A lateral jaw movement reflex

Periodontal Jaw Muscle Reflexes in the Albino Rat

Pressing or tapping stimulation on the maxillary incisor activated the jaw muscles, especially the masseter muscle, in an anesthetized albino rat. Periodontal jaw muscle reflexes involved two reflex pathways passing through the mesencephalic tract nucleus and the main sensory nucleus of the trigeminal nerve. The mechanoreceptors in periodontal ligaments contributed to an increase in the jaw muscle activity.

Effects of food consistency on the pattern of extrinsic tongue muscle activities during mastication in freely moving rabbits

The effects of physical characteristics of foods on the coordination of extrinsic tongue muscle activities during natural mastication were evaluated. Electromyograms of tongue-retractor (styloglossus, SG) and tongue-protractor (genioglossus, GG) muscles as well as the jaw-movement trajectories were recorded during raw rice and chow pellet chewing in the freely moving rabbit. Each masticatory cycle included a jaw closing (Cl) phase consisting of a fast-closing (FC) and a slow-closing (SC) phase, and a jaw opening (Op) phase. The duration of the Cl and SC phases was found to be much larger while the duration of the FC phase was much smaller during rice chewing than pellet chewing. The jaw movements during rice chewing had smaller amplitudes of the gape and lateral excursion of the jaw as compared with those during pellet chewing. The SG muscle had a double-peaked burst activity in each masticatory cycle with one peak during the Op phase (the SG1 burst) and the other during the Cl phase (the SG2 burst). They were significantly larger during pellet chewing as compared with rice chewing, but the duration of the SG2 burst was significantly longer during rice chewing than pellet chewing. The offset of the SG2 burst was delayed during rice chewing as compared with that during pellet chewing. There was little difference in the activity pattern of the GG burst between the foods. Our present results suggest that the SG muscle activity could be modified by the sensory feedback possibly to adapt to environmental demands during chewing.

Extrinsic tongue and suprahyoid muscle activities during mastication in freely feeding rabbits

To evaluate the coordination of tongue and suprahyoid muscle activities during natural mastication, electromyograms (EMGs) of jaw-closer, jaw-opener, suprahyoid (mylohyoid, MH), tongue-retractor (styloglossus, SG) and tongue-protractor (genioglossus, GG) muscles were recorded as well as the jaw-movement trajectories in vertical and horizontal axes in awake rabbits. Each masticatory cycle had three components including the fast-closing (FC), slow-closing (SC) and opening (Op) phases. The duration of the SC phase was much longer during pellet chewing while the durations of the FC and Op phases were much shorter during pellet chewing than bread or banana chewing. The jaw movements during banana chewing had a small amplitude of lateral excursion and a large amplitude of gape as compared with those during pellet and bread chewing. The MH muscle exhibited double-peaked EMG bursts during the Op phase. The MH bursts in the late part of the Op phase were dominant on the non-chewing side during pellet and bread chewing. The SG muscle also exhibited double-peaked EMG bursts. During pellet and bread chewing, the SG bursts during the SC phase were significantly larger on the chewing side than the non-chewing side. These bursts were also dominant during pellet chewing as compared with banana chewing. There was little difference in the GG bursts between the chewing and non-chewing sides or among the foods. Our results suggest that patterns of the MH and SG muscle activity are affected by the peripheral inputs and/or chewing patterns while those of the GG muscle activity was less modulated regardless of the consistency of foods.

Coordination of jaw and extrinsic tongue muscle activity during rhythmic jaw movements in anesthetized rabbits

To clarify the jaw-closer and tongue-retractor muscle activity patterns during mastication, electromyographic activity of the styloglossus (SG) as a tongue-retractor and masseter (Mass) as a jaw-closer muscles as well as jaw-movement trajectories were recorded during cortically evoked rhythmic jaw movements (CRJMs) in anesthetized rabbits. The SG and Mass muscles were mainly active during the jaw-closing (Cl) phase. The SG activity was composed of two bursts in one masticatory cycle; one had its peak during the jaw-opening (Op) phase (SG1 burst) and the other during the Cl phase (SG2 burst). The Mass activity during the Cl phase was dominant on the working side (opposite to the stimulating side) while the SG1 and SG2 bursts were not different between the sides. When the wooden stick was inserted between the molar teeth on the working side during CRJMs, the facilitatory effects on the SG1 and SG2 bursts on both sides were noted as well as those on the Mass bursts, but the effects on the SG1 burst seemed to be weak as compared with those on the Mass and SG2 bursts. The difference in the burst timing between the sides was noted only in the SG1 burst. When the trigeminal nerves were blocked, the peak and area of the SG and Mass burst decreased during CRJMs, and the facilitatory effects of the wooden stick application on the muscles were not noted. The results suggest that the jaw and tongue muscle activities may be adjusted to chew the food and make the food bolus.

Regulation of masticatory force during cortically induced rhythmic jaw movements in the anesthetized rabbit

To examine the relationships between masticatory force, electromyogram (EMG) of masticatory muscles, and jaw movement pattern, we quantitatively evaluated the effects of changing hardness of a chewing substance on these three variables. Cortically induced rhythmic jaw movements of a crescent-shaped pattern were induced by electrical stimulation of the cerebral cortical masticatory area in the anesthetized rabbit. The axially directed masticatory force was recorded with a small force-displacement transducer mounted on the ground surface of the lower molars. EMGs were recorded from the masseter and digastric muscles simultaneously with jaw movements. Five test strips of polyurethane foam of different hardness were prepared and inserted between the upper molar and the transducer during the movements. The peak, impulse, and buildup speed of the masticatory force increased with strip hardness, whereas duration of the exerted force did not vary with strip hardness. The integrated activity and duration of the masseteric EMG bursts also increased with strip hardness. The integrated EMG activity of the digastric bursts was weakly related to strip hardness, whereas the duration was not. The minimum gape increased with strip hardness, but the maximum gape did not. The horizontal excursion of the jaw did not vary in a hardness-dependent manner, although it was greater in the cycles with strip application than in the cycles without strip application. Deprivation of periodontal sensation by cutting the nerves to the teeth reduced the buildup speed of the force, maximum gape, net gape, and horizontal jaw movements. The denervation also elongated the force duration and that of masseteric EMG bursts. However, the rate of the hardness-dependent changes in the above parameters did not alter after denervation. The latency of the masseteric EMG response to strip application was evaluated before and after denervation. In both conditions, it was > or = 6 ms in approximately 70% of the cycles and <6 ms in the remaining approximately 30%, which cannot be explained by a simple reflex mechanism. On the basis of the analysis of correlation coefficients, the masseteric integrated EMG seemed to be a good indicator of the peak and impulse of the masticatory force both before and after denervation. We conclude that periodontal afferents would be responsible for a quick buildup of masticatory force and that afferents other than those from periodontal tissue would contribute to the hardness-dependent change of masticatory force during cortically induced rhythmic jaw movements.

The effects of food consistency on jaw movement and posterior temporalis and inferior orbicularis oris muscle activities during chewing in children

The possible effects of food consistency on the number of chews and the lapse of time in a chewing sequence, the jaw-movement pattern and velocity, and jaw and lip muscle activity during chewing were investigated. Fifteen healthy children with good occlusion were selected. First, each subject freely chewed hard (HJ) and soft (SJ) types of jelly without specifying the chewing side. The number of chews and elapsed time in a masticatory sequence (from the start of chewing to the completion of the final swallow) were measured. Second, the subjects performed right- and left-sided chewing of the same food. The electromyograms (EMG) of posterior temporalis (PT) and inferior orbicularis oris (OI) muscles on the right and left sides and associated jaw movement records were sampled. The HJ was chewed more times and with a longer time until finally swallowed (p < or = 0.0007) than the SJ. The HJ chewing also showed broader masticatory loops (p < or = 0.0199) in the frontal view and higher peak activities (p < or = 0.0007) for the PT muscle. The closing phase was longer when chewing the HJ than SJ, but the opening and intercuspal phases remained stable. More lateral excursion of the jaw was seen when chewing the HJ, but the jaw-movement trajectories in the sagittal and vertical directions were not affected by the change in consistency of the food. The jaw-closing velocities for the HJ chews were significantly slower (p < or = 0.0351) than those for the SJ chews in three directions. The HJ chews also revealed a longer duration between the onset of EMG burst for the PT muscle and the beginning of the centric occlusion (p < or = 0.0146). The OI muscle showed increased activity in accord with jaw opening, and consistent reciprocal cyclic activity with the PT muscle in terms of temporal associations (r > or = 0.5250; p < or = 0.0495). The OI muscle started to burst at a later part of the intercuspal phase, and frequently showed secondary activity in the jaw-closing and intercuspal phases. The peak activity for the ipsilateral OI muscle was significantly higher (p < or = 0.0106) than that for the contralateral OI muscle for both the HJ and SJ. The OI muscle activity, however, did not differ between the hard and soft jellies.(ABSTRACT TRUNCATED AT 400 WORDS)

Short electromyographic bursts in the rabbit digastric muscle during the jaw-closing phase

Masseter and digastric muscle activities and jaw movement trajectories were recorded in freely moving rabbits during eating. The patterns in these trajectories and activities were similar to those described in previous studies on restrained animals. Although the duration of masticatory sequences, which started with food intake followed by grinding movements and ended by swallowing, varied, the total number of chewing cycles in a chain of masticatory sequences was consistent (1043 +/- 51, mean +/- SD; n = 5, for chow pellets) among the animals tested. When animals ate hard foods, extra bursts in the digastric electromyograms occurred frequently in the jaw-closing phase. The digastric activities were rather short (6.1 +/- 1.0 ms; n = 100) and the amplitude of these digastric short bursts (DSBs) was much larger (1.69 +/- 0.81 mV; n = 100) than in the opening phase (0.56 +/- 0.33 mV; n = 100), which actually depressed the jaw. When a soft food (bread) was tested, this activity was not observed. The proportion of occurrences of the DSB in a chewing cycle was high at the slow-closing phase, indicating that the DSBs were due to tooth contacts during food crushing. Of 1035 chewing cycles examined in the five animals, 124 were associated with a DSB and 415 cycles with a masseter inhibitory period (MIP). The proportion of the occurrences of the MIP was significantly larger than that of the DSBs. Of 124 DSBs, 85 (68.5 per cent) coincided with an MIP. Four were not associated with clear MIPs, although there was masseter activity at the time of the DSBs. The other 35 DSBs were out of phase with the masseter bursts, although still in a closing phase. The durations of the MIPs accompanied by a DSB were significantly longer than those not so associated. The DSB may be a reflex response mediated by periodontal mechanoreceptors when the upper and lower teeth come together while chewing hard food. The reflex arc for the DSB may be independent of that for the MIP, and the threshold for the DSB may be higher.

Mastication and its control by the brain stem

This review describes the patterns of mandibular movements that make up the whole sequence from ingestion to swallowing food, including the basic types of cycles and their phases. The roles of epithelial, periodontal, articular, and muscular receptors in the control of the movements are discussed. This is followed by a summary of our knowledge of the brain stem neurons that generate the basic pattern of mastication. It is suggested that the production of the rhythm, and of the opener and closer motoneuron bursts, are independent processes that are carried out by different groups of cells. After commenting on the relevant properties of the trigeminal and hypoglossal motoneurons, and of internuerons on the cortico-bulbar and reflex pathways, the way in which the pattern generating neurons modify sensory feedback is discussed.

Reflex excitation of masticatory muscles induced by algesic chemicals applied to the temporomandibular joint of the cat

Algesic chemicals (7 per cent NaCl, KCl and histamine) applied to this joint of anaesthetized cats evoked reflexes in the ipsilateral anterior digastric, temporalis and genioglossus muscles. Whereas the application of isotonic saline was only briefly and weakly effective, and only consistently so in the genioglossus, a single application of each chemical could produce a sustained increase in electromyographic activity of all the muscles. The excitatory effects usually lasted 30 s or more, and the onset latency and latency to peak activity were usually less than 10 s and 20 s, respectively. These reflex excitatory effects and their temporal characteristics are consistent with recent findings of the effects of these algesic chemicals on trigeminal brainstem nociceptive neurones, and provide support for concepts of temporomandibular joint pain and dysfunction that are based on reflexly induced increases in masticatory muscle activity.

The effect of fentanyl on the lick/chew response in rabbits

Tooth pulp stimulation in the rabbit produces a complex rhythmic lick/chew response (LCR). Because of the effect of fentanyl on the afferent and efferent components of oral function we examined its effect on this response. Intravenous fentanyl attenuates the LCR in a dose dependent manner with the ED50 occurring at 0.1 mg/kg. Concurrently a different response to tooth stimulation appears consisting of a slow, non-rhythmic lateral jaw movement (LJR). The effects of fentanyl on both LCR and LJR are blocked by naloxone (0.1 mg/kg). The dopamine antagonist, chlorpromazine (10 mg/kg iv) also has the same qualitative effect as fentanyl on both responses.

Crossed and uncrossed central effects of muscle spindle afferents from the lateral pterygoid muscle of the guinea pig

Physiological evidence is presented for the presence of stretch reflexes in the lateral pterygoid (Pt) muscle of the guinea pig. The central reflex effects of excitation of Pt stretch reflex afferents were also investigated. Passive lateral jaw displacement, which resulted in stretch of the Pt muscle on the side of jaw movement and stretch of the zygomatico-mandibularis (Z) muscle on the side contralateral to the movement, evoked increased EMG activity in these muscles. Stimulation of the trigeminal mesencephalic nucleus (mes V) evoked monosynaptic reflexes in both the Pt and Z nerves. Tonic stretch of the Pt muscle facilitated the monosynaptic reflex in the Pt nerve evoked by stimulation of mes V. Tonic vibration of the Pt muscle facilitated the mes V evoked monosynaptic reflex in the nerves to the ipsilateral Pt and contralateral Z muscles. Conversely, tonic vibration of the Z muscle facilitated the monosynaptic reflex evolved by mes V stimulation in the contralateral Pt and ipsilateral Z nerve. The results support the view that muscle spindles exist in the Pt and Z muscles and that there is a monosynaptic stretch reflex for both the Pt and Z muscles with cell bodies located in the mes V nucleus. It was also shown that the ipsilateral Pt muscle and the contralateral Z muscle act as synergists in the production of lateral jaw movements and that the organization of the stretch reflexes originating from the Pt and Z muscles support their synergistic action.

Phasic and rhythmic responses of the oral musculature to mechanical stimulation of the rat palate

Activity of the anterior and posterior digastric muscles, the intrinsic tongue muscles and the masseter muscles was recorded electromyographically. Muscle-activity patterns depended on the site of stimulation, the strength of the stimulus and its duration. Five types of reflex patterns were distinguished: type A: on-response in the digastric muscles (latency about 7 ms) and in the intrinsic tongue muscles (latency about 15 ms); type B: on-off response in the digastric and intrinsic tongue muscles; type C: on-response followed by long-lasting low-level activity in the digastric and intrinsic tongue muscles; type D: on-off response and long-lasting low-level activity in the digastric and tongue muscles; type E: rhythmic activity in all muscles under investigation preceded by one of the other reflex types. In the type-E response, the frequency of the bursts was about 4.3 Hz; the masseter bursts were not in phase with those of the digastric muscles; the intrinsic tongue muscles showed constant activity with superimposed rhythmic bursts. By increasing the stimulus strength or the stimulus duration, there was a gradual shift from simple, phasic, reflex responses to more complex reflex patterns. This holds for all palatal sites. However, rhythmic activity was most easily elicited in the region of the incisive papilla, less easily in the antemolar region; no rhythmic activity was found in the intermolar region. Thus stimulus duration is a crucial factor in eliciting rhythmic oral activity in the rat.

Jaw movement-related activity and reflexly induced changes in the lateral pterygoid muscle of the monkey Macaca fascicularis

The muscle was exposed in 12 lightly anaesthetized monkeys so that electromyographic recordings could be made from the superior and inferior heads. The separately recorded activity in each head was initially examined in relation to activity recorded simultaneously in other jaw muscles or to jaw movements produced actively by the animals or passively induced by the experimenter. Subsequently, the reflex effects of stimuli applied to various oral-facial sites were examined. In general, the superior head was active in association with jaw-closing muscle activity and related movements produced by the animal whereas the inferior head was mainly active in jaw-opening. Both heads increased their activity with passively-induced jaw-opening movements, especially if the jaw-opening was combined with a horizontal deviation of the mandible. Both heads showed reflexly-induced excitation and silent periods as a result of stimuli applied to oral-facial sites, the most effective sites being palatal, lingual, labial and buccal mucosa and the teeth, stimulation of which, as well as stimuli applied to mandibular joint and cutaneous afferents, could also reflexly induce silent periods in both heads. The various effects may be related to mechanisms for protection and stabilization of the mandibular joint and masticatory muscles.