RESUMO
Skeletal muscle has a broad range of biomechanical functions, including power generation and energy absorption. These roles are underpinned by the force-velocity relationship, which comprises two distinct components: a concentric and an eccentric force-velocity relationship. The concentric component has been extensively studied across a wide range of muscles with different muscle properties. However, to date, little progress has been made in accurately characterising the eccentric force-velocity relationship in mammalian muscle with varying muscle properties. Consequently, mathematical models of this muscle behaviour are based on a poorly understood phenomenon. Here, we present a comprehensive assessment of the concentric force-velocity and eccentric force-velocity relationships of four mammalian muscles (soleus, extensor digitorum longus, diaphragm and digastric) with varying biomechanical functions, spanning three orders of magnitude in body mass (mouse, rat and rabbits). The force-velocity relationship was characterised using a hyperbolic-linear equation for the concentric component a hyperbolic equation for the eccentric component, at the same time as measuring the rate of force development in the two phases of force development in relation to eccentric lengthening velocity. We demonstrate that, despite differences in the curvature and plateau height of the eccentric force-velocity relationship, the rates of relative force development were consistent for the two phases of the force-time response during isovelocity lengthening ramps, in relation to lengthening velocity, in the four muscles studied. Our data support the hypothesis that this relationship depends on cross-bridge and titin activation. Hill-type musculoskeletal models of the eccentric force-velocity relationship for mammalian muscles should incorporate this biphasic force response. KEY POINTS: The capacity of skeletal muscle to generate mechanical work and absorb energy is underpinned by the force-velocity relationship. Despite identification of the lengthening (eccentric) force-velocity relationship over 80 years ago, no comprehensive study has been undertaken to characterise this relationship in skeletal muscle. We show that the biphasic force response seen during active muscle lengthening is conserved over three orders of magnitude of mammalian skeletal muscle mass. Using mice with a small deletion in titin, we show that part of this biphasic force profile in response to muscle lengthening is reliant on normal titin activation. The rate of force development during muscle stretch may be a more reliable way to describe the forces experienced during eccentric muscle contractions compared to the traditional hyperbolic curve fitting, and functions as a novel predictor of force-velocity characteristics that may be used to better inform hill-type musculoskeletal models and assess pathophysiological remodelling.
Assuntos
Contração Muscular , Músculo Esquelético , Humanos , Ratos , Camundongos , Animais , Coelhos , Conectina , Contração Muscular/fisiologia , Músculo Esquelético/fisiologia , Terapia por Exercício , Diafragma , MamíferosRESUMO
NEW FINDINGS: What is the central question of this study? Can we manipulate muscle recruitment to differentially enhance skeletal muscle fatigue resistance? What is the main finding and its importance? Through manipulation of muscle activation patterns, it is possible to promote distinct microvascular growth. Enhancement of fatigue resistance is closely associated with the distribution of the capillaries within the muscle, not necessarily with quantity. Additionally, at the acute stages of remodelling in response to indirect electrical stimulation, the improvement in fatigue resistance appears to be primarily driven by vascular remodelling, with metabolic adaptation of secondary importance. ABSTRACT: Exercise involves a complex interaction of factors influencing muscle performance, where variations in recruitment pattern (e.g., endurance vs. resistance training) may differentially modulate the local tissue environment (i.e., oxygenation, blood flow, fuel utilization). These exercise stimuli are potent drivers of vascular and metabolic change. However, their relative contribution to adaptive remodelling of skeletal muscle and subsequent performance is unclear. Using implantable devices, indirect electrical stimulation (ES) of locomotor muscles of rat at different pacing frequencies (4, 10 and 40 Hz) was used to differentially recruit hindlimb blood flow and modulate fuel utilization. After 7 days, ES promoted significant remodelling of microvascular composition, increasing capillary density in the cortex of the tibialis anterior by 73%, 110% and 55% for the 4 Hz, 10 and 40 Hz groups, respectively. Additionally, there was remodelling of the whole muscle metabolome, including significantly elevated amino acid turnover, with muscle kynurenic acid levels doubled by pacing at 10 Hz (P < 0.05). Interestingly, the fatigue index of skeletal muscle was only significantly elevated in 10 Hz (58% increase) and 40 Hz (73% increase) ES groups, apparently linked to improved capillary distribution. These data demonstrate that manipulation of muscle recruitment pattern may be used to differentially expand the capillary network prior to altering the metabolome, emphasising the importance of local capillary supply in promoting exercise tolerance.
Assuntos
Fadiga Muscular , Músculo Esquelético , Ratos , Animais , Músculo Esquelético/fisiologia , Capilares/fisiologia , Adaptação Fisiológica , Estimulação ElétricaRESUMO
Muscle spindle abundance is highly variable within and across species, but we currently lack any clear picture of the mechanistic causes or consequences of this variation. Previous use of spindle abundance as a correlate for muscle function implies a mechanical underpinning to this variation, but these ideas have not been tested. Herein, we use integrated medical imaging and subject-specific musculoskeletal models to investigate the relationship between spindle abundance, muscle architecture and in vivo muscle behaviour in the human locomotor system. These analyses indicate that muscle spindle number is tightly correlated with muscle fascicle length, absolute fascicle length change, velocity of fibre lengthening and active muscle forces during walking. Novel correlations between functional indices and spindle abundance are also recovered, where muscles with a high abundance predominantly function as springs, compared to those with a lower abundance mostly functioning as brakes during walking. These data demonstrate that muscle fibre length, lengthening velocity and fibre force are key physiological signals to the central nervous system and its modulation of locomotion, and that muscle spindle abundance may be tightly correlated to how a muscle generates work. These insights may be combined with neuromechanics and robotic studies of motor control to help further tease apart the functional drivers of muscle spindle composition.
Assuntos
Fusos Musculares , Músculo Esquelético , Humanos , Locomoção , Fibras Musculares Esqueléticas/fisiologia , Fusos Musculares/fisiologia , Músculo Esquelético/fisiologia , Caminhada/fisiologiaRESUMO
The motor unit comprises a variable number of muscle fibres that connect through myelinated nerve fibres to a motoneuron (MN), the central drivers of activity. At the simplest level of organisation there exist phenotypically distinct MNs that activate corresponding muscle fibre types, but within an individual motor pool there typically exists a mixed population of fast and slow firing MNs, innervating groups of Type II and Type I fibres, respectively. Characterising the heterogeneity across multiple levels of motor unit organisation is critical to understanding changes that occur in response to physiological and pathological perturbations. Through a comprehensive assessment of muscle histology and ex vivo function, mathematical modelling and neuronal tracing, we demonstrate regional heterogeneities at the level of the MN, muscle fibre type composition and oxygen delivery kinetics of the rat extensor digitorum longus (EDL) muscle. Specifically, the EDL contains two phenotypically distinct regions: a relatively oxidative medial and a more glycolytic lateral compartment. Smaller muscle fibres in the medial compartment, in combination with a greater local capillary density, preserve tissue O2 partial pressure (PO2 ) during modelled activity. Conversely, capillary supply to the lateral compartment is calculated to be insufficient to defend active muscle PO2 but is likely optimised to facilitate metabolite removal. Simulation of in vivo muscle length change and phasic activation suggest that both compartments are able to generate similar net power. However, retrograde tracing demonstrates (counter to previous observations) that a negative relationship between soma size and C-bouton density exists. Finally, we confirm a lack of specificity of SK3 expression to slow MNs. Together, these data provide a reference for heterogeneities across the rat EDL motor unit and re-emphasise the importance of sampling technique.
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Fibras Musculares Esqueléticas , Músculo Esquelético , Animais , Capilares , Neurônios Motores , Fibras Musculares Esqueléticas/fisiologia , Músculo Esquelético/fisiologia , RatosRESUMO
Mammalian motor systems adapt to the demands of their environment. For example, muscle fibre types change in response to increased load or endurance demands. However, for adaptations to be effective, motoneurons must adapt such that their properties match those of the innervated muscle fibres. We used a rat model of chronic functional overload to assess adaptations to both motoneuron size and a key modulatory synapse responsible for amplification of motor output, C-boutons. Overload of extensor digitorum longus (EDL) muscles was induced by removal of their synergists, tibialis anterior muscles. Following 21 days survival, EDL muscles showed an increase in fatigue resistance and a decrease in force output, indicating a shift to a slower phenotype. These changes were reflected by a decrease in motoneuron size. However, C-bouton complexes remained largely unaffected by overload. The C-boutons themselves, quantified by expression of vesicular acetylcholine transporter, were similar in size and density in the control and overload conditions. Expression of the post-synaptic voltage-gated potassium channel (KV 2.1) was also unchanged. Small conductance calcium-activated potassium channels (SK3) were expressed in most EDL motoneurons, despite this being an almost exclusively fast motor pool. Overload induced a decrease in the proportion of SK3+ cells, however, there was no change in density or size of clusters. We propose that reductions in motoneuron size may promote early recruitment of EDL motoneurons, but that C-bouton plasticity is not necessary to increase the force output required in response to muscle overload.
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Canais de Potássio Cálcio-Ativados , Canais de Potássio de Abertura Dependente da Tensão da Membrana , Animais , Mamíferos , Neurônios Motores/fisiologia , Músculo Esquelético/inervação , Ratos , Proteínas Vesiculares de Transporte de AcetilcolinaRESUMO
KEY POINTS: Spinal treatment can restore diaphragm function in all animals 1 month following C2 hemisection induced paralysis. Greater recovery occurs the longer after injury the treatment is applied. Through advanced assessment of muscle mechanics, innovative histology and oxygen tension modelling, we have comprehensively characterized in vivo diaphragm function and phenotype. Muscle work loops reveal a significant deficit in diaphragm functional properties following chronic injury and paralysis, which are normalized following restored muscle activity caused by plasticity-induced spinal reconnection. Injury causes global and local alterations in diaphragm muscle vascular supply, limiting oxygen diffusion and disturbing function. Restoration of muscle activity reverses these alterations, restoring oxygen supply to the tissue and enabling recovery of muscle functional properties. There remain metabolic deficits following restoration of diaphragm activity, probably explaining only partial functional recovery. We hypothesize that these deficits need to be resolved to restore complete respiratory motor function. ABSTRACT: Months after spinal cord injury (SCI), respiratory deficits remain the primary cause of morbidity and mortality for patients. It is possible to induce partial respiratory motor functional recovery in chronic SCI following 2 weeks of spinal neuroplasticity. However, the peripheral mechanisms underpinning this recovery are largely unknown, limiting development of new clinical treatments with potential for complete functional restoration. Utilizing a rat hemisection model, diaphragm function and paralysis was assessed and recovered at chronic time points following trauma through chondroitinase ABC induced neuroplasticity. We simulated the diaphragm's in vivo cyclical length change and activity patterns using the work loop technique at the same time as assessing global and local measures of the muscles histology to quantify changes in muscle phenotype, microvascular composition, and oxidative capacity following injury and recovery. These data were fed into a physiologically informed model of tissue oxygen transport. We demonstrate that hemidiaphragm paralysis causes muscle fibre hypertrophy, maintaining global oxygen supply, although it alters isolated muscle kinetics, limiting respiratory function. Treatment induced recovery of respiratory activity normalized these effects, increasing oxygen supply, restoring optimal diaphragm functional properties. However, metabolic demands of the diaphragm were significantly reduced following both injury and recovery, potentially limiting restoration of normal muscle performance. The mechanism of rapid respiratory muscle recovery following spinal trauma occurs through oxygen transport, metabolic demand and functional dynamics of striated muscle. Overall, these data support a systems-wide approach to the treatment of SCI, and identify new targets to mediate complete respiratory recovery.
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Diafragma , Traumatismos da Medula Espinal , Animais , Humanos , Cinética , Oxigênio , Nervo Frênico , Ratos , Ratos Sprague-Dawley , Recuperação de Função Fisiológica , Medula EspinalRESUMO
KEY POINTS: Heart failure is characterised by limb and respiratory muscle impairments that limit functional capacity and quality of life. However, compared with heart failure with reduced ejection fraction (HFrEF), skeletal muscle alterations induced by heart failure with preserved ejection fraction (HFpEF) remain poorly explored. Here we report that obese-HFpEF induces multiple skeletal muscle alterations in the rat hindlimb, including impaired muscle mechanics related to shortening velocity, fibre atrophy, capillary loss, and an impaired blood flow response to contractions that implies a perfusive oxygen delivery limitation. We also demonstrate that obese-HFpEF is characterised by diaphragmatic alterations similar to those caused by denervation - atrophy in Type IIb/IIx (fast/glycolytic) fibres and hypertrophy in Type I (slow/oxidative) fibres. These findings extend current knowledge in HFpEF skeletal muscle physiology, potentially underlying exercise intolerance, which may facilitate future therapeutic approaches. ABSTRACT: Peripheral skeletal muscle and vascular alterations induced by heart failure with preserved ejection fraction (HFpEF) remain poorly identified, with limited therapeutic targets. This study used a cardiometabolic obese-HFpEF rat model to comprehensively phenotype skeletal muscle mechanics, blood flow, microvasculature and fibre atrophy. Lean (n = 8) and obese-HFpEF (n = 8) ZSF1 rats were compared. Skeletal muscles (soleus and diaphragm) were assessed for in vitro contractility (isometric and isotonic properties) alongside indices of fibre-type cross-sectional area, myosin isoform, and capillarity, and estimated muscle PO2 . In situ extensor digitorum longus (EDL) contractility and femoral blood flow were assessed. HFpEF soleus demonstrated lower absolute maximal force by 22%, fibre atrophy by 24%, a fibre-type shift from I to IIa, and a 17% lower capillary-to-fibre ratio despite increased capillary density (all P < 0.05) with preserved muscle PO2 (P = 0.115) and isometric specific force (P > 0.05). Soleus isotonic properties (shortening velocity and power) were impaired by up to 17 and 22%, respectively (P < 0.05), while the magnitude of the exercise hyperaemia was attenuated by 73% (P = 0.012) in line with higher muscle fatigue by 26% (P = 0.079). Diaphragm alterations (P < 0.05) included Type IIx fibre atrophy despite Type I/IIa fibre hypertrophy, with increased indices of capillarity alongside preserved contractile properties during isometric, isotonic, and cyclical contractions. In conclusion, obese-HFpEF rats demonstrated blunted skeletal muscle blood flow during contractions in parallel to microvascular structural remodelling, fibre atrophy, and isotonic contractile dysfunction in the locomotor muscles. In contrast, diaphragm phenotype remained well preserved. This study identifies numerous muscle-specific impairments that could exacerbate exercise intolerance in obese-HFpEF.
Assuntos
Insuficiência Cardíaca , Animais , Contração Muscular , Músculo Esquelético , Obesidade , Qualidade de Vida , Ratos , Volume SistólicoRESUMO
OBJECTIVE: Adequacy of the microcirculation is essential for maintaining repetitive skeletal muscle function while avoiding fatigue. It is unclear, however, whether capillary remodelling after different angiogenic stimuli is comparable in terms of vessel distribution and consequent functional adaptations. We determined the physiological consequences of two distinct mechanotransductive stimuli: (1) overload-mediated abluminal stretch (OV); (2) vasodilator-induced shear stress (prazosin, PR). METHODS: In situ EDL fatigue resistance was determined after 7 or 14 days of intervention, in addition to measurements of femoral artery flow. Microvascular composition (muscle histology) and oxidative capacity (citrate synthase activity) were quantified, and muscle PO2 calculated using advanced mathematical modelling. RESULTS: Compared to controls, capillary-to-fiber ratio was higher after OV14 (134%, p < .001) and PR14 (121%, p < .05), although fatigue resistance only improved after overload (7 days: 135%, 14 days: 125%, p < .05). In addition, muscle overload improved local capillary supply indices and reduced CS activity, while prazosin treatment failed to alter either index of aerobic capacity. CONCLUSION: Targeted capillary growth in response to abluminal stretch is a potent driver of improved muscle fatigue resistance, while shear stress-driven angiogenesis has no beneficial effect on muscle function. In terms of capillarity, more is not necessarily better.
Assuntos
Antagonistas de Receptores Adrenérgicos alfa 1 , Capilares , Atividade Motora , Músculo Esquelético , Neovascularização Fisiológica , Prazosina , Antagonistas de Receptores Adrenérgicos alfa 1/farmacologia , Animais , Fenômenos Biomecânicos/efeitos dos fármacos , Fenômenos Biomecânicos/fisiologia , Capilares/efeitos dos fármacos , Capilares/crescimento & desenvolvimento , Capilares/fisiologia , Estimulação Elétrica , Masculino , Microcirculação/efeitos dos fármacos , Microcirculação/fisiologia , Microvasos/efeitos dos fármacos , Microvasos/fisiologia , Modelos Animais , Atividade Motora/fisiologia , Fadiga Muscular/efeitos dos fármacos , Fadiga Muscular/fisiologia , Músculo Esquelético/irrigação sanguínea , Músculo Esquelético/efeitos dos fármacos , Músculo Esquelético/fisiologia , Neovascularização Fisiológica/efeitos dos fármacos , Neovascularização Fisiológica/fisiologia , Consumo de Oxigênio/efeitos dos fármacos , Consumo de Oxigênio/fisiologia , Prazosina/farmacologia , Ratos , Ratos WistarRESUMO
Krogh's Nobel prize for insightful studies into the physiology of capillaries heralded a revolution in understanding that continues today. The view of passive conduits has been replaced by capillaries recognised as a key element in haemodynamic control, offering both a site where changes in tissue demand are sensed and a driver of integrated vascular responses. In addition, the capillary bed is known to play an important role in metabolic, hormonal and immune homeostasis. Not surprisingly, therefore, microvascular dysfunction is a hallmark of many central and peripheral diseases, leading to widespread morbidity and mortality. Consequently, there is growing interest in how best to specifically target this organ-system by means of effective angiotherapies. Underpinning a lot of our current understanding of capillary physiology has been a recognition of functional heterogeneity among different microvascular beds. In addition, there is increasing awareness of the role that spatial heterogeneity plays in determining both physiological and pathological outcomes that has led to an appreciation that quality, rather than just quantity of microvascular supply is important. This has required a re-appraisal of the methods used to determine both the extent and topology of the capillary network, with the benefit of facilitating new ways of exploring dynamic regulation of capillary supply and its potential consequences.
Assuntos
Cardiologia/história , Hemodinâmica , Músculo Esquelético/metabolismo , Músculos/metabolismo , Oxigênio/química , Animais , Capilares/fisiologia , Cardiologia/métodos , Difusão , História do Século XX , Humanos , Modelos Biológicos , Modelos Teóricos , Oxigênio/metabolismo , Consumo de Oxigênio , PerfusãoRESUMO
INTRODUCTION: The morphological characteristics of skeletal muscles innervated caudal to a spinal cord injury (SCI) undergo dramatic phenotypic and microvascular changes. METHOD: Female Sprague-Dawley rats received a severe contusion at thoracic level 9/10 and were randomly assigned to locomotor training, epidural stimulation, or a combination of the treatment groups (CB). Fiber type composition and capillary distribution were assessed in phenotypically distinct compartments of the tibialis anterior. RESULTS: Spinal cord injury induced a shift in type II fiber phenotype from oxidative to glycolytic (P < 0.05) as well as capillary loss within the oxidative core and glycolytic cortex; the CB treatment best maintained capillary supply within both compartments. DISCUSSION: The angiogenic response of CB training improved capillary distribution across the muscle; capillary distribution became spatially more homogeneous and mean capillary supply area decreased, potentially improving oxygenation. There is an important role for weight-bearing training in maintaining the oxidative phenotype of muscle after SCI. Muscle Nerve 59:370-379, 2019.
Assuntos
Capilares/patologia , Terapia por Exercício/métodos , Condicionamento Físico Animal/métodos , Traumatismos da Medula Espinal/patologia , Traumatismos da Medula Espinal/reabilitação , Animais , Atrofia , Estimulação Elétrica , Espaço Epidural , Feminino , Imuno-Histoquímica , Locomoção , Fibras Musculares de Contração Rápida/metabolismo , Fibras Musculares de Contração Rápida/patologia , Músculo Esquelético/irrigação sanguínea , Músculo Esquelético/patologia , Neovascularização Fisiológica , Projetos Piloto , Ratos , Ratos Sprague-Dawley , Recuperação de Função FisiológicaRESUMO
NEW FINDINGS: What is the central question of this study? Mammalian muscle is typically heterogeneous in fibre-type distribution, with distinct regional variation in composition. The effects this might have on mechanical performance are largely unknown. What is the main finding and its importance? Contractile properties vary regionally within a heterogeneous muscle. The mixed extensor digitorum longus muscle has phenotypically distinct compartments that differ in their isometric twitch kinetics, the optimal cycle frequency for maximal power generation and fatigue resistance. The mechanisms underpinning the decline in performance during fatigue differ between compartments. Regional variation in mechanical performance suggests that regions of the extensor digitorum longus muscle might be differentially recruited during locomotion, depending upon functional demand. Fibre-type composition is heterogeneous, and distribution varies spatially in many muscles, indicating that there might be regional variation in recruitment and mechanical output. The rat extensor digitorum longus muscle is composed of predominantly fast-twitch fibres and exhibits a gradient in phenotype, resulting in oxidative medial (areal composition 24.3% type I/IIa) and glycolytic lateral (92.4% type IIx/IIb) compartments. Here, we investigated the variation in mechanical performance between the medial and lateral compartments during isometric, isotonic and cyclical contractions. Isometric tetanic stress and force-velocity relationships were similar in both compartments, but isometric twitch kinetics were slower in the medial compared with the lateral compartment. The medial compartment also had a lower optimal cycle frequency for maximal net power generation (11 versus 15 Hz; P < 0.05) attributable to slower isometric kinetics, resulting in a lower level of activation and reduced net work generation at higher cycle frequencies, compared with the lateral compartment. The more oxidative, medial compartment had higher fatigue resistance, maintaining net power 26% longer than the lateral compartment. The predominant mechanisms underpinning the decrease in net power varied between the compartments, resulting from an increase in the work to extend the muscle and from a reduction in work during shortening in the medial and lateral compartments, respectively. Regional variation in mechanical performance and resistance to fatigue within a mixed muscle suggests that a differential recruitment pattern is likely during locomotion, with the medial compartment being used during slow-speed locomotion and the lateral compartment during burst activities.
Assuntos
Contração Isométrica/fisiologia , Contração Isotônica/fisiologia , Fadiga Muscular/fisiologia , Fibras Musculares Esqueléticas/fisiologia , Animais , Fenômenos Biomecânicos/fisiologia , Masculino , Contração Muscular/fisiologia , Fibras Musculares de Contração Rápida/fisiologia , Músculo Esquelético/fisiologia , Ratos , Ratos WistarRESUMO
Introduction: The accuracy of musculoskeletal models and simulations as methods for predicting muscle functional outputs is always improving. However, even the most complex models contain various assumptions and simplifications in how muscle force generation is simulated. One common example is the application of a generalised ("generic") force-velocity relationship, derived from a limited data set to each muscle within a model, uniformly across all muscles irrespective of whether those muscles have "fast" or "slow" contractile properties. Methods: Using a previously built and validated musculoskeletal model and simulation of trotting in the mouse hindlimb, this work examines the predicted functional impact of applying muscle-specific force-velocity properties to typically fast (extensor digitorum longus; EDL) and slow-contracting (soleus; SOL) muscles. Results: Using "real" data led to EDL producing more positive work and acting significantly more spring-like, and soleus producing more negative work and acting more brake-like in function compared to muscles modelled using "generic" force-velocity data. Extrapolating these force-velocity properties to other muscles considered "fast" or "slow" also substantially impacted their predicted function. Importantly, this also further impacted EDL and SOL function beyond that seen when changing only their properties alone, to a point where they show an improved match to ex vivo experimental data. Discussion: These data suggest that further improvements to how musculoskeletal models and simulations predict muscle function should include the use of different values defining their force-velocity relationship depending on their fibre-type composition.
RESUMO
The material properties of some bones are known to vary with anatomical location, orientation and position within the bone (e.g., cortical and trabecular bone). Details of the heterogeneity and anisotropy of bone is an important consideration for biomechanical studies that apply techniques such as finite element analysis, as the outcomes will be influenced by the choice of material properties used. Datasets detailing the regional variation of material properties in the bones of the skull are sparse, leaving many finite element analyses of skulls no choice but to employ homogeneous, isotropic material properties, often using data from a different species to the one under investigation. Due to the growing significance of investigating the cranial biomechanics of the rabbit in basic science and clinical research, this study used nanoindentation to measure the elastic modulus of cortical and trabecular bone throughout the skull. The elastic moduli of cortical bone measured in the mediolateral and ventrodorsal direction were found to decrease posteriorly through the skull, while it was evenly distributed when measured in the anteroposterior direction. Furthermore, statistical tests showed that the variation of elastic moduli between separate regions (anterior, middle and posterior) of the skull were significantly different in cortical bone, but was not in trabecular bone. Elastic moduli measured in different orthotropic planes were also significantly different, with the moduli measured in the mediolateral direction consistently lower than that measured in either the anteroposterior or ventrodorsal direction. These findings demonstrate the significance of regional and directional variation in cortical bone elastic modulus, and therefore material properties in finite element models of the skull, particularly those of the rabbit, should consider the heterogeneous and orthotropic properties of skull bone when possible.
Assuntos
Osso Esponjoso , Crânio , Animais , Coelhos , Elasticidade , Módulo de Elasticidade , Cabeça , Análise de Elementos Finitos , Fenômenos BiomecânicosRESUMO
Across the human body, skeletal muscles have a broad range of biomechanical roles that employ complex proprioceptive control strategies to successfully execute a desired movement. This information is derived from peripherally located sensory apparatus, the muscle spindle and Golgi tendon organs. The abundance of these sensory organs, particularly muscle spindles, is known to differ considerably across individual muscles. Here we present a comprehensive data set of 119 muscles across the human body including architectural properties (muscle fibre length, mass, pennation angle and physiological cross-sectional area) and statistically test their relationships with absolute spindle number and relative spindle abundance (the residual value of the linear regression of the log-transformed spindle number and muscle mass). These data highlight a significant positive relationship between muscle spindle number and fibre length, emphasising the importance of fibre length as an input into the central nervous system. However, there appears to be no relationship between muscles architecturally optimised to function as displacement specialists and their provision of muscle spindles. Additionally, while there appears to be regional differences in muscle spindle abundance, independent of muscle mass and fibre length, our data provide no support for the hypothesis that muscle spindle abundance is related to anatomical specialisation.
Assuntos
Fusos Musculares , Músculo Esquelético , Humanos , Fusos Musculares/fisiologia , Músculo Esquelético/fisiologia , Mecanorreceptores/fisiologia , Propriocepção/fisiologia , Movimento/fisiologiaRESUMO
Identifying structural limitations in O2 transport is primarily restricted by current methods employed to characterize the nature of physiological remodeling. Inadequate resolution or breadth of available data has impaired development of routine diagnostic protocols and effective therapeutic strategies. Understanding O2 transport within striated muscle faces major challenges, most notably in quantifying how well individual fibers are supplied by the microcirculation, which has necessitated exploring tissue O2 supply using theoretical modeling of diffusive exchange. With capillary domains identified as a suitable model for the description of local O2 supply and requiring less computation than numerically calculating the trapping regions that are supplied by each capillary via biophysical transport models, we sought to design a high-throughput method for histological analysis. We present an integrated package that identifies optimal protocols for identification of important input elements, processing of digitized images with semiautomated routines, and incorporation of these data into a mathematical modeling framework with computed output visualized as the tissue partial pressure of O2 (Po2) distribution across a biopsy sample. Worked examples are provided using muscle samples from experiments involving rats and humans. NEW & NOTEWORTHY Progress in quantitative morphometry and analytical modeling has tended to develop independently. Real diagnostic power lies in harnessing both disciplines within one user-friendly package. We present a semiautomated, high-throughput tool for determining muscle phenotype from biopsy material, which also provides anatomically relevant input to quantify tissue oxygenation, in a coherent package not previously available to nonspecialist investigators.
Assuntos
Músculo Esquelético/metabolismo , Músculo Esquelético/fisiologia , Músculo Estriado/metabolismo , Músculo Estriado/fisiologia , Oxigênio/metabolismo , Adulto , Animais , Capilares/metabolismo , Capilares/fisiologia , Humanos , Masculino , Microcirculação/fisiologia , Modelos Teóricos , Consumo de Oxigênio/fisiologia , Ratos , Adulto JovemRESUMO
In order to successfully perform motor tasks such as locomotion, the central nervous system must coordinate contractions of antagonistic and synergistic muscles across multiple joints. This coordination is largely dependent upon the function of proprioceptive afferents (PAs), which make monosynaptic connections with homonymous motoneurons. Homonymous pathways have been well studied in both health and disease but their collateral fibers projecting to heteronymous, synergistic muscles receive relatively less attention. This is surprising given that PA collaterals have significant effects on the excitability of heteronymous motoneurons, and that their synaptic terminal density is activity dependent. It is likely that the relative lack of literature is due to the lack of a preparation which allows synergistic heteronymous pathways to be assessed in vivo. Here, we describe a method to simultaneously evoke homonymous and heteronymous (synergistic) monosynaptic reflexes (MSRs) and study their modulation by descending pathways in adult rats. Through stimulation of the medial plantar nerve, we were able to produce an H reflex in the intrinsic foot (IF) muscles of the hind paw with a latency of 10.52 ± 3.8 ms. Increasing the stimulus intensity evoked a robust signal with a monosynaptic latency (11.32 ± 0.35 ms), recorded in the ipsilateral gastrocnemius (Gs). Our subsequent analyses suggest that Gs motoneurons were activated via heteronymous afferent collaterals from the medial plantar nerve. These reflexes could be evoked bilaterally and were modulated by conditioning stimuli to the cortex (Cx) and reticular formation. Interestingly, cortical stimulation was equally efficient at modulating both ipsilateral and contralateral reflexes, indicating that cortical modulation of lumbar sensory afferents lacks the laterality demonstrated by studies of cortical muscle activation. This technique represents a novel, relatively simple way to assess heteronymous afferent pathways in normal motor control as well as in models of motor disorders where adaptive and maladaptive plasticity of PAs and descending systems affects functional outcomes.