The Cross-Bridge Model

The Mechanism of Muscular Contraction 2.1. The Cross-Bridge Model 

FIGURE 1. A contractile unit showing the thin filaments emanating from the Z disks and the centrally located thick filaments. 

If one looks more closely at the situation, however, one cannot escape the feeling that the cross-bridge model has shifted the problem of finding the mechanism of contraction from the level of the myofilaments to the properties of the cross-bridges. These have to perform the (Maxwellian) 

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. 

From the above survey of the experimental evidence it should be apparent that the cross-bridge model cannot be considered as inviolable, and there is sufficient reason to look for some other viable models which can resolve all puzzling aspects of muscular contraction in a consistent manner. 

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Figure 14.12 The swinging cross-bridge model of muscle contraction driven by ATP hydrolysis, (a) A myosin cross-bridge (green) binds tightly in a 45 conformation to actin (red), (b) The myosin cross-bridge is released from the actin and undergoes a conformational change to a 90 conformation (c), which then rebinds to actin (d). The myosin cross-bridge then reverts back to its 45° conformation (a), causing the actin and myosin filaments to slide past each other. This whole cycle is then repeated.

Figure 14.17 A sequence of events combining the swinging cross-bridge model of actin and myosin filament sliding with structural data of myosin with and without bound nucleotides.

The Sliding Filament Cross-Bridge Model Is the Foundation on Which Current Thinking About Muscle Contraction Is Built... 

Spudich, J. A. 2001. The myosin swinging cross-bridge model. Nature Rev. Mol Cell Biol 2 387-392. 

Huxley s model (1957) for muscle contraction states that the mechanical and enzymatic characteristics can be described by the overall apparent attachment and detachment rates of the cross-bridge. Contraction was described as a transition between free and attached states (Huxley, 1957). This simple yet elegant two-state model produces several specific predictions about the relationships between force production, ATPase rates, and shortening as a function of these two rate constants. Although this model is somewhat oversimplified given the actual number of biochemical states, and several of its assumptions may not strictly hold in... 

Zahalak, G. I. (2000). The two-state cross-bridge model of muscle is an asymptotic limit of multi-state models, Journal of Theoretical Biology, 204 67-82. 

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). 

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