Cross-bridge movement

Huxley, H. E., Reconditi, M., Stewart, A., and Irving, T. (2003). X-ray interference evidence concerning the range of cross-bridge movement, and backbone contributions to the meridional pattern. Adv. Exp. Med. Biol. 538, 233-241. 

Time-resolved x-ray diffraction of frog muscle confirmed movement of the cross-bridges... 

How can hydrolysis of ATP produce macroscopic movement Muscle contraction essentially consists of the cychc attachment and detachment of the S-1 head of myosin to the F-actin filaments. This process can also be referred to as the making and breaking of cross-bridges. The attachment of actin to myosin is followed by conformational changes which are of particular importance in the S-1 head and are dependent upon which nucleotide is present (ADP or ATP). These changes result... 

The angle of the cross-bridge changes, producing movement of the actin filament. 

Each cross-bridge undergoes its own cycle of movements, independently of the other cross-bridges (Figure... 

To understand how a muscle contracts, consider the Interactions between one myosin head (among the hundreds In a thick filament) and a thin (actin) filament as diagrammed In Figure 3-25. During these cyclical Interactions, also called the cross-bridge cycle, the hydrolysis of ATP Is coupled to the movement of a myosin head toward the Z disk, which corresponds to the (+) end of the thin filament. Because the thick filament Is bipolar, the action of the myosin heads at opposite ends of the thick filament draws the thin filaments toward the center of the thick filament and therefore toward the center of the sarcomere (Figure 19-23). This movement shortens the sarcomere until the ends of the thick filaments abut the Z disk or the (—) ends of the thin filaments overlap at the center of the A band. Contraction of an Intact muscle results from the activity of hundreds of myosin heads on a single thick filament, amplified by the hundreds of thick and thin filaments In a sarcomere and thousands of sarcomeres In a muscle fiber. 

A number of pieces of evidence weaken the case of the cross-bridge model. The movement of the cross-bridges occurs, during contraction, not only in the region of the overlap between the myofilaments but also in the overlap-free region and continues even after the contraction is over. Thus the movement of the cross-bridges can occur even without the involvement of the actin and without a concomitant generation of force. 

The solution to the great puzzle of the unexplained energy in terms of the H stored at the cross-bridges also helps to explain the fast response of a muscle. The continued movement of the cross-bridges in the overlap free zone and after the end of the contraction is explicable in terms of the inertial hydrodynamic flow associated with proto-osmosis. This approach also does not require any long duration contact of a cross-bridge with the actins. The dissociation of ADP and P can occur at any time after the actual contact is made. 

A cross-bridge from the thick filament causes the movement by cyclic attachment to and detachment from the thin filament. Specifically, the cross-bridge detaches from the thin filament, reaches forward, reattaches to the thin filament and contracts sliding the thin filament past the thick filament. 

Figure 8.51. Stereo view in space-filling representation of scallop cross-bridge (SI) without light chains but with neutral residues in light gray, aromatics in black, other hydrophobics in gray, cuid charged residues in white. This perspective of the XZ plane, looking in the negative Z-direction, provides the best view of the movement of the amino-terminal domain that is positioned as a flap over the head of the lever arm in A and moves to be a free standing pedicle in...

Figure 8.52. The same perspective for the stereo view (cross-eye) of scallop cross-bridge (SI) as shown in Figure 8.51, except that it is in backbone representation and thereby allows better delineation of domain movements. In A the amino-terminal domain is shown as a flap over the head of the lever arm in the near-rigor state, whereas it separates as a free-standing pedicle in B, the ATP state. The amino-terminal domain movement also becomes apparent by the changing G53 location at its leading edge.

Modeling Contraction Dynamics. A. F. Huxley developed a mechanistic model to explain the structural changes at the sarcomere level that were seen under the electron microscope in the late 1940s and early 1950s. Because of its complexity, however, this (cross-bridge) model is rarely, if ever, used in studies of coordination. Instead, an empirical model, proposed by A. V. Hill, is used in virtually all models of movement to account for the force-length and force-velocity properties of muscle (Hill, 1938) (Fig. 6.21). 

For the damage identification a FE-model was built the box-girders were modeled with shell elements, the massive bridge ends with volumetric elements and the asphalt layer and non-structural parts with added masses. With regard to the supports at one of the intermediate pillars, there is a hinge the other supports are of the pendulum type. At these pendulum supports, the horizontal movement is somewhat restricted due to the reinforcement bars that cross the contact areas. 

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