Masseter EMG activity during sleep and sleep bruxism.

The masseter muscle is involved in the complex and coordinated oromotor behaviors such as mastication during wakefulness. The masseter electromyographic (EMG) activity decreases but does not disappear completely during sleep: the EMG activity is generally of low level and inhomogeneous for the duration, amplitude and intervals. The decreased excitability of the masseter motoneurons can be determined by neural substrates for NREM and REM sleep. The masseter EMG activity is increased in association with the level of arousal fluctuations within either sleep state. In addition, there are some motor events such as REM twitches, swallowing and rhythmic masticatory muscle activity (RMMA), whose generation might involve the additional activation of specific neural circuits. Sleep bruxism (SB) is characterized by exaggerated occurrence of RMMA. In SB, the rhythmic activation of the masseter muscle can reflect the rhythmic motor inputs to motoneurons through, at least in part, common neural circuits for generating masticatory rhythm under the facilitatory influences of transient arousals. However, it remains elusive as to which neural circuits determine the genesis of sleep bruxism. Based on the available knowledge on the masseter EMG activity during sleep, this review presents that the variety of the masseter EMG phenotypes during sleep can result from the combinations of the quantitative, spatial and temporal neural factors eventually sending net facilitatory inputs to trigeminal motoneurons under sleep regulatory systems.

Specific increase in non-functional masseter bursts in subjects aware of tooth-clenching during wakefulness.

Previous studies have reported that subjective awareness of a tooth-clenching habit is associated with increased jaw motor activity (Rao SM, Glaros AG, J Dent Res. 1979;58:1872). The aim of this study was to test the hypothesis that subjects with clenching awareness exhibit different motor expressions specific to non-functional oromotor activity under laboratory conditions without psychological or sensory effects. Polygraphic and audio-video recordings were made for a 30-min period of silent reading by 33 subjects without oro-facial pain. Oro-facial behaviours (e.g. swallowing, lip movements) were scored according to the polygraphic and audio-video records and masseter bursts were quantitatively analysed. Subjective psychological/sensory measures were also recorded before and/or after the polygraphic recording using a visual analogue scale. The subjects were classified into two groups one with 15 subjects who were aware of having a tooth-clenching habit and another with 18 who were not aware of any such habit. There were no differences between the groups with respect to the number of functional oro-facial behaviours or subjective psychological/sensory measures. Masseter bursts unrelated to functional oro-facial behaviours occurred more frequently in subjects with awareness [median (range) = 23 (2-187) bursts] than in those without [9.0 (0-36); P = 0.01], while neither burst activity [12.3 (1.8-34.5) % of maximum voluntary clenching and 10.1 (6.5-25.1) %, respectively] nor duration [1.17 (0.2-2.2) s and 1.28 (0.3-4.1) s, respectively] differed between the groups. The occurrence of functional oro-facial behaviours or other body behaviours (e.g. limb and body movements) did not differ between the two groups. These findings suggest that the increased masseter activity in subjects with tooth-clenching awareness is characterized by a specific increase in non-functional masseter bursts.

Patterns of masseter muscle activities during sleep in guinea pigs.

Basic neurophysiological mechanisms for sleep bruxism remain unknown. Analyses of masseter muscle activity during sleep in guinea pigs have shown that the duration and activity of masseter bursts differ between non-rapid eye movement (NREM) and rapid eye movement (REM) sleep and that some repetitive burst episodes do occur. Furthermore, masseter bursts occurred in association with a transient heart rate increase. These results suggest that various patterns of masseter bursts occur in association with transient arousal activity during sleep in guinea pigs.

Intracellular pH determination by a 31P-NMR technique. The second dissociation constant of phosphoric acid in a biological system.

To evaluate the accuracy of pH determination by 31P-NMR, factors which influence the pK value of phosphate were appraised on the basis of the titration of 1 mM phosphate buffer solution. When the method is used for the determination of cytoplasmic pH, ionic strength is the major factor causing shifts of apparent pK (pK') value, and the magnitude of the shift can be predicted from the ionic strength calculated by means of the Debye-Hückel equation. Ions (Na+, K+, Mg2+, and Ca2+) and salivary protein affected the pK' value by 0.1 to 0.3 units in solution with a given ionic strength depending on the species of ion. The form of the titration curve varied with temperature. Based on these results, the value of 6.75 was obtained with the uncertainty of 0.12 for the intracellular pK' of frog muscle at 24 degrees C.

Neuronal activity in the putamen and the globus pallidus of rabbit during mastication.

The pattern of jaw movements is changed during a masticatory sequence from ingestion of food to its deglutition. The masticatory sequence is divided into three distinct stages in the rabbit. However, the neural mechanism involved in the alteration of the masticatory stages is still unknown. This study was designed to determine whether neuronal activity in the putamen and globus pallidus is related to the alteration of the masticatory stages. Fifty-three percent of the recorded neurons showed significant alterations of activity during mastication. Of these neurons, 16% changed their firing frequency throughout the masticatory sequence (sequence-related neurons) and 84% changed their firing frequency with the transition of the masticatory stages (stage-related neurons). The stage-related neurons were classified into two groups based on their neuronal activity patterns observed during mastication, i.e. simple type and complex type. The former are the neurons that were either facilitated or inhibited once during mastication, and the latter are those showing the facilitation or inhibition twice or more during mastication. Complex-type neurons were observed more frequently in the globus pallidus than in the putamen. These results suggest that the basal ganglia is involved in mastication and may related to the transition between the masticatory stages.

Influence of food thickness and hardness on possible feed-forward control of the masseteric muscle activity in the anesthetized rabbit.

The facilitatory masseteric muscle response (FMR) elicited by polyurethane foam strip application between the opposing molars during cortically-induced rhythmic jaw movements (CRJMs) was induced earlier than masticatory force onset. The occurrence of this early response of the FMR (e-FMR) could not be explained by a simple reflex mechanism. One possible mechanism of the e-FMR is the involvement of a feed-forward control mechanism of the masticatory jaw movement. In the present study, experimentally designed polyurethane foam strips with various thickness and hardness were applied during CRJMs and analyzed in terms of how the e-FMR was modulated by the food hardness and thickness. The FMR onset was not related to the strip thickness or the strip hardness. However, the magnitude of the e-FMR increased in a thickness and a hardness-dependent manner. The sensory information of the food properties in the masticatory cycle may make the FMR adequate to chewing of the food in the following cycle, and such modulation may help chewing rhythms remain stable.

A He-Ne laser position-detector for recording jaw movements: principle of operation and application in animal experiments.

A new equipment for recording jaw movements in animals by use of a He-Ne laser and a phototransistor (PhT) array is described. The principle of operation of this equipment is that the beam angle of the He-Ne laser is continuously controlled to irradiate the center of the PhT array by a feedback mechanism composed of a microcomputer (CPU) and XY scanners. The driving currents sent from the CPU to the XY scanners are utilized for monitoring two-dimensional jaw movements. The linearity of this recording system is preserved both vertically and horizontally over a range of 11 mm from the center of the scanning field of the laser beam within less than 1.5%, which is satisfactory for monitoring rabbit jaw movements. When this equipment was applied to the rabbit, there was no sign of disturbance to the animal during chewing carrots.

Neural activity of chorda tympani mechanosensitive fibers during licking behavior in rats.

The chorda tympani nerve, supplying the anterior two-thirds of the tongue, contains gustatory and mechanosensitive afferent fibers. We have analyzed discharge patterns in rats of various fibers recorded from dissected nerve filaments during licking behavior of which 4 were taste-sensitive and 12 mechanosensitive. The incidence of these two types were estimated electrophysiologically under anesthesia and their conduction velocity measured. Recordings in freely moving animals showed that the mechanosensitive fibers innervating the dorsal part of the tongue gave two burst discharges per lick, suggesting that contact of the tongue with the upper incisors and/or lip occurred during tongue protrusion and retraction. The fibers from the tip of the tongue showed one burst discharge per lick, which was the response to contact with a drinking spout. No rhythmical discharges synchronized with lick signals were observed in the fibers from the lateral part of the tongue or the taste-sensitive fibers. Such mechanoreceptor discharges were difficult to detect in recordings from the whole chorda tympani nerve. This masking of responses was due mainly to activation of a small number of mechanosensitive fibers by licking-induced mechanical stimulation. The lubricating action of saliva also decreased mechanoreceptor sensitivity. Despite their small number, the mechanosensitive fibers had axons with faster conduction velocities (larger diameter) than the taste-sensitive fibers. This was probably the reason why dissected nerve bundles more frequently showed mechanical than taste responses in conscious rats.

Frequency-dependent modulation of rhythmic human jaw movements.

Modulation of jaw movement patterns by alteration of the chewing rhythm was analyzed in nine subjects. Jaw movements were monitored by an electronic transducer system. The subjects were asked to chew gum either at voluntarily determined rates or at rates guided by a light flash that varied from 2 to 7 Hz at 1-Hz intervals. The results can be summarized as follows: Chewing was generally conducted at rates below 3 Hz when the rhythm was voluntarily determined. When the rhythm was guided, however, the maximum chewing rate could occasionally be raised up to 6 Hz; the duration of the chewing cycle was more closely related to changes in the duration of the opening and occlusion phases than to that of the closing phase; at chewing rates below 3 Hz, the maximum gape did not appreciably change with the chewing rhythm, while it decreased linearly at rates above 3 Hz; and at chewing rates below 3 Hz, the jaw opening velocity increased with speed of chewing, whereas it decreased or sometimes fluctuated over a wide range at rates above 3 Hz. It is concluded that, during normal mastication at rates below 3 Hz, chewing rhythm is altered by changing both the velocity of jaw opening and the duration of the occlusion phase.

Effects of the interpolated vibratory stimulation on the interdental dimension discrimination in normal and joint defect subjects.

No significant differences in the interdental dimension discrimination (IDD) ability were found between normal and joint defect subjects. Furthermore, vibration onto the mandible gave similar effects on the nature of IDD in both subject groups. Thus, the muscle spindles of masticatory muscles may contribute to IDD, but temporomandibular receptors may not.

Influence of clenching intensity on bite force balance, occlusal contact area, and average bite pressure.

It has been difficult for investigators to simultaneously and reliably evaluate bite force in the intercuspal position with the area and location of occlusal contacts. This study was designed to investigate the variations in these parameters with respect to two factors: three levels of clenching and the preferred chewing side. Human subjects with normal occlusion were examined with a recently developed system (Dental Prescale Occluzer, Fuji Film, Tokyo, Japan). The three levels of clenching intensity were assessed by masseteric EMG activity and included the maximum voluntary contraction, and 30% and 60% of the maximum. The results indicated that the bite force and occlusal contact area on the whole dental arch increased with clenching intensity. In contrast, the average bite pressure, obtained by dividing the bite force by the contact area, remained unchanged regardless of the clenching intensity. As the clenching intensity increased, the medio-lateral position of the bite force balancing point shifted significantly (P<0.01) from the preferred chewing side toward the midline. The antero-posterior position remained stable in a range between the distal third of the first molar and the mesial third of the second molar. The bite force and occlusal contact area, which were mainly on the molars, increased with the clenching intensity, whereas the proportions of these two variables on each upper tooth usually did not change significantly. The exception was the second molar on the non-preferred chewing side. When comparisons were made between pairs of specific upper teeth of same name, usually no significant difference was found in bite force or occlusal contact area, regardless of the clenching level. Again, the exception to this observation was the second molar on the preferred chewing side, which had a larger area at the 30% clenching level. The results in normal subjects suggest that as the clenching intensity increases in the intercuspal position, the bite force adjusts to a position where it is well-balanced. This adjustment may prevent damage and overload to the teeth and temporomandibular joints.

Tongue, jaw, and lip muscle activity and jaw movement during experimental chewing efforts in man.

The electromyographic (EMG) activity of the human genioglossus (GG) muscle during chewing efforts is not fully understood. In this study, the EMG activity of the human GG muscle during unilateral gum chewing was illustrated and correlated with the activities in the anterior temporalis (AT), the anterior digastric (DG), and the inferior orbicularis oris (OI) muscles. GG muscle activity was measured with customized surface electrodes, while other muscles were recorded with conventional surface electrodes. EMG activities during tongue displacement and the articulation of long vowels, recorded by the customized electrodes, were consistent with the recordings obtained by fine wire electrodes placed in the GG muscle. Jaw displacement was monitored by means of a kinesiograph with a transducer attached to the mandibular central incisors. Mean normalized GG muscle activity showed an onset in the last one-fifth of the intercuspal phase, gradually increasing during jaw-opening, and at its greatest immediately before the maximum jaw-opening position. It then decreased during jaw-closing and ceased in intercuspation but showed a small rebound in the third fifth of the intercuspal phase. The GG muscle burst showed phase lags with the DG and OI muscles and an opposite phase with the AT muscle (all P < 0.0001). All correlations were statistically significant (all P < 0.0001, r values between 0.88 and 0.97). The results suggest central coordination of the timing of the activities of the jaw, lip, and tongue muscles in chewing.

Adjustment of the occlusal vertical dimension in the bite-raised guinea pig.

In humans, the inappropriate occlusal vertical dimension (OVD) causes several orofacial disorders, such as bruxism and pain in the masticatory muscles and temporomandibular joint. However, it is still unclear how strictly the OVD is adjusted. To answer this question, we studied the temporal change of the OVD in bite-raised young guinea pigs. The OVD was raised by fixation of a bite-raising appliance to the lower incisors, and increased by either 3 or 1.5 mm at the first molars. After the space produced between the upper and lower molars was filled within 10 days due to eruption of the molars, the appliance was removed. In the bite-raised animals, the raised OVD was reduced until it attained that observed in the control animals, after which the OVD increased according to cranial growth. These results show that the OVD is developmentally changed and strictly controlled.

An involvement of trigeminal mesencephalic neurons in regulation of occlusal vertical dimension in the guinea pig.

Although the occlusal vertical dimension (OVD) is strictly controlled, the neuronal mechanism of its regulation is still unclear. We hypothesize that neurons in the trigeminal mesencephalic nucleus (MesV) play an important role in the regulation of the OVD, because the MesV receives the projection from jaw-closing muscle spindles and periodontal mechanoreceptors. We measured the temporal OVD change in the guinea pig to study the effects of MesV lesions on the OVD. OVD-raised animals without MesV lesions showed a rapid OVD decrease to the same level as that in naïve controls, followed by an OVD increase after the OVD-raising appliance was removed. In contrast, OVD-raised animals with MesV lesions showed only a slight decrease in the OVD for 15 days after removal of the appliance, and then the OVD increased. The time-course of OVD development in normal-bite animals with MesV lesions was similar to that of naïve controls. These results suggest that MesV neurons are involved in OVD regulation.

Characteristics of mastication in the anodontic mouse.

Teeth and periodontal mechanoreceptors play important roles in regulating jaw movements during mastication. However, little is known concerning how jaw movements develop without tooth eruption. To answer this question, we studied masticatory behavior in the osteopetrotic mouse, where tooth eruption does not occur and periodontal mechanoreceptors are missing. A masticatory sequence of the osteopetrotic mouse was divided into two stages: incision and chewing. Incision is characterized by small amplitude and rapid (7 Hz) open-close jaw movements, while slow (5 Hz) and large amplitude open-close jaw movements characterize chewing. The frequency and properties of jaw movements were comparable with those in the normal mouse, though the osteopetrotic mouse had a higher cycle number during incision than did the normal mouse. These results indicate that conversion from sucking to mastication occurs in the anodontic mouse, and the central pattern generator producing the masticatory rhythm develops almost normally without tooth eruption.

Oral kinesthesia in patients with Duchenne muscular dystrophy.

In trials on normal subjects and patients with Duchenne muscular dystrophy (DMD), interdental dimension discrimination (IDD) was tested by assessing the ability of subjects to discriminate between pairs of sticks of different dimensions held between the upper and lower teeth. The IDD ability of the DMD patients was significantly inferior to that of the normal subjects. Further, DMD patients tended to overestimate the dimension of the first stick of each pair even more than did normal subjects. Vibration applied to the mandible seemed to lessen such inaccuracies of oral kinesthesia in the DMD patients. These results are compatible with the idea that muscle receptors, especially muscle spindles in jaw closing muscles, are mainly responsible for IDD.

Reconstruction of cerebral cortical veins using silicone tubing. Technical note.

A technique for reconstruction of a cerebral cortical vein which has been sacrificed during an interhemispheric or subtemporal surgical approach is described. The method involves the use of silicone tubing. The author's clinical experience in six patients is summarized.

Preparation and evaluation of a time-controlled release capsule made of ethylcellulose for colon delivery of drugs.

A novel ethylcellulose (EC) capsule which releases drug with a time-controlled fashion has been prepared. This capsule is composed of four parts, drug container, swellable substance, capsule body and cap. At the bottom of the body, micropores are made. As water penetrates through these micropores, the swellable substance such as low substituted hydroxypropyl cellulose (L-HPC) swells. When the cap made of water-insoluble macromolecular substance such as EC cannot persist the swelling pressure, the EC cap disintegrates and the drug in the container is released from the capsule. The lag-time is utilized for the delivery of drug to the colon. The release time of the drug from the capsule was measured both in vitro and in vivo experiments. In the case of an in vitro experiment, after 12mg of fluorescein as a model drug and 238mg of starch were filled into the container, caps having different thickness were attached to the capsule body and release study was performed. The release time of the drug was mainly dependent on the thickness of the cap. Using test capsules of which mean cap thickness were 39.1 +/- 2.3 (SE)microns, 63.1 +/- 5.0 microns and 75.6 +/- 4.1 microns, the in vivo release time was estimated after administration to beagle dogs. As a parameter, the peak time (tmax) when plasma fluorescein concentration reached to its maximum level was determined for the estimation of the release time of the drug from the capsule in the gastrointestinal tract. The in vivo tmax was well correlated with the cap thickness.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of rhythmic jaw movements of the rabbit.

The jaw-movement pattern of the rabbit during chewing foods of different consistency was analysed. The basic movement trajectory on the frontal plane showed a crescent loop, irrespective of the food type. Difference in food consistency exerted only minor effects on the maximum gape, extent of lateral excursion and the chewing rate. These parameters of jaw movement and also the duration of a single masticatory cycle (TCL) appeared to change with physical properties other than the hardness of food. On the other hand, the number of chewing cycles in a masticatory sequence increased with increasing the hardness of food. The TCL ranged between 250 and 300 ms (frequency: 3.3-4 Hz). When a single masticatory cycle was divided into three phases (opening, closing and power phases), the TCL depended more on the duration of the opening phase than that of other two phases.

Changes in facial skin temperature associated with chewing efforts in man: a thermographic evaluation.

Eleven healthy male adults chewed hard and soft chewing-gums for 5 min. A thermographic record of the face on the chewing side was made at the beginning of, during and after the effort. Facial temperature distributions during open/close cyclic unloaded jaw movements were recorded at a later date. The dimensions of the zones whose temperatures were 1.4 degrees C or more higher than the central temperature during the experiment were determined. There was a linear increase in the dimensions of these zones after the chewing. In contrast, the cyclic jaw movements did not result in significant increases. Chewing the hard gum produced significantly higher temperature rises than did the soft in the masseter area. After the chewing effort, the temperature fell gradually, but did not return to the initial state even after 30 min. The overall decreasing pattern of the temperature distribution for chewing the soft gum was similar to that for the hard gum. The facial temperature associated with chewing efforts rose in accordance with the resistance offered by the chewing-gums.