Digastric Muscle Activity Research Articles

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

ObjectiveRhythmic 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. MethodsEleven 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. ResultsLong-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. ConclusionsThese results suggest that the masticatory CPG was activated by electrical CT stimulations independently of the motoneuron inhibitory system during REM sleep.

Functional involvement of the sternohyoid muscle during breathing and swallowing in rats.

The sternohyoid muscle depresses the hyoid bone, but it is unclear whether the muscle contributes to respiratory and swallowing mechanisms. This study aimed to clarify whether the sternohyoid muscle participates in the respiration and swallowing reflex and how the activity is modulated in two conditions: with airway stenosis and with a fixed sternohyoid muscle length. Electromyographic activity in the sternohyoid, digastric, thyrohyoid, and diaphragm muscles was recorded in anesthetized rats. The sternohyoid muscle activity was observed in the inspiratory phase and during swallowing, and was well coordinated with digastric and thyrohyoid muscle activity. With airway stenosis, the respiratory activity per respiratory cycle was facilitated in all assessed muscles but the facilitation of activity per second occurred only in the digastric, thyrohyoid, and sternohyoid muscles. With airway stenosis, the swallowing activity was facilitated only in the digastric muscle but not in the thyrohyoid and sternohyoid muscles. Swallowing activity was not observed in the sternohyoid muscle in the condition with the sternohyoid muscle length fixed, although increased inspiratory activity remained. The current results suggest that 1) the sternohyoid muscle is slightly activated in the inspiratory phase, 2) the effect of airway stenosis on respiratory function may differ between the upper airway muscles and diaphragm, and 3) swallowing activity in the sternohyoid muscle is not dominantly controlled by the swallowing central pattern generator but instead occurs as a myotatic reflex.NEW & NOTEWORTHY We found that the sternohyoid muscle was activated in the inspiratory phase. However, increased airway resistance had different effects on the extrathoracic muscles than on the diaphragm. The swallowing activity of the sternohyoid disappeared when the muscle length was fixed. These findings suggest that the sternohyoid muscle may be activated not by the swallowing central pattern generator but as a myotatic reflex.

Influence of restricted mastication on swallowing function.

Oral food processing is an important part of daily food intake. A major part of this process is mastication, which prepares a bolus of food for swallowing by mechanically crushing and grinding ingested food between the teeth using rhythmic movements. Masticatory dysfunction is common in the elderly and in some neurological disorders and can have serious negative health consequences. This study investigated the effect of restricted mastication, achieved by experimentally reducing the duration of mastication, on masticatory patterns and subsequent swallowing function. Thirty healthy men (25 ± 3 years old) were instructed to chew gum jelly with a free mastication duration (G100), a half and a quarter duration of G100. Masseter and digastric electromyographic (EMG) activity was recorded to assess mastication and swallowing activity, respectively. In addition, the acceleration of the thyroid cartilage ridge was measured with an accelerometer. The root mean square (RMS) of muscle EMG activity in the masseter and digastric muscles, the number of masseter EMG bursts, time to peak and total duration of each masseter EMG burst, swallowing duration and laryngeal elevation latency were analysed. Restricting masticatory duration reduced the number of mastication cycles (p < .001), prolonged the time to peak (p < .001) and total duration of masseter EMG bursts (p < .001) and resulted in an overall increased RMS score of masseter muscle activity (p = .017). Furthermore, restricted masticatory duration led to a decrease in both swallowing duration (p = .001) and laryngeal elevation latency (p = .012), with a significant increase in the RMS score of digastric muscle activity (p < .001). Under the experimental conditions of restricted mastication, several adaptation features were observed, including changes in masticatory cycle characteristics and swallowing duration. Thus, although the overall masticatory efficiency was reduced, these adaptations allowed healthy individuals to still swallow safely.

Predicting masticatory muscle activity and deviations in mouth opening from non-invasive temporomandibular joint complex functional analyses.

A quantitative approach to predict expected muscle activity and mandibular movement from non-invasive hard tissue assessments remains unexplored. This study investigated the predictive potential of normalised muscle activity during various jaw movements combined with temporomandibular joint (TMJ) vibration analyses to predict expected maximum lateral deviation during mouth opening. Sixty-six participants underwent electrognathography (EGN), surface electromyography (EMG) and joint vibration analyses (JVA). They performed maximum mouth opening, lateral excursion and anterior protrusion as jaw movement activities in a single session. Multiple predictive models were trained from synthetic observations generated from the 66 human observations. Muscle function intensity and activity duration were normalised and a decision support system with branching logic was developed to predict lateral deviation. Performance of the models in predicting temporalis, masseter and digastric muscle activity from hard tissue data was evaluated through root mean squared error (RMSE) and mean absolute error. Temporalis muscle intensity ranged from 0.135 ± 0.056, masseter from 0.111 ± 0.053 and digastric from 0.120 ± 0.051. Muscle activity duration varied with temporalis at 112.23 ± 126.81 ms, masseter at 101.02 ± 121.34 ms and digastric at 168.13 ± 222.82 ms. XGBoost predicted muscle intensity and activity duration and scored an RMSE of 0.03-0.05. Jaw deviations were successfully predicted with a MAE of 0.9 mm. Applying deep learning to EGN, EMG and JVA data can establish a quantifiable relationship between muscles and hard tissue movement within the TMJ complex and can predict jaw deviations.

A Comparison Between Skeletal Class II and Class III Malocclusion Patients in Terms of the Masticatory Muscles' Activity: A Cross-Sectional Study.

Background This study aimed to determine if individuals with skeletal Class II and skeletal Class III malocclusions had different levels of masticatory muscle activity. Materials and methods This cross-sectional study, conducted at the University of Damascus, investigated the myoelectric activity of perioral muscles in patients with Class II and III malocclusions. The sample size of 60 patients was determined according to a prior sample size calculation. Patients were selected based on specific inclusion and exclusion criteria and divided into Class II and III groups. Electromyography was used to monitor the activity of various muscles, including the temporalis, masseter, orbicularis oris, buccinator, mentalis, and digastric muscles. Results The study found similar muscle activity within the same group in the temporalis, masseter, buccinator, digastric, and orbicularis oris muscles. No significant differences were observed between the Class II and III groups for several oral and perioral muscles (P > 0.05). However, the mean activity of the digastric muscle was significantly greater in the Class II group (P < 0.05), whereas the mean activity of the mentalis muscle was smaller in the Class II group (P < 0.05). Conclusions Perioral muscles influence facial complex development and jaw relationship, affecting orthodontic treatment. Digastric muscle activity is greater in Class II patients, while mental muscle activity is smaller in Class III patients. Further studies are needed for older age groups and other skeletal malocclusion types.

Effect of orthodontic appliances on masticatory muscle activity

There is limited and conflicting evidence regarding the impact of orthodontic appliances on masticatory muscle activity (MMA), jaw function, and oral parafunctional behaviours. Investigations in this field frequently utilise surface electromyography (sEMG) to analyse changes in muscle contractile activity. Fixed orthodontics appliances have been associated with significant reductions in MMA post archwire activation. Conversely, elevations in masseter and digastric muscle activity have been reported in response to removable functional appliances. Research into the effect of clear aligner therapy (CAT) on MMA is still in its infancy with few studies published. There is a lack of agreement on whether MMA increases or diminishes in response to CAT though there is evidence this relationship may change over time due to muscle adaptation. The impact of CAT on temporomandibular disorder (TMD) symptoms is unclear, particularly how pre-existing risk factors for TMD may alter and individual's response.

Exploratory Study on Central Sensitization and Bioelectrical Activity of the Selected Masticatory Muscles in Subjects with Myopia

Myopia is one of the most common eye disorders involving abnormal focusing of light rays. The studies recognize the association between the stomatognathic and visual systems. This compound may have a neurological basis associated with disorders such as central sensitization. The main aim of this study was to evaluate the influence of central sensitization on the bioelectrical activity of selected muscles of the masticatory organ in subjects with myopia. Selected masticatory and cervical spine muscles were analyzed using an eight-channel BioEMG III electromyograph. Central sensitization was analyzed using the central sensitization inventory. Statistical analysis revealed significantly higher scores on the central sensitization inventory in subjects with axial myopia compared to subjects without refractive error. Repeated positive correlations were observed in the sternocleidomastoid muscle activity and negative correlations in the digastric muscle activity during open and closed eyes in myopic subjects. Subjects with myopia have an increased score in the central sensitization inventory. The increase in the central sensitization inventory score is connected with the changes within the electromyographic activity of the masticatory and neck muscles. The effect of central sensitization on masticatory muscle activity in myopic subjects requires further study.

Electromyographic Analysis of Masticatory and Accessory Muscles in Subjects With Implant-Supported Fixed Prostheses: A Three-Arm Comparative Clinical Study.

Aim This study compares the electromyographic (EMG) activity of the masticatory and accessory muscles in patients with natural teeth and those wearing full-mouth fixed prostheses supported by implants. Method In this study, 30 subjects of 30-69 years performed static and dynamic EMG measurements of masticatory and accessory muscles (masseter, anterior temporalis, SCM, and anterior digastric) and were divided into three groups: Group 1 (G1, Control, Dentate), comprising 10 subjects with 14 or more natural teeth(30-51 years of age); Group 2 (G2, single arch implant-supported fixed prosthesis) composed of 10 patients with unilateral edentulism who were successfully rehabilitated with implant-supported fixed prostheses in the maxilla or mandible, restoring occlusion to 12-14 teeth per arch; (39-61 years of age); and Group 3 (G3, full mouth implant-supported fixed prosthesis) with10 subjects with completely edentulous arches with full mouth implant-supported fixed prosthesis with 12 occluding pairs of teeth (46-69 years of age). The left and right masseter, anterior temporalis, superior sagittal, and anterior digastric muscles were examined at rest, as well as maximum voluntary clenching (MVC), swallowing, and unilateral chewing. On muscle bellies, disposable, pre-gelled silver/silver chloride bipolar surface electrodes were parallel to muscle fibers. BIO-PAKeight® channels recorded electrical muscle activity (Bio-EMG III, BioResearch Associates, Inc. Brown Deer, WI). Results Full mouth embed upheld fixed prostheses patients had higher resting EMG activity than dentate and single curve implants. Full mouth embeds supported fixed prostheses and dentate patients had significantly different temporalis and digastric muscle mean EMG activity. Dentate people used their temporalis and masseter muscles more during the MVC than those with single-curve embedded upheld fixed prostheses limiting natural teeth or full-mouth implants. No event had the crucial item. Neck muscle differences were insignificant. All groups had higher SCM and digastric EMG activity during MVC than at rest. The single curve embed upheld fixed prosthesis group's temporalis and masseter muscles were significantly more active during gulping than the dentate and entire mouth groups. Single curve and entire mouth gulping SCM muscle EMG activity were similar. Digastric muscular EMG activity differed significantly between those with full-arch or partial-arch fixed prostheses and dentures. When instructed to bite one side, the masseter and temporalis front muscle mean EMG activity increased on the unrestricted side. Unilateral biting and temporalis muscle activation were comparable between groups. For the masseter muscle, the mean EMG was also higher on the functioning side, with no truly large differences between the three groups except for right-side biting when comparing the dentate and full mouth embed upheld fixed prosthesis groups and the single curve and full mouth groups. Conclusion The temporalis muscle activity difference was statistically significant in the full mouth implant-supported fixed prosthesis group.The three groups' static (clenching) sEMG analysis showed non-significant temporalis and masseter muscle activity increases. Full mouth swallowing increased digastric muscle activity. All three groups had similar unilateral chewing muscle activity except for the working side masseter muscle.

Masseter and digastric muscle activity evaluation using a novel electromyogram that utilizes elastic sheet electrodes.

To evaluate the reproducibility and reliability of a novel electromyogram (EMG) device with a flexible sheet sensor for measuring muscle activity related to mastication and swallowing. We developed a new EMG device made of elastic sheet electrodes to measure the masseter and digastric muscle activities for evaluating mastication and swallowing. To examine the measurement reproducibility of the new EMG device, masseter muscle activity was analyzed using the intraclass correlation coefficient (ICC). Further, we measured the maximum amplitude, duration, integrated value, and signal-to-noise ratio (SNR) using the new EMG device and conventional EMG devices and evaluated the reliability using ICC and Bland-Altman analysis. We confirmed high ICC (1,1) and ICC (2,1) scores (0.92 and 0.88, respectively) while measuring the reproducibility of the new EMG device. When compared to the active electrode EMG device, we found a high correlation for the maximum amplitude (0.90), duration (0.99), integrated values (0.90), and SNR (0.75), with no observation of significant fixed errors. Moreover, the regression coefficient was not significant for any of the evaluation items and no proportional error was observed. Compared with the passive electrode EMG device, the maximum amplitude and duration were highly correlated (0.73 and 0.89). In addition, the SNR exhibited a significant fixed error. In contrast, the regression coefficient was not significant for any of the evaluation items and no proportional error was observed. Our results suggest that the new EMG device can be used to reliably and reproducibly evaluate muscle activity during mastication and swallowing.

Changes in cortical, cardiac, and respiratory activities in relation to spontaneous rhythmic jaw movements in ketamine-anesthetized guinea pigs.

It has been reported that rhythmic jaw movements (RJMs) spontaneously occur in ketamine-anesthetized animals. The present study investigated the physiological processes that occur during the cortical, cardiac, and respiratory events which contribute to the genesis of RJMs in animals after supplemental ketamine injections. Fourteen guinea pigs were prepared to allow electroencephalographic, electrocardiographic, and electromyographic activities to be recorded from the digastric muscle, measurement of jaw movements, and nasal expiratory airflow under ketamine-xylazine anesthesia. Rhythmic jaw movements spontaneously occurred with rhythmic digastric muscle contractions, 23-29minutes after injection of supplemental ketamine (12.5 and 25.0mgkg-1 , intravenously). The cycle length of RJMs did not differ significantly between the two doses of ketamine (mean±SD: 12.5mgkg-1 , 326.5±60.0ms; 25mgkg-1 , 278.5±45.1ms). Following injection of ketamine, digastric muscle activity, heart and respiratory rates, and cortical beta power significantly decreased, while cortical delta and theta power significantly increased. These changes were significantly larger in animals given 25.0mgkg-1 of ketamine than in those given 12.5mgkg-1 . With the onset of RJMs, the levels of these variables returned to pre-injection levels, regardless of the dose of ketamine administered. These results suggest that, following supplemental ketamine injections, spontaneous RJMs occur during a specific period when the pharmacological effects of ketamine wear off, and that these RJMs are characterized by stereotypical changes in cardiac, respiratory, and cortical activities.

Effects of Wearing a Custom-Made Mouthguard during Static Exercise on Masticatory Muscle Activity in Athletes with Cerebral Palsy

Aim: Oral health is closely related to the general condition of athletes. Oral diseases can decrease their performance by affecting their concentration during games and practice sessions. Appropriate education is therefore required regarding the importance of dental support. We introduced periodic dental examinations and fabricated custom-made mouthguards (CMGs) for athletes with cerebral palsy (CP). This study aimed to determine the effects of wearing a CMG on masticatory muscle activities in CP athletes while measuring center of gravity (COG) sway. Materials and Methods: Thirteen athletes with CP (male, n=12; mean age, 27.3 ± 8.96 y) and 10 healthy male controls (mean age, 28.5 ± 1.35 y) participated in the present study. The CMG comprised 2-mm-thick polyolefin sheeting. All participants underwent dental checks and occlusal contact areas were measured with and without a CMG. We simultaneously measured gravitational sway at the COG with the eyes open, as well as masticatory (masseter and digastric) muscle activities with or without a CMG. Data were statistically analyzed using two-way mixed analyses of variance. Results: Indices for decayed, missing and filled teeth did not significantly differ between athletes and controls. However, the occlusal contact area significantly increased in athletes when wearing a CMG. Although COG sway slightly differed between having eyes open and closed in athletes, masticatory muscle activity increased and COG sway decreased. Conclusion: The findings suggest that wearing a CMG might change the modality of masticatory muscle activity in athletes with CP and might help to improve balance during static exercise.

Biting intentions modulate digastric reflex responses to sudden unloading of the jaw.

Reflex responses in jaw-opening muscles can be evoked when a brittle object cracks between the teeth and suddenly unloads the jaw. We hypothesized that this reflex response is flexible and, as such, is modulated according to the instructed goal of biting through an object. Study participants performed two different biting tasks when holding a peanut half stacked on a chocolate piece between their incisors. In one task, they were asked to split the peanut half only (single-split task), and in the other task, they were asked to split both the peanut and the chocolate in one action (double-split task). In both tasks, the peanut split evoked a jaw-opening muscle response, quantified from electromyogram (EMG) recordings of the digastric muscle in a window 20-60 ms following peanut split. Consistent with our hypothesis, we found that the jaw-opening muscle response in the single-split trials was about twice the size of the jaw-opening muscle response in the double-split trials. A linear model that predicted the jaw-opening muscle response on a single-trial basis indicated that task settings played a significant role in this modulation but also that the presplit digastric muscle activity contributed to the modulation. These findings demonstrate that, like reflex responses to mechanical perturbations in limb muscles, reflex responses in jaw muscles not only show gain-scaling but also are modulated by subject intent.

The role of masticatory muscles in the continuous loading of the mandible

Muscles are considered to play an important role in the ongoing daily loading of bone, especially in the masticatory apparatus. Currently, there are no measurements describing this role over longer periods of time. We made simultaneous and wireless in vivo recordings of habitual strains of the rabbit mandible and masseter muscle and digastric muscle activity up to ∼25 h. The extent to which habitually occurring bone strains were related to muscle-activity bursts in time and in amplitude is described. The data reveal the masseter muscle to load the mandible almost continuously throughout the day, either within cyclic activity bouts or with thousands of isolated muscle bursts. Mandibular strain events rarely took place without simultaneous masseter activity, whereas the digastric muscle only played a small role in loading the mandible. The average intensity of masseter-muscle activity bouts was strongly linked to the average amplitude of the concomitant bone-strain events. However, individual pairs of muscle bursts and strain events showed no relation in amplitude within cyclic loading bouts. Larger bone-strain events, presumably related to larger muscle-activity levels, had more constant principal-strain directions. Finally, muscle-to-bone force transmissions were detected to take place at frequencies up to 15 Hz. We conclude that in the ongoing habitual loading of the rabbit mandible, the masseter muscle plays an almost non-stop role. In addition, our results support the possibility that muscle activity is a source of low-amplitude, high-frequency bone loading.

Assessment of jaw movements by magnetic sensor in relation to topographies of orofacial behaviour in freely moving rats: Studies with the dopamine D 1-like receptor agonists SKF 83822 vs SKF 83959

This study applies new magnetic sensor-electromyographic technology for recording jaw movements in freely moving rats to analyse topographies of orofacial movement that occur in association with individual elements of behaviour under challenge with two dopamine D 1-like receptor agonists, SKF 83822 ([ R/S]-6-chloro-7, 8-dihydroxy-3-allyl-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1 H-3-benzazepine) and SKF 83959([ R/S]-3-methyl-6-chloro-7, 8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1 H-3-benzazepine). Grooming of the snout/face involved primarily dominant-mouth opening jaw movements with small activation of digastric muscles; subsequent grooming of the flank/trunk was characterised by repetitive, uniform jaw movements with small activation of digastric and masseter muscles. In contrast, grooming of the fingers and tail typically involved high-frequency jaw movements with variable vertical jaw movements and/or strong activation of masseter muscles. Vacuous chewing involved two distinct patterns of jaw movements: a dominant-closing pattern, with strong activation of masseter muscles, and a dominant-opening pattern, with slight activation of masseter muscles. SKF 83822 stimulates dopamine D 1-like receptors and activates adenylate cyclase but not phosphoinositide hydrolysis, while SKF 83959 stimulates dopamine D 1-like receptors and activates phosphoinositide hydrolysis but not adenylate cyclase. These agonists exerted differential effects on jaw movements, as SKF 83959 induced more jaw movements per episode of syntactic grooming than SKF 83822, while SKF 83822 induced more jaw movements during non-syntactic grooming than SKF 83959. Magnetic sensor technology in freely moving animals resolved distinct topographies of orofacial movement and informs on their relationship to other behaviours in the rodent repertoire and to dopamine D 1-like receptor function.

The influence of the occlusal vertical dimension on masticatory muscle activities and hyoid bone position in complete denture wearers

Long lasting usage of complete dentures causes lower occlusal vertical dimension, producing potential detrimental consequences. The aim of this study was to investigate how changes in vertical dimension during denture exchange affect muscular activity and hyoid bone position. Twenty-five edentulous, otherwise healthy patients (14 females, 11 males) aged 70.5 +/- 9 years, wearing their dentures over 5 years but no longer than 15 years (mean 9.8 +/- 5.2), were studied. New dentures were fabricated and the occlusal vertical dimension was recorded on cephalometric radiographs. Cephalograms were analyzed according to Ricketts. The relation of the hyoid bone position to the mandible was estimated. An evaluation of muscular activity was performed using the Biopak Electromyography Recording System synchronized with the T-Scan II Occlusal Analysis System. The occlusal vertical dimension was higher with the new dentures compared with the old dentures. The transition to new dentures was accompanied by a change of the vertical position of the hyoid bone. Digastric muscle activity was lower with the new dentures in comparison with the old dentures. Increase of the occlusal vertical dimension in complete denture wearers affects the hyoid bone position and masticatory muscle activity.

Intracortical inhibition in the human trigeminal motor system

Objective To investigate the presence and features of short-interval intracortical inhibition (SICI) in the human trigeminal motor system. Methods Surface electromyogram (EMG) was recorded from left and right digastric muscles in 7 subjects, along with additional experiments with intramuscular EMG in 2 subjects. Focal transcranial magnetic stimulation (TMS) was used to activate the motor cortex of one hemisphere and elicit motor evoked potentials (MEPs) in digastric muscles on each side, at rest and while subjects activated the muscles at 10% maximal EMG. Paired or single TMS pulses were delivered in blocks of trials, while conditioning TMS intensity and interstimulus interval (ISI) were varied. Results At rest, paired TMS (3-ms ISI) with conditioning intensities 0.8–0.9× active motor threshold ( T A) reduced the digastric MEP amplitude to a similar extent bilaterally. Conditioning at 0.5–0.7 T A did not significantly reduce the MEP. MEP amplitude was reduced to a similar extent in both digastric muscles by ISIs between 1 and 4 ms (0.8 T A). Voluntary bilateral activation of digastric muscles reduced the effectiveness of conditioning TMS compared to the resting state, with no differences between sides. The similarity of the responses in both digastric muscles was not due to EMG cross-talk (estimated to be ∼10% in surface records and ∼2% in intramuscular records), as the intramuscular records showed the same pattern as the surface records. Conclusions The effects of paired-pulse TMS on digastric are similar to those reported for contralateral hand muscles, and are consistent with activation of SICI circuits in M1 by conditioning TMS. Our evidence further suggests that the corticomotor representations of left and right digastric muscles in M1 of a single hemisphere receive analogous inhibitory modulation from SICI circuits. Significance SICI has been demonstrated in the face area of motor cortex controlling the trigeminal motor system in normal subjects. This method can be used to investigate abnormalities of SICI in movement disorders affecting the masticatory muscles in humans.

Daily jaw muscle activity in freely moving rats measured with radio‐telemetry

The jaw muscle activity of rats has been investigated for specific tasks. However, the daily jaw muscle use remains unclear. The purpose of the present study was to examine daily jaw muscle activity, and its variability over time, in the rat (n = 12) by the use of radio-telemetry. A telemetric device was implanted for the continuous recording of masseter muscle and digastric muscle activity. Daily muscle use was characterized by calculating the total time that each muscle was active (duty time), the number of bursts, and the average length of bursts. All parameters were estimated for activities exceeding various levels (5-90%) of the day's peak activity. Daily muscle use remained constant for 4 wk. At the low-activity level, the duty time and burst number of the digastric muscle were significantly (P < 0.01) higher than those of the masseter muscle, whereas the opposite was true at the high-activity level (P < 0.05). No significant intermuscular correlation was observed between the number of bursts of the masseter and digastric muscles, but the interindividual variation of both muscles changed, depending on the level of activation. These findings suggest that the masseter muscle and the digastric muscle show a differential active pattern, depending on the activity level.

Phosphorylation of Extracellular Signal-Regulated Kinase in medullary and upper cervical cord neurons following noxious tooth pulp stimulation

The phosphorylated Extracellular Signal-regulated Kinase (pERK) and Fos expression and masticatory muscle activity were analyzed in rats with capsaicin-induced acute inflammation of the tooth pulp in order to clarify the role of the spinal trigeminal nucleus and upper cervical spinal cord in tooth pulp pain. Digastric and masseteric muscle activities were significantly increased following capsaicin injection into the molar tooth pulp but not after vehicle treatment. The pERK-like immunoreactive (LI) neurons were observed in the subnuclei interpolaris–caudalis transition (Vi/Vc) zone, the paratrigeminal nucleus (Pa5) and the superficial laminae of the caudal Vc/C2 zone. The pERK expression was detected as early as 2 min and peaked at 5 min after capsaicin or vehicle injection. The pERK expression in the Vi/Vc zone and Pa5 was bilateral, whereas it was predominantly ipsilateral in the caudal Vc/C2 zone. The capsaicin treatment of the whisker pad produced pERK expression in the rostro-caudal middle portion of the ipsilateral Vc, but small number of pERK-LI cells were observed after vehicle treatment. The pERK expression was similar in the Vi/Vc zone following capsaicin injection into the upper or lower molar tooth pulp, whereas the pERK expression was in the lateral portion of the caudal Vc/C2 zone after upper molar injection and restricted to the medial portion of the Vc/C2 zone after the lower molar capsaicin. These data were confirmed with Western blots. There were differences in the distribution of Fos protein-like immunoreactive (LI) cells and pERK-LI cells following tooth pulp stimulation. After capsaicin application into the upper molar tooth pulp, no pERK-LI cells were observed in the ventral part of the Vi/Vc zone, whereas many Fos protein-LI cells were expressed in this region. The difference in the distribution pattern of pERK- and Fos protein-LI cells in the Vi/Vc zone suggests their differential temporal expression profiles after capsaicin. The present findings suggest that tooth-pulp-driven neurons in the spinal trigeminal nucleus are involved in tooth pulp pain through activation of the intracellular signal transduction pathway that involves earlier ERK phosphorylation and subsequent Fos expression.

Digastric muscle activities in anoxic infant rats

The digastric muscle acts for both feeding (including mastication and swallowing) and respiration. In this study, we examined whether or not the muscle activity is detectable during anoxia in developing rats. Rats at 4 different ages, days 5, 10, 16, and 24, were exposed to 100% N2 under pentobarbital or ketamine-xylazine anesthesia, and the electromyograms of digastric muscles (dEMG) and the diaphragm (diaEMG) were examined simultaneously. Prior to the anoxic exposure, at all ages, the dEMG was similar to or less apparent than the diaEMG, which was detected at each inspiratory movement. In anoxia, we first observed dEMG activity, mostly sporadic (days 5 and 10) or mostly tonic (days 16 and 24), when diaEMG activity was temporarily suppressed (we termed it Phase 1). Second, synchronous phasic or tonic dEMG and phasic diaEMG were recorded temporarily before terminal apnea (we termed it Phase 2). These phenomena were also obtained in vagotomized rats (all ages) or in rats injected with the N-methyl-D-aspartate receptor antagonist MK-801 (dizocilpine (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine hydrogen maleate) (days 16 and 24). In conclusion, our results suggest that in anoxia, dEMG activity is detectable during diaEMG suppression in early anoxia, irrespective of the developmental age, the anesthetic (pentobarbital or ketamine-xylazine), vagotomy, or MK-801 injections.

Effects of activator on masticatory muscle activity during daytime and sleep.

The purpose of this study was to investigate masticatory muscle activity with and without the use of an activator during daytime and sleep, and further to focus on the changes in muscle activity produced by the daytime use. The subjects in this study were 10 healthy males (mean age: 27.6 years). A portable electromyogram (EMG) recording device was used to record the activity from the right temporal, masseter and digastric muscles. After recording, the integrated EMG values (microV s) were measured. The muscle activity was lower during sleep than during daytime, irrespective of the use of the activator. In sleep-time, temporal and digastric muscle activity was significantly decreased, although masseter muscle activity presented no significant differences. With the activator in use, the digastric muscle activity tended to increase in comparison with the elevator muscles during daytime and sleep. Although the activity of both elevator muscles was diminished by use of the activator during sleep in all subjects, some subjects showed an increase during daytime. These results suggested that the activator should be used, if possible, not only during sleep, but also during daytime and clenched on consciously to obtain the adaptation and development of the masticatory muscles for the 're-training of the muscles' at a new favourable mandibular position.