Complexity of ruminant masticatory evolution

The evolution of robust jaws, hypsodont teeth, and large chewing muscles among grazing ruminants is a quintessential example of putative morphological adaptation. However, the degree of correlated evolution (i.e., to what extent the grazer feeding apparatus represents an evolutionary module), especially of soft and hard tissues, remains poorly understood. Recent generation of large datasets and phylogenetic information has made testing hypotheses of correlated evolution possible. We, therefore, test for correlated evolution among various traits of the ruminant masticatory apparatus including tooth crown height, jaw robustness, chewing muscle size, and characters of the molar occlusal surfaces, using phylogenetic and nonphylogenetic comparative methods as well as phylogenetic evolutionary model selection. We find that the large masseter muscles of grazing ruminants evolved with the inclusion of grass in the diet, an increase in the proportion of occlusal enamel bands oriented parallel to the chewing stroke, and possibly hypsodonty. We suggest that the masseter evolved under two evolutionary regimes: i) selection for higher masticatory forces during chewing and ii) flattening of the tooth profile, which resulted in reduced tooth guidance and, thus, a requirement for more chewing muscle activity during each chewing stroke, in agreement with previous research. The linear jaw metrics (depth of the mandibular angle, mandibular angle width, and length of the superficial masseteric scar) all show correlated evolution with hypsodonty and the proportion of enamel bands oriented parallel to the chewing stroke. We suggest that changes in the shape of the mandible represent the combined effects of selection for a reorientation of the chewing stroke, so as to emphasize horizontal translation of the teeth, and accommodation of high‐crowned teeth. Our analyses show that the ruminant feeding apparatus is an evolutionary mosaic with its various components showing both correlated and independent evolution. J. Morphol. 275:1093–1102, 2014. © 2014 Wiley Periodicals, Inc.     

Electromyography and mechanics of mastication in the albino rat.

The masticatory apparatus in the albino rat was studied by means of electromyography and subsequent estimation of muscular forces. The activity patterns of the trigeminal and suprahyoid musculature and the mandibular movements were recorded simultaneously during feeding. The relative forces of the individual muscles in the different stages of chewing cycles and biting were estimated on the basis of their physiological cross sections and their activity levels, as measured from integrated electromyograms. Workinglines and moment arms of these muscles were determined for different jaw positions. In the anteriorly directed masticatory grinding stroke the resultants of the muscle forces at each side are identical; they direct anteriorly, dorsally and slightly lingually and pass along the lateral side of the second molar. Almost the entire muscular resultant force is transmitted to the molars while the temporo-mandibular joint remains unloaded. A small transverse force, produced by the tense symphyseal cruciate ligaments balances the couple of muscle resultant and molar reaction force in the transverse plane. After each grinding stroke the mandible is repositioned for the next stroke by the overlapping actions of three muscle groups: the pterygoids and suprahyoids produce depression and forward shift, the suprahyoids and temporal backward shift and elevation of the mandible while the subsequent co-operation of the temporal and masseter causes final closure of the mouth and starting of the forward grinding movement. All muscles act in a bilaterally symmetrical fashion. The pterygoids contract more strongly, the masseter more weakly during biting than during chewing. The wide gape shifts the resultant of the muscle forces more vertically and moreposteriorly. The joint then becomes strongly loaded because the reaction forces are applied far anteriorly on the incisors. The charateristic angle between the almost horizontal biting force and the surface of the food pellet indicates that the lower incisors produce a chisel-like action. Tooth structure reflects chewing and biting forces. The transverse molar lamellae lie about parallel to the chewing forces whereas perpendicular loading of the occlusal surfaces is achieved by their inclination in the transverse plane. The incisors are loaded approximately parallel to their longitudinal axis, placement that avoids bending forces during biting. It is suggested that a predominantly protrusive musculature favors the effective force transmission to the lower incisors, required for gnawing. By grinding food across transversely oriented molar ridges the protrusive components of the muscles would be utilized best. From the relative weights of the masticatory muscles in their topographical relations with joints, molars and incisors it may be concluded that the masticatory apparatus is a construction adapted to optimal transmission of force from muscles to teeth.     

The activity of jaw elevator muscles during peanut chewing in patients with temporomandibular joint and muscle pain dysfunction syndrome

A synchronized system of EMG and jaw motion tracking device was used to observe some chewing parameters of jaw elevator muscles in 15 patients with temporomandibular joint and muscle pain dysfunction syndrome (TMJ) and 15 normal subjects. Duration of tooth contact (DTC), duration of muscle contraction before tooth contact (DMC), total duration of muscle contraction (DTM) and velocity of jaw movement during peanut chewing were observed. Symptoms of the TMJ patients included pain and tenderness at joints and muscles, and limitation and clicking at joints during jaw movements. It was found that the TMJ patients needed more numerous breaking off strokes before trituration at the occlusal level. There was a longer DMC in the earlier trituration period and TMJ patients had longer DMC than in normals. No difference was found between right and left side chewing or between temporalis and masseter muscles. DTM in the TMJ group was only slightly longer than in normals and the difference between early and late chewing periods was statistically not significant. DTC was only slightly shorter in the TMJ group while the difference between early and late chewing periods in both groups was significant. The average and maximum closing velocities were significantly lower in the TMJ group in both right and left chewing. The difference in the opening phase was not as significant. It was concluded that DMC and jaw closing velocity are more sensitive parameters than DTM and DTC on the diagnosis of TMJ dysfunction with or without occlusal interference. DTM and DTC are parameters more closely related to the influence of occlusal factors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Pharyngeal mastication and food transport in the carp (Cyprinus carpio L.): A cineradiographic and electromyographic study

Cyprinids constitute the largest fish family and are characterized by their pharyngeal teeth. The masticatory mechanism is still poorly understood. The complex of structures that determine the movements of pharyngeal teeth and chewing pad in the carp (Cyprinus carpio L.) is analyzed. Activities in 16 head muscles of a free-swimming carp were recorded. X-ray cinerecordings, synchronized with electromyograms, were made of the intake, transport, mastication, and deglutition of radiopaque food pellets. Metal markers allowed a detailed movement analysis. Masticatory cycles are bilaterally synchronous and show distinct crushing and grinding patterns. Direct masticatory muscles that suspend and connect the pharyngeal bones steer and stabilize the masticatory movements. Baudelot's ligament, between skull and pectoral girdle, is applied as fulcrum, effects a crucial shift of the rotation axis of the pharyngeal jaw, and transforms crushing into grinding; simultaneous abduction lengthens the grinding stroke. Body muscles supply indirectly the power for mastication; they also appear to be regulated more distantly. The epaxial muscles lift the skull and thereby the levators of the pharyngeal bones, thus transmitting high forces to the teeth. They also stretch the levator of the bone as soon as occlusion is reached and thus optimize its production of forces during grinding. The hypaxial muscles retract the pharyngeal bones indirectly during grinding and power the teeth in sliding. The chewing pad, previously assumed to be motionless, rotates rostroventrad with the skull and intensifies grinding. Respiration and mastication are mutually related. The extensive movements of the pharyngeal bones are permitted only by the simultaneous expansion of the buccopharynx and a slide-coupling in the branchial floor. Muscular pads that line the pharynx are shown to transport food toward the teeth. The constrictor pharyngis effects deglutition. Natural food, intestinal contents, and feces of the carp were analyzed with respect to the capacity for distinct masticatory operations. During the experiments pellets, barley, and worms were fed. The carp is specialized for polyphagy and this appears to be based on the profiles of the heterodont teeth rather than on drastic changes in the two preprogrammed activity patterns. Comparison of the pharyngeal jaw system in the carp and higher teleosts emphasizes the structural design for the application of large forces in this cyprinid.     

Influence of age and dental status on chewing behaviour studied by EMG recordings during consumption of various food samples

Objective: To evaluate the effects of age and dental status on chewing performance in humans. Design: Electromyography recordings (EMG) were made during chewing of six foods (rice, beef, cheese, crispy bread, apple, and peanuts) to compare the masticatory patterns of four subject groups with different ages and dental status. Subjects: Nineteen elders (mean age 67.2 years) classified into three categories according to their number of opposing post‐canine teeth pairs (i.e. functional units) and a control group of 10 young adults (mean age 26.5 years) with a high number of functional units. Main outcome measures: Number of chewing cycles, chewing time, total muscle activity and muscle activity per chew, burst and inter‐burst durations, maximum and mean voltages from EMG recordings. Results: Time‐related EMG parameters increased from young subjects to elderly subjects with high, middle and low dental status. Parameters related to EMG voltages per chew decreased in the same order among the different groups of subjects. These tendencies were observed for all the studied products. Subjects with weak muscle contraction may compensate for their poor chewing performance by lengthening both chewing cycle and sequence duration. Additional alterations in the chewing patterns were observed when age effect was associated with a dental status degradation in terms of number of functional units. Conclusion: Impairment in mastication for the elderly is due to both ageing and decreasing number of functional pairs of post‐canine teeth.     

The mammalian postorbital bar as a torsion-resisting helical strut

The mammalian skull is asymmetrically loaded during mastication because most of these animals chew on only one side at a time. This loading regime tends to twist the braincase relative to the rostral, tooth bearing part of the skull at the zone of potential weakness between the orbits. This torsional effect is exaggerated, and a postorbital bar is present, in those animals with very large masseter and pterygoid muscles. The lines of action of these muscles are oriented at large angles to the long axis of the skull in lateral view, providing large components of force that twist the skull segments relative to one another. When the temporalis is the dominant muscle, the torsional effect is usually less important, and the bar is absent, because this muscle acts at a smaller angle to the skull axis. The postorbital bar exhibits the predicted three dimensional spatial orientation required to resist torsional forces: it is a segment of an imaginary 45° helix that is wound around the skull, if the skull is idealized as a cylinder. This orientation is significant because, in general, maximum compressive and tensile shear stresses lie along 45° helices on a cylinder loaded in torsion; to resist torsion, material should be placed far from the axis of torsion and along a helix oriented at 45° to the deforming forces. Each half of a supraorbital ridge is also a segment of a 45° helix that is perpendicular to the helix passing through the postorbital bar. This model suggests that the postorbital bar is loaded in compression on the chewing side and in tension on the non-chewing side; the supraorbital ridge is loaded in tension on the chewing side and in compression on the non-chewing side.     

Experimental studies in human tooth wear. II

Studies of human tooth wear have been carried on for the past two years using a machine designed to approximate human chewing motions. During this time wear patterns that resemble those frequently found on the teeth of various American Indian skulls have been produced on casts of the definition. Noticeably among these patterns produced are examples of wear on the front end of the dental arches that result in an edge to edge bite. This type of wear was produced by wearing down casts of a modern dentition with a “normal” overbite. The forces applied to the casts mounted on the machine are variable over a wide range and numerous force combinations are possible. By noting these forces and the resulting vectors, the motions necessary to produce different wear patterns can be determined. This has especially aided in understanding the ways in which the oblique molar wear is produced.     

种植牙技术对老年牙列游离端缺失修复的疗效分析

目的:了解种植牙技术在老年人牙列游离端缺失修复中的应用价值。方法:对我院2012年3月至2014年3月收治的牙列游离端缺失修复患者进行抽样,选取76例患者随机分成两组,对照组予以义齿修复,实验组应用种植牙技术进行修复,对比两组患者咀嚼率、稳固率、成功率及满意度。结果:实验组咀嚼率、成功率、稳固率(91.18%、94.74%、92.11%)明显优于对照组(71.05%、84.21%、73.68%),且主诉美观效果、固位功能、舒适性(84.21%、94.74%、92.11%)明显优于对照组(63.16%、65.78%、71.05%),组间比较差异具有显著统计学意义(P0.05)。结论:在老年人牙列游离端缺失修复中,种植牙技术具有重要的应用价值,疗效确切,修复成功率高,值得临床大力推广与使用。     

Improved single- and multi-contact life-time testing of dental restorative materials using key characteristics of the human masticatory system and a force/position-controlled robotic dental wear simulator

This paper presents a new in vitro wear simulator based on spatial parallel kinematics and a biologically inspired implicit force/position hybrid controller to replicate chewing movements and dental wear formations on dental components, such as crowns, bridges or a full set of teeth. The human mandible, guided by passive structures such as posterior teeth and the two temporomandibular joints, moves with up to 6 degrees of freedom (DOF) in Cartesian space. The currently available wear simulators lack the ability to perform these chewing movements. In many cases, their lack of sufficient DOF enables them only to replicate the sliding motion of a single occlusal contact point by neglecting rotational movements and the motion along one Cartesian axis. The motion and forces of more than one occlusal contact points cannot accurately be replicated by these instruments. Furthermore, the majority of wear simulators are unable to control simultaneously the main wear-affecting parameters, considering abrasive mechanical wear, which are the occlusal sliding motion and bite forces in the constraint contact phase of the human chewing cycle. It has been shown that such discrepancies between the true in vivo and the simulated in vitro condition influence the outcome and the quality of wear studies. This can be improved by implementing biological features of the human masticatory system such as tooth compliance realized through the passive action of the periodontal ligament and active bite force control realized though the central nervous system using feedback from periodontal preceptors. The simulator described in this paper can be used for single- and multi-occlusal contact testing due to its kinematics and ability to exactly replicate human translational and rotational mandibular movements with up to 6 DOF without neglecting movements along or around the three Cartesian axes. Recorded human mandibular motion and occlusal force data are the reference inputs of the simulator. Experimental studies of wear using this simulator demonstrate that integrating the biological feature of combined force/position hybrid control in dental material testing improves the linearity and reduces the variability of results. In addition, it has been shown that present biaxially operated dental wear simulators are likely to provide misleading results in comparative in vitro/in vivo one-contact studies due to neglecting the occlusal sliding motion in one plane which could introduce an error of up to 49% since occlusal sliding motion D and volumetric wear loss V(loss) are proportional.     

Medial pterygoid muscle activity during the closing and compressive phases of human mastication

The lack of specific data correlating activity in the human medial pterygoid muscle with displacement of the jaw during mastication, and the hint of possible differences in function between certain mammalian species, prompted a study of unilateral mastication in six adult subjects. Muscle activity in the medial pterygoid, masseter, and anterior temporal muscles was recorded simultaneously with three-dimensional movement of an incisor point on the mandible. Signals from muscles and displacement transducer were sampled by a disc-based computer system programmed to analyze data averaged over 30 chewing cycles on each side and in some instances over 30 open-close and clench cycles. Patterns of medial pterygoid activity were consistent for the group as a whole, demonstrating activation of both muscles early in the closing cycle with strong ipsilateral muscle activity before and throughout the intercuspal phase of mastication. By contrast contralateral activity ceased during the crushing phase of the cycle, reappearing in some subjects just before the end of intercuspation. Medial pterygoid activity mirrored masseter and anterior temporal activity only during certain phases of the closing cycle, suggesting that these muscles should be considered as being selectively coactivated with, rather than synergists of, the major elevators of the jaw. The muscles were active during horizontal components of movement of the incisor teeth in chewing, but were inactive during the open-close and clench task despite vigorous contraction of the masseter muscles. Overall, the observations complement previous reports of medial pterygoid muscle activity in humans. They also confirm, for these muscles at least, a general similarity between man and the little brown bat, a relationship hitherto suspected but unsubstantiated.     

Coexpression of Notch3 and Rgs5 in the pericyte-vascular smooth muscle cell axis in response to pulp injury

Recent studies have shown that the pulp of human teeth contains a population of cells with stem cell properties and it has been suggested that these cells originate from pericytes. Molecules of the Notch signaling pathway regulate stem cell fate specification, while Rgs5 represents an excellent marker for pericytes. Pathological conditions such as dental trauma and carious lesion stimulate pulp stem cells to elaborate reparative dentin. Previous studies have shown that genes involved in the Notch pathway are activated in response to pulp injury in rodent and humans. To demonstrate the importance of pericytes as a source of stem cells during dental repair, we have studied Rgs5 and Notch3 mRNA expression by in situ hybridization in developing, adult intact and injured rodent teeth. Furthermore, we have examined the distribution of Notch3 protein in carious and injured human teeth using immunohistochemistry. Overlapping expression patterns of Rgs5 and Notch3 were observed during rodent tooth development as well as immediately after injury. Both genes were expressed in vascular structures during development and in perivascular and single capillary cells of injured teeth. However, the expression patterns of Rgs5 and Notch3 were different during tooth repair, with relatively extensive Rgs5 expression along the pericyte-vascular smooth muscle cell axis in central pulp arterioles. These results show co-expression of Rgs5 and Notch3 in pericytes of developing and injured teeth and furthermore indicate the importance of vascular-derived stem cells during pulp healing.     

A new model for calculating muscle forces from electromyograms

A muscle model is described that uses electromyogram (EMG), muscle length and speed of contraction to predict muscle force. Physiological parameters are the Hill constants and the shape of the twitch response to a single stimulus. The model was incorporated in a jaw model of the rabbit and tested by predicting the bite force produced by the jaw muscles during mastication. The time course of the calculated force appeared to match the bite force, measured in vivo by a strain gauge, applied to the bone below the teeth. The variation in peak strain amplitude from cycle to cycle correlated with the variation predicted by the model. The peak amplitude of the integrated EMGs of individual jaw muscles showed an average correlation with peak strain of 0.41. Use of the sum of the available peak amplitudes, weighted according to their effect upon the bite force increased the correlation to 0.46; the model predicted bite forces showed a correlation of 0.57 with the strain. The increase in correlation was statistically significant. The muscle forces were calculated using a minimum number of easily obtainable constants.     

The effects of increasing mandibular load on electrical activity in the mandibular closer muscles during feeding in the desert locust, Schistocerca gregaria

ABSTRACT. Electrical activity from individual mandibular closer muscle fibres of the desert locust Schistocerca gregaria was monitored during three feeding conditions: chewing soft grass, chewing soft grass against an artificial load, and chewing an incompressible grass. Spiking frequency during bursts of activity was found to increase under conditions of increased artificial, and natural load. These increases in spiking frequency occurred over a frequency range much lower than that necessary to produce a maximal tetanic contraction, and therefore correspond to increases in power output by the closer muscles in response to increases in load. A possible mechanism for this is described.     

Patterns of variation across primates in jaw-muscle electromyography during mastication

Biologists that study mammals continue to discuss the evolutionof and functional variation in jaw-muscle activity during chewing.A major barrier to addressing these issues is collecting sufficientin vivo data to adequately capture neuromuscular variation ina clade. We combine data on jaw-muscle electromyography (EMG)collected during mastication from 14 species of primates andone of treeshrews to assess patterns of neuromuscular variationin primates. All data were collected and analyzed using thesame methods. We examine the variance components for EMG parametersusing a nested ANOVA design across successive hierarchical factorsfrom chewing cycle through species for eight locations in themasseter and temporalis muscles. Variation in jaw-muscle EMGswas not distributed equally across hierarchical levels. Thetiming of peak EMG activity showed the largest variance componentsamong chewing cycles. Relative levels of recruitment of jawmuscles showed the largest variance components among chewingsequences and cycles. We attribute variation among chewing cyclesto (1) changes in food properties throughout the chewing sequence,(2) variation in bite location, and (3) the multiple ways jawmuscles can produce submaximal bite forces. We hypothesize thatvariation among chewing sequences is primarily related to variationin properties of food. The significant proportion of variationin EMGs potentially linked to food properties suggests thatexperimental biologists must pay close attention to foods givento research subjects in laboratory-based studies of feeding.The jaw muscles exhibit markedly different variance componentsamong species suggesting that primate jaw muscles have evolvedas distinct functional units. The balancing-side deep masseter(BDM) exhibits the most variation among species. This observationsupports previous hypotheses linking variation in the timingand activation of the BDM to symphyseal fusion in anthropoidprimates and in strepsirrhines with robust symphyses. The working-sideanterior temporalis shows a contrasting pattern with littlevariation in timing and relative activation across primates.The consistent recruitment of this muscle suggests that primateshave maintained their ability to produce vertical jaw movementsand force in contrast to the evolutionary changes in transverseocclusal forces driven by the varying patterns of activationin the BDM.     

Krapina, “Classic” Neanderthals,and the evolution of the European face

Except for the front end of the dental arch, tooth size remained at approximately the same level throughout the Middle Pleistocene. The Krapina Neanderthals at the end of the last interglacial differed from Homo erectus only in having larger front teeth. From that time on, tooth size in populations at the northern edge of the area of human occupation in the Old World has reduced approximately in proportion to the time elapsed. The “Classic” Neanderthals of western Europe, in fact, have teeth that are 15% smaller than those of the earlier Krapina Neanderthals and only 5% larger than the early Upper Palaeolithic. Reduction since the early Upper Palaeolithic has proceeded another full 20%. It is suggested that the development of heated stone cooking in the Mousterian, originally for the purpose of thawing frozen food, reduced the forces of selection that had previously maintained tooth size during the Middle Pleistocene. The operation of the Probable Mutation Effect, then produced the observed reductions.     

Myofascial force transmission: muscle relative position and length determine agonist and synergist muscle force.

Equal proximal and distal lengthening of rat extensor digitorum longus (EDL) were studied. Tibialis anterior, extensor hallucis longus, and EDL were active maximally. The connective tissues around these muscle bellies were left intact. Proximal EDL forces differed from distal forces, indicating myofascial force transmission to structures other than the tendons. Higher EDL distal force was exerted (ratio approximately 118%) after distal than after equal proximal lengthening. For proximal force, the reverse occurred (ratio approximately 157%). Passive EDL force exerted at the lengthened end was 7-10 times the force exerted at the nonlengthened end. While kept at constant length, synergists (tibialis anterior + extensor hallucis longus: active muscle force difference approximately -10%) significantly decreased in force by distal EDL lengthening, but not by proximal EDL lengthening. We conclude that force exerted at the tendon at the lengthened end of a muscle is higher because of the extra load imposed by myofascial force transmission on parts of the muscle belly. This is mediated by changes of the relative position of most parts of the lengthened muscle with respect to neighboring muscles and to compartment connective tissues. As a consequence, muscle relative position is a major codeterminant of muscle force for muscle with connectivity of its belly close to in vivo conditions.     

Biomechanics of the masticatory apparatus of Pteropus giganteus (Megachiroptera)

Jaw mechanics in Pteropus were studied by means of a three-dimensional model. The model included several parameters of muscle architecture, combined with quantified movement and electromyographical data. Estimates of the nature of the applied forces that act upon the mandible during a chewing cycle, and subsequent estimates of reaction forces at the bite point and joints during the powerstroke, were thus obtained for different food consistencies. The resultant muscle force (relative to the palate) shifts from upward and slightly backward at large gapes to upward and markedly backward at the end of closing. The resultant simultaneously moves anteriorly. During the powerstroke it retains a constant position and orientation along the thickened anterior edge of the coronoid process. The early stages of opening are guided by the slope of the teeth and mandibular fossa; during the remaining part of opening the working line of the resultant crosses the skull behind the joint and thus acquires an opening moment. The bite force has downward and forward components, and a slight transverse component. For a given applied muscular force its magnitude is larger in more posteriorly positioned bite points. Both joints are loaded, the contralateral one more than the ipsilateral. Food consistency affects magnitude and orientation of the applied force, and hence, magnitude and orientation of the bite force and magnitude of the joint reaction forces. The magnitude of masseter activity relative to temporalis activity appears to be the key factor for the orientation of the bite force, and hence for the mechanical optimal position of the food. The adaptive value of the general topography of the masticatory muscles in Pteropus is discussed.     

Oral disc muscles of anuran tadpoles

The presence and configurations of the small, extrinsic muscle fibers of the M. mandibulolabialis superior in the upper labium and the M. inferior in the lower labium are described for 24 species in 9 families. Although several species possess only the M. inferior, or both the M. inferior and superior, none has only the M. superior. Some species without tooth rows possess the M. inferior, and microhylid tadpoles have a single-stranded muscle different than the M. mandibulolabialis in each oral flap. The extrinsic oral muscles insert near the bottom of the inter-row valleys behind each tooth ridge and the bases of marginal papillae lateral to the ends of tooth rows. Muscles do not extend to marginal papillae beyond the most distal row of teeth. Muscle contraction is presumed to rotate the teeth distally so that they contact the substrate at the proper working angle. This action probably occurs at full extension of the oral disc just before disc closure starts.     

Differences in muscle sympathetic nerve response to isometric exercise in different muscle groups

The aim of this study was to examine the effects of muscle fibre composition on muscle sympathetic nerve activity (MSNA) in response to isometric exercise. The MSNA, recorded from the tibial nerve by a microneurographic technique during contraction and following arterial occlusion, was compared in three different muscle groups: the forearm (handgrip), anterior tibialis (foot dorsal contraction), and soleus muscles (foot plantar contraction) contracted separately at intensities of 20%, 33% and 50% of the maximal voluntary force. The increases in MSNA relative to control levels during contraction and occlusion were significant at all contracting forces for handgrip and at 33% and 50% of maximal for dorsal contraction, but there were no significant changes, except during exercise at 50%, for plantar contraction. The size of the MSNA response correlated with the contraction force in all muscle groups. Pooling data for all contraction forces, there were different MSNA responses among muscle groups in contraction forces (P = 0.0001, two-way analysis of variance), and occlusion periods (P = 0.0001). The MSNA increases were in the following order of magnitude: handgrip, dorsal, and plantar contractions. The order of the fibre type composition in these three muscles is from equal numbers of types I and II fibres in the forearm to increasing number of type I fibres in the leg muscles. The different MSNA responses to the contraction of different muscle groups observed may have been due in part to muscle metaboreflex intensity influenced by their metabolic capacity which is related to by their metabolic capacity which is related to the fibre type.     

Evolving Teeth Within a Stable Masticatory Apparatus in Orkney Mice

Mice from the Orkney archipelago exhibit an important diversity regarding molar shape. While on some islands mice display a usual dental pattern, teeth from other islands display additional cusplets and unusual phenotypes that may constitute case studies for evaluating the potential functional relevance of dental changes. We developed a multifaceted approach combining 2D and 3D geometric morphometrics, dental topography, dental wear, biomechanics, estimations of masticatory muscles force, and in vivo bite force on wild-derived lab descendants exemplifying the two extreme dental morphologies. The two strains differed in the geometry of the upper and lower tooth rows, and in the topography of the upper row only. Surprisingly, the most unusual tooth morphology appeared as the least complex because tooth simplification overwhelmed the signal provided by the occurrence of additional cusplets. No difference in bite force nor muscle force was evidenced, showing that the important change in dental morphology was accommodated without major changes in the rest of the masticatory apparatus. The evolution of unusual dental phenotypes was possibly fueled by drift and inbreeding in small and isolated populations on remote islands of the archipelago. No functional counter-selection impeded this diversification, since the unusual dental phenotypes did not disrupt occlusion and mastication.