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In the sliding filament model, the thick and thin filaments past each other, shortening the sarcomere. LEARNING OBJECTIVE [ edit ] Describe the sliding filament model of muscle contraction KEY POINTS [ edit ] ATP activates myosin, bringing it to a higher energy state, ready to form crossbridges with the thin actin filament. Myosin acts as a molecular ratchet by binding to an actin filament and changing shape, thereby pulling the actin filament toward the Aband. ATP binds again, destabilizing the myosin filament and enabling it to bind to another site along the actin filament, increasing the strength of contraction. All the myosin heads contract simultaneously, shortening all the sarcomeres, causing the muscle to contract. The myosin heads pull the Aband toward the Zlines at the end of each sarcomere, shortening the length of the Iband. TERMS [ edit ] crossbridge the bond between the globular head of a myosin molecule and the actin filament, repeatedly formed during muscle contraction to draw it into the A band of a sarcomere Aband contains the entire length of a single thick filament Iband the zone of thin filaments that is not superimposed by thick filaments Give us feedback on this content: FULL TEXT [ edit ] Sliding Filament Model of Contraction Muscles contract when sarcomeres shorten. The thin and thick filaments that compose sarcomeres do not shorten; instead, they slide past one another, causing the sarcomere to shorten while the filaments remain the same length. The sliding filament theory of muscle contraction is the binding of myosin to Register for FREE to stop seeing ads actin, forming crossbridges that generate filament movement . Sliding filament model When (a) a sarcomere (b) contracts, the Zlines move closer together and the I band gets smaller. The A band stays the same width and, at full contraction, the thin filaments overlap. Process of Movement Myosin is a molecular motor that acts like an active ratchet. Chains of actin proteins form high tensile passive 'thin' filaments that transmit the force generated by myosin to the ends of the muscle. Myosin also forms 'thick' filaments. Each myosin 'paddles' along an actin filament repeatedly binding, ratcheting, and letting go, sliding the thick filament over the thin filament. 1. ATP binds to myosin and is hydrolyzed by ATPase into ADP and phosphate. The energy released by this process activates the myosin head and cocks it into a highenergy, extended position. 2. The cocked myosin head binds to a newlyexposed active site on the thin filament, generating a crossbridge between actin and myosin. 3. Myosin releases the ADP and phosphate, returning to a lowenergy position, pulling the thin filament along; this movement is called a power stroke. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin. 4. The binding of ATP destabilizes the myosinactin bond, allowing myosin to detach from actin. While detached, ATP hydrolysis occurs, "recharging" the myosin head. If the actinbinding sites are still available, myosin can bind actin again. 5. The collective bending of numerous myosin heads (all in the same direction), combine to move the actin filament relative to the myosin filament. This results in muscle contraction. Contraction in all Sarcomeres The sarcomere consists of a central bidirectional thick filament flanked by two actin filaments, oriented in opposite directions. When each end of the myosin thick filament ratchets along the actin filament with which it overlaps, the two actin filaments are drawn closer together. The Ibands contract toward the Abands, thereby shortening the Iband as the myosin overlaps with the actin. Thus, the ends of the sarcomere are drawn in and the sarcomere shortens. Sarcomeres are connected by socalled 'Zlines', which anchor the ends of actin filaments in such a way that the filaments on each side of the Zline point in opposite directions (with reversed polarity). When a muscle fiber contracts, all sarcomeres contract simultaneously so that force is transmitted to the fiber ends.