Crossbridges, skeletal

The compliance in series with the active force. Force exerted by the activated elements must be transmitted or borne by whatever structural elements are in series with them. In skeletal muscle there is clearly a tendon in series but not so with smooth muscle. In smooth muscle, the total length of contractile apparatus is broken up into individual cells with intercalating extracellular connective structures. In addition, the portions of the crossbridges in series with the pulling site must also be stretched before force can rise to isometric levels. Taken together, the... 

Organization into macromolecular structures. There are no apparent templates necessary for the assembly of muscle filaments. The association of the component proteins in vitro is spontaneous, stable, and relatively quick. Filaments will form in vitro from the myosins or actins from all three kinds of muscle. Yet in vitro smooth muscle myosin filaments are found to be stable only in solutions somewhat different from in vivo conditions. The organizing principles which govern the assembly of myosin filaments in smooth muscle are not well understood. It is clear, however, a filament is a sturdy structure and that individual myosin molecules go in and out of filaments whose structure remains in a functional steady-state. As described above, the crossbridges sticking out of one side of a smooth muscle myosin filament are all oriented and presumably all pull on the actin filament in one direction along the filament axis, while on the other side the crossbridges all point and pull in the opposite direction. The complement of minor proteins involved in the structure of the smooth muscle myosin filament is unknown, albeit not the same as that of skeletal muscle since C-protein and M-protein are absent. 

In skeletal muscle, calcium binds to troponin and causes the repositioning of tropomyosin. As a result, the myosin-binding sites on the actin become uncovered and crossbridge cycling takes place. Although an increase in cytosolic calcium is also needed in smooth muscle, its role in the mechanism of contraction is very different. Three major steps are involved in smooth muscle contraction ... 

Contraction of smooth muscle is significantly slower than that of skeletal muscle. Furthermore, smooth muscle contraction is quite prolonged (3000 msec) compared to that in skeletal muscle (100 msec). The slow onset of contraction as well as its sustained nature is due to the slowness of attachment and detachment of the myosin crossbridges with the actin. Two factors are involved ... 

In smooth muscle, myosin crossbridges have less myosin ATPase activity than those of skeletal muscle. As a result, the splitting of ATP that provides energy to "prime" the crossbridges, preparing them to interact with actin, is markedly reduced. Consequently, the rates of crossbridge cycling and tension development are slower. Furthermore, a slower rate of calcium removal causes the muscle to relax more slowly. 

Under the electron microscope the myosin heads can sometimes be seen to be attached to the nearby thin actin filaments as crossbridges. When skeletal muscle is relaxed (not activated by a nerve impulse), the crossbridges are not attached, and the muscle can be stretched readily. The thin filaments are free to move past the thick filaments, and the muscle has some of the properties of a weak rubber band. However, when the muscle is activated and under tension, the crossbridges form more frequently. When ATP is exhausted (e.g., after death) muscle enters the state of rigor in which the crossbridges can be seen by electron... 

There is a great deal of literature on the kinetics of myosin SI (crossbridge) ATPase. Originally biochemical studies were restricted to rabbit skeletal myosin. However, recent studies have often been conducted on... 

Haselgrove, J. C., and Huxley, H. E. (1973). X-ray evidence for radial crossbridge movement and for the sliding filament model in actively contracting skeletal muscle. /. Mol. Biol. 77, 549-68. 

Kraft, T., Mattei, T., Radocaj, A., Piep, B., Nocula, C., Furch, M., and Brenner, B. (2002). Structural features of crossbridges in isometrically contracting skeletal muscle. Biophys.J. 82, 2536-2547. 

These mixing curves can be analyzed by a crossbridge model to predict the relative force-producing capability of the two myosins (Harris et al., 1994). The results suggest that smooth muscle myosin exerts 2.1 times the force per myosin cross-bridge compared to skeletal muscle myosin (Harris et al., 1994). 

Hai CM, Murphy RA (1992) Adenosine 5 -triphosphate consumption by smooth muscle as predicted by the coupled four-state crossbridge model. Biophys J 61 530-541 Harris DE, Warshaw DM (1993) Smooth and skeletal muscle actin are mechanically indistinguishable in the in vitro motility assay. Circ Res 72 219-224 Hartshorne DJ, Mrwa U (1982) Regulation of smooth muscle actomyosin. Blood V essels 19 1-18... 

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