An analysis of mandibular movement trajectories and masticatory muscle EMG activity during drinking in the guinea pig.

Electromyographic (EMG) activity of the anterior digastric, lateral pterygoid, and deep masseter muscles as well as the associated jaw movements during drinking were studied in the awake guinea pig. Drinking was characterized by rhythmic, vertically directed jaw movements with little or no associated lateral movements. The jaw opening phase of each cycle was associated with bilaterally synchronized EMG activity in the digastric and lateral pterygoid muscles, and the jaw closing phase with bilaterally synchronized activity in the masseter muscles. The mean EMG burst durations (+/- 1 S.E.) in the digastric and masseter muscles were 164.2 +/- 14.93 ms and 94.3 +/- 26.44 ms, respectively. The digastric muscle EMG burst duration was significantly correlated with drinking cycle time and with masseter muscle EMG onset; on the other hand, masseter muscle EMG burst duration was not correlated with cycle time. These patterns of EMG activity and jaw movement trajectories are similar to those induced by apomorphine in the ketamine-anesthetized guinea pig.

Genioglossus EMG activity during rhythmic jaw movements in the anesthetized guinea pig.

The electromyograph (EMG) activity of the left anterior digastric and the genioglossus muscles was studied in ketamine-anesthetized guinea pigs under 3 separate jaw movement paradigms. The first paradigm has been previously named spontaneous rhythmic jaw movements. These jaw movements occur 1-2 h after the onset of ketamine anesthesia. After spontaneous rhythmic jaw movements began, a single dose of apomorphine caused a new, second jaw movement paradigm to occur, apomorphine-induced rhythmic jaw movements. The final paradigm, cortically-evoked rhythmic jaw movements, was elicited by electrical stimulation of the masticatory area of the cerebral cortex. Genioglossus EMG activity was complex and highly variable in spontaneous rhythmic jaw movements; however, apomorphine-induced jaw movements were characterized by simultaneously occurring rhythmic EMG bursts of approximately 230 ms duration in both the digastric and genioglossus muscles. In 4 of 5 animals, genioglossus muscle activity onset preceded digastric muscle activity onset by approximately 20 ms. These results support the hypothesis that apomorphine-induced rhythmic jaw movements are an analog of lapping in the awake animal. In cortically-evoked rhythmic jaw movements, both digastric and genioglossus EMG activity were time-locked to the cortical electrical stimulation, with an onset latency of approximately 11 ms for the digastric EMG activity and of 16 ms for the genioglossus EMG activity. These results support the hypothesis that both trigeminal and hypoglossal motoneuron pools are closely coupled in certain coordinative movement patterns.

Angiotensin II-induced rhythmic jaw movements in the ketamine-anesthetized guinea pig.

The EMG activity of the left anterior digastric muscle as well as associated jaw movements were studied in ketamine-anesthetized guinea pigs that had received i.v. infusions of angiotensin II (ANG-II). Rhythmic jaw movements with two distinct movement profiles were associated with ANG-II infusion. One movement profile was typified by vertical jaw opening and closing movements with little or no associated horizontal movement. The second rhythmical jaw movement profile was unlike the first in that jaw closing was accompanied by a significant horizontal deflection of the jaw. Both jaw movement profiles were similar in that little or no horizontal movement occurred during jaw opening. Tongue protrusions were also observed during jaw opening in both cases. The results show that ANG-II induces rhythmic jaw movements in anesthetized guinea pigs. ANG-II-induced jaw movement profiles and digastric muscle EMG activity are similar to those seen after an i.v. injection of apomorphine in the anesthetized guinea pig, and to those associated with lapping in the awake animal.

Evidence of sex-specific differences in masticatory jaw movement patterns.

The complexity of human oral functional movements has not been studied in detail quantitatively, and only recently have studies begun to evaluate whether such movements contain sex-specific characteristics. Therefore, the purposes of this study were: (1) to quantify in detail the jaw movements and associated masticatory electromyographic activity occurring during gum chewing, and (2) to explore these data for evidence of sex specificity. Fourteen male and 17 female subjects participated in the study. Approximately 11 right- and 11 left-sided chewing cycles and associated masticatory electromyographic activity were sampled from each subject. The samples were quantified into 165 variables per chewing cycle, averaged to create a single multivariate vector for each subject, and then analyzed by a step-wise discriminant analysis. With a combination of 6 variables, a jackknifed cross-validation test found the probability of correct classification to be 93.5%. These findings support the hypothesis that masticatory jaw movements contain sex-specific features.

Intermittency in mastication and apomorphine-induced gnawing.

Rhythmic behaviors like mastication, gnawing, and locomotion, are characterized by temporal segmentation or intermittency. That is, they frequently occur as a series of short bursts interrupted by pauses rather than as one long uninterrupted burst. The function of intermittency as well as the mechanisms that produce it are unknown. Biogenic amine systems may play a role in producing intermittency; however, experimental work to confirm this is only in its infancy. The current study evaluates the structure of intermittency associated with mastication and apomorphine-induced gnawing in the guinea pig. Thirteen free-roaming animals were videotaped while masticating or gnawing. Eight animals were given 0.5 mg/kg i.m. apomorphine and videotaped while gnawing. The remaining five animals received no apomorphine injections, but were taped while feeding on alfalfa pellets. Custom software was used to score instances of maximum jaw closures in videotaped mastication and gnawing sequences. The time between successive maximum jaw closures, called the interocclude interval (IOI), was calculated for all scored sequences. A cutoff IOI value of 0.26 s differentiated pauses (IOI values equal or greater than 0.26 s) from chews or gnaws (IOI values less than 0.26 s). Two or more successive chews or gnaws, without intervening pauses, defined behavior bursts. Chew, gnaw, and burst durations were quantified and compared. Chew and gnaw durations were similar. However, chewing bursts were significantly longer than gnawing bursts. The significance of these results is presented in light of previous neurophysiological work on rhythmic jaw movements and intermittency.

Licking rate adaptations to increased mandibular weight in the adult rat.

In this experiment, chronic mandibular loading was used to study adaptation in licking rate. Twenty-four 115-day-old rats were randomly divided into two groups. The experimental group received 1.791 +/- 0.083 g submandibular gold implants, and the control group received 0.179 +/- 0.009 g submandibular acrylic implants. The animals were videotaped while lapping on two separate occasions preoperatively, and once every week postoperatively for 12 weeks. The videotapes were used to obtain licking rates for each animal at each taping session. The findings showed that licking rate decreased significantly after surgery for both groups; however, the decrease was similar for both the experimental and control groups. This indicates that licking rate was affected by the experimental design, but not specifically by the weight of the gold implant.

Temporal dynamics of human masticatory sequences.

Many motor behaviors produced by humans and other mammals are temporally segmented. That is, sequences of rhythmic or repetitive behavior occur as a series of brief, 2- to 4-s bouts separated from each other by pauses or posture adjustments. Little is known about the physiological mechanisms underlying temporal segmentation, although several hypotheses have been advanced. Experimental and modeling studies are currently underway to gain insight into this phenomenon. One of the problems hampering advancement is the lack of relatively simple behavior models that can be studied in both humans and other mammals. We have recently reported that temporal segmentation occurs in guinea pig chewing sequences. Thus, it seems logical to explore whether temporal segmentation occurs in human chewing sequences as well. Toward this end, the current study evaluated the temporal dynamics of chewing sequences in humans. Thirteen subjects were videotaped on campus eating areas during lunch-time. Inter-occlude intervals, i.e., time between maximum jaw closures, were calculated using a custom computer program, which also recorded whether the interval represented a chew or a pause in chewing. Chewing rate, pause durations, and chewing burst durations, i.e., duration of continuous chewing uninterrupted by pauses, were calculated. Median chewing burst duration for the sample was 2.91 s. This corroborates other studies' findings of 3-s temporal segmentation in repetitive movements. We conclude that automatic chewing sequences contain temporal segmentation. Future work is required to gain insight into whether the physiological mechanisms of this time-based phenomenon are similar among different species.

Validity of a brief questionnaire in screening asymptomatic subjects from subjects with tension-type headaches or temporomandibular disorders.

Clinical investigations of temporomandibular disorders require objective, repeatable methods for screening diseased subjects from non-diseased control subjects. This study evaluated whether information gathered from a short, public domain questionnaire was useful in distinguishing temporomandibular disorder subjects (n = 216) from non-temporomandibular disorder controls (n = 69) and tension-type headache subjects (n = 22). The questionnaire consisted of eight questions relating to jaw pain (i.e., location of pain, precipitating factors, and temporal pattern of pain) and five questions relating to jaw function (i.e., joint noises, locking, and difficulty in opening). There were five possible answers to each question which ranged from 0 (no symptoms) to 4 (unbearable or constant symptoms). The total scores for the eight pain questions and the five jaw function questions were used to determine the questionnaire's sensitivity and specificity in each group, and ROC curves were plotted to identify the best cutoff point for disease presence or absence. Results showed that the questionnaire reliably distinguished between the control group and temporomandibular disorder group with 90.3%-97.7% sensitivity and 95.7%-100% specificity at cutoff values between 5 and 9. These results support the use of the questionnaire as a primary screening tool for general practice and as a supplementary screening tool for clinical temporomandibular disorder studies. However, results also showed that the questionnaire was unable to distinguish easily between TMD subjects and temporalis region tension-type headache subjects.

Evidence that angiotensin II enhances apomorphine-induced jaw movements via AT1 receptors in the ventrolateral striatum: studies by magnet-sensing system in freely moving rats.

The role of angiotensin AT(1) receptors in the ventrolateral striatum in modulating apomorphine-induced jaw movements was studied using a magnet-sensing system combined with an intracerebral drug microinjection technique in freely moving rats. Bilateral injections of angiotensin II (1 and 2 micro g/0.2 micro l in each side) into the ventrolateral striatum, which alone did not significantly elicit jaw movements, dose-dependently enhanced apomorphine (1 mg/kg i.v.)-induced repetitive jaw movements. The enhancement of apomorphine-induced jaw movements by angiotensin II (2 micro g) was dose-dependently antagonized by the angiotensin AT(1) receptor antagonist losartan (15 and 30 mg/kg i.p.), given 3 h before, while losartan (30 mg/kg i.p.) alone did not significantly affect the apomorphine (1 mg/kg)-induced jaw movements. These results indicate that angiotensin II enhances apomorphine-induced jaw movements via stimulation of angiotensin AT(1) receptors located in the ventrolateral striatum.

Comparison of chin and jaw movements during gum chewing.

STATEMENT OF PROBLEM: Knowledge of mastication is based on studies that use jaw tracking equipment in nonroutine settings. Ethologists would argue that such data probably does not reflect routine masticatory function. If jaw movements could be tracked noninvasively, then the hypothesis that jaw tracking equipment and nonroutine settings alter mastication could be investigated. PURPOSE: This study quantitatively evaluated the relationship between chin and jaw movements during a gum-chewing task. MATERIAL AND METHODS: Masticatory chin and jaw movements of 50 subjects were tracked in the x-, y-, and z-axes for 15 seconds, which resulted in approximately 15 chewing cycles obtained per subject. For each chewing cycle, magnitude and timing of displacement, velocity, and acceleration extrema in each axis were computed for both jaw and chin movement data. Extrema means were calculated for each 15-second trial. The respective means representing chin versus jaw movements were compared with linear regression and correlation analyses. RESULTS: All mean extrema were significantly correlated (r range 0. 30-0.99; P <.05). Magnitude correlations were larger than timing correlations for acceleration extrema. In contrast, magnitude correlations were smaller than timing correlations for displacement extrema. The highest correlation occurred for chewing rate. CONCLUSIONS: Chin and jaw movements were correlated during chewing; however, only chewing rate was highly predictable from chin movement data.

Relationship between anteroposterior maxillomandibular morphology and masticatory jaw movement patterns.

The causal relationships between oral function and craniomandibular morphology are poorly understood. The aim of this study was to determine whether quantifiable features of masticatory jaw movements and associated EMG activity correlated with variation in morphology as defined by the ANB angle. Thirty-six healthy subjects with no previous orthodontic treatment, asymptomatic masticatory muscles, and asymptomatic temporomandibular joints participated. While subjects chewed gum, jaw movement data and surface EMG data were digitized and then quantified into a 300 variable vector for each subject. ANB angle measurements were calculated from digitized tracings of lateral cephalographs. Step-wise linear regression and discriminant analyses were used to determine the relationship between the ANB angle and a subset of the variables defining jaw movement patterns and EMG patterns. A linear combination of seven jaw movements and EMG variables accounted for over 75% of the variation in the ANB angle (adjusted x R2 = 0.78, P <.001). A jackknifed cross-validation of the discriminant analysis, which was forced to use the same seven variables as the regression analysis, resulted in correct classification of 14 of 20 skeletal Class I, 7 of 9 skeletal Class II, and 7 of 7 skeletal Class III subjects. These results suggest that there is an association between anteroposterior skeletal morphology, as quantified by the ANB angle, and masticatory jaw movement patterns, as quantified in this study.