Why animals have different muscle fibre types

Animals have different muscle fibre types: slow fibres with a low maximum velocity of shortening (Vmax) and fast fibres with a high Vmax. An advantage conferred by the use of different fibre types during locomotion has been proposed solely on the basis of their in vitro properties. Isolated muscle experiments show that force generation, mechanical power production and efficiency are all functions of V/Vmax, where V is the velocity of muscle shortening. But it is not known whether animals actually use the different fibres at shortening velocities that are optimal for mechanical power production and efficiency. Here we compare the V of muscle fibres during locomotion with their Vmax. This comparison shows that during slow locomotion, the slow fibres shorten at a velocity that gives peak mechanical power and efficiency and the fast fibres shorten at their optimal velocity when powering maximal movements. Our results also show that maximal movements are impossible without fast fibres because the slow ones cannot shorten rapidly enough.     

Wing bone stresses in free flying bats and the evolution of skeletal design for flight.

The primary mechanical functions of limb bones are to resist deformation, and hence provide stiff levers against which muscles can act, and to be sufficiently strong to prevent breaking under static or dynamic loads which arise from normal and accidental activities. If bones perform these functions with a minimum amount of material, the energetic costs associated with building, maintaining and transporting the skeleton will be minimized. Appropriate skeletal architecture for minimizing mass while maximizing strength depends on forces imposed on structural elements. In the evolutionary acquisition of flight in the bat lineage, the forelimb skeleton must have come to experience locomotor-forces that differed from those engendered by the terrestrial locomotion of non-flying bat relatives. Here we successfully measure in vivo strain on the wing bones of flying mammals. Our data demonstrate that torsion and shear are unique and crucial features of skeletal biomechanics during flight, and suggest that the evolution of skeletal design in bats and other flying vertebrates may be driven by the need to resist these loads.     

Agonist-induced potentiation of acetylcholine sensitivity in denervated skeletal muscle

The entire surface membrane of denervated skeletal muscle is sensitive to the neuromuscular transmitter, acetylcholine (ACh), whereas in innervated muscle only the junctional area is sensitive. It has been proposed that this difference is due to a 'trophic' effect exerted by ACh in innervated muscle to keep the extrajunctional regions of the surface membrane insensitive to its depolarising action. Several studies have demonstrated an agonist-induced potentiation of ACh sensitivity, followed by desensitisation, at the endplate region of normal muscles. The potentiation has been attributed to a cooperative action of ACh on the receptors. Desensitisation of the extrajunctional regions of denervated muscles by ACh has also been described. We now provide evidence that the transmitter itself potentiates the ACh contracture and depolarisation responses of the denervated muscles of the rat in vitro and that it produces this effect by increasing the number of available ACh receptors on the surface membrane.     

Convergent evolution in mechanical design of lamnid sharks and tunas

The evolution of 'thunniform' body shapes in several different groups of vertebrates, including whales, ichthyosaurs and several species of large pelagic fishes supports the view that physical and hydromechanical demands provided important selection pressures to optimize body design for locomotion during vertebrate evolution. Recognition of morphological similarities between lamnid sharks (the most well known being the great white and the mako) and tunas has led to a general expectation that they also have converged in their functional design; however, no quantitative data exist on the mechanical performance of the locomotor system in lamnid sharks. Here we examine the swimming kinematics, in vivo muscle dynamics and functional morphology of the force-transmission system in a lamnid shark, and show that the evolutionary convergence in body shape and mechanical design between the distantly related lamnids and tunas is much more than skin deep; it extends to the depths of the myotendinous architecture and the mechanical basis for propulsive movements. We demonstrate that not only have lamnids and tunas converged to a much greater extent than previously known, but they have also developed morphological and functional adaptations in their locomotor systems that are unlike virtually all other fishes.     

A common somitic origin for embryonic muscle progenitors and satellite cells

In the embryo and in the adult, skeletal muscle growth is dependent on the proliferation and the differentiation of muscle progenitors present within muscle masses. Despite the importance of these progenitors, their embryonic origin is unclear. Here we use electroporation of green fluorescent protein in chick somites, video confocal microscopy analysis of cell movements, and quail-chick grafting experiments to show that the dorsal compartment of the somite, the dermomyotome, is the origin of a population of muscle progenitors that contribute to the growth of trunk muscles during embryonic and fetal life. Furthermore, long-term lineage analyses indicate that satellite cells, which are known progenitors of adult skeletal muscles, derive from the same dermomyotome cell population. We conclude that embryonic muscle progenitors and satellite cells share a common origin that can be traced back to the dermomyotome.     

Cell lineages generating axial muscle in the zebrafish embryo

Cell lineage may contribute to determining the numbers, positions and types of cells formed during embryogenesis. In vitro clonal analyses show that vertebrate cells can autonomously maintain lineage commitments to single fates and that terminal development may include an invariant sequence of cell divisions. In addition, in vivo studies with Xenopus led to the proposal that clonal restrictions to spatial 'compartmental' domains arise during early development, analogous to what is observed in insects. In the zebrafish, individual gastrula cells generate clones of progeny that are confined within single tissues, but spatial restrictions have not been described. We now have examined the in vivo terminal cell lineages of zebrafish axial muscles. We obtained no evidence either for strict developmental regulation of division pattern or for spatial compartmentation within muscle lineages.     

Fast and slow myosin within single skeletal muscle fibres of adult rabbits.

There is good evidence for the coexistence of different myosin types both in developing muscles and in Purkinje cells from adult chicken hearts. In skeletal muscle fibres of adult animals, however, coexistence of fast (FM) and slow (SM) myosin has only been demonstrated after long-term electrical stimulation. The term 'promiscuity' has recently been coined to describe the coexistence of different myosin isoenzymes within a single fibre. Using novel, refined immunological methods we demonstrate here the presence of both FM and SM within single fibres of the musculus tibialis anterior of adult rabbits. Essentially identical results were also obtained with other muscles. Our findings imply that the genes coding for FM and SM can be expressed simultaneously within the same cell throughout an animal's entire life, and not only during development or after artificial electrical stimulation.     

Insulin stimulates sugar transport in giant muscle fibres of the barnacle

Insulin stimulates sugar transport in vertebrate skeletal muscle but the mechanism of insulin action is unknown. It has been reported that Na transport in giant muscle fibers of the barnacle (Balanus nubilis) is sensitive to insulin but no one has examined the sensitivity of sugar tansport to insulin in this preparation. We show here that insulin does, indeed, stimulate sugar transport in barnacle muscle. The great advantage of barnacle muscle over all other muscles used so far for investigating the mechanism of insulin action is its large size, which facilitates measurements on single cells and permits the experimenter to control the intracellular environment of the muscle fibre by the technique of internal dialysis. Using single muscle fibres it is possible to show that acceleration of sugar transport by insulin is associated with a fall in ionized Ca, a fall in cyclic AMP and a rise in cyclic GMP. Working with internally dialysed muscle fibres we find that insulin only increases sugar transport when the dialysis solution contains ATP. In the absence of insulin, sugar transport is dialysed muscle is increased by a rise in ionized Ca, a fall in cyclic AMP and, when the internal Ca is elevated, by a rise in cyclic GMP.     

Minimally invasive high-speed imaging of sarcomere contractile dynamics in mice and humans

Sarcomeres are the basic contractile units of striated muscle. Our knowledge about sarcomere dynamics has primarily come from in vitro studies of muscle fibres and analysis of optical diffraction patterns obtained from living muscles. Both approaches involve highly invasive procedures and neither allows examination of individual sarcomeres in live subjects. Here we report direct visualization of individual sarcomeres and their dynamical length variations using minimally invasive optical microendoscopy to observe second-harmonic frequencies of light generated in the muscle fibres of live mice and humans. Using microendoscopes as small as 350 microm in diameter, we imaged individual sarcomeres in both passive and activated muscle. Our measurements permit in vivo characterization of sarcomere length changes that occur with alterations in body posture and visualization of local variations in sarcomere length not apparent in aggregate length determinations. High-speed data acquisition enabled observation of sarcomere contractile dynamics with millisecond-scale resolution. These experiments point the way to in vivo imaging studies demonstrating how sarcomere performance varies with physical conditioning and physiological state, as well as imaging diagnostics revealing how neuromuscular diseases affect contractile dynamics.     

Tracking kinesin-driven movements with nanometre-scale precision

Several enzyme complexes drive cellular movements by coupling free energy-liberating chemical reactions to the production of mechanical work. A key goal in the study of these systems is to characterize at the molecular level mechanical events associated with individual reaction steps in the catalytic cycles of single enzyme molecules. Ideally, one would like to measure movements driven by single (or a few) enzyme molecules with sufficient temporal resolution and spatial precision that these events can be directly observed. Kinesin, a force-generating ATPase involved in microtubule-based intracellular organelle transport, will drive the unidirectional movement of microscopic plastic beads along microtubules in vitro. Under certain conditions, a few (less than or equal to 10) kinesin molecules may be sufficient to drive either bead movement or organelle transport. Here we describe a method for determining precise positional information from light-microscope images. The method is applied to measure kinesin-driven bead movements in vitro with a precision of 1-2 nm. Our measurements reveal basic mechanical features of kinesin-driven movements along the microtubule lattice, and place significant constraints on possible molecular mechanisms of movement.     

Early motor activity drives spindle bursts in the developing somatosensory cortex

Sensorimotor coordination emerges early in development. The maturation period is characterized by the establishment of somatotopic cortical maps, the emergence of long-range cortical connections, heightened experience-dependent plasticity and spontaneous uncoordinated skeletal movement. How these various processes cooperate to allow the somatosensory system to form a three-dimensional representation of the body is not known. In the visual system, interactions between spontaneous network patterns and afferent activity have been suggested to be vital for normal development. Although several intrinsic cortical patterns of correlated neuronal activity have been described in developing somatosensory cortex in vitro, the in vivo patterns in the critical developmental period and the influence of physiological sensory inputs on these patterns remain unknown. We report here that in the intact somatosensory cortex of the newborn rat in vivo, spatially confined spindle bursts represent the first and only organized network pattern. The localized spindles are selectively triggered in a somatotopic manner by spontaneous muscle twitches, motor patterns analogous to human fetal movements. We suggest that the interaction between movement-triggered sensory feedback signals and self-organized spindle oscillations shapes the formation of cortical connections required for sensorimotor coordination.     

Mutations in DMRT3 affect locomotion in horses and spinal circuit function in mice

Locomotion in mammals relies on a central pattern-generating circuitry of spinal interneurons established during development that coordinates limb movement. These networks produce left-right alternation of limbs as well as coordinated activation of flexor and extensor muscles. Here we show that a premature stop codon in the DMRT3 gene has a major effect on the pattern of locomotion in horses. The mutation is permissive for the ability to perform alternate gaits and has a favourable effect on harness racing performance. Examination of wild-type and Dmrt3-null mice demonstrates that Dmrt3 is expressed in the dI6 subdivision of spinal cord neurons, takes part in neuronal specification within this subdivision, and is critical for the normal development of a coordinated locomotor network controlling limb movements. Our discovery positions Dmrt3 in a pivotal role for configuring the spinal circuits controlling stride in vertebrates. The DMRT3 mutation has had a major effect on the diversification of the domestic horse, as the altered gait characteristics of a number of breeds apparently require this mutation.     

A lethal mutation in mice eliminates the slow calcium current in skeletal muscle cells

Contraction of a vertebrate skeletal muscle fibre is triggered by electrical depolarization of sarcolemmal infoldings termed transverse-tubules (t-tubules), which in turn causes the release of calcium from an internal store, the sarcoplasmic reticulum (SR). The mechanism that links t-tubular depolarization to SR calcium release remains poorly understood. In principle, this link might be provided by the prominent slow calcium current that has been described in skeletal muscle cells of adult frogs and rats. However, blocking this current does not abolish the depolarization-induced contractile responses of frog muscle, and the function of this slow calcium current is unknown. Here we describe measurements of calcium currents in developing skeletal muscle cells of normal rats and mice, and of mice with muscular dysgenesis, a mutation that causes excitation-contraction (E-C) coupling to fail. We find that a slow calcium current is present in skeletal muscle cells of normal animals but absent from skeletal muscle cells of mutant animals. The effect of the mutation is specific to the slow calcium current of skeletal muscle; a fast calcium current is present in developing skeletal muscle cells of both normal and mutant animals, and slow calcium currents are present in cardiac and sensory neurones of mutant animals. We believe this to be the first report of a mutation affecting calcium currents in a multicellular organism. The effects of the mutation raise important questions about the relationship between the slow calcium current and skeletal muscle E-C coupling.     

木薯淀粉对栉孔扇贝闭壳肌凝胶特性的改善作用

为了解决扇贝产品单一,精深加工比例低的问题,以栉孔扇贝闭壳肌为研究对象,对其营养成分进行了测定,并研究了不同添加量的木薯淀粉对扇贝肉糜凝胶强度、质构特性、持水性、白度、蛋白组成及微观结构的影响。结果表明,栉孔扇贝闭壳肌中水分、粗蛋白、粗脂肪和灰分质量分数分别为82.15%、16.23%、2.05%和1.24%,除色氨酸外,16种水解氨基酸的总量为13.204%。随着木薯淀粉添加量的增加,扇贝闭壳肌肉糜的凝胶强度、持水性、硬度和咀嚼度呈现先升高后降低的趋势,但对其弹性、黏聚性和白度没有显著性影响。电泳图谱结果表明,添加质量分数为1%的木薯淀粉,贝糜凝胶蛋白中肌球蛋白重链的条带最窄,说明淀粉可促进贝糜蛋白的交联;光学显微镜和扫描电镜结果表明,木薯淀粉添加量为1%或2%时,贝糜凝胶的网络结构更为致密,孔洞更小。本研究表明,栉孔扇贝是一种高蛋白的水产资源,可用来生产贝肉糜凝胶,且1%的木薯淀粉添加量对贝糜凝胶品质具有较好的改善作用。希望本研究能为扇贝精深加工提供一定的理论依据。     

Telemetered in vivo strain analysis of locomotor mechanics of brachiating gibbons

The slender elongated form that is characteristic of the forelimb long bones of gibbons (Hylobates) has long been attributed to their functional adaptation to habitual armswinging locomotion, although potential selective advantages of this morphology for brachiation have yet to be demonstrated. If the forces exerted on the limb skeleton during brachiation indeed differ greatly from those of other locomotor modes, then the changes in skeletal loading accompanying a shift in locomotor behaviour could favour alterations in skeletal morphology in brachiating lineages. In vivo skeletal strain patterns recorded by using radiotelemetry during brachiation indicate that the forelimb bones of the gibbon are loaded in substantial tension and show reduced bending and compression in comparison with those of other mammals. We suggest that this unique loading regime could have contributed to the evolution of the distinctive morphology of hylobatid limbs.     

仿生蟹的行走方式与自适应控制策略

论述基于稳定性的八足行走问题，分析生物蟹的典型行走方式和机械性能。建立五自由度单腿机构模型，确定二圆柱关节参数规划轨迹和三自由度关节参数修正实现仿生蟹行走的控制策略。设计了仿生蟹的步行走方式，提出了基于约束的自适应行走方式，使仿生蟹具有更优良的行走性能。应用坐标变换和微分变换建立了步行走的控制方程，并给定仿生蟹的结构参数，完成了仿真计算和性能曲线绘制，计算结果证明了球铰和控制策略在仿生机器人中应用的可行性。     

Dystrophic heart failure blocked by membrane sealant poloxamer

Dystrophin deficiency causes Duchenne muscular dystrophy (DMD) in humans, an inherited and progressive disease of striated muscle deterioration that frequently involves pronounced cardiomyopathy. Heart failure is the second leading cause of fatalities in DMD. Progress towards defining the molecular basis of disease in DMD has mostly come from studies on skeletal muscle, with comparatively little attention directed to cardiac muscle. The pathophysiological mechanisms involved in cardiac myocytes may differ significantly from skeletal myofibres; this is underscored by the presence of significant cardiac disease in patients with truncated or reduced levels of dystrophin but without skeletal muscle disease. Here we show that intact, isolated dystrophin-deficient cardiac myocytes have reduced compliance and increased susceptibility to stretch-mediated calcium overload, leading to cell contracture and death, and that application of the membrane sealant poloxamer 188 corrects these defects in vitro. In vivo administration of poloxamer 188 to dystrophic mice instantly improved ventricular geometry and blocked the development of acute cardiac failure during a dobutamine-mediated stress protocol. Once issues relating to optimal dosing and long-term effects of poloxamer 188 in humans have been resolved, chemical-based membrane sealants could represent a new therapeutic approach for preventing or reversing the progression of cardiomyopathy and heart failure in muscular dystrophy.     

基于AnyBody仿真及有限元建模下的八段锦运动生物力学研究

以AnyBody仿真骨骼肌多体逆向动力学三元素肌肉收缩力模型及有限元法构建八段锦动作的三维胫骨模型,全面、客观地计算不同动作最大肌力峰值时刻的积分肌电、力矩、肌肉收缩等生物力学特性. 将10例健康青年八段锦志愿者的身高、体质量、髋宽、髋深、膝宽、踝宽等形态学数据作为约束条件,用AnyBody7.1.2软件导出八段锦8个动作下肢最大肌力指标,依据胫骨最大应力约束条件,用Minics 10.01、Geomagic studio 2013、ANSYS 19.2软件建构三维数据,添加AnyBody中的3个方向数据,分析并计算不同动作最大力的应力传导大小、分布规律. 研究结果表明:八段锦是左右对称性运动,双侧同块肌肉积分肌电值、肌肉力值无显著性差异,有利于锻炼双下肢稳定性及协调性,运动中人体需要通过移动来保持平衡,膝关节在垂直轴方向力最大,伸展力矩值最大;做八段锦运动过程中,髋关节主导发力肌肉为臀中肌,膝关节主导发力肌肉为比目鱼肌、股外侧肌、腓肠肌,踝关节主导发力肌肉为拇长屈肌;表面肌电验证是评价人体肌肉激活程度的主要方式,八段锦运动中胫骨前肌、腓肠肌、股直肌、股二头肌的肌肉活性仿真结果与积分肌电值具有较高的一致性.以AnyBody软件分析下肢动作特征和有限元软件建构的胫骨模型,适用于八段锦动作的动态研究,可为计算八段锦运动时动力学变化提供新思路与实践价值参考.      

Abnormal expression of inducible nitric oxide synthase in Duchenne muscular dystrophy muscles

Duchenne muscular dystrophy (DMD) is a fatal genetic disease for the youth and children. 8 biopsies of DMD patients were determined and demonstrated that the membrane_binding nitric oxide synthase was enriched in normal skeletal muscles and was little in DMD muscles. The results from Western blot and immunohistochemistry showed that inducible nitric oxide synthase (iNOS) was overexpressed in DMD muscle fibers, while a small amount of highly localized iNOS can be found in normal fibers. Based on these findings, it is proposed that the mechanism of progressive injury in DMD muscle might be associated with the abnormal expression of iNOS.     

Relationship between the partitioning properties of motoneuron recruitment and the needle feeling propagation

Histological and physiological studies indicated that most skeletal muscles can be divided into a series of relatively independent sub-volumes: "neuromuscular compartments" and the partitioning property of the muscle result in the localization of muscle reflex. In the present experiment we studied the recruitment properties of medial gastracnemius (MG) muscle motoneuron pool of decerebrate cat with two kinds of local mechanical stimulation: local stretch and acupuncture-like stimuli. The results indicate that: (ⅰ) there is an obvious property of localization of recruitment activity, only the MNs which innervate the stimulated compartment were recruited by weaker and shorter stimuli; (ⅱ) recruitment activity spread to those MNs which supply the adjacent and distal compartments during the strength of stimulation or duration of the stimulation was increased; and (ⅲ) the recruitment property of muscle activity elicited by the local mechanical stimulation is thought similar to that of "needle feeling" along the meridian pathway during stimulation of acupuncture point.