Mastication and its control by the brain stem

Masticatory motion is controlled in humans by a limited set of muscle synergies

The masticatory motion, whereby food introduced into the mouth is processed into a bolus suitable for swallowing, can be divided into successive masticatory cycles, each comprising downward and subsequent upward movements of the mandible. The present study deals with the problem of the existence of muscle synergies in mastication, that is whether some of the muscles involved in mastication receive common motor drives, rather than controlled individually. Evidence for muscle synergy during mastication is scarce, partly due to the difficulties in simultaneous recording of the electromyographic (EMG) activities from all the muscles involved. Thus, we analyzed the variability of the mandibular motion during mastication rather than to examine the EMG patterns, based on the hypothesis that a motion elicited by a limited set of muscle synergies can be approximated as a superposition of the same number of independent motions. Mandibular motion paths were recorded from 8 healthy males (25-31 years), who chewed gum or gummy candy. A morphometric technique, which describes the shape of a closed curve by using normalized elliptic Fourier descriptors and reduces the variance of the shape by using principal component analysis, was applied to analyze the variability of the mandibular motion paths. We found three independent variations of the motion paths, whose linear combinations accounted for an average of 93% (range, 88-96%) of the total variance. The extracted variations were similar among the subjects. These findings provide indirect evidence for the existence of a limited set of muscle synergies for mastication in humans.

Threshold for Detection of Incisal Forces Is Increased by Jaw Movement

Current knowledge regarding the sensitivity of the teeth to forces is based on psychophysical experiments that measured touch detection thresholds under static jaw conditions. It is not known whether jaw movements alter the perception of forces applied to the teeth, but, based on limb movement studies, it is hypothesized that the perception of mechanoreceptor outputs will be downwardly modulated by jaw movements. We predicted that, compared with static jaw conditions, rhythmic jaw movements would be associated with significantly higher psychophysical thresholds for the detection of incisally applied forces. In eight participants, mechanical pulses were delivered to an incisor during static jaw holding or during cyclic jaw opening and closing. Analogous to findings in human limbs, the psychophysical salience of periodontal mechanoreceptor feedback was downwardly modulated by physiologically relevant movements; detection thresholds for mechanical pulses applied to a central incisor were significantly higher during jaw-closing movements than during static jaw positioning.

The influence of the occlusal vertical dimension on masticatory muscle activities and hyoid bone position in complete denture wearers

PURPOSE

Long lasting usage of complete dentures causes lower occlusal vertical dimension, producing potential detrimental consequences. The aim of this study was to investigate how changes in vertical dimension during denture exchange affect muscular activity and hyoid bone position.

MATERIAL AND METHODS

Twenty-five edentulous, otherwise healthy patients (14 females, 11 males) aged 70.5 +/- 9 years, wearing their dentures over 5 years but no longer than 15 years (mean 9.8 +/- 5.2), were studied. New dentures were fabricated and the occlusal vertical dimension was recorded on cephalometric radiographs. Cephalograms were analyzed according to Ricketts. The relation of the hyoid bone position to the mandible was estimated. An evaluation of muscular activity was performed using the Biopak Electromyography Recording System synchronized with the T-Scan II Occlusal Analysis System.

RESULTS

The occlusal vertical dimension was higher with the new dentures compared with the old dentures. The transition to new dentures was accompanied by a change of the vertical position of the hyoid bone. Digastric muscle activity was lower with the new dentures in comparison with the old dentures.

CONCLUSIONS

Increase of the occlusal vertical dimension in complete denture wearers affects the hyoid bone position and masticatory muscle activity.

Influence of age on mastication: effects on eating behaviour

The present review covers current knowledge about the ageing of oral physiology related to mastication and its effects on eating behaviour. Mastication is the first process undergone by a food during feeding. It has a key role in the maintenance of nutritional status in two respects. First, the perceptions of food's sensory properties elicited during chewing and swallowing are one of the major determinants of the pleasure which drives us to eat; second, the properties of the swallowed bolus are affected by oral conditions and this may modulate the subsequent phases of digestion. Ageing in healthy dentate subjects induces moderate changes in oral physiology. Changes in neuromuscular activity are partly compensated by changes in chewing behaviour. No clear age effect is seen in texture perception, although this does impact on food bolus properties. In contrast, great alterations in both chewing behaviour and food bolus properties are observed when ageing is associated with a compromised dentition, general health alterations and drug intake. Eating behaviour is far more complex than just chewing behaviour and the concerns of the elderly about food cannot be explained solely by oral physiology. Discrepancies are often noticed with older subjects between various objective measurements of oral performance and corresponding measures of self-perception. In addition, although more foods are recognised as hard to chew with increasing age, there is no clear shift in preference towards food that is easy to chew. Food choices and food consumption are also driven by memory, psychology and economic factors. Advances in the understanding of food choice in the elderly need a sustained collaborative research effort between sensory physiologists, nutritionists, and food scientists.

Indications for jaw gape-related control of relative muscle activation in sequent chewing strokes

Jaw muscle activity ratios in unilateral isometric biting differ from ratios of unilateral chewing but approach the latter if the jaw gape in biting is made as small as the minimum interocclusal distance (MID) of chewing. Especially, the masseter working/balancing side ratio (W/B-ratio) becomes as asymmetric as in chewing, because of reduction in balancing side (BS) masseter activity. This behaviour of ratios might reflect a 'chewing-specific' motor strategy induced when isometric biting is performed with a 'chewing-like' gape. If this hypothesis applies, activity ratios should be associated with MIDs of sequent chewing strokes in a similar manner as with incremented jaw gapes in isometric biting. To test this prediction, bilateral surface electromyograms of masseter and anterior temporalis muscles and incisor movements were recorded during unilateral chewing in 52 subjects. W/B-ratios of masseter and temporalis activities and temporalis/masseter-ratios on both sides were calculated. The ratios were related to MIDs of consecutive chewing cycles. Three of the four ratios were associated with masticatory MID in the same manner as with jaw gape in isometric biting. In particular with decreasing MID, the masseter W/B-ratio increased from 1.5 to 2.2 (P < 0.01). This increase in asymmetry was attributed to a stronger decrease in masseter activity on the BS than on the working side. We conclude that relative jaw muscle activation is associated with interocclusal distance in a similar way in isometric biting and in chewing. This analogy supports the idea of a common jaw gape-related neuromuscular strategy facilitated by afferent signalling of interocclusal distance.

Molarless condition suppresses proliferation but not differentiation rates into neurons in the rat dentate gyrus

The dentate gyrus (DG) of the hippocampal complex is one of the few areas of the rodent brain where neurogenesis continues throughout adulthood. We investigated the effects of the molarless condition on cell proliferation, rate of differentiation into neurons in the subgranular zone of the DG, and plasma corticosterone levels. The molarless condition decreased cell proliferation in the DG and increased plasma corticosterone levels. Approximately 80% of newly generated cells differentiated into neurons and the remaining 20% of the cells differentiated into astrocytes. These ratios were not significantly different between control and molarless rats. In conclusion, the rates of neurogenesis and gliogenesis in the DG are suppressed by the molarless condition, and this suppression might be associated with the increased corticosteroid levels in molarless subjects.

Stomatognathic adaptive motor syndrome is the correct diagnosis for temporomandibular disorders

Temporomandibular disorder is a generic and inadequate conception to be used as a diagnosis. It fails to express the etiology or the pathophysiology and it is mainly associated with the anatomical site. Moreover, the clinical condition presents a mandibular motor problem and not a joint problem. The hypothesis presents the new diagnosis stomatognathic motor adaptive syndrome, which comprehend a motor response and the adaptive processes it induces. Inadequate occlusal contacts cause the mandible to shift in order to reach an ideal intercuspal position. The condylar displacements are proportional to such movements. Temporomandibular joint (TMJ) receptors respond to the capsular mechanical stress and the information reaches the trigeminal sensory nuclei. The mandibular modified position seems to be relevant information and may interfere with catecholaminergic neurotransmission in basal ganglia. The main motor responses comprise increased jaw muscle tone, decreased velocity of movements and incoordination. The overload of muscle function will produce adaptive responses on many stomatognathic structures. The muscle adaptive responses are hypertonia, pain, fatigue and weakness. Temporomandibular joint presents tissue modification, disc alteration and cracking noise. Periodontium show increased periodontal membrane, bone height loss and gingival recession. Teeth manifest increased wear facets, abfraction and non-accidental fractures. The periodontal and teeth adaptive processes are usually identified as occlusal trauma. The altered stomatognathic functions will show loss of velocity during mastication and speech. Fatigue, weakness in jaw muscle and difficulties to chew hard food are related to hypertonia. Incoordination between stomatognathic muscles groups is found, causing involuntary tongue/cheek biting and lateral jaw movements on speech. Otologic complaints, as aural fullness and tinnitus, are related to the tensor tympani muscle, innervated by the trigeminal nerve.

The development of jaw motion for mastication

The developmental course of early chewing has rarely been studied, although such knowledge is essential for understanding childhood feeding and swallowing disorders. The goal of this investigation was to quantitatively describe age- and consistency-related changes in jaw kinematics during early chewing development. An optical-motion tracking system was used to record jaw movements during chewing in 3-dimensions in 11 typically-developing participants longitudinally from 9-30 months of age. Age related changes in jaw movement were described for both puree and regular consistencies. The findings demonstrated that the development of rotary jaw motion, jaw motion speed, and management of consistency upgrades are protracted across the first two years of life. Young children did not differentiate their jaw closing speeds for puree and regular consistencies until 18-24 months of age, at which age the speed of movement was significantly slower for the puree than for the regular consistency. Horizontal jaw closing speed decreased significantly with age for the puree consistency, but not for the regular consistency. The emergence of a rotary chew pattern was not observed at the ages studied.

Brain stem control of the phases of swallowing

The phases of swallowing are controlled by central pattern-generating circuitry of the brain stem and peripheral reflexes. The oral, pharyngeal, and esophageal phases of swallowing are independent of each other. Although central pattern generators of the brain stem control the timing of these phases, the peripheral manifestation of these phases depends on sensory feedback through reflexes of the pharynx and esophagus. The dependence of the esophageal phase of swallowing on peripheral feedback explains its absence during failed swallows. Reflexes that initiate the pharyngeal phase of swallowing also inhibit the esophageal phase which ensures the appropriate timing of its occurrence to provide efficient bolus transport and which prevents the occurrence of multiple esophageal peristaltic events. These inhibitory reflexes are probably partly responsible for deglutitive inhibition. Three separate sets of brain stem nuclei mediate the oral, pharyngeal, and esophageal phases of swallowing. The trigeminal nucleus and reticular formation probably contain the oral phase pattern-generating neural circuitry. The nucleus tractus solitarius (NTS) probably contains the second-order sensory neurons as well as the pattern-generating circuitry of both the pharyngeal and esophageal phases of swallowing, whereas the nucleus ambiguus and dorsal motor nucleus contain the motor neurons of the pharyngeal and esophageal phases of swallowing. The ventromedial nucleus of the NTS may govern the coupling of the pharyngeal phase to the esophageal phase of swallowing.

Effects of a liquid diet on temporomandibular joint mechano-receptors

Load during mastication is an important factor for the development and maintenance of mechano- receptor properties. The purpose of this study was to examine property changes in the rat TMJ mechano-receptors under conditions of liquid diet feeding and low articular load during the growth period. The hypothesis was that alterations in mastication of liquid diet might increase TMJ mechano-receptor sensitivity. Sixty-six two-week-old male Wistar rats were divided into two groups: a control group that was fed on whole pellets, and an experimental group that was fed a liquid diet. Electrophysiological recordings from the TMJ units were obtained from the trigeminal ganglion when the rats were 5, 7, and 9 weeks old. In the experimental group, TMJ mechanoreceptor sensitivity increased, because the firing threshold gradually decreased and the maximum instantaneous frequency gradually increased. In conclusion, functional properties of TMJ mechano-receptors under low articular loading conditions cannot mature normally within the growth period.

Asymmetric control of inspiratory and expiratory phases by excitability in the respiratory network of neonatal mice in vitro

Rhythmic motor behaviours consist of alternating movements, e.g. swing-stance in stepping, jaw opening and closing during chewing, and inspiration-expiration in breathing, which must be labile in frequency, and in some cases, in the duration of individual phases, to adjust to physiological demands. These movements are the expression of underlying neural circuits whose organization governs the properties of the motor behaviour. To determine if the ability to operate over a broad range of frequencies in respiration is expressed in the rhythm generator, we isolated the kernel of essential respiratory circuits using rhythmically active in vitro slices from neonatal mice. We show respiratory motor output in these slices at very low frequencies (0.008 Hz), well below the typical frequency in vitro (approximately 0.2 Hz) and in most intact normothermic mammals. Across this broad range of frequencies, inspiratory motor output bursts remained remarkably constant in pattern, i.e. duration, peak amplitude and area. The change in frequency was instead attributable to increased interburst interval, and was largely unaffected by removal of fast inhibitory transmission. Modulation of the frequency was primarily achieved by manipulating extracellular potassium, which significantly affects neuronal excitability. When excitability was lowered to slow down, or in some cases stop, spontaneous rhythm, brief stimulation of the respiratory network with a glutamatergic agonist could evoke (rhythmic) motor output. In slices with slow (<0.02 Hz) spontaneous rhythms, evoked motor output could follow a spontaneous burst at short (<or=1 s) or long (approximately 60 s) intervals. The intensity or timing of stimulation determined the latency to the first evoked burst, with no evidence for a refractory period greater than approximately 1 s, even with interburst intervals >60 s. We observed during inspiration a large magnitude (approximately 0.6 nA) outward current generated by Na(+)/K(+) ATPase that deactivated in 25-100 ms and thus could contribute to burst termination and the latency of evoked bursts but is unlikely to control the interburst interval. We propose that the respiratory network functions over a broad range of frequencies by engaging distinct mechanisms from those controlling inspiratory duration and pattern that specifically govern the interburst interval.

Mastication-induced modulation of the jaw-opening reflex during different periods of mastication in awake rabbits

The present study aimed to determine if sensory inputs from the intraoral mechanoreceptors similarly contributed to regulating the activity of the jaw-opening muscles throughout the masticatory sequence. We also aimed to determine if sensory inputs from the chewing and non-chewing sides equally regulated the activity of the jaw-opening muscles. Electromyographic (EMG) activities of jaw muscles (digastric and masseter) and jaw movements were recorded in awake rabbits. The entire masticatory sequence was divided into preparatory, rhythmic-chewing and preswallow periods, based on jaw muscles activity and jaw movements. The jaw-opening reflex (JOR) was evoked by unilateral low-intensity stimulation of the inferior alveolar nerve (IAN) on either the chewing or non-chewing side. Amplitude of the JOR was assessed by measuring peak-to-peak EMG activity in the digastric muscle, and was compared among the masticatory periods and between the chewing and non-chewing sides. The JOR was strongly suppressed during the jaw-closing phase in the rhythmic-chewing and preswallow periods, but this effect was transiently attenuated during the late part of the jaw-opening phase in these periods. However, modulation of the JOR varied from strong suppression to weak facilitation during the preparatory period. The patterns of JOR modulation were similar on the chewing and non-chewing sides in all masticatory periods. The results suggest that the sensory inputs from the intraoral mechanoreceptors regulate the activity of the jaw-opening muscles differently during the preparatory period compared with the other masticatory periods. Sensory inputs from both the chewing and non-chewing sides similarly regulate the activity of the jaw-opening muscles.

Babbling, chewing, and sucking: oromandibular coordination at 9 months

PURPOSE

The ontogeny of mandibular control is important for understanding the general neurophysiologic development for speech and alimentary behaviors. Prior investigations suggest that mandibular control is organized distinctively across speech and nonspeech tasks in 15-month-olds and adults and that, with development, these extant forms of motor control primarily undergo refinement and rescaling. The present investigation was designed to evaluate whether these coordinative infrastructures for alimentary behaviors and speech are evident during the earliest period of their co-occurrence.

METHOD

Electromyographic (EMG) signals were obtained from the mandibular muscle groups of 15 typically developing 9-month-old children during sucking, chewing, and speech.

RESULTS

Unlike prior investigations of 12- and 15-month-olds and adults, 9-month-olds' analyses of peak correlations among agonist and antagonist comparisons of mandibular EMG data revealed weak coupling during sucking, chewing, and babble; associated lag values for antagonist muscle groups indicated greater synchrony during alimentary behaviors and less synchrony during babble. Unlike the speech data of 15-month-olds, 9-month-olds exhibited consistent results across speech subtasks.

CONCLUSION

These findings were consistent with previous results in which mandibular coordination across behaviors was more variable for younger age groups, whereas the essential organization of each behavior closely reflected that seen in older infants and adults.

Jaw elevator silent periods in complete denture wearers and dentate individuals

Functional meaning and underlying mechanisms of jaw elevator silent period (SP) have still not been completely understood. Since complete denture wearers (CDWs) have no periodontal receptors in their jaws, the aim was to examine SPs in CDWs and to compare it with dentate individuals (DIs). Thirty six DIs (skeletal/occlusal Class I) and 24 eugnath CDWs participated. EMG signals were registered using the EMGA-1 apparatus from the left and the right side anterior temporalis (ATM) and masseter muscles (MM). Ten registrations of an open-close-clench (OCC) cycle were obtained for each individual. DIs had the average latency between 12.5 and 12.9 ms and always one single short inhibitory pause (IP) with complete inhibition of motoneurons (20.1-21.1 ms). On the other hand, in CDWs various types of SPs emerged: single or single prolonged SPs, double SPs, SPs with three IPs, periods of depressed muscle activity following the first, or the second IP, SPs with relative inhibition of motoneurons or even in several registrations the SP was missing. Unless more than one IP emerged, complete duration of inhibitory pauses (CDIP) was measured. CDIP varied from 37.17 to 42.49 ms. Average latencies were from 16.22 to 16.76 ms. Based on the results of this study it is obvious that both, the duration and the latencies were significantly longer in CDWs than in DIs (p<0.05), which can be explained by different mechanisms responsible for the muscle reflex behaviour.

Task specificity in early oral motor development

This article addresses a long-standing clinical and theoretical debate regarding the potential relationship between speech and nonspeech behaviors in the developing system. The review is motivated by the high popularity of nonspeech oral motor exercises (NSOMEs), including alimentary behaviors such as chewing, in the treatment of speech disorders in young children. The similarities and differences in the behavioral characteristics, sensory requirements, and task goals for speech and nonspeech oromotor behaviors are compared. Integrated theoretical paradigms and empirical data on the development of early oromotor behaviors are discussed. Although the efficacy of NSOMEs remains empirically untested at this time, studies of typical developmental speech physiology fail to support a theoretical framework promoting the use of NSOMEs. Well-designed empirical studies are necessary, however, to establish the efficacy of NSOMEs for specific clinical population and treatment targets.

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Severe tooth wear is discussed from the perspective of appreciating the aetiologic background of the problem. A multifactorial basis is acknowledged for the extensively worn dentition and efforts to eliminate or minimize these underlying factors are important. The management of a case of severe incisal and occlusal wear can adopt different approaches. The author proposes the rationale and advantages for involving orthodontic preparation as part of the occlusal rehabilitation.

First bite for hardness judgment as haptic exploratory procedure

This study examines whether the modulation of biting behavior while subjects are engaged in food texture judgment can be explained as an intra-oral exploratory procedure optimized for recognizing a specified sensory attribute. Subjects were asked to compare two cheese samples for "the force required to penetrate the sample with the molar teeth" (the definition for "hardness" used in this study). Based on this definition, we hypothesize that the subjects targeted the first peak of the bite time-force profile (i.e. the intra-oral phenomenon of the initial fracture) as an essential property for judgment. We observed significant elongation of the first peak in the judgmental biting, compared to the biting without judgment, for all subjects. Shortening of the second peak (teeth-to-teeth contact) duration and decrease of the second peak force were also observed for all subjects. These active biting modulations suggested that the first peak was targeted for judgment, whereas the second peak was not targeted. The sample with greater maximum force or time-integral of the bite force at the first peak was also judged as requiring greater force; these agreements were statistically significant. This result confirmed that the parameters related to the first peak were targeted as judgmental cues. We concluded that the biting behavior in hardness judgment functions as the exploratory procedure and was optimized for encoding the target sensory properties.

Neurobiology of orofacial proprioception

Primary sensory fibers innervating the head region derive from neurons of both the trigeminal ganglion (TG) and mesencephalic trigeminal nucleus (MTN). The trigeminal primary proprioceptors have their cell bodies in the MTN. Unlike the TG cells, MTN neuronal somata are centrally located within the brainstem and receive synaptic inputs that potentially modify their output. They are a crucial component of the neural circuitry responsible for the generation and control of oromotor activities. Gaining an insight into the chemical neuroanatomy of the MTN is, therefore, of fundamental importance for the understanding of neurobiology of the head proprioceptive system. This paper summarizes the recent advances in our knowledge of pre- and postsynaptic mechanisms related to orofacial proprioceptive signaling in mammals. It first briefly describes the neuroanatomy of the MTN, which is involved in the processing of proprioceptive information from the face and oral cavity, and then focuses on its neurochemistry. In order to solve the puzzle of the chemical coding of the mammalian MTN, we review the expression of classical neurotransmitters and their receptors in mesencephalic trigeminal neurons. Furthermore, we discuss the relationship of neuropeptides and their corresponding receptors in relaying of masticatory proprioception and also refer to the interactions with other atypical neuromessengers and neurotrophic factors. In extension of previous inferences, we provide conclusive evidence that the levels of transmitters vary according to the environmental conditions thus implying the neuroplasticity of mesencephalic trigeminal neurons. Finally, we have also tried to give an integrated functional account of the MTN neurochemical profiles.

Reflex control of human jaw muscles

The aim of this review is to discuss what is known about the reflex control of the human masticatory system and to propose a method for standardized investigation. Literature regarding the current knowledge of activation of jaw muscles, receptors involved in the feedback control, and reflex pathways is discussed. The reflexes are discussed under the headings of the stimulation conditions. This was deliberately done to remind the reader that under each stimulation condition, several receptor systems are activated, and that it is not yet possible to stimulate only one afferent system in isolation in human mastication experiments. To achieve a method for uniform investigation, we need to set a method for stimulation of the afferent pathway under study with minimal simultaneous activation of other receptor systems. This stimulation should also be done in an efficient and reproducible way. To substantiate our conviction to standardize the stimulus type and parameters, we discuss the advantages and disadvantages of mechanical and electrical stimuli. For mechanical stimulus to be delivered in a reproducible way, the following precautions are suggested: The stimulus delivery system (often a probe attached to a vibrator) should be brought into secure contact with the area of stimulation. To minimize the slack between the probe, the area to be stimulated should be taken up by the application of pre-load, and the delivered force should be recorded in series. Electrical stimulus has advantages in that it can be delivered in a reproducible way, though its physiological relevance can be questioned. It is also necessary to standardize the method for recording and analyzing the responses of the motoneurons to the stimulation. For that, a new technique is introduced, and its advantages over the currently used methods are discussed. The new method can illustrate the synaptic potential that is induced in the motoneurons without the errors that are unavoidable in the current techniques. We believe that once stimulation, recording, and analysis methods are standardized, it will be possible to bring out the real "wiring diagram" that operates in conscious human subjects.

Effect of bolus size on chewing, swallowing, oral soft tissue and tongue movement

Simultaneous 3D articulograph and ultrasonograph techniques were used to monitor the chewing, swallowing, oral soft tissue and tongue movements of six subjects whilst consuming varying numbers (1, 2 and 4) of a confectionery product. Modifying the number of confectionery units had a variable effect on chewing, swallowing and oral soft tissue movements during the start of the chewing sequence. The distance, range and velocity of chin movement were significantly lower when 1 confectionery unit was consumed compared with 2 and 4 during the middle stage of the chewing sequence. Significant differences in modioli movement were observed during the initial stages of chewing allowing the identification of a working and non-working side, whilst no differences in thyroid cartilage movement were noted over the whole chewing sequence. Increasing the number of confectionery units caused a significant increase in the index of tongue movement during the end of the eating sequence, although the mean index of tongue movement over the total chewing sequence remained constant. Jaw movement correlated with tongue movement, where for all samples, gape decreased linearly as the chewing sequence progressed, reflected in a similar decline in the index of tongue movement. The lack of differences in the index of tongue movement observed over the start and middle of the chewing sequence as a consequence of the number of confectionery units consumed suggested that the change in the physical properties of the bolus had a greater effect on the index of tongue movement than the change in bolus size.

Oromandibular dystonia involving the lateral pterygoid muscles: four cases with different complexity

The report describes oromandibular dystonia (OMD) in four women with involuntary activity of the lateral pterygoid muscles (LP), causing incapacitating protrusive and lateral jaw movements and displacements, and treatment with botulinum toxin type A (BTX). For initial survey and treatment control, OMD was analyzed with several, independent, and standardized methods. OMD severity and functional difficulties were evaluated subjectively and scored from videotapes. Jaw movements were assessed graphically with a magnetic tracking system, and electromyographical activity (EMG) of LP was recorded with needle electrodes using an intraoral approach, whereas activity of masseter muscles was recorded with surface electrodes. EMG-guided BTX injections (25-40 units Botox per muscle) into the muscles were performed with cannula electrodes. Compared with reference values for LP, OMD was associated with a markedly increased level of spontaneous activity, but almost normal level of maximum voluntary activity. The central pattern generator for mastication seemed to override the dystonic activity, as all patients were able to chew despite some distortion. BTX reduced both the spontaneous and the maximum activity for 3-9 months. Concomitantly, a marked reduction of the OMD severity, mandibular movements and functional disturbances were also present with the best effect in localized OMD with late onset.

Volitional control of food transport and bolus formation during feeding

When healthy individuals eat solid food, chewed food is usually transported to the oropharynx where it accumulates before swallowing (stage II transport). We tested the hypothesis that this transport process can be altered by volition. Eight healthy young subjects ate 8 g pieces of cookie with barium while movements were recorded with videofluorography. There were two trials for each subject, each with different instructions: 1) without command: to eat the cookie in his/her usual manner; 2) with command: to chew the cookie, give a signal when ready to swallow, and then swallow on command of the investigator. We measured the number of chewing cycles, the duration of each stage in the feeding sequence, and the position of the leading edge of the barium at time of command and at swallow onset. Sequence duration was longer with than without command (P=0.02), primarily because of an increase in the number of chewing cycles (P=0.02). The leading edge was typically higher in the foodway at the time of swallow onset with than without command (P=0.06). Under the command condition, stage II transport was delayed, and transport to the valleculae was inhibited. Volition alters swallow initiation in both the timing and location of the food bolus relative to the airway.

Interactive relationship between the mechanical properties of food and the human response during the first bite

Biting is an action that results from interplay between food properties and the masticatory system. The mechanical factors of food that cause biting adaptation and the recursive effects of modified biting on the mechanical phenomena of food are largely unknown. We examined the complex interaction between the bite system and the mechanical properties. Nine subjects were each given a cheese sample and instructed to bite it once with their molar teeth. An intra-oral bite force-time profile was measured using a tactile pressure-measurement system with a sheet sensor inserted between the molars. Time, force, and impulse for the first peak were specified as intra-oral parameters of the sample fracture. Mechanical properties of the samples were also examined using a universal testing machine at various test speeds. Besides fracture parameters, initial slope was also determined as a mechanical property possibly sensed shortly after bite onset. The bite profile was then examined based on the mechanical parameters. Sample-specific bite velocities were identified as characteristic responses of a human bite. A negative correlation was found between bite velocity and initial slope of the sample, suggesting that the initial slope is the mechanical factor that modifies the consequent bite velocity. The sample-specific bite velocity had recursive effects on the following fracture event, such that a slow velocity induced a low bite force and high impulse for the intra-oral fracture event. We demonstrated that examination of the physiological and mechanical factors during the first bite can provide valuable information about the food-oral interaction.

Eating-induced facial myoclonic dystonia probably due to a putaminal lesion

Myoclonic dystonia is considered a form of dystonia. We present the unusual case of a 36-year-old woman with HIV infection, who developed left facial myoclonic dystonia, triggered by eating in the setting of probable progressive multifocal leukoencephalopathy involving the contralateral basal ganglia.

The role of periodontal mechanoreceptors in mastication

The aim of this review is to discuss what is known about the reflex control of the human masticatory system by the periodontal mechanoreceptors and to put forward a method for standardised investigation. To deliver mechanical stimulus in a reproducible way, the following precautions are suggested: the stimulus should be brought into secure contact with the area of stimulation, and slack between the probe and the area to be stimulated should be taken up by the application of a preload. It is also important to ensure that there is minimal simultaneous activation of receptor systems other than the periodontal mechanoreceptors. It is also necessary to standardise the method for recording and analysing the response.

Chewing in patients with severe neurological impairment

OBJECTIVE AND DESIGN

As degenerative disease, infarction and hemorrhage in the CNS may compromise chewing, the aim was to classify and analyse such chewing disturbances. The case series included clinical and electromyographic recordings from 10 patients (ages 12-78 years) with neurological disorders.

RESULTS

Classifications were involuntary munching (two women with dystonia which was abolished during mastication), ceased chewing function (three men with impaired volitional motor control and spasticity from locked-in syndrome, restricted chewing range (two men and one woman with reduced jaw opening due to paradoxical activity after brainstem lesions), and distorted chewing pattern (two men with dystonia resulting in blockings during chewing).

CONCLUSION

The effect of the neurological impairment illustrated the complex control of mastication and the interaction between central and peripheral mechanisms, and the variation of the chewing disturbances was surprisingly great, even with similar diagnosis.

Force encoding by human periodontal mechanoreceptors during mastication

This overview summarises current knowledge on the force-encoding properties of periodontal mechanoreceptors supplying the human postcanine teeth and describe their signalling during chewing. Microneurographic experiments reveal that these receptors adapt slowly to maintained tooth loads. Similar to periodontal receptors at anterior teeth, about half respond to forces applied to more than one tooth and their receptive fields are broadly tuned to direction of force application. However, population analyses demonstrate that periodontal receptors supplying anterior and posterior teeth differ in their capacity to signal horizontal and vertical forces, respectively. Most periodontal receptors exhibit a strongly curved relationship between discharge rate and force amplitude, featuring the highest sensitivity to changes in force at forces below 1N for anterior teeth and 4N for posterior teeth. Also the dynamic sensitivity is markedly reduced at high forces. According to a quantitative model of responses in periodontal receptors based on these data, most receptors efficiently encode food contact during chewing, but due to the marked saturation tendencies at higher forces these receptors poorly encode the magnitude of the strong chewing forces and the force changes occurring at these high loads. Information provided by periodontal receptors is critical for the specification of manipulative forces used when food is positioned between the teeth and prepared for chewing. When the strong chewing forces are applied to crush the food, the receptors signal functionally important information about the mechanical properties of food as well as the spatial contact patterns between the food and the dentition.

Jaw–neck dysfunction in whiplash-associated disorders

This paper reports data from recent studies on integrative jaw-neck motor control in healthy subjects and disturbed jaw-neck behaviour in whiplash-associated disorders (WAD). The results show that neck function is an integral part of natural jaw behaviour, and that neck injury can impair jaw function and therefore disturb eating behaviour. We also show preliminary results from implementation of a new approach for rehabilitation of jaw-neck dysfunction and pain in WAD.

Influence of jaw gape on EMG of jaw muscles and jaw-stretch reflexes

The influence of jaw gapes on jaw-stretch reflexes and jaw muscles activity was studied in order to test the sensitivity of human muscle spindle afferents in various jaw muscles. Twelve healthy men (mean age+/-S.E.M.: 25.0+/-1.2yr) participated in the study. Short-latency excitatory reflex responses were evoked by a custom-made stretch device with the subjects biting on a jaw-bar with their front teeth. Surface electromyographic (sEMG) recordings from right masseter (MAR), and right temporalis (TAR), intramuscular EMG (imEMG) recordings from right lateral pterygoid (LPR) and right anterior digastric (ADR) muscles were made. The reflex at different jaw gapes of 6, 10, 14, 18, 22, 26, 30, 34, and 38mm were examined in random order with standard stretch conditions of 1mm displacement and 10ms ramp time. Twenty sweeps of the reflex were recorded at each level with at least 5s interval between each sweep with online monitoring of the visual feed back at 15% of maximum voluntary contraction (MVC) of each jaw gape from MAR. The results showed that the peak-to-peak amplitude of the jaw-stretch reflex in MAR was significantly higher at 14mm compared to 30, 34, and 38mm (P<0.038), whereas the reflex amplitude in TAR increased with jaw gape until a maximum at 34mm. There was no significant effect of jaw gape in LPR muscles (P=0.825) and no obvious stretch reflex was observed in ADR. When the amplitude was normalised to the pre-stimulus EMG at each jaw gape, the highest normalised amplitude was observed at 14mm jaw gape in MAR, however there was no significant effect of jaw gape on the normalised amplitude in TAR and LPR. In addition, masseter EMG at MVC significantly decreased with the increase of the gapes, i.e. biting at 6, 14, and 18mm gapes had a significantly higher MVC compared to 26, 30, 34, and 38mm (ANOVA: P<0.013). It is concluded that the jaw gapes influence the sensitivity of the human muscle spindle afferents in jaw-closing muscles with a distinct peak, which is within normal jaw gapes during function.

Periodontal anaesthetisation decreases rhythmic synchrony between masseteric motor units at the frequency of jaw tremor

This study links the reduction in jaw physiological tremor around 8 Hz following periodontal mechanoreceptor (PMR) anaesthetisation to changes in coherence between masseteric motor unit discharges. We have recorded single motor unit activity from two separate sites in the right masseter muscle during a low level tonic contraction, both prior to and during anaesthetisation of the peri-incisal PMRs. Anaesthetisation of PMRs decreased coherent activity between motor units circa 8 Hz, and decreased synchrony between the same motor unit pairs. It is proposed that tremor-generating inputs that cause rhythmic synchronisation of masseteric motor units arise from, or are amplified by the PMRs.

Intrinsic membrane properties and morphological characteristics of interneurons in the rat supratrigeminal region

The membrane properties and morphological features of interneurons in the supratrigeminal area (SupV) were studied in rat brain slices using whole-cell patch clamp recording techniques. We classified three morphological types of neurons as fusiform, pyramidal, and multipolar and four physiological types of neurons according to their discharge pattern in response to a 1-sec depolarizing current pulse from -80 mV. Single-spike neurons responded with a single spike, phasic neurons showed an initial burst of spikes and were silent during the remainder of the stimulus, delayed-firing (DF) neurons exhibited a slow depolarization and delay to initial spike onset, and tonic (T) neurons showed maintained a discharge throughout the stimulus pulse. In a subpopulation of neurons (10%), membrane depolarization to around -44 mV produced a rhythmic burst discharge (RB) that was associated with voltage-dependent subthreshold membrane oscillations. Both these phenomena were blocked by the sodium channel blocker riluzole at a concentration that did not affect the fast transient spike. Low doses of 4-AP, which blocks low-threshold K+ currents, transformed bursting into low-frequency tonic discharge. In contrast, bursting occurred with exposure to cadium, a calcium-channel blocker. This suggests that persistent sodium currents and low-threshold K+ currents have a role in intrinsic burst generation. Importantly, RB cells were most often associated with multipolar neurons that exhibited either a DF or a T discharge. Thus, the SupV contains a variety of physiological cell types with unique morphologies and discharge characteristics. Intrinsic bursting neurons form a unique group in this region. .

Three-dimensional finite element modelling of muscle forces during mastication

This paper presents a three-dimensional finite element model of human mastication. Specifically, an anatomically realistic model of the masseter muscles and associated bones is used to investigate the dynamics of chewing. A motion capture system is used to track the jaw motion of a subject chewing standard foods. The three-dimensional nonlinear deformation of the masseter muscles are calculated via the finite element method, using the jaw motion data as boundary conditions. Motion-driven muscle activation patterns and a transversely isotropic material law, defined in a muscle-fibre coordinate system, are used in the calculations. Time-force relationships are presented and analysed with respect to different tasks during mastication, e.g. opening, closing, and biting, and are also compared to a more traditional one-dimensional model. The results strongly suggest that, due to the complex arrangement of muscle force directions, modelling skeletal muscles as conventional one-dimensional lines of action might introduce a significant source of error.

Forces applied by anterior and posterior teeth and roles of periodontal afferents during hold-and-split tasks in human subjects

Hold-and-split tasks were performed by 20 subjects (12 females and 8 males) using the right central incisors, canines, 2nd premolars, and 1st molars, respectively. Half a peanut was positioned on a transducer-equipped plate and the subject was instructed to hold the plate with the peanut between two antagonistic teeth, and not using more force than necessary. After ca. 3 s the subject was instructed to split the peanut in a natural manner. Each session consisted of a series of three in which the subject performed the hold-and-split task five times for each tooth. Thus, in total, data were obtained from 60 trials for each subject. The magnitude of the forces and the force rates used to split the peanut increased distally along the dental arch. However, the duration of the split phase was similar for the various teeth examined. During anesthesia of the periodontal ligament (four subjects), no significant changes were seen in the split phase. The forces used to hold the peanut between the teeth also increased distally along the dental arch: 0.60 N for the incisor, 0.77 N for the canine, 1.15 N for the 2nd premolar, and 1.74 N for the 1st molar. The difference in hold forces for the various teeth can be explained by the different sensitivity characteristics of the periodontal afferents innervating anterior and posterior teeth. During periodontal anesthesia, the magnitude and variability of the hold forces increased for all types of teeth, thus supporting the suggestion that periodontal afferent information is used in the regulation of the level of forces used to hold and manipulate morsels between the teeth.

Impaired mastication modifies the dynamics of bolus formation

Mastication is a complex sensory-motor activity whereby a food product is transformed into a bolus. Consumers mainly perceive the sensory properties of the food during the intra-oral manipulation of the product. Consequently, the quality of the chewing process could have consequences on the perception of sensory properties and food choice. By focusing on meat products, this study aimed to analyze the influence of dental status on (i) dynamic adaptation of the chewing behavior (evaluated by electromyography) to the changes in texture during bolus formation and (ii) bolus properties (mechanical resistance and saliva incorporation) obtained from meat of different initial textures. Two groups of subjects (dentate subjects and denture wearers), known to present highly different chewing efficiency, were compared. For both groups, salivary flow rates were evaluated at rest and after stimulation by chewing (paraffin and meat). The salivary flow rates, assessed during chewing of a nonedible matrix (paraffin), were a good predictor of salivary flow rates induced by meat chewing for both groups of subjects. Salivary flow rates were not affected by the dental status. In contrast, the chewing behavior varied between groups. For denture wearers, the chewing pattern was strongly impaired and not adapted to the changes in meat structure during bolus formation. Denture wearers swallowed less fragmented boli than dentate subjects, but boli had a similar level of moisture for both groups of subjects.

Inclination of the occlusal plane is associated with the direction of the masticatory movement path

Using lateral cephalograms and a jaw movement-recording system, the relationship between the masticatory movement path and dentofacial morphology was investigated in 17 subjects (9 males and 8 females, mean age 23.5 years) without a history of orthodontic treatment. The masticatory movement path was measured at the right and left lower first molar while the subjects chewed gum. The angle between the Frankfort horizontal plane and the masticatory axis (FH-masticatory angle), defined as the axis passing the opening and closing turning point on the sagittal masticatory path, was also measured. The correlation between the angular measurements derived from the lateral cephalogram and the FH-masticatory angle was then investigated. A positive correlation was observed in the FH-masticatory, occlusal plane (P < 0.05), and mandibular plane (P < 0.01) angles. Furthermore, it was found that the angle between the masticatory axis and the occlusal plane (69.1 +/- 4.2 degrees) remained constant even as the masticatory axis showed a tendency to incline forward as the mandibular plane angle became steeper; the rates of change of the FH-masticatory and the occlusal plane angles were approximately 1:1. This finding suggests that the masticatory movement path is closely associated with the occlusal plane.