To understand muscle you must take it apartle

Striated muscle is an elegant system for study at many levels. Much has been learned about the mechanism of contraction from studying the mechanical properties of intact and permeabilized (or skinned) muscle fibers. Structural studies using electron microscopy, X-ray diffraction or spectroscopic probes attached to various contractile proteins were possible because of the highly ordered sarcomeric arrangement of actin and myosin. However, to understand the mechanism of force generation at a molecular level, it is necessary to take the system apart and study the interaction of myosin with actin using in vitro assays. This reductionist approach has lead to many fundamental insights into how myosin powers muscle contraction. In addition, nature has provided scientists with an array of muscles with different mechanical properties and with a superfamily of myosin molecules. Taking advantage of this diversity in myosin structure and function has lead to additional insights into common properties of force generation. This review will highlight the development of the major assays and methods that have allowed this combined reductionist and comparative approach to be so fruitful. This review highlights the history of biochemical and biophysical studies of myosin and demonstrates how a broad comparative approach combined with reductionist studies have led to a detailed understanding of how myosin interacts with actin and uses chemical energy to generate force and movement in muscle contraction and motility in general

History-Dependent Mechanical Properties of Permeabilized Rat Soleus Muscle Fibers

AbstractPermeabilized rat soleus muscle fibers were subjected to repeated triangular length changes (paired ramp stretches/releases, 0.03 l0,±0.1 l0 s−1 imposed under sarcomere length control) to investigate whether the rate of stiffness recovery after movement increased with the level of Ca2+ activation. Actively contracting fibers exhibited a characteristic tension response to stretch: tension rose sharply during the initial phase of the movement before dropping slightly to a plateau, which was maintained during the remainder of the stretch. When the fibers were stretched twice, the initial phase of the response was reduced by an amount that depended on both the level of Ca2+ activation and the elapsed time since the first movement. Detailed analysis revealed three new and important findings. 1) The rates of stiffness and tension recovery and 2) the relative height of the tension plateau each increased with the level of Ca2+ activation. 3) The tension plateau developed more quickly during the second stretch at high free Ca2+ concentrations than at low. These findings are consistent with a cross-bridge mechanism but suggest that the rate of the force-generating power-stroke increases with the intracellular Ca2+ concentration and cross-bridge strain

The Different Muscle-Energetics during Shortening and Stretch

The helical shape of the thin filaments causes their passive counterclockwise rotation during muscle stretch that increases tensile stress and torque at first by unwinding and then by winding up the four anchoring Z-filaments. This means storage of energy in the series elastic Z-filaments and a considerable decrease of the liberated energy of heat and work to (h—wap), where h is the heat energy and wap the stretch energy induced from outside by an apparatus. The steep thin filament helix with an inclination angle of 70° promotes the passive rotation during stretch, but impedes the smooth sliding of shortening by increased friction and production of frictional heat. The frictional heat may be produced by the contact with the myosin cross-bridges: (1) when they passively snap on drilling thin filaments from cleft to cleft over a distance 2 × 2.7 nm = 5.4 nm between the globular actin monomers in one groove, causing stepwise motion; or (2) when they passively cycle from one helical groove to the next (distance 36 nm). The latter causes more heat and may take place on rotating thin filaments without an effective forward drilling (“idle rotation”), e.g., when they produce “unexplained heat” at the beginning of an isometric tetanus. In an Appendix to this paper the different states of muscle are defined. The function of its most important components is described and rotation model and power-stroke model of muscular contraction is compared