Mechanical Properties Of Muscle Fibers Research Articles

Effect of calcium ion on the morphology structure and compression elasticity of muscle fibers from honeybee abdomen

Muscle contraction activated by calcium ion is the key to reveal that honeybee abdomen can achieve various physiological activities through flexible exercises and contributes to a powerful mechanical function of muscle fibers. To investigate the stimulating effect of calcium ion on muscle fibers of honeybee abdomen, atomic force microscopy was used to measure the morphology structure and mechanical properties of muscle fibers from honeybee abdomen in different calcium ion solutions. The periodic morphology structure of muscle fibers stimulated by different calcium ion concentration changed greatly, and the sarcomere length contracted from 6.53 μm to 4.29 μm as the calcium ion concentration increased from 0.11 mM to 10 mM. The mechanical measurement showed that the elastic modulus of Z-line reached the maximum, followed by M-line, overlap zone and I-band in sequence at the same calcium ion concentration, and was approximately 3.636, 2.450, 2.284, 2.748 times that of I-band from 0.11 mM to 10 mM calcium ion concentration. Combining the experimental analysis, the calcium ion threshold range was obtained based on the response surface method. This work adequately elucidates biological structure and biomechanics of muscle fibers from honeybee abdomen and could provide reference for other similar muscle system.

Mechanical properties of respiratory muscles.

Striated respiratory muscles are necessary for lung ventilation and to maintain the patency of the upper airway. The basic structural and functional properties of respiratory muscles are similar to those of other striated muscles (both skeletal and cardiac). The sarcomere is the fundamental organizational unit of striated muscles and sarcomeric proteins underlie the passive and active mechanical properties of muscle fibers. In this respect, the functional categorization of different fiber types provides a conceptual framework to understand the physiological properties of respiratory muscles. Within the sarcomere, the interaction between the thick and thin filaments at the level of cross-bridges provides the elementary unit of force generation and contraction. Key to an understanding of the unique functional differences across muscle fiber types are differences in cross-bridge recruitment and cycling that relate to the expression of different myosin heavy chain isoforms in the thick filament. The active mechanical properties of muscle fibers are characterized by the relationship between myoplasmic Ca2+ and cross-bridge recruitment, force generation and sarcomere length (also cross-bridge recruitment), external load and shortening velocity (cross-bridge cycling rate), and cross-bridge cycling rate and ATP consumption. Passive mechanical properties are also important reflecting viscoelastic elements within sarcomeres as well as the extracellular matrix. Conditions that affect respiratory muscle performance may have a range of underlying pathophysiological causes, but their manifestations will depend on their impact on these basic elemental structures.

An Integrative Analysis of the Muscle Myosin Motor Using Genetic and Transgenic Tools

We use Drosophila melanogaster to investigate the mechanisms by which myosin heavy chain isoforms contribute to muscle-specific cyto-architectures and contractile properties. We expressed various myosin isoforms in transgenic indirect flight muscles. Along with our collaborators, we examined 1) ATPase, in vitro actin sliding, step size and structure of the isolated myosin, 2) ultrastructure and mechanical properties of muscle fibers, and 3) locomotory abilities of the transgenic organisms. We found that isoform-specific differences in myosin cause relatively small structural variations in muscle assembly, but are critical to myofibril stability and function.

Effects of Eccentric Exercise on Joint Stiffness and Muscle Connectin (Titin) Isoform in the Rat Hindlimb

We investigated the effects of repeated eccentric exercise for rat medial gastrocnemius muscle on ankle joint stiffness and muscle connectin (titin) isoform composition (longer form, alpha-connectin; shorter form, beta-connectin). Male Wistar rats were trained on a custom-made, isokinetic dynamometer (eccentric-exercise group, n = 6; sham-operated group, n = 6). The exercise session consisted of 20 eccentric contractions elicited by submaximal electric stimulations under anesthesia. The contracting muscle was forcibly lengthened by an isokinetic dorsi-flexion of the ankle joint (velocity, 30 degrees/s; range of motion, 45 degrees). Rats in the eccentric-exercise group were trained every two days for 20 days (10 sessions in total). The static passive resistive torque (PRT) of 45 degrees at the ankle joint was used as a measure of the joint stiffness, and was determined before and after the experimental period. After 10 sessions of eccentric exercise, the wet weight of medial gastrocnemius muscle significantly increased (P < 0.05), whereas the static PRT significantly decreased (P < 0.05) in the eccentric-exercise group, when compared to the sham-operated group. Myosin-ATPase staining showed a decrease in the number of type IIb/IId fibers (P < 0.001) and an increase in the number of type IIa fibers (P < 0.05). However, no significant difference was seen in the connectin (titin) isoform composition between the eccentric-exercise group and the sham-operated group, suggesting that the reduction in PRT was not due to change in resting mechanical properties of muscle fibers.

Mutation of the myosin converter domain alters cross-bridge elasticity.

Elastic distortion of a structural element of the actomyosin complex is fundamental to the ability of myosin to generate motile forces. An elastic element allows strain to develop within the actomyosin complex (cross-bridge) before movement. Relief of this strain then drives filament sliding, or more generally, movement of a cargo. Even with the known crystal structure of the myosin head, however, the structural element of the actomyosin complex in which elastic distortion occurs remained unclear. To assign functional relevance to various structural elements of the myosin head, e.g., to identify the elastic element within the cross-bridge, we studied mechanical properties of muscle fibers from patients with familial hypertrophic cardiomyopathy with point mutations in the head domain of the beta-myosin heavy chain. We found that the Arg-719 --> Trp (Arg719Trp) mutation, which is located in the converter domain of the myosin head fragment, causes an increase in force generation and fiber stiffness under isometric conditions by 48-59%. Under rigor and relaxing conditions, fiber stiffness was 45-47% higher than in control fibers. Yet, kinetics of active cross-bridge cycling were unchanged. These findings, especially the increase in fiber stiffness under rigor conditions, indicate that cross-bridges with the Arg719Trp mutation are more resistant to elastic distortion. The data presented here strongly suggest that the converter domain that forms the junction between the catalytic and the light-chain-binding domain of the myosin head is not only essential for elastic distortion of the cross-bridge, but that the main elastic distortion may even occur within the converter domain itself.

Determining structure/function relationships for sarcomeric myosin heavy chain by genetic and transgenic manipulation of Drosophila.

Drosophila melanogaster is an excellent system for examining the structure/function relationships of myosin. It yields insights into the roles of myosin in assembly and stability of myofibrils, in defining the mechanical properties of muscle fibers, and in dictating locomotory abilities. Drosophila has a single gene encoding muscle myosin heavy chain (MHC), with alternative RNA splicing resulting in stage- and tissue-specific isoform production. Localization of the alternative domains of Drosophila MHC on a three-dimensional molecular model suggests how they may determine functional differences between isoforms. We are testing these predictions directly by using biophysical and biochemical techniques to characterize myosin isolated from transgenic organisms. Null and missense mutations help define specific amino acid residues important in actin binding and ATP hydrolysis and the function of MHC in thick filament and myofibril assembly. Insights into the interaction of thick and thin filaments result from studying mutations in MHC that suppress ultrastructural defects induced by a troponin I mutation. Analysis of transgenic organisms expressing engineered versions of MHC shows that the native isoform of myosin is not critical for myofibril assembly but is essential for muscle function and maintenance of muscle integrity. We show that the C-terminus of MHC plays a pivotal role in the maintenance of muscle integrity. Transgenic studies using headless myosin reveal that the head is important for some, but not all, aspects of myofibril assembly. The integrative approach described here provides a multi-level understanding of the function of the myosin molecular motor.

The action of muscles in swimming fish.

This review will focus on the mechanical performance of the trunk muscles of fish in relation to their important role in the generation of lateral undulations during swimming. In most fish, the different muscle fibre types are located in separate regions in the trunk (e.g. Boddeke, Slijper & Van der Stelt, 1959). This makes fish muscle particularly suitable for investigating how different fibre types are used during locomotion. Not only can the activity of homogeneous fibre bundles be detected fairly easily by electromyography (e.g. Bone, 1966), but bundles of fibres of the same type may also be isolated for mechanical experiments which mimic the in vivo activation and strain variation (e.g. Altringham & Johnston, 1990a). The following questions will be addressed in particular. First, a swimming fish produces waves of curvature along its trunk with its muscles. What is the nature of the associated net bending moments along the trunk and how much power is produced along the trunk (section 2)? Second, what are the mechanical properties of muscle fibres of fish and what is their efficiency (sections 3 and 4)? How are the muscle fibres at different positions in the trunk of a swimming fish recruited during various swimming modes? What is their in vivo work and power output (section 5)? Space limits do not allow consideration of the structural design of the myomeres, nor the complex innervation patterns of fish muscles.