Coordination of Cortically Induced Rhythmic Jaw and ...

JOIJRNALOF NEUROPHYSIOLOGY Vol. 69. No. 2, February 1993. I’rrntcd

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Coordination of Cortically Induced Rhythmic Movements in the Rabbit Z. J. LIU, Y. MASUDA, T. INOUE, H. FUCHIHATA, A. SUMIDA, Faculty ofDentistry, Osaka University, Suita, Osaka 565, Japan

Jaw and Tongue

K. TAKADA,

AND

T. MORIMOTO

the biological motor systems. To elucidate the organization of this coordination, movements of these two organs have I. Rhythmic movements of the jaw, tongue, and hyoid that been recorded during chewing in various animals, includwere induced by stimulation of the cortical masticatory area ing the bat (Greet and DeVree 1984), rabbit ( Ardran et al. (CMA) were recorded cineradiographically in the anesthetized 1958; Cortopassi and Muhl 1990), opossum (Crompton et rabbit. Jaw movements were also recorded by a laser position deal. 1977) tenrec (Oron and Crompton 1985), cat (Thextector. 2. The evoked jaw movements were classified into four types: ton 1981; Thexton and McGarrick 1988, 1989), macaque small circular (type A), large circular (type B), large vertical (type (Frank et al. 1984) and human ( Abd-El-Malek 1955). C), and crescent-shaped (type D). Among these, types B and D These studies showed that when the tongue moves and resembled the jaw movements of the food transport cycle and changes its shape in a narrow intraoral spaceduring chewthose of the chewing cycle in a masticator-y sequence. ing, its movement is coordinated well with jaw movements 3. Each type of jaw movement was associated with a particular both in the spatial and temporal aspects. Furthermore, the pattern of tongue and hyoid movements. In general, the tongue movement pattern of the jaw and tongue was found to vary protruded during jaw opening and retracted during jaw closure. according to the masticator-y sequence and also to the conThe hyoid generally moved upward and forward during jaw opensistency of the food. The basic pattern of rhythmic jaw ing but downward and backward during jaw closure. movements ( RJMs) during mastication is currently 4. Electromyograms ( EMGs) were recorded from jaw muscles [ masseter (Ma) and digastric (Di) muscles], extrinsic tongue thought to be produced by the central pattern (or rhythm) muscles [ styloglossus (Sg) and genioglossus (Gg) muscles], and generator (CPG) in the brain stem (Dellow and Lund hyoid muscles [ sternohyoid (Sh) and geniohyoid (Gh) muscles] 1971; reviewed by Lund 1976; Lund and Enomoto 1988; during cortically induced rhythmic jaw and tongue movements Nakamura et al. 1979). The coordination of the move(CRJTMs). These muscles were classified into two groups: grozlp ments of the jaw and tongue may also be programmed in I was activated mainly in the jaw opening phase, and group2 was the CPG (Jiich et al. 1985). The variability of the moveactivated mainly in the jaw closing and power phases. The Di, Gg, ment pattern of these two organs during natural masticaand Gh were included in the former, and the Ma, Sg, and Sh were tion could be ascribed to the peripheral sensory inputs to included in the latter. the CPG, which vary with the change in physical properties 5. The timings of EMG activation to a jaw movement cycle were relatively constant for the muscles ofgroup 1, irrespective of of food, the processof mastication, the location of food in the types of CRJTMs, whereas those for the muscles of group 2 the mouth, and other factors (Lowe 1980). If RJMs and altered considerably with the different types of CRJTMs. rhythmic movements of the tongue were induced without 6. Relationships of the integrated muscle activity between the the actual feeding of food, the basic pattern of the coordinaDi and Gg and between the Di and Gh were significant, whereas tion of these two organs could be clarified. Fictive masticathose between the Ma and Sg and between the Ma and Sh tion induced by electrical stimulation of the cerebral cortiwere not. cal masticatory area (CMA) is suitable for this purpose be7. When a small strip of polyurethane form of various degrees cause it is induced without the feeding of food and thus is of hardness was inserted between the opposing molars during not influenced by the variable sensory inputs arising during CRJTMs, EMG activity of the muscles of group 2 increased with chewing of the food. Furthermore, various patterns of the hardness of the strip. On the other hand, EMG activities of the RJMs, some of which resemblethose found in natural masmuscles ofgrozlp 1 were less affected by the same intraoral stimuli. 8. Two conclusions were reached: first, physiological propertication, could be induced depending on the stimulated site ties of the CRJTMs and cortically induced rhythmic movements in the CMA of the rabbit (Bremer 1923; Lund et al. 1984; of the hyoid were essentially similar to those observed in natural Morimoto et al. 1989). One of the purposes of the present mastication. This fictive mastication might thus be regarded as a study is to elucidate the basic pattern of organization of the suitable model for simulating natural mastication. Second, synmovements of the jaw and tongue in cortically induced ficchronous activity of the muscles of group I was related to the tive mastication. synchronization of rhythmic movements among these three orThe coordination of movements of the jaw and tongue is gans, whereas activities of the muscles in group 2 were related to ascribed to harmonious activity of the muscles attached to the differences in the movement patterns of these three organs these organs. Because the tongue partly attaches to the among the four types of CRJTMs. hyoid, activity of the hyoid muscles is also related to this coordination. Few studies have been conducted on the relaINTRODUCTION tionship among the activities of these musclesduring mastiCoordination of jaw and tongue movements in mastica- cation (Crompton 1989; Oron and Crompton 1985; Thexton 1984; Thexton and Crompton 1989; Weijs and Dantion may be one of the most intricate mechanisms within SUMMARY

AND

CONCLUSIONS

0022-3077/93

$2.00 Copyright

0 1993 The American Physiological Society

569

570

LIU

tuma 1975, 198 1). Because various patterns of the RJMs are induced by CMA stimulation, examination of the differences in electromyographic (EMG) activities of the jaw, tongue, and hyoid muscles among these patterns will clarify the mechanism coordinating these muscle activities. The second purpose of this study is, therefore, to examine the relationship among the EMG activities associated with various patterns of cortically induced RJMs and rhythmic tongue movements ( CRJTMs). When a test substance is inserted between the opposing molars during cortically induced fictive mastication, the patterns of RJMs and associated activity of the jaw-closing muscles are found to be modified (Lavigne et al. 1987; Morimoto et al. 1989). However, it is not clear how the tongue and hyoid muscles respond to such intraorally applied stimuli. Furthermore, the responses of these muscles and those of the jaw to the changes in the physical properties of the inserted substance have yet to be analyzed. The third purpose of this study is to make such an analysis of how the jaw, tongue, and hyoid muscles respond to insertion of the test substance with its various physical properties between the opposing molars duri’ng fictive mastication. The mechanism for coordinating the movements of the jaw, tongue, and hyoid both in spatial and temporal aspects in mastication is discussed on the basis of the results of EMG studies. METHODS

Surgical procedures Thirty-five male rabbits, weighing 2.3-3.5 kg, were used. The animals were initially anesthetized with intravenous administration of alpha chloralose (60 mg/ kg) and urethane (500 mg/kg) via an ear vein. Sodium thiamylal (Isozol, Yoshitomi Pharmaceutical, 25% solution, 0.5 ml/kg) was added every 25 min, or a mixture of halothane and oxygen was supplemented. During the surgery, anesthesia was maintained at such a level that no apparent cornea1 reflex nor spontaneous eye movements occurred. After tracheal cannulation, artificial ventilation was conducted at a rate of 40 breaths/min. The heart rate and the blood pressure of the femoral artery were in the range between 2 10 and 240 beats/ min and between 60 and 110 mmHg, respectively. Small screws were attached to the mentum to hold a phototransistor array for

ET AL.

tracing jaw movements by means of a Helium-Neon laser position detector (Morimoto et al. 1984). The animal’s head was fixed in a stereotaxic apparatus by means of three skull screws in position such that the lambda was 1.5 mm below the bregma (Sawyer et al. 1954). For intracortical microstimulation of the CMA, the right cortical surface was exposed between 0 and 8 mm anterior to the bregma and mediolaterally between 2 mm lateral to the midline and the lateral edge of the cranium. The underlying dura was opened, and the exposed area was covered with warm liquid paraffin ( 37°C). The cisterna magna was opened to drain the cerebrospinal fluid to reduce the brain pulsations. The rectal temperature was maintained between 36 and 38°C with a heating pad, and the electrocardiogram was monitored continuously.

Cortical stimulation Intracortical microstimulation was given to the right CMA by a glass-coated metal electrode having an impedance of l-3 MQ at 1 kHz (the tip being 20-30 pm diam). The ground electrode was placed on the cranium 5 mm posterior to the bregma, and the stimulating electrode was vertically inserted from the dorsal surface. Repetitive electrical stimuli with square pulses with 33-ms intervals (30 Hz) and 0.2-ms durations were applied at an intensity of 40- 100 PA for -7 s. The anesthesia during the experiment was controlled at a level slightly lighter than that during surgery. The heart rate was -240 beats/min. The lowest arterial pressure of the femoral artery was -80 mmHg and the highest one was in the range between 120 and 135 mmHg. Pupillary constriction appeared when the light was on. No obvious changes in the heart rate, respiration, pupillary size, or arterial pressure were induced by CMA stimulation. For convenience in describing the location of the effective sites producing CRJTMs, the level anterior to the bregma is indicated by “A,” that lateral to the midline by “L,” and the depth from the dorsal surface by “D.”

Cineradiograph

ev

Three rabbits were prepared for cineradiographic recordings of the movements of the jaw, tongue, and hyoid. Radioopaque markers were implanted under anesthesia by the following procedure. I) After drilling a small hole at the interproximal space of the adjacent central incisors of the upper and lower jaws, a lead ball 2 mm diam was fixed in the hole with self-curing resin. 2) A stainless steel wire 0.8 mm diam and 1.0 mm in length was inserted into the tongue at three portions, i.e., 1 mm behind the tongue tip, the commissure of the lingual frenum, and the middle

FIG. 1. The jaw, tongue, and hyoid muscles used for recording electromyographic (EMG) activities. A: group I muscles that were activated predominantly in the jaw-opening phase. B: group 2 muscles that were activated predominantly in the jaw-closing and power phases. Ma, masseter; Di, digastricus; Sg, styloglossus; Gg, genioglossus; Sh, sternohyoideus; Gh, geniohyoideus.

COORDINATION TABLE

1.

OF JAW AND TONGUE

Physicalparametersof test strips

Specimen

Hardness, Hs

Compressive Strength, kgfs/cm2

Impact Resilience, %

1 2 3 4 5

27 47 67 84 91

76 195 293 517 768

57 64 56 65 53

of the intermolar eminence. The interval between these tongue markers was - 15 mm. The three portions were selected as the reference points of the anterior, middle, and posterior parts of the tongue, respectively. 3) A small piece of wire identical to the above mentioned tongue markers was inserted into a small hole drilled at the middle of the hyoid body. The X-ray was conducted from the left side of the animal. Sagittal images were recorded during CRJTMs through a set of cineradiographic instruments composed of a medical X-ray tube apparatus (UH-6GE-3 IV, focus size 0.6 X 6 mm/ 1 X 1 mm, Hitachi Medical, Japan), an image intensifier (5-in., Phillips, Holland), and a tine camera (35 mm-4 ni, Photo-Sonics, Burbank, CA, USA). Images were photographed on 35-mm CFS film ( Estar-AH base, Eastman Kodak, USA) at 64 frames per second (70 KV, 60 mA, 4 ms and 150 FID). During cineradiography, jaw movements on the frontal plane were also monitored with the laser position detector and stored on tape. Immediately after radiographic recordings, EMGs were recorded simultaneously with jaw movements during stimulation of the same cortical sites as those used for radiography. When the induced jaw movements were

A

571

MOVEMENTS

found to be similar to those recorded during radiography, the movements of the jaw, tongue, and hyoid were compared with the associated EMG activities of the relevant muscles. The tine films were later replayed on an analyzing projector (Vanguard XR-35, Vanguard Instrument, Melville, NY) in the sequence of the frames recorded. Movement trajectories of the three tongue markers in the CRJTMs were examined separately with respect to the maxilla, mandible, and hyoid. For the maxilla, the markers attached to the interincisal space of the maxilla and the clear line of the hard palate were used as a reference point and a reference line, respectively. When the movements were displayed relative to the mandible, the interincisal marker of the mandible was used as the reference point, and the line connecting the lower margin of the mandibular marker and the lowest point of the mandibular ridge was defined as the reference line. The hyoid image and the hyoid marker were used as the references of hyoid movements.

EMG Pairs of enamel-coated copper wires ( 150 pm diam, 5 mm interpolar distance) with bared tips 1.5 mm in length were inserted into the left jaw muscles [ masseter (Ma) and digastric (Di) muscles], left extrinsic tongue muscles [ styloglossal (Sg) and genioglossal (Gg) muscles] and left hyoid muscles [ sternohyoid (Sh) and geniohyoid (Gh) muscles] for recording EMG activity (Fig. 1). The Di is anatomically classified as a suprahyoid muscle but is included here in the jaw muscles because it is regarded physiologically as a dominant antagonist of the jaw-closing muscles. The Sg was composed of two bellies in the rabbit, dorsal and ventral ones. Only the ventral belly was used for recording. All the muscles except the Ma were exposed by a submandibular approach. The mylohyoid mus-

C 8.5

8.0

7.5

7.0

6.5

FIG. 2. Distribution of the sites producing rhythmic jaw and tongue movements by cerebral cortical masticatory area (CMA) stimulation and the patterns of evoked jaw movements. A : schematic representation of the coronal section through the level 3 mm anterior to the bregma (A3) of the right hemisphere. B: coronal section of the area enclosed by a rectangle in -4. Dots: sites producing rhythmic jaw and tongue movements. C: patterns of evoked jaw movements. Note that small and large circular movements were produced from the dorsal part of the effective area, whereas large vertical and crescent-shaped movemenfs were produced from the ventral part. Also note that the direction of the jaw-closing path of the circular movement is opposite to the other movements.

LIU

572

ET AL.

A2

A3

A5

A4

A6

DO.5

no movement

03.5 20 15 10 5 0 45678mm

4 5 6 7 8 ITTITT 45678mm

L

L

L

3 5 6 7 8 mm

35678mm

L

q

type

A

n typeD

L

FIG. 3. Relationship between stimulation sites and patterns of the evoked jaw movements in the regions anteroposteriorly between A2 and A6, mediolaterally between L4 and L8, and dorsoventrally between DO and D4. In this figure, the level anterior to the bregma is represented by A, that deep from the cortical surface by D, and that lateral to the midline by L. In each graph, ordinate: number of cases examined among 20 animals, abscissa: mediolateral levels of stimulation. Note that the patterns of evoked jaw movements are produced in a site-dependent manner, as shown in Fig. 2.

cle was dissected sagitally along the hypoglossal nerve to expose the Gh and Gg. The inserted electrodes were covered with a mixture of liquid paraffin and Vaseline. The EMG signals were amplified, filtered (30 Hz-3 kHz), and displayed simultaneously with jaw movements on an oscilloscope and stored on magnetic tape using a 14-channel data recorder (DFR-6 1430, Sony-Magnescale).

Intraoral

interval of 3 min. Jaw movements were recorded by means of the laser position detector simultaneously with EMG activity of the jaw, tongue, and hyoid muscles before, during, and after the insertion of the strip. The movements of the tongue were not recorded radiographitally during the intraoral stimulation experiments, but video recordings were made. No quantitative analysis was performed on the video images.

stimulation

Five test strips of polyurethane foam ( 14 mm long, 5 mm wide and 2 mm thick) with different degrees of hardness were prepared. The physical properties of the strips are listed in Table 1. The procedures for giving intraoral stimulation by means of these strips have been described elsewhere (Morimoto et al. 1989). In brief, the left buccal tissues were incised from the angle of the mouth to the anterior edge of the Ma to expose the first and second molars. The incised edges of both the maxillary and mandibular segments were sutured separately and were anesthetized locally by injection of xylocaine (2%). A test strip was inserted and held horizontally by an experimenter between the left opposing first and second molars using a forceps during 10 cycles of CRJTMs. Each of the test strips was applied 5 times in random order at an

Data processing and statistical analysis The data stored on magnetic tape were later replayed and analyzed using a signal processor (7T- 18, Nihondenki-Sanei). The extent of the anteroposterior movements of the three tongue markers and the hyoid marker was compared between types B and D of CRJTMs. Because the CRJTMs were regular and had stereotyped movements, their trajectories were stable except for the few initial cycles evoked after the start of cortical stimulation. Accordingly, three stable cycles were employed for tracing and analyzing the jaw, tongue, and hyoid movements. For EMG recordings, the following variables were analyzed: 1) the duration of EMG burst activities, 2) the timing of the EMG activity in relation to the phase of a masticatory cycle, and 3) the

COORDINATION

Type

A

Relative

to Maxilla

Relative

to Mandible

OF

JAW

AND

TONGUE

MOVEMENTS

573

TYPe c

TYPe B

Type D

1Omm

OR

OR

FIG. 4. Jaw, tongue, and hyoid movements in each type of cortically induced fictive mastication. The figures illustrate the sagittal view from the left side of the animal. The tcjp figures demonstrate the movements relative to the maxilla and the bottom figures those relative to the mandible. The paths shown by the solid and dotted lines indicate the movement trajectories during the jaw-opening phase and the combined jaw-closing and power phase, respectively. 1, upper incisor marker; 2, lower incisor marker; 3, mandibular reference line connecting the lowest point of the lower incisor marker and the lowest border of the mandible; 4, path of the mandibular movement; 5, 6, and 7, paths of the anterior, middle, and posterior tongue markers, respectively; 8, path of the hyoid movement. For the convenience of illustration, the hyoid position was moved forward from its actual position. R, positions of the markers at rest.

integrated activity of each muscle. The mean values were calculated for each variable from the data of five masticator-y cycles for each type of CRJTM. A nonpaired t test was performed for comparison of the aforementioned three variables among different CRJTM patterns. In addition, correlation coefficients were computed between the integrated activities of the jaw muscles and those of the tongue or hyoid muscles for the same CRJTM pattern. A paired t test was employed to determine whether there were significant differences between the values obtained before and during intraoral stimulation. The values measured during the intraoral stimulation were expressed as a percentage of those determined before stimulation. Statistical significance was determined at the P < 0.05 (significant) and P < 0.01 (highly significant) levels.

Histology At the end of the experiments, the animals were deeply anesthetized and microlesions were made at a few points in the CMA to use as references for mapping. Then the animals were perfused with saline through the ascending aorta, followed by 10% formalin. After 24 h postfixation, the brain was removed from the skull, cut frontally at 50 pm, and stained with cresyl violet. RESULTS

Jaw movements

Twenty animals were used for mapping the CMA. More than 50 sites were stimulated in each animal to define the area producing RJMs and their patterns by using currents of < 100 Examnles of the distribution of the effective

sites and patterns of cortically induced RJMs at A3 are shown in Fig. 2, in which the movements were induced by a stimulus intensity about a few microamperes above the threshold. In this animal, the effective sites were distributed mediolaterally between L6.5 and LS. 5 and dorsoventrally between D 1.O and D4.5 (Fig. 2 B) . Although there were individual differences in the distribution of the effective sites, they were generally found in the region lying anteroposteriorly between A2 and A6 and mediolaterally between L5.0 and L8.0. The sites with the lowest threshold were mostly found in layer V. 2. Comparisonof anteroposteriormovementof tongue and hyoid markersbetweentype B and type D TABLE

Type

Relative Ant. Mid Post. Hyoid Relative Ant. Mid. Post. Hyoid

to Mandible tongue marker tongue marker tongue marker marker to Maxilla tongue marker tongue marker tongue marker marker

All values in mm. 005 . . **p-c001 . .

Ant.,

B

Type

D

Mean

SD

Mean

SD

P

13.8 10.1 11.3 7.3

1.33 0.77 0.99 0.89

3.5 5.4 6.5 5.9

0.61 0.82 1.62 1.35

** ** ** *

7.6 6.8 7.3 4.1

1.87 0.80 1.42 0.34

2.3 3.5 5.4 3.6

0.67 1.71 0.92 0.31

** ** ** *

Post., posterior.

*P