised that decreased muscular loading will result in a decrease in the growth factor expression and mandible growth. Fourteen 21-day-old Wistar strain male rats were randomly divided into two groups and fed on either a hard or soft diet for 14 weeks. An implanted radio-telemetric device recorded continuously muscle activity of the superficial masseter muscle. Chondroblast proliferation in the condylar cartilage was identified by insulin-like growth factor-1 receptor (IGF-1r) immunostaining. Furthermore, an X-ray was taken for cephalometric analysis. In the soft-diet group, the duty time of the superficial masseter muscle at higher activity levels was significantly lower than that in the hard-diet group. This decrease in muscular loading of the jaw system was accompanied by: a significant reduction in (i) articular cartilage thickness, (ii) expression of IGF-1r immunopositive cells and (iii) mandible ramus height. In conclusion, a decrease in masticatory demand during the growth period leads to insufficient mandibular development.

Jaw-motor effects of experimental jaw-muscle pain and stress in patients with deep bite and matched control subjects.

OBJECTIVE: The effect of experimental jaw-muscle pain and stress on masticatory muscle activity in TMD-patients has been discussed. Furthermore, associations between TMD and deep bite patients have been studied. Accordingly in the present study, comparison of EMG responses at rest, maximal clenching, during evoked pain and stress between deep bite patients and controls was investigated. DESIGN: In 30 deep bite patients and in 30 sex-/age-matched controls with neutral occlusion EMG activity was recorded bilaterally from masseter and anterior temporalis muscles before and during evoked pain and before and during a stress task. Evoked pain was induced by injections of glutamate into the masseter (local pain) and brachioradialis (remote pain) muscles and resting EMG activity was recorded before and after 1, 2, 3, 4, 5 and 10min. A precision task was used to simulate a stressful condition and EMG activity was recorded twice during the task. Maximal EMG activity was recorded during maximal clenching. RESULTS: Resting and maximal EMG activity were significantly different between groups and age with no gender differences. EMG activity during local pain and during the precision task were significantly different between groups, gender, age and time, whereas no time effect was found for the EMG activity during remote pain. CONCLUSIONS: Patients with deep bite have significantly different jaw motor responses to painful stimulation of the trigeminal region and manual precision tasks suggesting a differential integration of both somatosensory and behavioural stimuli.