Muscle proprioceptors in adult rat: mechanosensory signaling and synapse distribution in spinal cord.

The characteristic signaling and intraspinal projections of muscle proprioceptors best described in the cat are often generalized across mammalian species. However, species-dependent adaptations within this system seem necessary to accommodate asymmetric scaling of length, velocity, and force information required by the physics of movement. In the present study we report mechanosensory responses and intraspinal destinations of three classes of muscle proprioceptors. Proprioceptors from triceps surae muscles in adult female Wistar rats anesthetized with isoflurane were physiologically classified as muscle spindle group Ia or II or as tendon organ group Ib afferents, studied for their firing responses to passive-muscle stretch, and in some cases labeled and imaged for axon projections and varicosities in spinal segments. Afferent projections and the laminar distributions of provisional synapses in rats closely resembled those found in the cat. Afferent signaling of muscle kinematics was also similar to reports in the cat, but rat Ib afferents fired robustly during passive-muscle stretch and Ia afferents displayed an exaggerated dynamic response, even after locomotor scaling was accounted for. These differences in mechanosensory signaling by muscle proprioceptors may represent adaptations for movement control in different animal species.NEW & NOTEWORTHY Muscle sensory neurons signal information necessary for controlling limb movements. The information encoded and transmitted by muscle proprioceptors to networks in the spinal cord is known in detail only for the cat, but differences in size and behavior of other species challenge the presumed generalizability. This report presents the first findings detailing specializations in mechanosensory signaling and intraspinal targets for functionally identified subtypes of muscle proprioceptors in the rat.

Independent AMP and NAD signaling regulates C2C12 differentiation and metabolic adaptation

The balance of ATP production and consumption is reflected in adenosine monophosphate (AMP) and nicotinamide adenine dinucleotide (NAD) content and has been associated with phenotypic plasticity in striated muscle. Some studies have suggested that AMPK-dependent plasticity may be an indirect consequence of increased NAD synthesis and SIRT1 activity. The primary goal of this study was to assess the interaction of AMP- and NAD-dependent signaling in adaptation of C2C12 myotubes. Changes in myotube developmental and metabolic gene expression were compared following incubation with 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and nicotinamide mononucleotide (NMN) to activate AMPK- and NAD-related signaling. AICAR showed no effect on NAD pool or nampt expression but significantly reduced histone H3 acetylation and GLUT1, cytochrome C oxidase subunit 2 (COX2), and MYH3 expression. In contrast, NMN supplementation for 24 h increased NAD pool by 45 % but did not reduce histone H3 acetylation nor promote mitochondrial gene expression. The combination of AMP and NAD signaling did not induce further metabolic adaptation, but NMN ameliorated AICAR-induced myotube reduction. We interpret these results as indication that AMP and NAD contribute to C2C12 differentiation and metabolic adaptation independently (AU)

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Absence of morphological and molecular correlates of sarcopenia in the macaque tongue muscle styloglossus.

INTRODUCTION: Equivocal decline of tongue muscle performance with age is compatible with resistance of the tongue to sarcopenia, the loss of muscle volume and function that typically occurs with aging. To test this possibility we characterized anatomical and molecular indices of sarcopenia in the macaque tongue muscle styloglossus (SG). METHODS: We quantified myosin heavy chain (MHC), muscle fiber MHC phenotype and size and total and phosphorylated growth- and atrophy-related proteins by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), immunoblot and immunohistochemistry (IHC) in the SG in twenty-four macaque monkeys (Macaca rhesus, age range 9months to 31years) categorized into Young (<8years of age), Middle-aged (15-21years of age) and Old (>22years of age) groups. RESULTS: In Young, Middle and Old age groups, by SDS-PAGE MHCI comprised ~1/3 and MHCII ~2/3 of total MHC. MHCI relative frequency was lower and MHCII higher in Middle versus Young (p=0.0099) and Middle versus Old (p=0.052). Relative frequencies of MHC fiber phenotype were not different by age but were different by phenotype (rates 233, 641 and 111 per 1000 fibers for MHCI, MHCII and MHCI-II respectively, p=0.03). Few or no fibers were positive for developmental MHC. Mean cross-sectional area (CSA) was not different among the three age groups for MHCII and MHCI-II; however MHCI fibers tended to be larger in Middle versus Old and Young (mean=2257µm2,1917µm2 (p=0.05) and 1704µm2 (p=0.06), respectively). For each age group, mean CSA increased across MHC phenotype (lowest mean CSA for MHCI and highest mean CSA for MHCII). Spearman analysis demonstrated age-related increases in total p70 ribosomal protein S6 kinase (P70), phosphorylated P70421/424, phosphorylated P38 mitogen-activated protein kinase and muscle atrophy F-Box, a trend to age-related decrease in total extracellular signal-regulated kinase (ERK), and no age-related change in total protein kinase B (Akt/PKB), phosphorylated Akt, phosphorylated ERK, phosphorylated c-Jun N-terminal kinase (JNK46) and phosphorylated P70389. CONCLUSION: Common anatomical and molecular indices of sarcopenia are absent in our sample of macaque SG. Relative frequencies of MHCII protein and phenotype are preserved with age. Although MAFbx expression increases with age, this is not associated with fiber atrophy, perhaps reflecting compensatory growth signaling by p70. The resistant nature of the styloglossus muscle to sarcopenia may be related to routine activation of tongue muscles in respiration and swallowing and the preservation of hypoglossal motoneuron number with age.

Independent AMP and NAD signaling regulates C2C12 differentiation and metabolic adaptation.

The balance of ATP production and consumption is reflected in adenosine monophosphate (AMP) and nicotinamide adenine dinucleotide (NAD) content and has been associated with phenotypic plasticity in striated muscle. Some studies have suggested that AMPK-dependent plasticity may be an indirect consequence of increased NAD synthesis and SIRT1 activity. The primary goal of this study was to assess the interaction of AMP- and NAD-dependent signaling in adaptation of C2C12 myotubes. Changes in myotube developmental and metabolic gene expression were compared following incubation with 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) and nicotinamide mononucleotide (NMN) to activate AMPK- and NAD-related signaling. AICAR showed no effect on NAD pool or nampt expression but significantly reduced histone H3 acetylation and GLUT1, cytochrome C oxidase subunit 2 (COX2), and MYH3 expression. In contrast, NMN supplementation for 24 h increased NAD pool by 45 % but did not reduce histone H3 acetylation nor promote mitochondrial gene expression. The combination of AMP and NAD signaling did not induce further metabolic adaptation, but NMN ameliorated AICAR-induced myotube reduction. We interpret these results as indication that AMP and NAD contribute to C2C12 differentiation and metabolic adaptation independently.

Decrease of myofiber branching via muscle-specific expression of the olfactory receptor mOR23 in dystrophic muscle leads to protection against mechanical stress.

BACKGROUND: Abnormal branched myofibers within skeletal muscles are commonly found in diverse animal models of muscular dystrophy as well as in patients. Branched myofibers from dystrophic mice are more susceptible to break than unbranched myofibers suggesting that muscles containing a high percentage of these myofibers are more prone to injury. Previous studies showed ubiquitous over-expression of mouse olfactory receptor 23 (mOR23), a G protein-coupled receptor, in wild type mice decreased myofiber branching. Whether mOR23 over-expression specifically in skeletal muscle cells is sufficient to mitigate myofiber branching in dystrophic muscle is unknown. METHODS: We created a novel transgenic mouse over-expressing mOR23 specifically in muscle cells and then bred with dystrophic (mdx) mice. Myofiber branching was analyzed in these two transgenic mice and membrane integrity was assessed by Evans blue dye fluorescence. RESULTS: mOR23 over-expression in muscle led to a decrease of myofiber branching after muscle regeneration in non-dystrophic mouse muscles and reduced the severity of myofiber branching in mdx mouse muscles. Muscles from mdx mouse over-expressing mOR23 significantly exhibited less damage to eccentric contractions than control mdx muscles. CONCLUSIONS: The decrease of myofiber branching in mdx mouse muscles over-expressing mOR23 reduced the amount of membrane damage induced by mechanical stress. These results suggest that modifying myofiber branching in dystrophic patients, while not preventing degeneration, could be beneficial for mitigating some of the effects of the disease process.

Activation of p38 in C2C12 myotubes following ATP depletion depends on extracellular glucose

Muscle cells adjust their glucose metabolism in response to myriad stimuli, and particular attention has been paid to glucose metabolism after contraction, ATP depletion, and insulin stimulation. Each of these requires translocation of GLUT4 to the cell membrane, and may require activation of glucose transporters by p38. In contrast, AICAR stimulates glucose transport without activation of p38, suggesting that p38 activation may be an indirect consequence of accelerated glucose transport or metabolism. This study was designed to investigate the contribution of AMPK and p38 to ATP homeostasis and glucose metabolism to test the hypothesis that p38 reflects glycolytic activity rather than controls glucose uptake. Treating mature myotubes with rotenone caused transient ATP depletion in 15 min with recovery by 120 min, associated with increased lactate production. Both ACC and p38 were rapidly phosphorylated, but ACC remained phosphorylated while p38 phosphorylation declined as ATP recovered. AMPK inhibition blocked ATP recovery, lactate production, and phosphorylation of p38 and ACC. Inhibition of p38 had little effect. AICAR induced ACC phosphorylation, but not lactate production or p38 phosphorylation. Finally, removing extracellular glucose potentiated rotenone-induced AMPK activation, but reduced lactate generation, ATP recovery and p38 activation. Thus, glucose metabolism is highly sensitive to ATP homeostasis via AMPK activity, but p38 activity is dispensable. Although p38 is strongly phosphorylated during ATP depletion, this appears to be an indirect consequence of accelerated glycolysis

Activation of p38 in C2C12 myotubes following ATP depletion depends on extracellular glucose.

Muscle cells adjust their glucose metabolism in response to myriad stimuli, and particular attention has been paid to glucose metabolism after contraction, ATP depletion, and insulin stimulation. Each of these requires translocation of GLUT4 to the cell membrane, and may require activation of glucose transporters by p38. In contrast, AICAR stimulates glucose transport without activation of p38, suggesting that p38 activation may be an indirect consequence of accelerated glucose transport or metabolism. This study was designed to investigate the contribution of AMPK and p38 to ATP homeostasis and glucose metabolism to test the hypothesis that p38 reflects glycolytic activity rather than controls glucose uptake. Treating mature myotubes with rotenone caused transient ATP depletion in 15 min with recovery by 120 min, associated with increased lactate production. Both ACC and p38 were rapidly phosphorylated, but ACC remained phosphorylated while p38 phosphorylation declined as ATP recovered. AMPK inhibition blocked ATP recovery, lactate production, and phosphorylation of p38 and ACC. Inhibition of p38 had little effect. AICAR induced ACC phosphorylation, but not lactate production or p38 phosphorylation. Finally, removing extracellular glucose potentiated rotenone-induced AMPK activation, but reduced lactate generation, ATP recovery and p38 activation. Thus, glucose metabolism is highly sensitive to ATP homeostasis via AMPK activity, but p38 activity is dispensable. Although p38 is strongly phosphorylated during ATP depletion, this appears to be an indirect consequence of accelerated glycolysis.

Absence of developmental and unconventional myosin heavy chain in human suprahyoid muscles.

INTRODUCTION: Contradictory reports of the myosin heavy chain (MHC) composition of adult human suprahyoid muscles leave unresolved the extent to which these muscles express developmental and unconventional MHC. METHODS: By immunohistochemistry, separation sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)-Coomassie, separation SDS-PAGE-Western blot, and mRNA PCR, we tested for conventional MHCI, MHCIIA, MHCIIX, developmental MHC embryonic and MHC neonatal, and unconventional MHC alpha-cardiac, MHC extraocular, and MHC slow tonic in adult human anterior digastric (AD), geniohyoid (GH), and mylohyoid (MH) muscles. RESULTS: By separation SDS-PAGE-Coomassie and Western blot, only conventional MHC are present. By immunohistochemistry all muscle fibers are positive for MHCI, MHCIIA, or MHCIIX, and fewer than 4 fibers/mm(2) are positive for developmental or unconventional MHC. By PCR, mRNA of MHCI and MHCIIA dominate, with sporadically detectable MHC alpha-cardiac and without detectable mRNA of other developmental and unconventional MHC. CONCLUSIONS: We conclude that human suprahyoid muscles AD, GH, and MH are composed almost exclusively of conventional MHC isoforms.

High-frequency electrical stimulation reveals a p38-mTOR signaling module correlated with force-time integral.

High-frequency electrical stimulation (HFES) leads to muscle hypertrophy, and attention has been drawn to the high forces involved. However, both mechanical and metabolic stresses occur simultaneously, and both stimuli influence signaling cascades related to protein synthesis. This study aimed to identify the immediate signaling correlates of contraction-induced force and metabolic stresses under the hypothesis that HFES induces growth-related signaling through mechanical stimulation. Force-time integral (FTI) signaling in mouse tibialis anterior muscle was examined by separately manipulating the time of contraction to emphasize the metabolic aspect or the force of contraction to emphasize the mechanical aspect. When FTI was manipulated by changing the total time of activation, phosphorylation of p54 JNK, ERK and p70S6k(T421/S424) was independent of FTI, while phosphorylation of acetyl-CoA carboxylase and p38 correlated with FTI. When FTI was manipulated by changing the force of contraction, p54 JNK, ERK and p70S6k(T421/S424) were again independent of FTI, while phosphorylation of p38 and FAK correlated with FTI. Factor analysis identified a p38-mTOR signaling module that correlated with FTI in both experiments. The consistent link among p38, mTOR and FTI suggests that they form a connected signaling module sensitive to the mechanical aspects of FTI, separate from markers of metabolic load.

Practical limits on muscle synergy identification by non-negative matrix factorization in systems with mechanical constraints.

Statistical decomposition, including non-negative matrix factorization (NMF), is a convenient tool for identifying patterns of structured variability within behavioral motor programs, but it is unclear how the resolved factors relate to actual neural structures. Factors can be extracted from a uniformly sampled, low-dimension command space. In practical application, the command space is limited, either to those activations that perform some task(s) successfully or to activations induced in response to specific perturbations. NMF was applied to muscle activation patterns synthesized from low dimensional, synergy-like control modules mimicking simple task performance or feedback activation from proprioceptive signals. In the task-constrained paradigm, the accuracy of control module recovery was highly dependent on the sampled volume of control space, such that sampling even 50% of control space produced a substantial degradation in factor accuracy. In the feedback paradigm, NMF was not capable of extracting more than four control modules, even in a mechanical model with seven internal degrees of freedom. Reduced access to the low-dimensional control space imposed by physical constraints may result in substantial distortion of an existing low dimensional controller, such that neither the dimensionality nor the composition of the recovered/extracted factors match the original controller.

Neuromechanic: a computational platform for simulation and analysis of the neural control of movement.

Neuromusculoskeletal models solve the basic problem of determining how the body moves under the influence of external and internal forces. Existing biomechanical modeling programs often emphasize dynamics with the goal of finding a feed-forward neural program to replicate experimental data or of estimating force contributions or individual muscles. The computation of rigid-body dynamics, muscle forces, and activation of the muscles are often performed separately. We have developed an intrinsically forward computational platform (Neuromechanic, www.neuromechanic.com) that explicitly represents the interdependencies among rigid body dynamics, frictional contact, muscle mechanics, and neural control modules. This formulation has significant advantages for optimization and forward simulation, particularly with application to neural controllers with feedback or regulatory features. Explicit inclusion of all state dependencies allows calculation of system derivatives with respect to kinematic states and muscle and neural control states, thus affording a wealth of analytical tools, including linearization, stability analyses and calculation of initial conditions for forward simulations. In this review, we describe our algorithm for generating state equations and explain how they may be used in integration, linearization, and stability analysis tools to provide structural insights into the neural control of movement.

Changes in growth-related kinases in head, neck and limb muscles with age.

Sarcopenia coincides with declines in several systemic processes that signal through the MAP kinase and Akt-mTOR-p70S6k cascades typically associated with muscle growth. Effects of aging on these pathways have primarily been examined in limb muscles, which experience substantial activity and neural changes in addition to systemic hormonal and metabolic changes. Head and neck muscles are reported to undergo reduced sarcopenia and disuse with age relative to limb muscles, suggesting muscle activity may contribute to maintaining mass with age. However many head and neck muscles derive from embryonic branchial arches, rather than the somites from which limb muscles originate, suggesting that developmental origin may be important. This study compares the expression and phosphorylation of MAP kinase and mTOR networks in head, neck, tongue, and limb muscles from 8- and 26-month old F344 rats to test the hypothesis that physical activity and developmental origin contribute to preservation of muscle mass with age. Phosphorylation of p38 was exaggerated in aged branchial arch muscles. Phosphorylation of ERK and p70S6k T421/S424 declined with age only in the biceps brachii. Expression of p70S6k declined in all head and neck, tongue and limb muscles although no change in phosphorylation of p70S6k on T389 could be resolved. A systemic change that results in a loss of p70S6k protein expression may reduce the capacity to respond to acute hypertrophic stimuli, while the exaggerated p38 signaling in branchial arch muscles may reflect more active muscle remodeling.

Directional constraint of endpoint force emerges from hindlimb anatomy.

Postural control requires the coordination of force production at the limb endpoints to apply an appropriate force to the body. Subjected to horizontal plane perturbations, quadruped limbs stereotypically produce force constrained along a line that passes near the center of mass. This phenomenon, referred to as the force constraint strategy, may reflect mechanical constraints on the limb or body, a specific neural control strategy or an interaction among neural controls and mechanical constraints. We used a neuromuscular model of the cat hindlimb to test the hypothesis that the anatomical constraints restrict the mechanical action of individual muscles during stance and constrain the response to perturbations to a line independent of perturbation direction. In a linearized neuromuscular model of the cat hindlimb, muscle lengthening directions were highly conserved across 10,000 different muscle activation patterns, each of which produced an identical, stance-like endpoint force. These lengthening directions were closely aligned with the sagittal plane and reveal an anatomical structure for directionally constrained force responses. Each of the 10,000 activation patterns was predicted to produce stable stance based on Lyapunov stability analysis. In forward simulations of the nonlinear, seven degree of freedom model under the action of 200 random muscle activation patterns, displacement of the endpoint from its equilibrium position produced restoring forces, which were also biased toward the sagittal plane. The single exception was an activation pattern based on minimum muscle stress optimization, which produced destabilizing force responses in some perturbation directions. The sagittal force constraint increased during simulations as the system shifted from an inertial response during the acceleration phase to a viscoelastic response as peak velocity was obtained. These results qualitatively match similar experimental observations and suggest that the force constraint phenomenon may result from the anatomical arrangement of the limb.

Sarcomeric myosin expression in the tongue body of humans, macaques and rats.

Expression of developmental and unconventional myosin heavy chain (MHC) isoforms in some adult head and neck muscles is thought to reflect specific contractile demands of muscle fibers active during kinematically complex movements. Mammalian tongue muscles are active during oromotor behaviors that encompass a wide range of tongue movement speeds and tongue shape changes (e.g. respiration, oral transport, swallowing, rejection), but the extent to which tongue muscles express developmental and unconventional MHC is not known. Quantitative PCR was used to determine the mRNA content of conventional MHC-beta, MHC-2a, MHC-2b and MHC-2x, the developmental isoforms embryonic MHC and neonatal MHC and the unconventional isoforms atrial/cardiac-alpha MHC (MHC-alpha), extraocular MHC, masseter MHC and slow tonic MHC in tongue body muscles of the rat, macaque and human. In all species, conventional MHC isoforms predominate. MHC-2b and MHC-2x account for 98% of total MHC mRNA in the rat. MHC-2a, MHC-2x and MHC-beta account for 94% of total MHC mRNA in humans and 96% of total MHC mRNA in macaque. With the exception of MHC-alpha in humans (5%), developmental and unconventional MHC mRNA represents less than 0.3% of total MHC mRNA. We conclude that in these species, there is limited expression of developmental and unconventional MHC and that diversity of tongue body muscle fiber contractile properties is achieved primarily by MHC-beta, MHC-2a, MHC-2x and MHC-2b. Whether expression of MHC-alpha mRNA in tongue is unique to humans or present in other hominoids awaits further investigation.

Stretch-induced ERK2 phosphorylation requires PLA2 activity in skeletal myotubes.

Mechanical stretch rapidly activates multiple signaling cascades, including phospholipases and kinases, to stimulate protein synthesis and growth. The purpose of this study was to determine whether PLA2 activation contributes to stretch-induced phosphorylation of ERK2 in skeletal muscle myotubes. Myotubes derived from neonatal C57 mice were cultured on silicone membranes and subjected to brief cyclic stretch. Inhibition of PLA2 prevented ERK2 phosphorylation, while inhibition of prostaglandin or leukotriene synthesis did not. ERK2 phosphorylation was also blocked by genistein and PD98059, implicating the canonical raf-MEK-ERK cassette. It appears that PLA2, but not further metabolism of arachidonic acid, is required for stretch-induced activation of ERK2. Exposure to exogenous arachidonic acid had no effect on ERK2 phosphorylation, but exposure to lysophosphatidylcholine, the other metabolite of PLA2, caused a dose-dependent increase in ERK2 phosphorylation. These results suggest that stretch-induced activation of ERK2 may result from an interaction between PLA2 derived lysophosphatidylcholine and membrane receptors.

Reduction of neuromuscular redundancy for postural force generation using an intrinsic stability criterion.

Postural control requires the coordination of multiple muscles to achieve both endpoint force production and postural stability. Multiple muscle activation patterns can produce the required force for standing, but the mechanical stability associated with any given pattern may vary, and has implications for the degree of delayed neural feedback necessary for postural stability. We hypothesized that muscular redundancy is reduced when muscle activation patterns are chosen with respect to intrinsic musculoskeletal stability as well as endpoint force production. We used a three-dimensional musculoskeletal model of the cat hindlimb with 31 muscles to determine the possible contributions of intrinsic muscle properties to limb stability during isometric force generation. Using dynamic stability analysis we demonstrate that within the large set of activation patterns that satisfy the force requirement for posture, only a reduced subset produce a mechanically stable limb configuration. Greater stability in the frontal-plane suggests that neural control mechanisms are more highly active for sagittal-plane and for ankle joint control. Even when the limb was unstable, the time-constants of instability were sufficiently great to allow long-latency neural feedback mechanisms to intervene, which may be preferential for movements requiring maneuverability versus stability. Local joint stiffness of muscles was determined by the stabilizing or destabilizing effects of moment-arm versus joint angle relationships. By preferentially activating muscles with high local stiffness, muscle activation patterns with feedforward stabilizing properties could be selected. Such a strategy may increase intrinsic postural stability without co-contraction, and may be useful criteria in the force-sharing problem.

Asymmetric interjoint feedback contributes to postural control of redundant multi-link systems.

Maintaining the postural configuration of a limb such as an arm or leg is a fundamental neural control task that involves the coordination of multiple linked body segments. Biological systems are known to use a complex network of inter- and intra-joint feedback mechanisms arising from muscles, spinal reflexes and higher neuronal structures to stabilize the limbs. While previous work has shown that a small amount of asymmetric heterogenic feedback contributes to the behavior of these systems, a satisfactory functional explanation for this non-conservative feedback structure has not been put forth. We hypothesized that an asymmetric multi-joint control strategy would confer both an energetic and stability advantage in maintaining endpoint position of a kinematically redundant system. We tested this hypothesis by using optimal control models incorporating symmetric versus asymmetric feedback with the goal of maintaining the endpoint location of a kinematically redundant, planar limb. Asymmetric feedback improved endpoint control performance of the limb by 16%, reduced energetic cost by 21% and increased interjoint coordination by 40% compared to the symmetric feedback system. The overall effect of the asymmetry was that proximal joint motion resulted in greater torque generation at distal joints than vice versa. The asymmetric organization is consistent with heterogenic stretch reflex gains measured experimentally. We conclude that asymmetric feedback has a functionally relevant role in coordinating redundant degrees of freedom to maintain the position of the hand or foot.

Inter-joint coupling effects on muscle contributions to endpoint force and acceleration in a musculoskeletal model of the cat hindlimb.

The biomechanical principles underlying the organization of muscle activation patterns during standing balance are poorly understood. The goal of this study was to understand the influence of biomechanical inter-joint coupling on endpoint forces and accelerations induced by the activation of individual muscles during postural tasks. We calculated induced endpoint forces and accelerations of 31 muscles in a 7 degree-of-freedom, three-dimensional model of the cat hindlimb. To test the effects of inter-joint coupling, we systematically immobilized the joints (excluded kinematic degrees of freedom) and evaluated how the endpoint force and acceleration directions changed for each muscle in 7 different conditions. We hypothesized that altered inter-joint coupling due to joint immobilization of remote joints would substantially change the induced directions of endpoint force and acceleration of individual muscles. Our results show that for most muscles crossing the knee or the hip, joint immobilization altered the endpoint force or acceleration direction by more than 90 degrees in the dorsal and sagittal planes. Induced endpoint forces were typically consistent with behaviorally observed forces only when the ankle was immobilized. We then activated a proximal muscle simultaneous with an ankle torque of varying magnitude, which demonstrated that the resulting endpoint force or acceleration direction is modulated by the magnitude of the ankle torque. We argue that this simple manipulation can lend insight into the functional effects of co-activating muscles. We conclude that inter-joint coupling may be an essential biomechanical principle underlying the coordination of proximal and distal muscles to produce functional endpoint actions during motor tasks.

Limited expression of slow tonic myosin heavy chain in human cranial muscles.

Recent reports of slow tonic myosin heavy chain (MHCst) in human masticatory and laryngeal muscles suggest that MHCst may have a wider distribution in humans than previously thought. Because of the novelty of this finding, we sought to confirm the presence of MHCst in human masticatory and laryngeal muscles by reacting tissue from these muscles and controls from extraocular, intrafusal, cardiac, appendicular, and developmental muscle with antibodies (Abs) ALD-58 and S46, considered highly specific for MHCst. At Ab dilutions producing minimal reaction to muscle fibers positive for MHCI, only extraocular, intrafusal, and fetal tongue tissue reacted with Ab S46 had strong immunoreaction in an appreciable number of muscle fibers. In immunoblots, Ab S46, but not Ab ALD-58, labeled adult extraocular muscles; no other muscles were labeled with either Ab. We conclude that, in humans, Ab S46 has greater specificity for MHCst than does Ab ALD-58. We suggest that reports of MHCst in human masticatory and laryngeal muscles reflect false-positive identification of MHCst due to cross-reactivity of Ab ALD-58 with another MHC isoform.

Immunohistochemical characterization of slow and fast myosin heavy chain composition of muscle fibres in the styloglossus muscle of the human and macaque (Macaca rhesus).

OBJECTIVE: Muscle fibre contractile diversity is thought to be increased by the hybridization of multiple myosin heavy chain (MHC) isoforms in single muscle fibres. Reports of hybrid fibres composed of MHCI and MHCII isoforms in human, but not macaque, tongue muscles, suggest a human adaptation for increased tongue muscle contractile diversity. Here we test whether hybrid fibres composed of MHCI and MHCII are unique to human tongue muscles or are present as well in the macaque. METHODS: MHC composition of the macaque and human styloglossus was characterized with antibodies that allowed identification of three muscle fibre phenotypes, a slow phenotype composed of MHCI, a fast phenotype composed of MHCII and a hybrid phenotype composed of MHCI and MHCII. RESULTS: The fast phenotype constitutes 68.5% of fibres in the macaque and 43.4% of fibres in the human (P<0.0001). The slow phenotype constitutes 20.2% of fibres in the macaque and 39.3% of fibres in the human (P<0.0001). The hybrid phenotype constitutes 11.2% of fibres in the macaque and 17.3% of fibres in the human (P=0.0002). Macaques and humans do not differ in fiber size (cross-sectional area, diameter). However, measures of fibre size differ by phenotype such that fast>hybrid>slow (P<0.05). CONCLUSION: These data demonstrate differences in the relative percent of muscle fibre phenotypes in the macaque and human styloglossus but also demonstrate that all three phenotypes are present in both species. These data suggest a similar range of mechanical properties in styloglossus muscle fibres of the macaque and human.