Characterization of brux-like movements in the laboratory rat by optoelectronic mandibular tracking and electromyographic techniques

Pergamon

Archs oral Biol. Vol. 42, No. 1, pp. 33-43, 1997 © 1997Publishedby ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S0003-9969(96)00093-3 0003-9969/97$17.0o+ o.oo

CHAPACTERIZATION OF BRUX-LIKE MOVEMENTS IN THE LABORATORY RAT BY OPTOELECTRONIC M A N D I B U L A R TRACKING A N D ELECTROMYOGRAPHIC TECHNIQUES K E N N E T H E. BYRD Department of Anatomy, MS-259, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis, IN 46202-5120, U.S.A. (Accepted 9 October 1996)

Summary--High-resolution optoelectronic mandibular tracking and fine-wireelectrornyographic (EMG) data from the anterior temporalis muscles of laboratory rats (Rattus norvegicus) were collected during mastication (chewing) and bruxing/thegosis (grinding/sharpening of teeth) in order to test for task-related activity patterns of the anterior temporalis. Analyses of the collected data revealed that masticatory and bruxing/thegosis cycles displayed significantly different patterns of movement trajectories, displacemenL, duration, velocity, and acceleration in all three spatial dimensions (frontal vertical, frontal horizontal and sagittal horizontal). Activity patterns in the anterior temporalis during masticatory and bruxing/thegosis behaviours were also significantly different from each other. High-resolution analyses revealed that the masticatory cycle had both opening-burst and closing-burst phasic patterns of anterior temporalis activity while the bruxing/thegosis cycle displayed only opening-burst phasic patterns. The opening- and closing-burst attributes of anterior temporalis phasic activity patterns in relation to physiological centric occlusion also revealed significant differences between masticatory and bruxing/thegosis behaviours. These data demonstrate that the anterior temporalis muscle of the laboratory rat does indeed display task-related activity patterns depending upon the manifested oral behaviour. The task-related shifts of EMG patterns in the anterior temporalis between masticatory bruxing/ thegosis bekaviours in the same animal suggests a complex neurophysiological substrate that coordinates the three-dimensional expression of phasic activity patterns in the muscle. The radically different nature of masticatory and bruxing/thegosis cycles and their associated EMG patterns in the anterior temporalis suggest the possible existence of a bruxing/thegosis pattern generator in addition to the masticatory one. Careful, high-resolution analyses of these rat behaviours by combined optoelectronic/ EMG techniques suggest that the rat model for human bruxism may prove useful in future studies. © 1997 Published by Elsevier Science Ltd. All rights reserved. Key words: mastication, bruxing/thegosis, mandibular tracking, electromyography.

INTRODUCTION

as per Karoly's postulation (Ramfjord and Ash, 1971). Human bruxism has a multifactorial aetiology: neuromuscular (Schhrer, 1974; Christensen, 1981; Hamada et al., 1982; Christensen and Mohamed, 1984; Holmgren et al., 1990; Holmgren and Sheikholeslam, 1994), psychogenic (Rush and Solberg, 1976; Rao and Glaros, 1979; BudtzJorgensen, 1980, 1981; Richmond et al., 1984; Hicks and Conti, 1989; Fischer and O'Toole, 1993), occlusal anatomy (Schfirer, 1974; Arnold, 1981; Houston et al., 1987; Seligman et al., 1988; Vanderas and Manetas, 1995), neurophysiological (Dubner et al., 1978; Pohto, 1979; Clark et al., 1980; Sunden-Kuronen, et al., 1983; Pratap-Chand and Gourie-Devi, 1985; Ellison and Stanziani, 1993; Giannaula and Parera, 1993) mechanisms have been described. It has been suggested that human bruxism may represent an atavistic remnant of oral behaviours from a time when grinding/sharpening of teeth for aggressive and/or defensive purposes

Despite the use of the laboratory rat (Rattus norvegicus) in numerous studies on the neurophysiology of mammalian mastication (Hiiemfie and Ardran, 1968; Vorontsov and Labas, 1968; Weijs, 1975; Weijs and Dantuma, 1975; Thomas and Peyton, 1983; Byrd, 1988a,b; Byrd and Chai, 1988), there have been no high-resolution, combined mandibular tracking and EMG studies on the precise activity patterns of rat ms.sticatory muscles during mastication (chewing) and brux-like (grinding/sharpening) mandibular movements. Raml~ord and Ash (1971) have defined human bruxism as 'non-functional' grinding and gnashing (eccentric bruxism) as well as intercuspal clenching (centric bruxism). It has also been suggested that all humans exhibit bruxism at some stage of their lives Abbreviation." EMG, electromyographic.

33

34

K.E. Byrd

was perhaps commonplace (Sch~irer, 1974; Dubner 1978) and actually functional in nature. Related to this concept are recent findings that human bruxism may be due, in part, to overactivity of the sympathetic nervous system (Takahama, 1961; Satoh and Harada, 1973; Kampe et al., 1986; Sjoholm, Piha and Lehtinen, 1995). The anterior temporalis muscle is an important effector of positional movements of the mandible during human bruxism (Holmgren et al., 1985; Sheikholeslam et al., 1986; Lyons and Baxendale, 1990; Holmgren et al., 1990). In laboratory rats, the muscle fibre orientation of the anterior temporalis (Fig. 1) approximates that of the human, which justifies its use as an animal model for the study of human bruxism. A detailed, high-resolution physiological study of activity patterns (EMG) in the rat anterior temporalis during both regular masticatory and brux-like movements should therefore provide important new data. Laboratory rats display both eccentric and centric bruxism. The tooth-sharpening (thegosis) behaviour of Every (1970) or the 'chattering' mandibular movements shown by nervous/stressed laboratory rats are familiar to any researcher who has worked with them. Recent research has shown that such tooth-sharpening/thegotic behaviour is widespread and serves an important function for proper utilization of the incisor teeth in all rodents, including rats (Druzinsky, 1995). To date, there have been no high-resolution, detailed data collections and analyses of these bruxing/thegosis behaviours; such

analyses are made possible by the proper application of combined optoelectronic mandibular tracking and electromyographic methods. These techniques (Luschei and Goodwin, 1974; Byrd et al., 1978; Byrd and Luschei, 1980; Larson et al., 1980; Byrd and Garthwaite, 1981; Byrd, 1981, 1984, 1988a,b) provide the high-resolution (l kHz sampling rate) data required for neurophysiological detection of task-dependent activity patterns within individual muscles of mastication. My purpose now was to provide such data for the laboratory rat and carefully, precisely characterize the nature of these bruxing/thegosis mandibular movements in comparison with regular masticatory movements. The null hypothesis tested was that there are no significant differences between the E M G patterns of the anterior temporalis during bruxing/thegosis and masticatory movements of the mandible.

et al.,

MATERIALS AND M E T H O D S

Five 39-day-old, male Sprague-Dawley rats were trained by positive reinforcement to accept restraint during data collection (Byrd, 1988a,b; Byrd and Chai, 1988). A non-invasive acrylic splint was used to attach the small light bulb needed for optoelectronic mandibular tracking; this splint contained a length of 0.76 mm dia. orthodontic wire that held the small light bulb 7 mm anterior to the mandibular incisors (Byrd, 1988a,b; Byrd and Chai, 1988). After confirming that the splint (1) fitted snugly against the gingiva of the mandibular incisors, (2)

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Brux-like movements in the rat allowed protrusion of the wire between the lips during full closure, 113)did not interfere with tongue movements, and (4) did not prevent proper occlusal contacts between the maxillary and mandibular teeth, it was secured to the gingival cuff of the mandibular incisors wit]a a single drop of butyl-cyanoacrylate Superglue (Nexaband, Raleigh, NC). This provided rigid attachment of the splint to the mandible and permitted adequate data collection for up to 4 h (Byrd, 1988a,b; Byrd and Chai, 1988). Before anaesthesia (an intramuscular injection of a mixture of 100 rng/ml ketamine, 20 mg/ml rompun, and 10 mg/ml acepromazine at a dose of 0.01 ml/10 g body wt) had worn off, paired bipolar stainless-steel fine-wire electrodes of 0.002 dia. (304 annealed, heavy poly-nylon insulation; California Fine Wire Co., Glover City, CA) and double-hook construction (Loeb and Gans, 1986) were inserted into 27 gauge x 1/2 inch hypodermic needles before being placed percutaneously into the anterior temporalis (Fig. 1). The bared-tip length of each electrode was 0.5 mm, confirmed under a dissecting microscope. The electrodes were placed in the anterior temporalis at a point 13 mm posterior to the palpated posterior edge of the zygomatic process of the maxillary bone (that portion immediately posterior to the infraorbital foramen) and 7 mm above the palpated zygomatic arch. Each rat was then positioned in front of paired, biaxial-light transducers and calibrated for data collection at centric occlusion, 5 mm, and 10 mm below centric occlusion (Byrd, 1988a,b; Byrd and Chai, 1988); 1 mm of vertical, horizontal, or sagittal movement of the light bulb equalled 0.383 V at an oscilloscope setting of 1 V per division. A mean linearity coefficient for optoelectronic mandibular tracking (Byrd, 198ga,b; Byrd and Chai, 1988) was calculated and determined as 0.976 for the collected data. Data were collected at 1 h after recovery from anaesthesia. Standardized food items were fed to each rat in the form of 2 × 3 mm pellets and data were collected during periods of actual mastication (masticatory cycles) and tooth grinding or 'chattering' (bruxing/thegosis cycles). In this study, the term thegosis (Every, 1975; Druzinsky, 1995) refers to the proces:~ of sharpening/honing the incisors that occurs during bruxing by the rats. Bruxing/thegosis cycles were elicited by repeatedly tapping two No. 3 scalpel handles above and behind each rat's head so that the noise source could not be observed. This procedure was continued until the desired behaviour was produced by the anxious rat. E M G signals were first amplified at a gain of 2000 (bandpass 1 Hz-30 kHz) by P511 preamplifiers (Grass Instruments, Quincy, MA) and also simultaneously displayed with both X - Y optoelectronic and E M G waveforms on separate storage oscilloscopes (models T912 and 5111A; Tektronix,

35

Beaverton, OR). Both optoelectronic (frontal vertical, frontal horizontal, and sagittal horizontal) and E M G signals from the anterior temporalis were simultaneously recorded on an F M instrumentation cassette tape-recorder (TEAC MR-30, Tokyo, Japan) at 9.5 cm/s before being digitized at 1 kHz by a microcomputer data acquisition/analysis system (Compaq 386 25e and DATAPAC hardware/ software package, R U N Technologies, Laguna Niguel, CA). The digitized waveforms were then edited and analysed by the DATAPAC software. Both masticatory and bruxing/thegosis optoelectronic and E M G waveforms were displayed one cycle at a time and data-sets were produced. From these data-sets, spatial and temporal attributes were calculated for both waveforms (see Figs 2-6 and Tables 1-4). Means and SD for latency/duration, displacement, velocity, and acceleration were all calculated using the DATAPAC software. All E M G electrode placements in the anterior temporalis were confirmed immediately after data collection by clipping the E M G wires, killing the rat, and carefully dissecting the muscle. In the five rats, electrode placement was found to be at a mean + SD locus of 13.24+ 0.32 mm posterior to the posterior edge of the zygomatic process of the maxillary bone and 7.16 mm + 0.30 mm superior to the zygomatic arch. All animal procedures were approved by licensed vivaria staff and were in full compliance with NIH Guiding Principles for Research Involving Animals and American Physiological Society standards.

RESULTS Figure 2 shows frontal-plane traces for both masticatory-cycle and bruxing/thegosis-cycle trajectories; orientation of these frontal-plane traces is consistent with earlier published depictions of masticatory movements (Byrd, 1981, 1984, 1988b). Both masticatory and bruxing events displayed definite lateral translation and were not the purely propalinal/anterior-posterior movements described in earlier, low-resolution studies (Hiiemfie and Ardran, 1968; Weijs, 1975; Weijs and Dantuma, 1975). Their respective attributes (Table 1) reflected the differences apparent in Fig. 2. Masticatory cycles typically alternated between right- and left-side chewing patterns while bruxing/thegosis were ipsilateral in nature. As shown in Table 1, masticatory cycles were significantly different from bruxing/thegosis cycle for duration, displacement, velocity, and acceleration in all three dimensions. Essentially, bruxing/thegosis cycles were very rapid, brief physiological events in comparison to masticatory cycles.

K. E. Byrd

36

1

MC 2

MC 3

MC 4

MC 5

I lmm

BTC 1

BTC 2

BTC 3

BTC 4

BTC 5

Fig. 2. Optoelectronic mandibular traces for five successive masticatory (MC) and bruxing/thegosis (BTC) cycles from the same animal. Orientation as in Byrd (1981, 1984, 1988b) and as if reader is inside the rat's mouth and looking anterior. Arrows indicate direction of mandibular movement and calibration angle = 1 mm in both dimensions. Note (1) definite lateral translation of both masticatory and bruxing/thegosis cycles; (2) alternating nature (chew on left, chew on right, chew on left) of masticatory cycles in contrast to ipsilateral bruxing/thegosis cycles; in this case, all left side; and (3) small area traversed by bruxing/thegosis cycles in contrast to masticatory cycles.

The patterns of anterior temporalis activity during masticatory and bruxing cycles were also significantly different from each other (see Figs 3 6). Waveforms from the muscle during both types of cycle showed a rhythmic, phasic pattern (Figs 3 and 5), but higherresolution analyses of the muscle activity during masticatory (Fig. 4) and bruxing (Fig. 6) cycles demonstrated important differences in their respective patterns. As shown in Fig. 4, the masticatory cycle displayed anterior temporalis activity waveforms with both opening- and closing-burst increases in E M G amplitude; in contrast, the very rapid bruxing/thego-

sis cycles showed only opening-burst activity (see Fig. 6). A careful inspection of the depicted masticatory cycles at both relatively low (Fig, 3) and high resolution (Fig. 4) revealed that the observed opening-burst activity patterns were not due to cross-talk (Loeb and Gans, 1986). As shown in the third burst of the left anterior temporalis ( L A T in Fig. 3), opening-burst activity during the masticatory cycle was most prominent towards the end of a masticatory sequence as the bolus was reduced. Statistical analyses of the opening-burst activity of the anterior temporalis during masticatory and

Table 1. Attributes of whole masticatory (MC) and bruxing/thegosis (BTC) cycles (:~ + SD) MC Duration (ms) FV disp. (mm) FV vel. (mm/ms) FV accel. (mm/ms 2) FH disp. (mm) FH vel. (mm/ms) FH accel. (mm/ms 2) SH disp. (mm) SH vel (mm/ms) SH accel. (mm/ms 2)

284.11 +54.85 5.06+0.64 0.01821 + 0.00302 0.00007 ___0.00003 1.45+0.40 0.00513 + 0.00140 0.00002 + 0.00001 2.27__+0.66 0.00813 + 0.00228 0.00003 __+0.00001

BTC (n (n (n (n (n (n (n (n (n (n

= = = = = = = = = =

101) 101) 101) 101) 101) 101) 101) 101) 101) 101)

62.87+26.87 0.80+0.22 0.01515 + 0.00823 0.00035 _ 0.00036 0.46+0.18 0.00860 + 0.00504 0.00020 + 0.00020 0.39+0.14 0.00700 + 0.00350 0.00015 + 0.00014

(n (n (n (n (n (n (n (n (n (n

= = = = = = = = = =

102) 102) 102) 102) 102) 102) 102) 102) 102) 102)

Duration, latency of cycles; FV disp., frontal vertical displacement of mandible; FV vel., frontal vertical velocity of mandible; FV accel., frontal vertical acceleration of mandible; FH disp., frontal horizontal displacement of the mandible; FH vel., frontal horizontal velocity of the mandible; FH accel., frontal horizontal acceleration of the mandible; SH disp., sagittal horizontal displacement of the mandible; SH vel., sagittal horizontal velocity of the mandible; SH accel., sagittal horizontal acceleration of the mandible.

Brux-like movements in the rat

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Fig. 3. Anteriar temporalis activity during three successive masticatory-cycle behaviours in a single animal. FV, frontal vertical optoelectronic waveform; RAT, right anterior temporalis EMG signal; and LAT, left anterior temporalis EMG signal. Note regular, phasic pattern of temporalis EMG pattern during masticatory cycles.

bruxing cycles confirmed that the muscle behaved very differently depending upon the type of oral behaviour being manifested (see Table 2). In other words, the mean _+ SD attributes (frontal vertical/ frontal horizantal/sagittal horizontal displacement, velocity, and acceleration) of the masticatory cycle during the approx. 60 ms of opening-burst activity were significantly different from their counterparts in the bruxing cycle (Table 2). Particularly noticeable were the greater frontal horizontal accelerations for the bruxmg cycle during opening-burst activity (Table 2). The closing-burst attributes of anterior temporalis activity during the masticatory cycle (Table 3) stood alone, as the bruxing/thegosis cycle did not demonstrate such patterns (compare Figs 4 and 6). Table 4 shows opening- and closing-burst attributes of anterior temporalis activity in relation to physiological centric occlusion--that point of maximum approximation between mandibular and maxillary teeth during masticatory and bruxing movements (see Byrd, 1988b; Byrd and Chai, 1988). Physiological centric occlusion is therefore identical with the term 'minimum gape' (Hiiem/ie and Crompton, 1985) commonly used in the compara-

tive literature. Significant differences between masticatory- and bruxing-cycle attributes during opening-burst activity in the anterior temporalis were present for the temporal initiation of the opening burst before physiological centric occlusion, initiation inferior from that centric occlusin, initiation medial/lateral from it, and initiation posterior from it (see Table 4).

DISCUSSION

Data presented here demonstrate conclusively that the anterior temporalis muscle of the laboratory rat does indeed display task-dependent activity patterns depending upon the manifested oral behaviour. In this case, radically different--both quantitative and qualitative--EMG patterns were detected during respective masticatory and bruxing/thegosis behaviours. The null hypothesis regarding the activity patterns of the anterior temporalis during the masticatory and bruxing/thegosis cycles is therefore rejected.

38

K.E. Byrd 4.9

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Fig. 4. Higher-resolution graphic of opening-burst and closing-burst activity of anterior temporalis during a single masticatory-cycle event. FV, frontal vertical optoelectronic waveform; PCO, physiological centric occlusion (maximum jaw approximation); RAT, right anterior temporalis EMG signal; OB, opening burst; and CB, closing burst. Note opening burst occurs during mandibular depression and closing burst occurs during mandibular elevation.

Opening-burst activity during masticatory and bruxing cycles As mentioned earlier in the Results section, the presence of opening-burst activity for the anterior temporalis during the fast-open phase (Hiiem~ie and Crompton, 1985) of the masticatory cycle was typically more prominent towards the end of a masticatory sequence as the bolus was reduced. This may be a response to increased viscosity of the bolus (mixture of food item fragments and salivary secretions) before actual deglutition and/or reflects mandibular positioning just before the act of swallowing the bolus. Opening-burst activity of the anterior temporalis muscle during the bruxing/thegosis cycle, however, probably reflects mandibular positioning and definite lateral/buccal translation during incisor sharpening (see Fig. 18 in Druzinsky, 1995). A careful inspection of opening-burst loci during the bruxing cycle depicted here in Fig. 6 showed that openingburst activity during that cycle began after minimum gape/physiological centric occlusion and through mandibular depression; opening-burst ac-

tivity during bruxing/thegosis typically ceased during early mandibular elevation (see Fig. 6).

EMG and functional patterns The observed task-related shifts of EMG patterns in the anterior temporalis suggest an exquisite neurophysiological substrate that coordinates the threedimensional expression of the muscle's latencies and phasic activity patterns. All evidence from previous research suggests that the mechanism(s) effecting such patterns are subcortical and involve a masticatory central pattern generator located in the mammalian brainstem (Bremer, 1923; Magoun et al., 1933; Rioch, 1934; Dellow and Lund, 1971; Lund, 1976, 1991; Delcomyn, 1980; Luschei and Goldberg, 1981; Lund et al., 1981, 1982, 1983; Chandler and Goldberg, 1982, 1988; Byrd, 1985). This masticatory central pattern generator can be modified by input from higher centres such as the motor cortex (Luschei and Goldberg, 1981). As demonstrated here, the radically different nature of the masticatory and bruxing/thegosis cycles and their associated EMG patterns in the anterior temporalis suggests the possible existence, in addition

Brux-like movements in the rat

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Fig. 5. Anterior temporalis activity during 12 successive bruxing/thegosis-cycle behaviours by the same animal shown in Figs 3 and 4. FV, frontal vertical optoelectronic waveform; RAT, right anterior temporalis EMG signal; LAT, left anterior temporalis EMG signal. Note rapid nature of bruxing/thegosiscycle phasic activity patterns in contrast to masticatory-cycle patterns shown in Fig. 3.

to the masticatory, of a bruxing/thegosis pattern generator in the laboratory rat. This hypothesis is supported by the occurrence of bruxism in humans in coma (Pratap-Chand and Gourie-Devi, 1985), which suggests that the neurophysiological and neuroanatomical systems for bruxism may also be similar to the masticatory central pattern generator and subcortical in nature. The task-related changes in activity patterns in the rat anterior temporalis shown here may also be due, in part, to selective recruitment of motor units, as previously described for cats (Hoffer et al., 1987; Chanaud and MacF'herson, 1991) and humans (Ter Haar Romeny e t a / . , 1982) during different motor behaviours. The oL,vious three-dimensional alterations of the muscle's activity patterns during masticatory and bruxing behaviours as shown here, however, strongly suggest that the behaviour-dependent activity patterns of the rat anterior temporalis are due to something more than just selective recruitment of motor units. The distribution of both fibre types and muscle spindles is heterogeneous within the masseter muscle (Eriksson and Thornell, 1983, 1987); this observation, coupled with the fact that motor-unit terA0B 42-I-C

ritories within the masseter are restricted to specific zones (McMillan and Hannam, 1991), suggests that similar conditions may exist in the rat anterior temporalis. If this is true, the behaviour-dependent nature of anterior temporalis activity patterns shown here may have an anatomical complexity that mirrors the neurophysiological one. The presence of neuromuscular compartments within the muscles of mastication, as documented by Herring and her co-workers (Herring et al., 1979, 1989, 1991; Wineski and Herring, 1985; Herring and Wineski, 1986; Anapol and Herring, 1989; Herring, 1992), further supports the idea that neurophysiological and anatomical complexities of the mammalian masticatory system are interrelated and play an important part in behaviour-dependent activity of the anterior temporalis. B r u x - l i k e behaviours in the rat

The mandibular movements/EMG patterns during the bruxing/thegosis cycle documented here are in broad agreement with earlier low-resolution findings by Pohto (1979), Sunden-Kuronen et al. (1983), and Shoji, Bruce and Siu (1994). Data presented here support the earlier discovery that E M G

K. E. Byrd

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msec. Fig. 6. Higher-resolution graphic of opening-burst activity of anterior temporalis during three successive events by the same animal shown in Figs 3 and 4. FV, frontal vertical optoelectronic waveform; RAT, right anterior temporalis EMG signal; PCO, physiological centric occlusion (maximum jaw approximation); and OB, opening burst. Note exclusive opening-burst phasic activity of anterior temporalis in contrast to opening- and closing-burst phasic activity during masticatory cycle as shown in Fig. 4.

bursts during n o r m a l (i.e., masticatory cycle) a n d brux-like (i.e., bruxing/thegosis cycle) m a n d i b u l a r m o v e m e n t s in the rat are significantly different from each o t h e r (Shoji et al., 1994). The fact t h a t bruxing cycles are m o s t frequent when the rat has anxiety, as s h o w n by P o h t o (1979) a n d also observed here, strongly supports the n o t i o n t h a t psychogenic/stress

factors ( R a o a n d Glaros, 1979; Budtz-Jorgensen, 1980, 1981; Hicks a n d Conti, 1989) play a n importa n t part in h u m a n bruxism as well. In h u m a n s , bruxism has been classically defined as being ' n o n - f u n c t i o n a l ' in nature ( R a m t ] o r d a n d Ash, 1971); however, anxiety/stress-induced bruxing/thegosis cycles a p p e a r to be functionally import-

Table 2. Opening burst attributes of anterior temporalis during masticatory (MC) and bruxing/thegosis (BTC) cycles 0~ _+SD) MC Duration (ms) FV disp. (mm) FV vel. (mm/ms) FV accel. (mm/ms 2) FH disp. (mm) FH vel. (mm/ms) FH accel. (mm/ms 2) SH disp. (mm) SH vel (mm/ms) SH accel. (mm/ms 2)

60.70 ± 1.97 ± 0.03492 ± 0.00065 ± 0.61 ± 0.01082 ± 0.00023 ± 0.86 ± 0.01576 ± 0.00034 ±

19.95 0.77 0.01251 0.00054 0.28 0.00385 0.00015 0.57 0.0100 0.00042

BTC (n (n (n (n (n (n (n (n (n (n

= 101) - 101) 101) = 101) = 101) = 101) = 101) = 101) = 101) = 101)

28.34 ± 16.03 0.67 ± 0.20 0.02755 ± 0.01149 0.00132 ± 0.00091 0.35 ± 0.17 0.01428 ± 0.00778 0.00068 + 0.00054 0.32-t-0.13 0.01213 ± 0.00519 0.00055 ± 0.00046

(n = (n = (n = (n = (n = (n (n = (n(n = (n -

102) 102) 102) 102) 102) 102) 102) 102) 102) 102)

Duration, latency of opening-burst activity of anterior temporalis during masticatory and bruxing/thegosis cycles; all other abbreviations as in Table 1 but in relation to opening-burst activity.

Brux-like movements in the rat

41

Table 3. Closing-burst attributes of anterior temporalis during masticatory cycles (.~ + SD) Duration (ms) FV disp. (ram) FV vel. (mm/ms) FV accel. (ram/ms 2) FH disp. (ram) FH vel. (mra/ms) FH accel. (rnm/ms 2) SH disp. (ram) SH vel (mm/ms) SH accel. (ram/ms 2)

103.14 + 18.57 4.87 _+0.65 0.04994 + 0.01074 0.00054 ± 0.00026 1.07 + 0.34 0.01069 + 0.00340 0.00011 + 0.00006 2.06 + 0.71 0.02100 ___0.00778 0.00022 _+0.00011

(n (n (n (n (n (n (n (n (n (n

= = = = = = = = = =

101) 101) 101) 101) 101) 101) 101) 101) 101) 101)

Duration, latency of closing-burst activity of anterior temporalis during masticatory cycles; all other abbreviations as in previous tables but for closing-burst activity. Table 4. Opening- and closing-burst attributes of anterior temporalis in relation to physiological centric occlusion (PCO) during masticatory (MC) and bruxing/thegosis (BTC) cycles (2+SD) Opening burst MC Init. Init. Init. lnit.

from PCO (ms) inferior from PC() (ram) medial/lateral from PCO (mm) posteriorfromPCO(mm)

163.82 _+26.11 5.03 +_0.62 1.15 _+0.33 2.13_+0,61

Closing burst BTC

(n = (n = (n = (n-

101) 101) 101) 101)

MC

37.16 _+ 16.25 (n 0.78 _+0.22 (n 0.41 _+0.17 (n 0.35_+0.12 (n

= = =

86) 86) 86) 86)

86.50 _+ 17.64 (n 4.87 _+0.65 (n 1.03 _+0.34 (n 1.98_+0.71 (n

= = = =

101) 101) 101) 101)

Init. from PCO, temporal initiation of muscle activity before point of physiological centric occlusion; lnit. inferior from PCO, inferior-most point of mandibular displacement during initiation of muscle activity in relation to physiological centric occlusion; lnit. medial/lateral from PCO, medial-most/lateral-most point of mandibular displacement during initiation of muscle activity in relation to physiological centric occlusion; Init. posterior from PCO, posterior-most point of mandibular displacement during initiation of muscle activity in relation to physiological centric occlusion. ant as a tooth-sharpening or 'thegotic' mechanism in many non-human animals including pigs, monkeys, and rats (Every, 1970, 1975; Druzinsky, 1995). In this context, the. eccentric bruxism shown here by rats during bruxing/thegosis movements are definitely not 'non-functional' . The masticatory cycles of human bruxers have been studied (Faulkner, 1989): they displayed: (1) a shorter interval between cycles, (2) irregular!y shaped envelopes of motion, (3) sudden changes in direction, and (4) a loss of the typical 'tear-drop' pattern seen in human chewing cycles. These similarities and differences between human bruxism and rat bruxing/thegosis cycles should be further investigated.

Neuropharmacological considerations

Bruxism has also been experimentally induced in laboratory rats using haloperidol (Sunden-Kuronen et al., 1983) and apomorphine combined with electrical stimulation (Pohto, 1979). These data, combined with research that indicated that increased bruxing in macaque monkeys is associated with increased concentrations of urinary and plasma cortisol during anxiety/stress (Budtz-Jorgensen, 1980, 1981), indicate that brux-like behaviours have, at least in part, a definite neuropharmacological substrate. The finding of bruxism in human patients following treatment with neuroleptic drugs (Ellison and Stanziani, 1993; Giannaula and Parera, 1993) further supports the role of neuroactive compounds in the manifestation of discrete oral behaviours.

Careful, precise high-resolution data for bruxing/ thegosis and masticatory-cycle behaviours as provided here should allow better utilization of animal models in future research in the area of human bruxism.

This research was supported by National Institute of Dental Research Grant DE-07830. Acknowledgements

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