Masticatory Sequence Research Articles

An investigation of the coupling between respiration, mastication, and swallowing in the awake rabbit.

1. The interrelationships between respiration, mastication, and swallowing were studied in awake rabbits previously prepared for the chronic recording of jaw movements and the electromyographic activity of jaw and throat muscles. These signals were recorded before, during, and after the mastication of rabbit chow, together with respiratory movements. 2. The onset of activity in the thyrohyoid muscle was used as a marker for swallowing. Measurements were made of cycle duration, and the phase relationship between the masticatory and respiratory rhythms was calculated. Deviations in masticatory and respiratory movements during swallowing were detected; the phases of the masticatory and respiratory cycles in which the deviations occurred were determined, and the intervals between the deviations and the swallowing marker were calculated. 3. Two characteristic swallowing patterns were observed that were called Interposed and Terminal swallows. Interposed swallows occurred within a masticatory sequence, while Terminal swallows ended the sequence. 4. In most rabbits, respiratory rate slowed during mastication. This brought the average frequency of the two movements closer together, and there was a weak but significant correlation between the masticatory and respiratory rates. However, respiration and mastication were not linked on a cycle-by-cycle basis, and the change in respiratory rate was found to be unrelated to masticatory rate. Further, there was no tendency for the masticatory and respiratory rhythms to adopt any particular phase relationship as swallowing was approached. Some weak and transient phase coupling of the two systems occurred after swallowing. 5. During swallowing, respiration was most often inhibited soon after the start of the inspiratory phase. Swallows within a masticatory sequence occurred most frequently during the early opening phase of the masticatory cycle, and swallows after the end of a sequence occurred with the jaw in the resting position. There was a strong tendency for the durations of the masticatory and respiratory cycles containing deviations to be longer than preceding control cycles, and the effect on respiration lasted several cycles. 6. For both Interposed and Terminal swallows, a tight temporal relationship was observed between deviations in respiration and the swallowing marker: most deviations tended to occur before swallowing. In contrast, the time of deviations in mastication relative to the swallowing marker depended on swallow type and was more variable. Finally, there was no link between the start of the pauses in the two rhythms. 7. We conclude that respiratory and masticatory processes are not strongly coupled.(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 in the rabbit: a description of movement and muscle activity.

1. Mastication was studied in awake rabbits that had previously been prepared for chronic recording of jaw movement and jaw muscle electromyography (EMG) under general anesthesia. Data were stored on tape and replayed for computer-assisted analysis. Most data was taken from the mastication of rabbit chow, but the basic features described in the paper also apply to eating of other foods. The series of movements was divided into cycles, and the phases that compose them, using peaks in the vertical movement, velocity, and acceleration. 2. The whole series of movements from ingestion to swallowing was termed the masticatory sequence. We found, as have others, that the sequence can be divided into three consecutive periods, based on the form of the movements. These were named the preparatory, reduction, and preswallowing series of cycles. 3. The results of earlier studies suggest that the food is transported back to the molar teeth during the preparatory series, ground up during the reduction series, and that the bolus is formed for swallowing during the preswallowing series. 4. The typical cycle of the preparatory series was called type I; it had two phases: opening (O) and fast closing (FC). The jaw-opening muscles were very active during O, but EMG bursts from the closer muscles were small or undefinable during FC. There was not much movement away from the midline in either phase, and the cycles were the shortest in the sequence. 5. The reduction series was predominantly composed of type II cycles. These were of intermediate duration and had three phases [O, a short FC, and a slow closing phase (SC)], during which the pellets were crushed between the molar teeth. The jaw moved toward the working side during FC. At or soon after the start of SC, the closer muscles became very active and continued to contract as the teeth were drawn toward the midline and slightly backwards. 6. The preswallowing series was made up of five-phase cycles that we called type III. These were the longest of the sequence. Jaw opening now occurred in three stages: O1 was the fall from tooth contact to approximately the postural position, O2 was a pause, and O3 was defined as the final movement to maximum opening. The digastric burst occurred during O3. There were again two closing phases, FC and SC, but closer bursts were smaller than in type II, and SC was shorter.(ABSTRACT TRUNCATED AT 400 WORDS)

Modifications of masticatory behavior after trigeminal deafferentation in the rabbit.

Bilateral trigeminal deafferentation was performed in the rabbit in order to assess the role of orofacial inputs in regulation of the pattern of jaw movements during chewing. After bilateral combined section of the maxillary and inferior alveolar nerves, the animals did not eat food by themselves in the first postoperative week. However, they could chew and swallow when food was inserted into the mouth by an experimenter. The pattern of jaw movements and associated EMG activities of masticatory muscles during chewing were modulated remarkably by deafferentation. These modifications include 1) decrease in the horizontal excursions of the mandible at the power phase, 2) decrease in the maximum gape, 3) insufficient occlusion at the power phase (or increase in the minimum gape), 4) irregular patterns of jaw movements, 5) facilitation of the chewing rate, 6) increase in the number of chewing cycles in a masticatory sequence (the process from acceptance of food to swallowing), and 7) decrease in jaw-closing muscle activities. The findings indicate that deafferentation of the trigeminal sensory branches reduced masticatory force. On the other hand, no significant change was seen in the animals with disruption of cutaneous sensations of the face due to bilateral section of the infraorbital and mental nerves. Intraoral sensations rather than extraoral sensations may thus be important for regulation of masticatory force and jaw movements during chewing. Jaw movements during chewing were also analyzed in the animals with either bilateral ablation of the cortical masticatory area (CMA) or bilateral lesion of the ventral posteromedial nucleus (VPM) of the thalamus in order to examine whether profound effects of trigeminal deafferentation are produced via the transcortical loop. The animals with lesion of either the CMA or VPM demonstrated disturbances in feeding behavior, including the dropping of ingested food from the mouth, elongation of a masticatory process, reduction in the chewing efficiency, etc. However, the pattern of jaw movements during chewing were essentially similar to that in the preoperative period. These results do not necessarily deny a contribution of the CMA to regulation of jaw movements but suggest that the transcortical feedback loop via the CMA and thalamic VPM nucleus would not primarily be responsible for pattern formation of jaw movements during chewing in the rabbit. Probably, the sensory feedback via the transcortical loop may indirectly facilitate activities of the brain stem CPG, which facilitates the chewing rhythm or enables masticatory sequences to be conducted smoothly.

Single-unit activity in bulbar reticular formation during food ingestion in chronic cats.

1. Single-unit activity was recorded from 215 neurons in the medial bulbar reticular formation during the masticatory sequence, from intake to deglutition, of 3 kinds of food (cat food pellets, canned fish, and milk) in 8 chronically prepared, unanesthetized, spontaneously respiring cats with their head fixed to a stereotaxic apparatus without pain or pressure. The firing patterns were compared to the simultaneously recorded EMGs of the jaw-closing and -opening muscles and to the jaw movement. 2. Fifty neurons changed their firing patterns during mastication. Nine neurons increased and one neuron decreased or stopped firing coincident with the masticatory sequence without an apparent rhythmical modulation of frequency corresponding with the masticatory rhythm (nonphasic group). The firing pattern of the remaining 40 neurons was modulated in phase with jaw movement (phasic group); 34 neurons either showed a spike burst or attained the highest firing frequency during the jaw-opening phase (opening type), while 6 neurons did so during the jaw-closing phase (closing type). The firing patterns of each neuron were essentially the same regardless of the kind of food ingested, except for 2 opening-type neurons that showed a rhythmical burst during mastication of solid food and tonic activity during lapping milk. 3. For 16 phasic neurons, there were significant correlations between some aspects of the firing pattern and a parameter of the movement during ingestion of solid food and/or milk. With one exception, these relationships did not appear to be due to sensory feedback. 4. We detected a monosynaptic excitatory projection from 3 opening-type neurons to the anterior digastric motoneurons, and monosynaptic inhibitory projections to the temporal or masseter motoneurons from 3 other opening-type neurons, by spike-triggered averaging of the full-wave rectified EMG of the jaw-closing and -opening muscles. No monosynaptic projections from the closing-type neurons or nonphasic group neurons to either jaw-opener or -closer motoneurons were detected. 5. The instantaneous firing frequency of all 3 inhibitor premotor neurons was positively correlated with the opening velocity, and the firing of 2 was also related to the jaw displacement.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of pain from the mandibular joint and muscles on masticatory motor behaviour in man

Habitual chewing of a coherent bolus was studied in 12 dentate subjects with painful mandibular-joint disorders and 12 healthy, dentate controls. Bilateral electromyograms of jaw elevators, and jaw movement, were recorded for three complete masticatory sequences. Computer analysis was used to classify chewing movements as continuous or discontinuous. Root-mean-square (r.m.s.), myoelectric signal amplitudes were computed for each of four jaw elevators. Although discontinuous chewing cycles were significantly more frequent in painful function ( p = 0.001), they also occurred in pain-free function, a finding which reduces their diagnostic significance. During painless and painful function, r.m.s. activities did not differ statistically when elevators acted as agonists on both the dominant and non-dominant chewing side ( p > 0.1). When used as antagonists, such as during jaw opening, the elevators had greater mean peak activities during painful than painless function ( p = 0.0001). Variability in maximum gape was greater during painful than painless function ( p = 0.001), but peak maximum gapes in complete masticatory sequences were not affected by pain, and neither were minimum interocclusal gapes. More frequent reshaping and repositioning of the bolus in the presence of pain could explain these differences between painful and pain-free function.

Changes in masticatory movement parameters within the chewing period in young dentate persons and patients rehabilitated with bridges supported by implants in the mandible.

Experiments were carried out to study changes in masticatory movements in the chewing sequence of young dentate persons and complete denture wearers rehabilitated with bridges on osseo-integrated implants (OIB) in the lower jaw. Two test foods were used in the dentate group (apple, bread) and bread in the OIB group. A computer-based opto-electronic system (Selspot) was employed to record and analyse all single chewing cycles, regarding parameters reflecting, chewing rhythm, mandibular velocity and displacement. The different parameters were tested both intra- and inter-individually versus the series of the single and overall chewing cycles in the masticatory sequences respectively. The results indicated that different subjects performed different patterns of mastication within the chewing period when chewing the same type of food. Different test foods (texture, size) also seemed to affect the chewing pattern in a different way, e.g. mandibular displacement showed an obvious reduction when chewing apple, probably due to the bigger size of the bolus. The inter- and intra-individual analysis showed in some parameters discrepancies implying problems in analysing the chewing pattern within the chewing sequence without using both methods.

Mastication in the musk shrew, Suncus murinus (mammalia, Soricidae).

High-speed cinematography shows that Suncus murinus (Crocidurinae) masticates fast (mean 5.5, 5-10 masticatory cycles per sec). Their grasping behavior is not stereotyped. The unilateral mandibular movements combine vertical, anteroposterior, and lateral displacements; and any masticatory sequence may include crushing, repositioning, shearing, and grinding components. Size and consistency of food influence the duration of individual chewing cycles. As food is transferred to the new working side, the chewing direction reverses, either near maximum closure or near maximum opening. An unfused mandibular symphysis permits tilting movements of the two halves of the mandible. Food may be squeezed between the lower incisors. The working side tilts outward during closing; this may improve shearing or grinding action. The closing phase is posteriorly directed. Thus, the masticatory movements of these shrews differ from those that have been described in many other mammals.

Computer-based assessment of habitual mastication

A computer-based system to assess mandibular movements and EMG masseter and temporal muscle activity during habitual mastication was applied to 12 healthy subjects and the chewing performance was tested with a standardized bolus of crisp bread. A Siemens Sirognatograph instrument was used to record mandibular movements while EMG signals were recorded with surface silver-silver electrodes. Software provided data of single chewing cycles in space and versus time, of mean mandibular displacement during opening and closing, mean velocity, and EMG activity. Clockwise and counterclockwise cycles were computed separately. The data obtained allow the following conclusions: Habitual mastication is a rhythmic event with different envelopes of motion that alternate characteristically in each subject. Each chewing cycle accomplishes several tasks but, depending on its envelope, is probably more oriented to a specific performance. A mean chewing cycle can be constructed for each individual. Its pattern depends on the distribution of different envelopes of motion that alternate during the masticatory sequence. During closure, a prevalently isotonic and a prevalently isometric phase of muscle contraction can be distinguished. Mean movement and EMG data for the whole group are not distributed at random, but according to a typical pattern. The system seems to be finalized to a well-balanced function and distribution of loads.

Relationship between jaw movement and food breakdown in human mastication.

Different quantities and sizes of peanuts were offered to six human subjects for mastication at two or three experimental sessions during which jaw movements and chewing cycle duration were measured. The amplitude of vertical movement and cycle duration depended on the position of a chew in a sequence of chews (masticatory sequence). Cycle duration also depended on the position of a given masticatory sequence during the first experimental session, the earlier sequences tending to be longer. Lateral jaw movements fluctuated about a reasonably constant value during a sequence and were unaffected by different food inputs. The amplitude of vertical movements increased markedly with an increase in food weight but was unaffected by change in the initial food particle size. These results were analyzed in the light of previous reports on the comminution of peanuts. It is hypothesized that observed jaw movements were unrelated to food particle size but that the amplitude of each vertical movement depended on the volume of food that was broken by the teeth during the next closing stroke. This food volume was provided and selected by bulk movements of the tongue during the preceding jaw opening.

A comparative electromyographic and kinesiographic study of deliberate and habitual mastication in man

Twelve healthy, fully-dentate subjects participated in experiments which included the continuous recording of surface electromyography and jaw movement during habitual and deliberate right-sided or left-sided chewing of a coherent bolus. Analogue data streams were converted to digital values. Root-mean-square (r.m.s.) muscle-activity traces were computed from raw electromyographic data. The working side was defined as the side from which the mandible approached the position of occlusal stoppage when in the most cranially directed part of the chewing cycle. In any given muscle, greater mean peak r.m.s. activities were found with ipsilateral than contralateral bolus replacement ( p < 0.01, s); such differences were more pronounced for the masseter than the anterior temporal muscles. During habitual chewing, mean peak r.m.s. activities exceeded the value established by deliberate mastication with ipsilateral bolus placement in 27 of 48 muscles; this may be because of more vigorous chewing during habitual performance. No subject was strictly unilateral in their preference for bolus placement and in 6 of the 12 subjects, there was a timed side-switching of the bolus within the masticatory sequence. The results also indicated that any averaging of data based upon time-amplitude alone would be inappropriate for habitual chewing because of the call for different working sides within a particular masticatory sequence. Thus a new data format based upon numerical representation of the electromyographic activity against time was introduced.

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.