Jaw Muscle Activity Research Articles

Separate but equal? Relating morphological divergence to functional performance in the evolution of primate chewing

A long‐standing hypothesis in studies of primate mastication argues that musculoskeletal form has changed markedly during primate evolution while masticatory behaviors, such as the jaw‐muscle activation sequence during chewing, has remained relatively similar across primates. If morphology has evolved within this conserved behavioral framework, then we predict that jaw‐muscle activity patterns exhibit limited variation across primates and show little association with jaw‐muscle and skull form among species.To examine these relationships, we compared jaw‐muscle electromyography (EMG) data to musculoskeletal measures of the masticatory apparatus across several primates including strepsirrhines, monkeys and humans and one non‐primate treeshrew.Contrary to the prediction of little association between jaw‐muscle activity and musculoskeletal form, we find strong associations between the timing and magnitude of jaw‐muscle EMGs and masticatory apparatus shapes. These associations fit in vivo observations and theoretical predictions linking patterns of jaw loading to morphologies providing increased load resistance. We hypothesize that several functional modules exist in the primate masticatory apparatus incorporating both neuromuscular activation and musculoskeletal form. Jaw‐muscle activity patterns appear to be an integrated and evolving part of the primate masticatory apparatus.Supported by NSF.

Bite Force and Influential Factors on Bite Force Measurements: A Literature Review

Maximum voluntary bite force is an indicator of the functional state of the masticatory system and the level of maximum bite force results from the combined action of the jaw elevator muscles modified by jaw biomechanics and reflex mechanisms. The measurement of bite force can provide useful data for the evaluation of jaw muscle function and activity. It is also an adjunctive value in assessing the performance of dentures. Technological advances in signal detection and processing have improved the quality of the information extracted from bite force measurements. However, these measurements are difficult and the reliability of the result depends on a number of factors, such as presence of pain and temporomandibular disorders, gender, age, cranio-facial morphology, and occlusal factors. In addition to these physiological factors, recording devices and techniques are important factors in bite force measurement. Therefore, one should be careful when comparing the bite force values reported in the research.

A Preliminary Analysis of the Relationship Between Jaw‐Muscle Architecture and Jaw‐Muscle Electromyography During Chewing Across Primates

The architectural arrangement of the fibers within a muscle has a significant impact on how a muscle functions. Recent work on primate jaw-muscle architecture demonstrates significant associations with dietary variation and feeding behaviors. In this study, the relationship between masseter and temporalis muscle architecture and jaw-muscle activity patterns is explored using Belanger's treeshrews and 11 primate species, including two genera of strepsirrhines (Lemur and Otolemur) and five genera of anthropoids (Aotus, Callithrix, Cebus, Macaca, and Papio). Jaw-muscle weights, fiber lengths, and physiologic cross-sectional areas (PCSA) were quantified for this preliminary analysis or collected from the literature and compared to published electromyographic recordings from these muscles. Results indicate that masseter architecture is unrelated to the superficial masseter working-side/balancing-side (W/B) ratio across primate species. Alternatively, relative temporalis architecture is correlated with temporalis W/B ratios across primates. Specifically, relative temporalis PCSA is inversely related to the W/B ratio for the anterior temporalis, indicating that as animals recruit a larger relative percentage of their balancing-side temporalis, they possess the ability to generate relatively larger amounts of force from these muscles. These findings support three broader conclusions. First, masseter muscle architecture may have experienced divergent evolution across different primate clades related to novel functional roles in different groups. Second, the temporalis may be functionally constrained (relative to the masseter) across primates in its functional role of creating vertical occlusal forces during chewing. Finally, the contrasting results for the masseter and temporalis suggest that the fiber architecture of these muscles has evolved as distinct functional units in primates.

Influence of neck rotation and neck lateroflexion on mandibular equilibrium

Neuromuscular interaction between neck and jaw muscles has been reported in several studies. However, the influence of experimentally modified posture of the neck on jaw muscle activity during isometric biting was not investigated so far. The aim of the present study was to test by the aid of simultaneous electromyographic and intraoral bite force measurements whether neck rotation and lateroflexion, in contrast to a straightforward neck position, change the isometric cocontraction patterns of masticatory muscles under identical submaximum bite forces of 50-200 N. Electric muscle activity of all masticatory muscles and changes of the reduction point (RP) of the resultant bite force vectors were examined. An anteroposterior displacement of the RPs could be observed for the rotated and lateroflexed neck position in comparison with the straightforward position. On the other hand, the results revealed no significant differences between bilateral muscle activation under the different test conditions. These findings suggest a force transmission between the neck and the masticatory system, but no essential activity changes in the masticatory muscles under short time posture modification of the neck.

Correspondence

To the Editor: RE: ‘Functional characteristics of the rat jaw muscles: daily muscle activity and fiber type composition’ by Kawai et al. Journal of Anatomy, Vol. 215, pp. 656–662. Kawai et al. present an interesting study of the relationship between electromyographic activity in jaw muscles and their fibre type composition (Kawai et al. 2009). The concept of functional adaptation has been widely accepted since I introduced it in the 1970s (Salmons & Sréter, 1976; Salmons, 1980; Salmons & Henriksson, 1981). The evidence comes entirely from muscles of the limbs and trunk, but it is reasonable to expect the same regulatory phenomena to be present in skeletal muscles of cranial origin even though they contain, and often co-express, a greater diversity of myosin heavy chain isoforms. The authors of this paper appear, however, to have misinterpreted the adaptive hypothesis. This states that sustained high levels of activity induce slow, fatigue-resistant properties; low or intermittent levels of activity allow the fibers to retain, or revert to, a native fast, fatigue-susceptible state. Accordingly, where there is a high proportion of type IIX fibres it would be associated not with the generation of large forces, as the authors propose, but with a low aggregate amount of activity. The high-frequency patterns needed to produce large forces do not differ from low-frequency patterns in their effect on myosin isoform composition (Hudlicka et al. 1982; Kernell et al. 1987; Sutherland et al. 2003). It is certainly true that the muscles of smaller animals have a lower proportion of slow fibres. This is associated with a higher threshold for the transition from fast to slow myosin isoforms (Salmons & Henriksson, 1981; Sutherland et al. 1998, 2006). The transition, although present in the rat, is correspondingly more difficult to demonstrate than in larger species (Jarvis et al. 1996). On this basis a better correlation between the proportion of type I fibres and muscle activity should be seen in a larger experimental animal, and has in fact been found in rabbit jaw muscles by van Wessel et al. (2005). This citation is curiously absent from the present paper. There is a good deal of published evidence to show that myosin heavy chain composition responds to the aggregate number of impulses reaching the muscle (see Salmons, 2009 for a recent review). Unfortunately this is not measured by the overall duration and amplitude of electromyographic activity over the same period: high-frequency bursts of short duration could deliver the same number of impulses as low-frequency trains of longer duration. If the authors were able to take this into account, the apparent inconsistencies in their data for the anterior temporalis muscle and the seeming departures from Henneman’s size principle would, I suggest, disappear.

Changes in rabbit jaw-muscle activity parameters in response to reduced masticatory load

Mechanical food properties influence the neuromuscular activity of jaw-closing muscles during mastication. It is, however, unknown how the activity profiles of the jaw muscles are influenced by long-term alterations in masticatory load. In order to elucidate the effect of reduced masticatory load on the daily habitual activity profiles of three functionally different jaw muscles, the electromyograms of the masseter, temporalis and digastric muscles were recorded telemetrically in 16 male rabbits between seven and 20 weeks of age. Starting at eight weeks of age the experimental animals were fed significantly softer pellets than the control animals. Daily muscle activity was quantified by the relative duration of muscle use (duty time), burst number and burst length in relation to multiple activity levels. The daily duty time and burst number of the masseter muscle were significantly lower in the experimental group than in the control group at 5% and 10% of the maximum activity during the two weeks following the change in food hardness. By contrast, altered food hardness did not significantly influence the activity characteristics of the temporalis and digastric muscles. The findings suggest that a reduction in masticatory load decreases the neuromuscular activity of the jaw-closing muscles that are primarily responsible for force generation during mastication. This decrease is most pronounced in the weeks immediately following the change in food hardness and is limited to the activity levels that reflect muscle contractions during chewing. These findings support the conclusion that the masticatory system manifests few diet-specific long-term changes in the activity profiles of jaw muscles.

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.

Cephalometric and electromyographic study of patients of East African ethnicity with and without anterior open bite

A review of the literature showed that there are disturbances of muscle activity and tongue function in patients with anterior open bite (AOB). However, most studies have had white subjects, even though open bite is more prevalent in black populations. In this pilot study, we examined jaw muscle activity and tongue pressure in East African black subjects. Ten subjects (4 boys, 6 girls; ages, 10.1-13.2 years) were recruited. A cephalometric comparison of subjects with and without AOB was performed. Surface electrodes were placed on the bilateral anterior temporalis, superficial masseter, superior orbicularis oris, and anterior digastric muscles. Electromyograms were recorded at rest, maximal voluntary clenching, chewing, and swallowing. Tongue pressure during rest and swallowing, and maximal bite force were also measured. There was no statistically significant difference between the 2 groups in cephalometric measures (except in amount of open bite and overbite) and maximum bite force. During chewing, both groups showed similar coordinated activity patterns in the muscles, but the AOB subjects tended to show more electromyographic activity in the muscles of the balancing side. During clenching, the AOB subjects exhibited lower electromyographic activity compared with the controls. AOB subjects also had greater anterior tongue pressure during swallowing. Data from tongue pressure at rest were inconclusive. East African blacks appear to have neuromuscular and skeletal predispositions to AOB, but their vertical incisor relationships are variable. Factors in addition to those investigated might be involved.

Effects of eccentric jaw exercise on temporal summation in jaw-closing muscles of healthy subjects

Eccentric jaw exercises has been known to cause muscle soreness but no studies have so far examined to what extent temporal summation mechanisms within the exercised muscles are changed. The purpose of the present study was to investigate the effects of an eccentric biting exercise on the temporal summation, mechanical pressure sensitivity and jaw muscle activity. A total of 15 healthy men participated in a two-session-experiment: In one session, they performed 30 min controlled eccentric jaw exercise and the other session served as a no-exercise control. Soreness sensations at rest and during maximal biting, pressure pain thresholds (PPTs) and electromyographic (EMG) activity during maximal jaw biting were recorded before (baseline), immediately after (Post-task), and 1 day after the exercise (1-day-after). The temporal summation ratio using intra-muscular electrical stimulation of the masseter was investigated at baseline and at 1-day-after. The eccentric jaw exercise was associated with significant increases in soreness sensation and decreased PPTs at Post-task and at 1-day-after. The EMG activity and biting force did not change. The summation ratio was significantly decreased at 1-day-after in both sessions. The present findings demonstrate that eccentric jaw exercise does not induce detectable changes in temporal summation. However, the summation ratio may have clinical utility to differentiate the location of sensitization.

Heterogeneous activity level of jaw-closing and -opening muscles and its association with arousal levels during sleep in the guinea pig

Exaggerated jaw motor activities during sleep are associated with muscle symptoms in the jaw-closing rather than the jaw-opening muscles. The intrinsic activity of antagonistic jaw muscles during sleep remains unknown. This study aims to assess the balance of muscle activity between masseter (MA) and digastric (DG) muscles during sleep in guinea pigs. Electroencephalogram (EEG), electroocculogram, and electromyograms (EMGs) of dorsal neck, MA, and DG muscles were recorded with video during sleep-wake cycles. These variables were quantified for each 10-s epoch. The magnitude of muscle activity during sleep in relation to mean EMG activity of total wakefulness was up to three times higher for MA muscle than for DG muscle for nonrapid eye movement (NREM) and rapid-eye-movement (REM) sleep. Although the activity level of the two jaw muscles fluctuated during sleep, the ratio of activity level for each epoch was not proportional. Epochs with a high activity level for each muscle were associated with a decrease in deltaEEG power and/or an increase in heart rate in NREM sleep. However, this association with heart rate and activity levels was not observed in REM sleep. These results suggest that in guinea pigs, the magnitude of muscle activity for antagonistic jaw muscles is heterogeneously modulated during sleep, characterized by a high activity level in the jaw-closing muscle. Fluctuations in the activity are influenced by transient arousal levels in NREM sleep but, in REM sleep, the distinct controls may contribute to the fluctuation. The above intrinsic characteristics could underlie the exaggeration of jaw motor activities during sleep (e.g., sleep bruxism).

Cranial Suture Morphology and its Relationship to Diet in Cebus

Cranial sutures are complex morphological structures. Four Cebus species (C. albifrons, C. apella, C. capucinus, C. olivaceus) are used here to test the hypothesis that sagittal suture complexity is enhanced in animals that eat materially challenging foods. These primates are ideal for such comparative studies because they are closely related and some are known to exhibit differences in the material properties of the foods they ingest and masticate. Specifically, Cebus apella is notable among members of this genus for ingesting food items of high toughness as well as consistently demonstrating a relatively robust cranial morphology. Consistent with previous studies, C. apella demonstrates significantly more robust mandibular and temporal fossa morphology. Also, C. apella possesses sagittal sutures that are more complex than congenerics. These data are used to support the hypothesis that cranial suture complexity is increased in response to consuming diets with more obdurate material properties. One interpretation of this hypothesis is that, compared to non-apelloids, total strain in the apelloid cranial suture connective tissue environment is elevated due to increased jaw muscle activity by increases in either force magnitudes or the number of chewing events. It is argued that greater masticatory function enhances the growth and modeling of cranial suture interdigitation. These data show that cranial suture complexity is one more hard tissue feature from the skull that might be used to inform hypotheses of dietary functional morphology.

Measuring biting behavior induced by acute stress in the rat

Previous reports have indicated that biting behavior is enhanced in rats that are subject to acute stress. Several methods have been proposed for studying this phenomenon, one of which is the electromyography (EMG) of the jaw muscles. In this study, we compared total EMG activity with the EMG activity related to biting behavior, as determined by video monitoring, before and after restraint stress. Wistar albino rats (150 g) were subject to surgery for electrode placement 48 h before measurements. The EMG activity of the masseter muscle on one side was recorded for 10 min before and immediately after 30 min of restraint stress. Restraint stress increased jaw muscle EMG activity, although some of the activity was related to behaviors other than biting. Recording the EMG activity of jaw muscles is useful for measuring the stress response in rats, but careful video monitoring is needed if biting behavior is to be studied in particular.

Jaw and hyolingual muscle activity patterns and bite forces in the herbivorous lizard Uromastyx acanthinurus

Uromastyx lizards are basal agamid lizards that are characterized by an herbivorous diet and a robust akinetic skull. Recently, lizards in general, and Uromastyx lizards in particular, have become a model system for mechanical analyses of skull function. However, quantitative models of skull function are hampered by a lack of knowledge on the activation patterns of jaw and hyolingual muscles. Moreover, in vivo bite force or strain data, essential to test the validity of these modelling efforts, are lacking. Here we provide data on the muscle activation patterns and bite forces in the lizard Uromastyx acanthinurus. Our results show that muscle recruitment patterns during intra-oral transport and swallowing, are qualitatively similar to those observed for other lizards. Whereas during the slow opening phase the hyolingual muscles show a pronounced activity, the fast opening phase is characterised by strong activity in the jaw opener and the tongue and hyoid retractors. During fast closing the jaw closers become active and at the end of this phase, the jaw closers become silent momentarily before showing a renewed and strong activity during the slow closing/power stroke phase. Measurements of bite forces indicate bite force levels similar to those of agamid lizards of similar size, and similar to those predicted based on recent modelling efforts. These data should allow further refinement and validation of recently published models of biting in lizards of the genus Uromastyx.

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.

Activity of jaw muscles in unilateral cross-bite without mandibular shift

Objectives To evaluate and compare electromyographic activity of jaw muscles at rest and during maximal voluntary clenching in maximal intercuspation between growing individuals with unilateral posterior cross-bite without functional mandibular lateral shift during closure and matched individuals with normal occlusion. These findings may help elucidate the impact of this occlusal condition on jaw muscle function and the therapeutic implications. Design 30 growing children (10–12 years) with unilateral cross-bite without functional mandibular lateral shift during closure (cases) and 30 age- and sex-matched children with normal occlusion (controls) were consecutively selected applying highly restrictive criteria. Electromyographic activity coming from the areas of right and left anterior temporalis, posterior temporalis, masseter, and supra-hyoid muscles was evaluated at rest and during clenching. The asymmetry index was calculated for each muscle area to quantify the degree of asymmetry between sides. The masseter/anterior temporalis ratio during clenching was also recorded. Results No significant differences were found between groups at rest. During clenching, activity of the cross-bite side masseter area was significantly lower in cases versus controls but no difference was found between cross-bite and non-cross-bite sides of this muscle. Conclusions Unilateral posterior cross-bite without functional mandibular lateral shift is associated with a lower electromyographic activity of masseter muscle area of the cross-bite side during maximal clenching in maximal intercuspation, compared to the children with normal occlusion. This result could suggest a functional adaptation that should be carefully considered. Long-term studies are needed to determine future repercussions and to evaluate the benefit of an early treatment.

Waking‐state oral parafunctional behaviors: specificity and validity as assessed by electromyography

In contrast to sleep-related oral parafunctional behaviors, little is known about waking oral parafunctional behaviors. The Oral Behaviors Checklist contains terms referring to a variety of non-observable behaviors that are reliable when prompted (e.g. 'clench') but validity data are absent. Our goal was to assess whether (i) each behavioral term is distinct electromyographically, and (ii) temporomandibular disorder (TMD) subjects differ from non-TMD subjects in their performance. Surface electromyographic (EMG) activity was used to measure bilateral masseter, temporalis, and suprahyoid muscles while subjects (27 patients with TMD; 27 healthy controls) performed ten oral behaviors without explanation. Electromyographic data were averaged between bilateral muscles and two trials. A multivariate construct (jaw muscle activity) was analyzed using Wilks lambda within multivariate analysis of variance (manova). Obvious behaviors (e.g. clench, read, tongue press) exhibited expected EMG patterns, and patients and controls produced identical profile plots of the EMG data. Of 10 tested behaviors, nine were found to be associated with significantly differing proportions of amplitudes across muscles and were thus unique. Behaviors with similar terms were associated with different EMG patterns. The present data support the specificity of behavioral terms and performances. Implications include causation related to TMD based on subtle behaviors that occur at a high frequency.

Telemetry System for Assessing Jaw-Muscle Function in Free-ranging Primates

In vivo laboratory-based studies describing jaw-muscle activity and mandibular bone strain during mastication provide the empirical basis for most evolutionary hypotheses linking primate masticatory apparatus form to diet. However, the laboratory data pose a potential problem for testing predictions of these hypotheses because estimates of masticatory function and performance recorded in the laboratory may lack the appropriate ecological context for understanding adaptation and evolution. For example, in laboratory studies researchers elicit rhythmic chewing using foods that may differ significantly from the diets of wild primates. Because the textural and mechanical properties of foods influence jaw-muscle activity and the resulting strains, chewing behaviors studied in the laboratory may not adequately reflect chewing behaviors of primates feeding in their natural habitats. To circumvent this limitation of laboratory-based studies of primate mastication, we developed a system for recording jaw-muscle electromyograms (EMGs) from free-ranging primates so that researchers can conduct studies of primate jaw-muscle function in vivo in the field. We used the system to record jaw-muscle EMGs from mantled howlers (Alouatta palliata) at Hacienda La Pacifica, Costa Rica. These are the first EMGs recorded from a noncaptive primate feeding in its natural habitat. Further refinements of the system will allow long-term EMG data collection so that researchers can correlate jaw-muscle function with food mechanical properties and behavioral observations. In addition to furthering understanding of primate feeding biology, our work will foster improved adaptive hypotheses explaining the evolution of primate jaw form.

Motor control of jaw muscles in chewing and in isometric biting with graded narrowing of jaw gape

When a certain bite force is applied during unilateral chewing, the combination of jaw elevator muscle activities is different than when a comparable force is applied in unilateral isometric biting, e.g. on a force transducer. Masticatory peak force is generated in a nearly isometric phase of the chewing cycle, with a jaw gape of about 1 mm. In contrast, peak force in isometric biting on force measuring equipment usually induces jaw gapes of 6 mm or even more. Therefore, we tested the hypothesis that the jaw gape influences relative activation of elevator muscles in unilateral isometric biting. We further examined whether such influence could explain the different activity combinations of chewing and isometric biting. In thirty asymptomatic males, masseter and temporalis activities were recorded during intermittent isometric biting with jaw gapes of 6, 5, 3, 2 and 1 mm and during unilateral chewing. Activity combinations were described by working/balancing ratios and by temporalis/masseter ratios. With decreasing jaw gape the working/balancing ratio of the posterior temporalis decreased (P < 0.002) while that of the masseter increased (P < 0.001). Likewise, the temporalis/masseter ratio on the balancing side increased (P < 0.001). With decreasing jaw gape, activity ratios of isometric biting approached ratios of chewing. We conclude that: (i) relative jaw muscle activation in isometric biting depends on the jaw gape, (ii) relative muscle activation in chewing resembles relative activation of isometric biting with a small 'chewing-like' gape. This suggests that characteristic activity combinations in chewing are mainly a result of the approximately isometric contraction during the slow closing phase of the chewing cycle.

Synaptic Transmission From the Supratrigeminal Region to Jaw-Closing and Jaw-Opening Motoneurons in Developing Rats

The supratrigeminal region (SupV) receives abundant orofacial sensory inputs and descending inputs from the cortical masticatory area and contains premotor neurons that target the trigeminal motor nucleus (MoV). Thus it is possible that the SupV is involved in controlling jaw muscle activity via sensory inputs during mastication. We used voltage-sensitive dye, laser photostimulation, patch-clamp recordings, and intracellular biocytin labeling to investigate synaptic transmission from the SupV to jaw-closing and jaw-opening motoneurons in the MoV in brain stem slice preparations from developing rats. Electrical stimulation of the SupV evoked optical responses in the MoV. An antidromic optical response was evoked in the SupV by MoV stimulation, whereas synaptic transmission was suppressed by substitution of external Ca2+ with Mn2+. Photostimulation of the SupV with caged glutamate evoked rapid inward currents in the trigeminal motoneurons. Gramicidin-perforated and whole cell patch-clamp recordings from masseter motoneurons (MMNs) and digastric motoneurons (DMNs) revealed that glycinergic and GABAergic postsynaptic responses evoked in MMNs and DMNs by SupV stimulation were excitatory in P1-P4 neonatal rats and inhibitory in P9-P12 juvenile rats, whereas glutamatergic postsynaptic responses evoked by SupV stimulation were excitatory in both neonates and juveniles. Furthermore, the axons of biocytin-labeled SupV neurons that were antidromically activated by MoV stimulation terminated in the MoV. Our results suggest that inputs from the SupV excite MMNs and DMNs through activation of glutamate, glycine, and GABAA receptors in neonates, whereas glycinergic and GABAergic inputs from the SupV inhibit MMNs and DMNs in juveniles.

How does pain affect jaw muscle activity? The Integrated Pain Adaptation Model

Pain and limitation of movement are two cardinal symptoms of temporomandibular disorders but it is unclear how one influences the other. The relationship between pain and movement is clinically significant but controversial with two major theories having been proposed: the Vicious Cycle Theory and the Pain Adaptation Model. The Vicious Cycle Theory proposes a vicious cycle between pain and muscle activity. This theory has little scientific basis but underpins many management strategies. The Pain Adaptation Model is more evidence-based and proposes that pain causes changes in muscle activity to limit movement and protect the sensory-motor system from further injury. The Pain Adaptation Model has many positive features but does not appear to explain the relation between pain and muscle activity in all situations. We propose that the relationship is influenced by the functional complexity of the sensory-motor system and the multidimensional nature of pain. This new Integrated Pain Adaptation Model states that pain results in a new recruitment strategy of motor units that is influenced by the multidimensional (i.e., biological and psychosocial) components of the pain experience. This new recruitment strategy aims to minimize pain and maintain homeostasis. This model emphasizes the individual reaction to pain and suggests a tailored approach towards management.