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The development of muscular force

In the early 1950s it became clear that there were two types of filaments in muscle and that neither extended the entire length of the muscle. Furthermore, it was found that the overall length of the filaments did not change during contraction. Those findings ruled out most of the then existing theories about the mechanism of muscle contraction and, based on shape or length changes in structural elements, provided the basis for the now widely accepted sliding-filament theory.

Shortening of the entire muscle occurs as the thin filaments on both sides of the A band slide farther into the A band. As the sliding progresses, the areas of the sarcomere containing only one type of filament—that is, the I band and the H zone—decrease in size because more and more of the thin and thick filaments overlap each other. On the other hand, the A band remains the same length because the thick filaments do not change in length except in extreme shortening. The distance from the Z line to the edge of the H zone also remains virtually the same length because that distance is determined by the length of the thin filaments. The sliding-filament theory must be expanded to explain certain aspects of contraction: how the force that moves the filaments past each other is generated and how ATP takes part in the process of contraction.

The sliding of the filaments is thought to result from the interaction of the cross bridges with thethin filaments during contraction. Each time a bridge links to a thin filament and operates in a specific way (see the paragraph below), it causes a small movement of the thin filament along the thick filament. Since muscles are able to shorten considerably, requiring sliding of the filaments through large distances, there must be repeated cycles of interaction between a given cross bridge and successive sites on the thin filament.

The myosin heads attached to the actin filament are thought to change their angle with respect to the thin filaments. This change leads to force development, the elastic element residing in some part of the cross bridge (i.e., the portion of myosin connecting the core of the thick filaments to actin) or in the actin-myosin junction itself. The precise connection between various chemical steps in the hydrolysis of ATP and force generation is still under investigation.Similarly, the precise nature of the structural change that corresponds to force generation is notfully understood—instead of the rotation of the whole myosin head about the point of contact between actin and myosin, a bending motion may take place within the head. Some theories suggest purely electrostatic interactions between thick and thin filaments without specific interaction between actin and the myosin heads. The many similarities, however, between factors that affect actin and myosin interactions in solution and those that modulate the behaviour of whole muscle fibres make models of contraction and force development based onspecific protein–protein interaction more fruitful.

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