Box 103

Functional Considerations: The Sliding Filament Model

The sliding filament model postulates that the ratchet-like movements of the myosin heads bound to actin produce the movement of the thin filaments relative to the thick filaments, which in turn causes the sarcomere to shorten. Although the sliding filament model can explain contraction in a single sarcomere, it cannot adequately explain the shortening of a myofibril of a muscle fiber. Obviously, if the activity just described were to occur simultaneously in adjacent sarcomeres, no contraction could occur. Equal and opposite forces would be exerted on either side of the Z line, and the contraction of any given sarcomere would be prevented by the contraction of its two immediate serial neighbors. Recent studies with ultrahigh-speed photography have demonstrated that an extremely small temporal delay occurs between the contraction of adjacent sarcomeres, so that a wave-like contraction actually occurs in each muscle fibril and, consequently, in each muscle fiber.

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