Left Masseter Muscles Research Articles

Peripheral mGluR5 antagonist attenuated craniofacial muscle pain and inflammation but not mGluR1 antagonist in lightly anesthetized rats

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300–400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 μl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10 s. After 30 μl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans’ blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans’ blue dye concentration. Intramuscular pretreatment with 5 mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans’ blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Correlation and cluster analysis of sensory, pain, and reflex thresholds to various stimulus modalities in symptom-free subjects

Objective In order to evaluate the possible relation between the psychophysical response and a motor reflex, sensory and pain thresholds to various stimuli were analyzed in combination with the occurrence threshold of the late masseteric exteroceptive suppression (ES2) period. Methods Twenty men and 20 women participated. The tactile detection threshold and the filament-prick pain detection threshold were measured on the cheek skin overlying the left masseter muscles. The pressure pain threshold and pressure pain tolerance threshold were measured at the left masseter muscle. The surface EMG was recorded from the left masseter muscle, while electrical stimuli with 13 fixed intensities were applied to the skin above the left mental nerve. The stimulation intensity at which the ES2 appeared for the first time and the lowest stimulus intensity at which the subjects reported to be painful were defined as the ES2 and pain threshold, respectively. Results There were significant positive correlations between the tactile detection threshold and the pain thresholds determined using the different stimulus modalities, and the ES2 threshold was also significantly correlated with the pain thresholds ( P < 0.05). Cluster analysis could significantly discriminate two distinct groups with high versus low tactile, pain and ES2 thresholds ( P < 0.05). Conclusions The present findings suggested that the ES2 reflex response has a relation with the individual sensory and pain sensitivity in symptom-free subjects. Significance Combined examination of sensory, pain, and ES2 thresholds might provide complementary information on the pathophysiology underlying orofacial pain.

Organisation of common inputs to motoneuron pools of human masticatory muscles

Objective To determine the pattern of organization of common inputs to the motoneuron pools of individual muscles in the masticatory system. Methods Six subjects bit on a rubber-coated wooden splint placed between the upper and lower incisor teeth. We recorded the surface electromyogram (EMG) of co-contracting masseter, temporalis and digastric muscles bilaterally during isometric jaw closing at 5%, 10%, 20% and 40% of maximal voluntary masseter EMG. Results The cross-correlograms of the EMGs of homologous muscle pairs indicate that there are common synaptic inputs to the motoneuron pools of the left and right masseter, and left and right digastric muscles, but not to left and right temporalis. The amplitude of the central peak in masseter and digastric correlograms increased with bite force. When the activity of ipsilateral muscle pairs was cross-correlated, central peaks were prominent for masseter–digastric and masseter–temporalis muscle pairs, and the peak amplitudes increased significantly with bite force. In contrast, no significant central peak was observed for temporalis–digastric muscle pairs at any level of voluntary biting. Conclusions We conclude that there is synchronous modulation of input bilaterally to the masseter muscles and to the digastric muscles but not to the temporalis muscles. There is synchronous modulation of input to ipsilateral masseter–digastric and masseter–temporalis muscle pairs but not to temporalis and digastric muscles. Significance The extent of common input to motoneuron pools of muscles acting around a common joint varies for different muscle pairs, and is not simply a function of whether the muscles of the pair are synergists or antagonists.

Influence of age and gender on the jaw-stretch and blink reflexes

The aim of this study is to investigate the influence of age and gender on jaw-stretch and blink reflexes (BR). Thirty "young" (26.5+/-0.7 years) and thirty "old" (47.8+/-1.8 years) healthy adults were included. Short-latency stretch reflex responses were evoked in the masseter and temporalis muscles by fast jaw-stretches, and BR in orbicularis oculi muscle were evoked by painful electrical pulses (0.5 ms duration), delivered by a concentric electrode placed on the left lower forehead close to the supraorbital foramen. For the jaw-stretch reflex, the pre-stimulus EMG activity in the old subjects was significantly lower than that of the young subjects in the right and left masseter and temporalis muscles (P<0.006), whereas there was no difference in the results between males and females. The normalized peak-to-peak amplitude of the EMG in the left masseter and left and right temporalis muscles was significantly lower in the old subjects compared with the young subjects (P<0.02). Females had significantly higher normalized peak-to-peak EMG amplitudes compared with males in the right masseter and left temporalis muscles (P<0.05). The old subjects had significantly lower root mean square (RMS) (P=0.01) and average (P<0.02) BR values in the right and left orbicularis oculi muscles, and lower area under the curve (AUC) (P=0.02) values in the left orbicularis oculi muscle compared with the young subjects. Female subjects had significantly lower AUC (P=0.02) in the left orbicularis oculi muscle compared with males. The old subjects had significantly later offset (P<0.003) and longer duration (P<0.001) in the left orbicularis oculi compared with the young subjects. The results of the present study demonstrated a significant effect of both age and gender on stretch and BR and suggested that these variables should be taken into consideration in the interpretation of brainstem reflexes in basic and clinical studies.

Neuropathie trigéminale motrice pure révélée par un syndrome algo-dysfonctionnel de l’appareil manducateur chez une patiente co-infectée par le VIH et le VHC

Pure trigeminal motor neuropathy (PTMN) is a rarely described condition. We report the case of a 41-year-old woman infected with the human immunodeficiency virus (HIV1) and hepatitis C virus who presented with weakness of left temporalis and masseter muscles and painful left temporomandibular joint dysfunction (TMD) a few months after cerebral toxoplasmosis revealing acquired immunodeficiency syndrome (AIDS). Magnetic resonance imaging revealed severe wasting and fat replacement of the left temporalis, pterygoid and masseter muscles and showed neither abnormalities in the left motor nucleus of the trigeminal nerve nor compression of the left trigeminal nerve. Electromyographic examination gave evidence of denervation in the left temporalis, masseter and pterygoid muscles and blink reflex studies were normal, confirming the diagnosis of PTMN which was probably secondary to HIV and HCV co-infection.

TMJ Inflammation Increases Fos Expression in the Nucleus Raphe Magnus Induced by Subsequent Formalin Injection of the Masseter or Hindpaw of Rats

The study was designed to examine the effect of persistent temporomandibular joint (TMJ) inflammation on neuronal activation in the descending pain modulatory system in response to noxious stimulus. Formalin was injected into the left masseter muscle or hindpaw of rats 10 days after injection of the left TMJ with saline or complete Freund's adjuvant (CFA). The results showed that 10-day persistent TMJ inflammation (induced by CFA) alone did not induce a significant increase in Fos-like immunoreactive (Fos-LI) neurons in the rostral ventromedial medulla (RVM) or locus coeruleus (LC), but that formalin injection of the masseter muscle or hindpaw induced a significant increase in Fos-LI neurons in the RVM and LC of rats with and without TMJ inflammation (P < 0.05). However, persistent TMJ inflammation significantly increased Fos-LI neurons in the nucleus raphe magnus (NRM) induced by subsequent formalin injection of the masseter muscle and hindpaw (70.2% increase and 53.8% increase, respectively, over the control TMJ-saline-injected rats; P < 0.05). The results suggest that persistent TMJ inflammation increases neuronal activity, in particularly in the NRM, by the plastic change of the descending pain modulatory system after ipsilateral application of a noxious stimulus to either orofacial area or a spatially remote body area.

咬筋内に発生した富細胞型神経鞘腫の１例

Schwannoma is a benign tumor originating from Schwann's sheath. Cellular schwannoma is one of the variants of schwannoma, which differs from conventional schwannoma and is extremely rare in the oral and maxillofacial region. We present a case of cellular schwannoma arising in the masseter muscle. The patient was a 58-year-old woman who was referred to our hospital because of swelling of the left cheek. An elastic firm mass, 30mm in diameter, was found in the left cheek. The mass was movable and slightly tender. MRI revealed a well-circumscribed mass within the left masseter muscle. Under a clinical diagnosis of a tumor in the left cheek, the lesion was extirpated under general anesthesia. The tumor was encapsulated and easily removed. It was solid, and the cut surface was yellow. Histopathological examination revealed that the tumor consisted mainly of a proliferation of spindle cells resembling Schwann's cells with an Antoni A type pattern. Verocay bodies were not observed. High cellularity and mitotic activity were characteristic features. A small area with an Antoni B type pattern was also observed. Immunohistochemically, the tumor cells were positive for S-100. On the basis of these findings, the tumor was diagnosed to be a cellular sahwannoma. Recurrence has not been observed for 1 year 4 months after surgery.

Ketamine attenuates glutamate-induced mechanical sensitization of the masseter muscle in human males

The purpose of the present study was to determine whether glutamate-induced mechanical sensitization of the masseter muscle in human volunteers involves activation of peripheral N-methyl-D-aspartate (NMDA) receptors. Healthy male volunteers (n=18) participated in this randomized, two-session study. During each session, the volunteers received two injections into the right masseter muscle. An initial injection of glutamate (1 M, 0.2 ml) alone was followed 30 min later by a second injection of glutamate alone or glutamate combined with ketamine (10 mM). Pressure pain threshold (PPT) was assessed over the right masseter muscle at and 2 cm above the injection site, as well as over the right temporalis muscle and left masseter muscle prior to the first injection. The PPT was reassessed at all four sites every 5 min from 10 to 30 min after the second injection and once again 60 min after the second injection. Glutamate-evoked muscle pain, pain area and the sensory pain response index of the McGill pain questionnaire were all significantly reduced by co-injection of ketamine. The mean PPT values were significantly decreased by approximately 10%, 10, 15 and 25 min after injection of glutamate, but only over the site of injection. Co-injection of ketamine with glutamate also completely blocked the glutamate-induced mechanical sensitization 15 min post-injection as compared with glutamate alone. The lack of spread of mechanical sensitization outside the area of glutamate injection is consistent with the view that glutamate-induced mechanical sensitization results from a peripheral mechanism. The attenuation of glutamate-induced mechanical sensitization by ketamine suggests that this effect is mediated, in part, through activation of peripheral NMDA receptors.

Gender difference in masseteric exteroceptive suppression period and pain perception

Objective Use of brain stem reflexes in the assessment of orofacial function requires insight into the influence of demographic factors such as gender. The aim of this study was to characterize possible gender differences in the relation between quantitative measures of the masseteric exteroceptive suppression (ES) reflex response and pain perception evoked by incrementally increasing electrical stimulation. Methods In 12 men and 12 women, the surface electromyogram was recorded from the left masseter muscle. Thirteen fixed stimulus intensities from 5 to 35 mA at 2.5 mA increments were applied to the skin above the left mental nerve. The stimulation intensity at which the late ES appeared first and the first intensity at which the subjects reported the stimulus intensity to be painful were defined as the reflex threshold (RT) and pain threshold (PT), respectively. Furthermore, data were analyzed using stimulus–response curves, and the reflex appearance levels (RAL), the saturation level, the slope from appearance to saturation of the reflex (SLP), and the pain appearance level (PAL) were determined. Results The PT was equal to or higher than the RT in 9 of the 12 men, but only in 4 of the 12 women. Further, women had significantly lower PAL, RAL, and SLP (12.7±0.8, 12.9±1.4, and 3.0±0.9 mA, respectively) compared to men (20.3±1.6 mA, 16.7±1.1 T, and 4.1±0.4, respectively) ( P<0.05). Conclusions The present results document that women have a lower reflex threshold and pain threshold to cutaneous electrical stimulation than men. Significance These findings suggest that gender differences may exist in the sensory–motor integration of primary afferent input from the orofacial region and that these differences should be considered in the design of future reflex studies.

WITHDRAWN: Peripheral mGluR5 antagonist attenuated craniofacial muscle pain and inflammation but not mGluR1 antagonist in lightly anesthetized rats

The present study investigated the role of peripheral group I metabotropic glutamate receptors (mGluRs) in MO-induced nociceptive behaviour and inflammation in the masseter muscles of lightly anesthetized rats. Experiments were carried out on male Sprague-Dawley rats weighing 300-400 g. After initial anesthesia with sodium pentobarbital (40 mg/kg, i.p.), one femoral vein was cannulated and connected to an infusion pump for intravenous infusion of sodium pentobarbital. The rate of infusion was adjusted to provide a constant level of anesthesia. Mustard oil (MO, 30 microl) was injected into the mid-region of the left masseter muscle via a 30-gauge needle over 10s. After 30 microl injection of 5, 10, 15, or 20% MO into the masseter muscle, the total number of hindpaw shaking behaviour and extravasated Evans' blue dye concentration in the masseter muscle were significantly higher in the MO-treated group in a dose-dependent manner compared with the vehicle (mineral oil)-treated group. Intramuscular pretreatment with 3 or 5% lidocaine reduced MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration. Intramuscular pretreatment with 5mM MCPG, non-selective group I/II mGluR antagonist, or MPEP, a selective group I mGluR5 antagonist, produced a significant attenuation of MO-induced hindpaw shaking behaviour and increases in extravasated Evans' blue dye concentration in the masseter muscle while LY367385, a selective group I mGluR1 antagonist, did not affect MO-induced nociceptive behaviour and inflammation in the masseter muscle. These results indicate that peripheral mGluR5 plays important role in mediating MO-induced nociceptive behaviour and inflammation in the craniofacial muscle.

Madarosis and Facial Alopecia Presumed Secondary to Botulinum A Toxin Injections

: Botulinum neurotoxins are considered relatively safe therapy for treatment of inappropriate muscle spasms. As seen in this report, undesirable consequences may result. : The author conducted a clinical interview and examination of the patient. : Unilateral left-sided madarosis and facial alopecia were observed in a patient undergoing botulinum A toxin injections for the treatment of left oromandibular dystonia. The patient had started treatment 18 months previously. He received injections every 3 months in the left masseter and left temporalis muscles. He first noted loss of his left sideburn 8 months into treatment. After 10 months of treatment, he noted that he no longer needed to shave on the left side of his face. Eighteen months after receiving his first botulinum toxin injection, madarosis of the temporal aspect of the left lower lid was observed. : This is the first case report documenting an idiosyncratic unilateral madarosis and facial alopecia as adverse side effects presumed secondary to botulinum A toxin injections.

Relationship between the direction of mandibular growth and masseter muscle conduction velocity

The purpose of this study was to investigate the relationship between muscle conduction velocity (MCV) of the masseter muscle and the direction of mandibular growth. Longitudinal cephalometric X-rays taken at the prepubertal and postpubertal periods of 16 Japanese girls were analyzed. MCV was calculated from the delay in myoelectric signals obtained by using multiple surface electrode arrays placed along the fibers of the left masseter muscle in the postpubertal period. The direction of mandibular growth was evaluated by superimposition of the lateral cephalometric X-rays at the prepubertal and postpubertal periods. The relationship between MCV and the direction of mandibular growth was analyzed statistically. MCV was significantly correlated with the vertical facial height at the postpubertal period and the direction of mandibular condyle growth. If the relationship between prepubertal and postpubertal of MCV is clarified, it might be possible to predict the direction of mandibular growth and the vertical facial proportions at the postpubertal period from MCV of the masseter muscle at the prepubertal period.

Exteroceptive suppression periods in masseteric EMG: Use of stimulus–response curves

The present study documented quantitative changes in the duration and the depth of the exteroceptive suppression (ES) period in the masseteric electromyogram (EMG) using stimulus-response (SR) curves. From six males and six females (20-41 years), the surface masseteric EMG was recorded bilaterally. Visual feedback from the left masseter muscle (MM) allowed a constant clenching level of 30% or 50% maximal voluntary contraction (MVC). Automatic triggering of the stimulus to the left mental nerve occurred when the background EMG remained constant. Eight to twelve stimulus intensities (range, 0-40 mA) were determined based on the subjects' sensory threshold (T). The late ES (ES2) occurred consistently and could be analyzed using SR curves. ES2 in the right MM appeared at 7.3+/-3.1T at 30% MVC, while it was significantly decreased to 5.2+/-2.2T at 50% MVC (P<0.05). The depth of ES2 suppression in the left MM was 69.7+/-5.7% at 30% MVC, and this increased significantly to 76.6+/-5.5% at 50% MVC (P<0.05). In addition, the depth of ES2 suppression in the left MM was significantly higher than that in the right MM at 50% MVC (70.0+/-7.4%, P<0.05). Consequently, it was confirmed that both the appearance and depth of ES may be affected by the level of background EMG. More importantly, the use of multiple stimulation intensities allowed development of reliable SR curves. These SR curves may be used to determine inter-individual differences in ES.

Identificação do lado de preferência mastigatória através de exame eletromiográfico comparado ao visual

O objetivo deste estudo foi verificar a freqüência da mastigação preferencialmente unilateral através da eletromiografia comparada à inspeção visual. Participaram deste estudo 29 indivíduos saudáveis, alunos do Curso de Odontologia - Uniararas, na faixa etária entre 18 e 25 anos, de ambos os gêneros, selecionados através dos critérios: dentição permanente completa ou apenas ausência dos 3os molares, sem sinais clínicos de disfunção craniomandibular ou doença periodontal. A análise eletromiográfica foi realizada nos músculos masseteres durante mastigação habitual de cenoura e parafilme. Considerou-se a presença de um lado de preferência mastigatória (LPM) quando os valores de amplitude de contração muscular obtidos em RMS (Root Mean Square) ou Valor Eficaz tivessem uma diferença de pelo menos 20% entre os músculos masseteres direito e esquerdo. A inspeção visual foi realizada por um único observador durante as aquisições eletromiográficas. Considerou-se a presença de um lado de preferência mastigatória quando a mastigação ocorria com uma freqüência de ciclos mastigatórios pelo menos 30% maior por um dos lados, após o registro de 20 ciclos mastigatórios consecutivos. Do total da amostra, 82,8% (24/29) e 72,4% (21/29) apresentaram um lado de preferência mastigatória durante a mastigação de cenoura e parafilme, respectivamente. A concordância entre a eletromiografia e a inspeção visual foi verificada em 83% das observações. Pode-se concluir que a maioria dos sujeitos examinados nessa pesquisa possuía um lado de preferência mastigatória durante mastigação habitual de cenoura e parafilme e que a eletromiografia pode ser usada como parâmetro para detecção do lado de preferência mastigatória.

Effects of a functional appliance on masticatory muscles of young adults suffering from muscle-related temporomandibular disorders.

The aim of this study was to investigate the effects of an original orthodontic functional appliance [function generating bite for deep bite correction (FGB-D)] on masticatory muscle activity in subjects suffering from muscle-related temporomandibular disorders (TMD). Electromyographic (EMG) analysis was performed on 33 young adults (nine men, 24 women) to evaluate the contractile symmetry of the right and left masseter and anterior temporalis muscles. The subjects were divided into three groups: a muscle-related TMD group requiring orthodontic treatment for deep bite correction (three men, eight women) and treated with FGB-D; a muscle-related TMD group not requiring orthodontic treatment (three men, eight women) and treated with a Michigan occlusal splint; and a TMD-free group (three men, eight women) as a control group. Records were made by surface EMG of maximum voluntary teeth clenching, with and without the functional appliance or occlusal splint in place, before and after 12 months of therapy. A torque index was derived from the surface EMG recordings to estimate lateral displacement of the mandible. The results show that the FGB-D corrects the torque index and thus the lateral displacement of the mandible.

Associations between pain and neuromuscular activity in the human jaw and neck muscles

The aim of this study was to test the effects of glutamate-evoked jaw or neck muscle pain on electromyographic (EMG) activity of jaw and neck muscles in humans. EMG recordings were made from left (MAL) and right (MAR) masseter muscles, and right sternocleidomastoid (SCM) and splenius (SP) muscles in three different head positions (head rest, head back, head right) or during maximal jaw clenching in 19 men. Glutamate (1 M) or isotonic saline was injected into MAR or SP, and induced pain was recorded on visual analogue scales. EMG activity in MAL and MAR was increased in the head back position compared to head rest and head right positions, whereas EMG activity in SCM and SP was progressively increased as the head was moved from rest position to head back to head right positions. Glutamate-evoked MAR pain was associated with increases in EMG activity in MAR, SCM and SP at rest but not in the head back or head right positions. Glutamate-evoked SP pain was associated with an increase in SP EMG activity at rest and a decrease in SCM EMG activity in the head right position. Decreases in jaw clench-related EMG activity were observed in MAL, MAR and SCM muscles only during glutamate-evoked MAR pain. Isotonic saline injections induced no pain or EMG changes. In conclusion, experimental neck pain is not associated with tonic increases in jaw EMG activity although jaw muscle pain can be linked to increases in neck EMG activity with the head and jaw at rest.

Glutamate evoked neck and jaw muscle pain facilitate the human jaw stretch reflex

Objective Although pain and neuromuscular function are clearly linked in several clinical conditions manifested in the craniofacial and cervical regions, it is unclear if pain in these regions influences reflexly evoked activity in the jaw or neck muscles in humans. The aim of the present study was to test the effects of glutamate-evoked jaw or neck muscle pain on the jaw stretch reflex recorded in both jaw and neck muscles. Methods Nineteen healthy men participated in the study. Electromyographic (EMG) recordings were made from the left masseter (MAL) and right masseter (MAR) muscles and the right sternocleidomastoid (SCM) and splenius (SP) muscles. Glutamate (1 M) or isotonic saline was injected into the MAR or right SP in random order and then the other solution was injected 1–3 weeks later. Pain intensity was scored on a 10 cm visual analogue scale. Stretch reflexes were evoked by standardized jaw stretches before, during and 15 min after the end of the experimental muscle pain. Twenty trials were averaged in each condition. Results Pain evoked by MAR or SP glutamate injections was associated with a significant increase in the stretch reflex amplitude recorded in both MAR and SCM. The onset and offset times and duration of the stretch reflex did not change in any muscle during the various pain conditions. Injection of isotonic saline into the MAR or SP did not produce any significant change in the reflex parameters in any of the muscles. Conclusions The results indicate the close interplay between the craniofacial and cervical regions in the neuromuscular changes that may result from musculoskeletal pain in either region. Significance The changes in neuromuscular activity documented in this study may be involved in the clinical occurrence of altered muscle activity in the orofacial and cervical regions as a result of deep tissue trauma and pain.

こう筋内に生じた動静脈奇形の１例

Arteriovenous malformation (AVM) rarely occurs in the head and neck region. AVM of the masseter muscle is extremely rare. We report a case of AVM of the masseter muscle. The patient was a 54-year-old woman with pain of the left masseter muscle region for about 1 week. She had a movable mass in the left masseter region. On clenching, the mass became erect. Enhanced CT scanning showed a high density region in the masseter muscle. MRI T 1 imaging showed that the tumor was the same signal intensity as muscle. On MRI T 2 images, signal intensity of the mass was greater than that of fat. Color Doppler imaging showed high intensity color flow at the base of the mass. Under the diagnosis of an erectile hemangioma of the masseter muscle, the mass was resected surgically with an Nd: YAG laser via submandibular and preauricular approaches on May 17, 2000. Hemorrhage (20 grams) was controlled by ligation of the feeding artery. Histologically, the mass consisted mainly of cavernous hemangiomatous proliferation of veins with arterialisation and a few structurally deformed arteries. The pathological diagnosis was arteriovenous malformation. The postoperative course was without complications. As of 29 months after operation, there has been no sign of recurrence.

Emotional stress and brux-like activity of the masseter muscle in rats.

The aim of this study was to further clarify the relationship between emotional stress and bruxism. In experiment 1, 60 male 9-week-old Wistar rats were divided into four groups: the emotionally stressed (ES), the emotionally non-stressed (NS), the electrically foot-shocked (FSd), and the non-foot-shocked (NSd). ES rats were confined in a communication box for one hour a day to observe the emotional responses of neighbouring FSd rats. On days 0, 1, 4, 8, and 12, the electromyographic activity of the ES and NS rats' left masseter muscles was recorded for one hour, three hours after confinement in the communication box. Brux-like activity appeared in the masseter muscle of the ES group on days 1, 4, 8, and 12, but not in the NS group. In experiment 2, 36 male Wistar rats, 9 weeks old, were divided into three groups: emotionally stressed rats treated with an anti-anxiety drug (DES), emotionally stressed rats treated with saline as a vehicle (VES), and 24 FSd rats. Stress and EMG procedures were the same as those in experiment 1. Brux-like episodes decreased in DES rats from day 1 and significant differences were found on days 4 (P < 0.01), 8 (P < 0.05), and 12 (P < 0.05), when compared with the VES group. These findings suggest that emotional stress induces brux-like activity in the masseter muscle of rats, which was reduced with anti-anxiety drugs.

Recurrent osseous choristoma of the masseter muscle: Case report

A 1222-year-old Turkish man was seen at the Dental Clinic on August 5, 1995 for evaluation of a painless, slowly enlarging, swelling of the left facial region of 5 years in duration (Fig 1). There were no significant findings in the medical or dental history and no history of trauma to the region. Palpation revealed a firm, slightly tender, well-circumscribed, mobile mass approximately 5 3 3 cm, covered by normal skin and apparently not attached to the underlying bone. The serum levels of calcium, calcitonin, and parathyroid hormone were all within normal range. Results of hematologic and urine analyses were also unremarkable. Radiographs revealed several well-circumscribed, radiopaque, spheric masses in the soft tissues of the left face; these did not appear to be attached to the mandible or maxilla (Figs 2, 3). Both T1and T2-weighted magnetic resonance images (MRI) were obtained to evaluate the vascularity of the lesion. These images also revealed the well-circumscribed, spheric osseous masses (Fig 4A), surrounded by hemorrhagic areas (Fig 4B). The lesion in aggregate was located entirely within the left masseter muscle. The preoperative clinical impression was that the lesions represented a benign osseous process, possibly teratomas, osteomas, calcified lymph nodes, osseous hamartomas, or calcified hematomas. The masses were surgically excised under general anesthesia through an extraoral approach (Fig 5), and the specimens were sent for microscopic examination. GROSS FINDINGS