Trends in the Actions of Mammalian Masticatory Muscles

The actions of the masticatory muscles of a variety of mammalsin which feeding behavior and the configuration of the masticatoryapparatus differ have been reported. The most common approachused in these studies involves (1) obtaining a good anatomicalperception of the musculature, (2) deriving a theoretical modelof the actions of these muscles during jaw movement, and (3)testing this model by recording muscle activity and jaw movementssimultaneously. A catalogue of the activity patterns in eleven species of mammalsduring food reduction reveals certain trends in the actionsof the masticatory muscles. Horizontal jaw movements are generatedprimarily by differential activities of the deep temporalis,superficial masseter, and medial pterygoid. Vertical movementsand the maintenance of tooth to food contact apparently areproduced by action of the superficial temporalis, deep masseter,and zygomaticomandibularis. Thus, horizontal movements are seeminglygenerated by muscles having fibers arranged in marked anteroposteriordirection, whereas vertical movements are generated by muscleshaving more or less vertically arranged fibers. The asymmetry of jaw movement and the muscular activity generatingit suggest that mastication involves an interactionbetween anunbalanced and flexible functional unit (muscles) and a balancedand stable structural unit (skull and teeth). Thus, any unbalancingof the structural unit results in a further unbalancing of themasticatory process.     

Jaw-muscle electromyography during chewing in Belanger's treeshrews (Tupaia belangeri)

We examined masseter and temporalis recruitment and firing patterns during chewing in five male Belanger's treeshrews (Tupaia belangeri), using electromyography (EMG). During chewing, the working-side masseters tend to show almost three times more scaled EMG activity than the balancing-side masseters. Similarly, the working-side temporalis muscles have more than twice the scaled EMG activity of the balancing-side temporalis. The relatively higher activity in the working-side muscles suggests that treeshrews recruit less force from their balancing-side muscles during chewing. Most of the jaw-closing muscles in treeshrews can be sorted into an early-firing or late-firing group, based on occurrence of peak activity during the chewing cycle. Specifically, the first group of jaw-closing muscles to reach peak activity consists of the working-side anterior and posterior temporalis and the balancing-side superficial masseter. The balancing-side anterior and posterior temporalis and the working-side superficial masseter peak later in the power stroke. The working-side deep masseter peaks, on average, slightly before the working-side superficial masseter. The balancing-side deep masseter typically peaks early, at about the same time as the balancing-side superficial masseter. Thus, treeshrews are unlike nonhuman anthropoids that peak their working-side deep masseters early and their balancing-side deep masseters late in the power stroke. Because in anthropoids the late firing of the balancing-side deep masseter contributes to wishboning of the symphysis, the treeshrew EMG data suggest that treeshrews do not routinely wishbone their symphyses during chewing. Based on the treeshrew EMG data, we speculate that during chewing, primitive euprimates 1) recruited more force from the working-side jaw-closing muscles as compared to the balancing-side muscles, 2) fired an early group of jaw-closing muscles followed by a second group of muscles that peaked later in the power stroke, 3) did not fire their working-side deep masseter significantly earlier than their working-side superficial masseter, and 4) did not routinely fire their balancing-side deep masseter after the working-side superficial masseter.     

Masseter electromyography during chewing in ring-tailed lemurs (Lemur catta)

We examined masseter recruitment and firing patterns during chewing in four adult ring-tailed lemurs (Lemur catta), using electromyography (EMG). During chewing of tougher foods, the working-side superficial masseter tends to show, on average, 1.7 times more scaled EMG activity than the balancing-side superficial masseter. The working-side deep masseter exhibits, on average, 2.4 times the scaled EMG activity of the balancing-side deep masseter. The relatively larger activity in the working-side muscles suggests that ring-tailed lemurs recruit relatively less force from their balancing-side muscles during chewing. The superficial masseter working-to-balancing-side (W/B) ratio for lemurs overlaps with W/B ratios from anthropoid primates. In contrast, the lemur W/B ratio for the deep masseter is more similar to that of greater galagos, while both are significantly larger than W/B ratios of anthropoids. Because ring-tailed lemurs have unfused and hence presumably weaker symphyses, these data are consistent with the symphyseal fusion-muscle recruitment hypothesis stating that symphyseal fusion in anthropoids provides increased strength for resisting forces created by the balancing-side jaw muscles during chewing. Among the masseter muscles of ring-tailed lemurs, the working-side deep masseter peaks first on average, followed in succession by the balancing-side deep masseter, balancing-side superficial masseter, and finally the working-side superficial masseter. Ring-tailed lemurs are similar to greater galagos in that their balancing-side deep masseter peaks well before their working-side superficial masseter. We see the opposite pattern in anthropoids, where the balancing-side deep masseter peaks, on average, after the working-side superficial masseter. This late activity of the balancing-side deep masseter in anthropoids is linked to lateral-transverse bending, or wishboning, of their mandibular symphyses. Subsequently, the stresses incurred during wishboning are hypothesized to be a proximate reason for strengthening, and hence fusion, of the anthropoid symphysis. Thus, the absence of this muscle-firing pattern in ring-tailed lemurs with their weaker, unfused symphyses provides further correlational support for the symphyseal fusion late-acting balancing-side deep masseter hypothesis linking wishboning and symphyseal strengthening in anthropoids. The early peak activity of the working-side deep masseter in ring-tailed lemurs is unlike galagos and most similar to the pattern seen in macaques and baboons. We hypothesize that this early activity of the working-side deep masseter moves the lower jaw both laterally toward the working side and vertically upward, to position it for the upcoming power stroke. From an evolutionary perspective, the differences in peak firing times for the working-side deep masseter between ring-tailed lemurs and greater galagos indicate that deep masseter firing patterns are not conserved among strepsirrhines.     

Quantitative assay of electromyograms during mastication in domestic cats (Felis catus)

Mastication has been studied by cinematography with synchronized electromyography (computer quantified and analyzed), while unanesthetized, freely feeding cats (Felis catus) were reducing equivalent-sized chunks of raw and cooked beef and cooked chicken. Cats reduce food on one side at a time, and their chewing cycles show both horizontal and anteroposterior deflections. Food objects are shifted from side to side by lateral jerks of the head and movements of the tongue. During the opening phase, the lower jaw is rotated relatively straight downward, and the digastric muscles are active in bilateral symmetry. Near the end of opening, the head jerks upward, both zygomaticomandibulares start to fire, and opening acceleration of the mandible decreases. Closing starts with horizontal displacement of the mandibular canines toward the working side, accompanied by asymmetrical activities from the working side deep temporalis and the balancing side medial pterygoid, as well as a downward jerk of the head. As closing proceeds, the mandibular canines remain near the working side and the working side zygomaticomandibularis and deep masseter are very active. Near the end of closing, the mandibular canine on the working side moves toward the midline, and adductors, digastrics, and lateral pterygoids of both sides are active. The adductors of the working side are generally more active than those of the balancing side. During a reduction sequence, the number and shape of the masticatory cycles, as well as movements of the head, during a reduction sequence are affected significantly by food type. As reduction proceeds, the duration of bite and the muscular activity (as characterized by number and amplitude of spikes) change significantly among muscles of the working and balancing sides. The adductors of the working side are generally most active when cats chew raw beef, less for cooked beef, and least for cooked chicken. In general, the adductor activity reflects food consistency, whereas that of the digastrics and lateral pterygoids reflects more the vertical and lateral displacements of the mandible. Statistical analysis documents that the methods of electrode insertion and test give repeatable results for particular sites in different animals. Thus, it should be possible to compare these results with those produced while other mammalas are masticating.     

Masticatory motor patterns in ungulates: a quantitative assessment of jaw-muscle coordination in goats, alpacas and horses

We investigated patterns of jaw-muscle coordination during rhythmic mastication in three species of ungulates displaying the marked transverse jaw movements typical of many large mammalian herbivores. In order to quantify consistent motor patterns during chewing, electromyograms were recorded from the superficial masseter, deep masseter, posterior temporalis and medial pterygoid muscles of goats, alpacas and horses. Timing differences between muscle pairs were evaluated in the context of an evolutionary model of jaw-muscle function. In this model, the closing and food reduction phases of mastication are primarily controlled by two distinct muscle groups, triplet I (balancing-side superficial masseter and medial pterygoid and working-side posterior temporalis) and triplet II (working-side superficial masseter and medial pterygoid and balancing-side posterior temporalis), and the asynchronous activity of the working- and balancing-side deep masseters. The three species differ in the extent to which the jaw muscles are coordinated as triplet I and triplet II. Alpacas, and to a lesser extent, goats, exhibit the triplet pattern whereas horses do not. In contrast, all three species show marked asynchrony of the working-side and balancing-side deep masseters, with jaw closing initiated by the working-side muscle and the balancing-side muscle firing much later during closing. However, goats differ from alpacas and horses in the timing of the balancing-side deep masseter relative to the triplet II muscles. This study highlights interspecific differences in the coordination of jaw muscles to influence transverse jaw movements and the production of bite force in herbivorous ungulates.     

Form and function of the masticatory musculature in the tree sloths, Bradypus and Choloepus

The tree sloths, Bradypus and Choloepus, show unusual masticatory specializations, compared to each other and to other mammals. Both have an incomplete zygomatic arch with descending jugal process, a complex superficial masseter, a large temporalis and medial pterygoid musculature, and a lateral pterygoid with two heads. In Choloepus the deep masseter and zygomaticomandibularis are typical when compared to other mammals. However, in Bradypus there is an ascending jugal process from which enlarged and vertically oriented deep masseter and zygomaticomandibularis muscles originate. Although both sloths are folivores, the anterior teeth in Choloepus are caniniform, while those of Bradypus have lost such elongation. In both sloths the glenoid cavity is similarly located; however, in Bradypus the craniomandibular joint is raised above the occlusal plane, and the pterygoid flanges are elongated. Prediction of the evolutionary sequence of cranial changes from Choloepus-like (primitive) to Bradypus-like (derived) morphology is based upon the most parsimonious model of masseter-medial pterygoid complex changes for masticatory efficiency improvement. The model proposes that the condylar neck in Bradypus was elongated and that this single change predicated a series of other structural changes. Mandibular movement patterns in both sloths showed anteromedially directed unilateral power strokes as in other mammals. Puncture-crushing, tooth-sharpening, and chewing cycles are distinct in Choloepus, less so in Bradypus. The masticatory rate is slow in sloths compared to other mammals of similar body size, averaging 590 ms per cycle for Choloepus and 510 ms for Bradypus.     

Symphyseal fusion and jaw-adductor muscle force: an EMG study

The purpose of this study is to test various hypotheses about balancing-side jaw muscle recruitment patterns during mastication, with a major focus on testing the hypothesis that symphyseal fusion in anthropoids is due mainly to vertically- and/or transversely-directed jaw muscle forces. Furthermore, as the balancing-side deep masseter has been shown to play an important role in wishboning of the macaque mandibular symphysis, we test the hypothesis that primates possessing a highly mobile mandibular symphysis do not exhibit the balancing-side deep masseter firing pattern that causes wishboning of the anthropoid mandible. Finally, we also test the hypothesis that balancing-side muscle recruitment patterns are importantly related to allometric constraints associated with the evolution of increasing body size. Electromyographic (EMG) activity of the left and right superficial and deep masseters were recorded and analyzed in baboons, macaques, owl monkeys, and thick-tailed galagos. The masseter was chosen for analysis because in the frontal projection its superficial portion exerts force primarily in the vertical (dorsoventral) direction, whereas its deep portion has a relatively larger component of force in the transverse direction. The symphyseal fusion-muscle recruitment hypothesis predicts that unlike anthropoids, galagos develop bite force with relatively little contribution from their balancing-side jaw muscles. Thus, compared to galagos, anthropoids recruit a larger percentage of force from their balancing-side muscles. If true, this means that during forceful mastication, galagos should have working-side/balancing-side (W/B) EMG ratios that are relatively large, whereas anthropoids should have W/B ratios that are relatively small. The EMG data indicate that galagos do indeed have the largest average W/B ratios for both the superficial and deep masseters (2.2 and 4.4, respectively). Among the anthropoids, the average W/B ratios for the superficial and deep masseters are 1.9 and 1.0 for baboons, 1.4 and 1.0 for macaques, and both values are 1.4 for owl monkeys. Of these ratios, however, the only significant difference between thick-tailed galagos and anthropoids are those associated with the deep masseter. Furthermore, the analysis of masseter firing patterns indicates that whereas baboons, macaques and owl monkeys exhibit the deep masseter firing pattern associated with wishboning of the macaque mandibular symphysis, galagos do not exhibit this firing pattern. The allometric constraint-muscle recruitment hypothesis predicts that larger primates must recruit relatively larger amounts of balancing-side muscle force so as to develop equivalent amounts of bite force. Operationally this means that during forceful mastication, the W/B EMG ratios for the superficial and deep masseters should be negatively correlated with body size. Our analysis clearly refutes this hypothesis. As already noted, the average W/B ratios for both the superficial and deep masseter are largest in thick-tailed galagos, and not, as predicted by the allometric constraint hypothesis, in owl monkeys, an anthropoid whose body size is smaller than that of thick-tailed galagos. Our analysis also indicates that owl monkeys have W/B ratios that are small and more similar to those of the much larger-sized baboons and macaques. Thus, both the analysis of the W/B EMG ratios and the muscle firing pattern data support the hypothesis that symphyseal fusion and transversely-directed muscle force in anthropoids are functionally linked. This in turn supports the hypothesis that the evolution of symphyseal fusion in anthropoids is an adaptation to strengthen the symphysis so as to counter increased wishboning stress during forceful unilateral mastication. (ABSTRACT TRUNCATED)     

Functional and evolutionary significance of the recruitment and firing patterns of the jaw adductors during chewing in Verreaux's sifaka (Propithecus verreauxi)

Jaw-muscle electromyographic (EMG) patterns indicate that compared with thick-tailed galagos and ring-tailed lemurs, anthropoids recruit more relative EMG from their balancing-side deep masseter, and that this muscle peaks late in the power stroke. These recruitment and firing patterns in anthropoids are thought to cause the mandibular symphysis to wishbone (lateral transverse bending), resulting in relatively high symphyseal stresses. We test the hypothesis that living strepsirrhines with robust, partially fused symphyses have muscle recruitment and firing patterns more similar to anthropoids, unlike those strepsirrhines with highly mobile unfused symphyses. Electromyographic (EMG) activity of the superficial and deep masseter, anterior and posterior temporalis, and medial pterygoid muscles were recorded in four dentally adult Verreaux's sifakas (Propithecus verreauxi). As predicted, we find that sifaka motor patterns are more similar to anthropoids. For example, among sifakas, recruitment levels of the balancing-side (b-s) deep masseter are high, and the b-s deep masseter fires late during the power stroke. As adult sifakas often exhibit nearly complete symphyseal fusion, these data support the hypothesis that the evolution of symphyseal fusion in primates is functionally linked to wishboning. Furthermore, these data provide compelling evidence for the convergent evolution of the wishboning motor patterns in anthropoids and sifakas.     

Reconstruction of the cranial musculature and masticatory function of the Pleistocene panamerican ground sloth Eremotherium laurillardi (Mammalia,Xenarthra, Megatheriidae)

Cranial musculature, dental function and mandibular movement patterns in Eremotherium laurillardi were reconstructed from the examination of crania and dentitions. Size, shape and pattern of muscle divisions were reconstructed from the examination of bony rugosities indicating muscle attachments. Details of masticatory muscle structure and function were based on dissections of the tree sloths Bradypus and Choloepus. Among sloths, masticatory muscles in E. laurillardi demonstrate a different synergist–antagonist pattern, reflecting greater emphasis on mediolateral mandibular movements. Eight cranial character complexes (anterior facial, zygomatic arch, superficial masseter, deep masseter–zygomaticomandibularis, pterygoid, temporal, occipital and occlusal) determined by interrelated contributions of each component made to group functions were identified. An elongate anterior face and predental spout in E. laurillardi allowed protrusion of a long narrow tongue at small degrees of gape, reflecting a probably ancestral xenarthran condition. Gape minimisation, in conjunction with the mediolaterally directed masticatory stroke in E. laurillardi, was a unique solution to increase masticatory efficiency by permitting molariform tooth shearing surfaces to remain in or near occlusion for a greater percentage of each chewing cycle.     

Temporalis function in anthropoids and strepsirrhines: an EMG study

The major purpose of this study is to analyze anterior and posterior temporalis muscle force recruitment and firing patterns in various anthropoid and strepsirrhine primates. There are two specific goals for this project. First, we test the hypothesis that in addition to transversely directed muscle force, the evolution of symphyseal fusion in primates may also be linked to vertically directed balancing-side muscle force during chewing (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). Second, we test the hypothesis of whether strepsirrhines retain the hypothesized primitive mammalian condition for the firing of the anterior temporalis, whereas anthropoids have the derived condition (Weijs [1994] Biomechanics of Feeding in Vertebrates; Berlin: Springer-Verlag, p. 282-320). Electromyographic (EMG) activities of the left and right anterior and posterior temporalis muscles were recorded and analyzed in baboons, macaques, owl monkeys, thick-tailed galagos, and ring-tailed lemurs. In addition, as we used the working-side superficial masseter as a reference muscle, we also recorded and analyzed EMG activity of the left and right superficial masseter in these primates. The data for the anterior temporalis provided no support for the hypothesis that symphyseal fusion in primates is linked to vertically directed jaw muscle forces during mastication. Thus, symphyseal fusion in primates is most likely mainly linked to the timing and recruitment of transversely directed forces from the balancing-side deep masseter (Hylander et al. [2000] Am. J. Phys. Anthropol. 112:469-492). In addition, our data demonstrate that the firing patterns for the working- and balancing-side anterior temporalis muscles are near identical in both strepsirrhines and anthropoids. Their working- and balancing-side anterior temporalis muscles fire asynchronously and reach peak activity during the power stroke. Similarly, their working- and balancing-side posterior temporalis muscles also fire asynchronously and reach peak activity during the power stroke. Compared to these strepsirrhines, however, the balancing-side posterior temporalis of anthropoids appears to have a relatively delayed firing pattern. Moreover, based on their smaller W/B ratios, anthropoids demonstrate a relative increase in muscle-force recruitment of the balancing-side posterior temporalis. This in turn suggests that anthropoids may emphasize the duration and magnitude of the power stroke during mastication. This hypothesis, however, requires additional testing. Furthermore, during the latter portion of the power stroke, the late activity of the balancing-side posterior temporalis of anthropoids apparently assists the balancing-side deep masseter in driving the working-side molars through the terminal portion of occlusion.     

A cineradiographic and electromyographic study of mastication in Tenrec ecaudatus

Regular chewing was studied in the specialized Malagasy insectivore Tenrec ecaudatus with the aid of precisely correlated electromyography of the main adductors, digastrics, and two hyoid muscles and cineradiography for which metallic markers were placed in the mandibles, tongue, and hyoid bone. During the power stroke the body of the mandible moves dorsally and medially. The medially directed component of movement at this time is greatly increased by simultaneous rotation of the mandible about its longitudinal axis. The highly mobile symphysis, spherical dentary condyle, loss of superficial masseter muscle and zygoma, and the simplified zalamnodont molars all appear to be related to the large amount of mandibular rotation that occurs during occlusion. The balancing side lateral pterygoid muscle (inferior head) apparently shifts the working side mandible laterally during the last part of opening and the first part of closing. The working side temporalis and the superficial masseter muscle are both responsible for the shift back to the midline. The temporalis is usually active to the same extent on the working and balancing sides during the power stroke. The level of activity (amplitude) of the temporalis and duration of the power stroke increase with harder foods. Whenever soft foods are chewed, the superficial masseter is only active on the working side; whenever foods of increasing hardness are chewed, its level of activity on the balancing side increases to approach that of the working side. Mandibular rotation is greatly reduced when hard foods are chewed.     

Further evaluation of an EMG technique for assessment of the deep cervical flexor muscles

A novel surface electromyographic (EMG) technique was recently described for the detection of deep cervical flexor muscle activity. Further investigation of this technique is warranted to ensure EMG activity from neighbouring muscles is not markedly influencing the signals recorded. This study compared deep cervical flexor (DCF) muscle activity with the activity of surrounding neck and jaw muscles during various anatomical movements of the neck and jaw in 10 volunteer subjects. DCF EMG activity was recorded with custom electrodes inserted via the nose and fixed by suction to the posterior mucosa of the oropharynx. Surface electrodes were placed over the sternocleidomastoid, anterior scalene, masseter and suprahyoid muscles. Positioned in supine, subjects performed isometric cranio-cervical flexion, cervical flexion, right and left cervical rotation, jaw clench and resisted jaw opening. Across all movements examined, EMG amplitude of the DCF muscles was greatest during neck movements that would require activity of the DCF muscles, particularly during cranio-cervical flexion, their primary anatomical action. The actions of jaw clench and resisted jaw opening demonstrated significantly less DCF EMG activity than the cranio-cervical flexion action (p < 0.05). Across all other movements, the neighbouring neck and jaw muscles demonstrated greatest EMG amplitude during their respective primary anatomical actions, which occurred in the absence of increased EMG amplitude recorded from the DCF muscles. The finding of substantial EMG activity of the DCF muscles only during neck actions that would require their activity, particularly cranio-cervical flexion, and not during actions involving the jaw, provide further assurance that the majority of myoelectric signals detected from the nasopharyngeal electrode are from the DCF muscles.     

The structure and function of the jaw muscles in the rat (Rattus norvegicus L.)

'Tonic' and 'phasic' muscle fibre types can be distinguished histologically, using either histochemical techniques or by staining for lipid with Sudan black B. As muscles of mastication not only move the lower jaw of the rat, a 'phasic' action, but also suspend it from the cranium, a 'tonic' activity, some indication of the contribution of the major muscles to these functions has been gained from an examination of the fibre content of transverse frozen sections stained with Sudan black B. The numbers of 'pale' ('phasic') and 'dark' ('tonic') fibres were counted using a montage at a magnification of 60. Results suggest that the anterior temporal, deep masseter and external pterygoid have an important tonic action in stabilizing the position of the lower jaw as well as contributing to the production of movement; and that the superficial masseter and posterior temporal, in particular, have an almost completely phasic action. These conclusions are entirely consistent with the probable functions of the muscles inferred from their anatomy.     

Contour analysis of masticatory jaw movements and muscle activity in Macaca mulatta

Frontal plane mandibular movements during mastication and the associated electromyographic (EMG) activity for left and right superficial masseter, posterior temporalis, anterior temporalis, and anterior belly of the digastric (ABD) were studied for two adult male Macaca mulatta by the new technique of “contour” analysis. Contour analysis allowed graphic and quantitative portrayal of multiple chew cycle patterns of mandibular movement and EMG activity during active mastication. A series of computer programs (ATS, ATSED, ATSXYZ) facilitated the collection, editing and definition, and finally processing of these masticatory data into contour plots. These preliminary data indicated the essential symmetry of mandibular movement patterns, high chew cycle variability inferior to occlusion, multiple centers of intense EMG activity for balancing-side superficial masseter, and no difference between working-side anterior and posterior temporalis EMG patterns. Maximum EMG amplitude was found in the area of buccal phase power stroke (BPS). Maximum EMG amplitude for ABD was located medial and inferior to occlusion; all other muscle maximum amplitudes were buccal and inferior to occlusion. The location of maximum EMG amplitudes for superficial masseter and ABD were closer to occlusion (more superior) during mastication of carrot than were maximum amplitudes during biscuit mastication. The absence of any detectable shift of EMG maximum amplitude location between biscuit and carrot for posterior and anterior temporalis suggested, along with the continuous EMG activity of working-side posterior temporalis, a secondary role for the temporalis (compensation for superficial masseter activity) during active mastication.     

Electrogenesis of muscular fibers in the rat masseter muscle proper

It was established by the method of intracellular lead of membrane rest potentials (MRP) and action potentials (AP) that fibers with high MRP and little overshot are mainly located in superficial layers of the rat masseter muscle proper whereas those with low MRP and high overshot principally in deep layers. Excitability of the cytoplasmic membrane of muscular fibers of both types proved to be related in electrical properties (critical level of depolarization, current threshold). It is suggested that the rat masseter muscle contains a great number of rapid phasic fibers in superficial layers and slow ones in deep layers.     

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.     

Control of jaw movements in two species of macropodines (Macropus eugenii and Macropus rufus)

The masticatory motor patterns of three tammar wallabies and two red kangaroos were determined by analyzing the pattern of electromyographic (EMG) activity of the jaw adductors and correlating it with lower jaw movements, as recorded by digital video and videoradiography. Transverse jaw movements were limited by the width of the upper incisal arcade. Molars engaged in food breakdown during two distinct occlusal phases characterized by abrupt changes in the direction of working-side hemimandible movement. Separate orthal (Phase I) and transverse (Phase II) trajectories were observed. The working-side lower jaw initially was drawn laterally by the balancing-side medial pterygoid and then orthally by overlapping activity in the balancing- and working-side temporalis and the balancing-side superficial masseter and medial pterygoid. Transverse movement occurred principally via the working-side medial pterygoid and superficial masseter. This pattern contrasted to that of placental herbivores, which are known to break down food when they move the working-side lower jaw transversely along a relatively longer linear path without changing direction during the power stroke. The placental trajectory results from overlapping activity in the working- and balancing-side adductor muscles, suggesting that macropods and placental herbivores have modified the primitive masticatory motor pattern in different ways.     

Anticipatory postural adjustments to arm movement reveal complex control of paraspinal muscles in the thorax

Anatomical and empirical data suggest that deep and superficial muscles may have different functions for thoracic spine control. This study investigated thoracic paraspinal muscle activity during anticipatory postural adjustments associated with arm movement. Electromyographic (EMG) recordings were made from the right deep (multifidus/rotatores) and superficial (longissimus) muscles at T5, T8, and T11 levels using fine-wire electrodes. Ten healthy participants performed fast unilateral and bilateral flexion and extension arm movements in response to a light. EMG amplitude was measured during 25 ms epochs for 150 ms before and 400 ms after deltoid EMG onset. During arm flexion movements, multifidus and longissimus had two bursts of activity, one burst prior to deltoid and a late burst. With arm extension both muscles were active in a single burst after deltoid onset. There was differential activity with respect to direction of trunk rotation induced by arm movement. Right longissimus was most active with left arm movements and right multifidus was most active with right arm movements. All levels of the thorax responded similarly. We suggest that although thoracic multifidus and longissimus function similarly to control sagittal plane perturbations, these muscles are differentially active with rotational forces on the trunk.     

Control of jaw movements in two species of macropodines (Macropus eugenii and Macropus rufus).

The masticatory motor patterns of three tammar wallabies and two red kangaroos were determined by analyzing the pattern of electromyographic (EMG) activity of the jaw adductors and correlating it with lower jaw movements, as recorded by digital video and videoradiography. Transverse jaw movements were limited by the width of the upper incisal arcade. Molars engaged in food breakdown during two distinct occlusal phases characterized by abrupt changes in the direction of working-side hemimandible movement. Separate orthal (Phase I) and transverse (Phase II) trajectories were observed. The working-side lower jaw initially was drawn laterally by the balancing-side medial pterygoid and then orthally by overlapping activity in the balancing- and working-side temporalis and the balancing-side superficial masseter and medial pterygoid. Transverse movement occurred principally via the working-side medial pterygoid and superficial masseter. This pattern contrasted to that of placental herbivores, which are known to break down food when they move the working-side lower jaw transversely along a relatively longer linear path without changing direction during the power stroke. The placental trajectory results from overlapping activity in the working- and balancing-side adductor muscles, suggesting that macropods and placental herbivores have modified the primitive masticatory motor pattern in different ways.