Sliding filament model
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The mechanism for muscle contraction evaded scientists for years and requires continued research and updating. The sliding filament theory describes a process used by muscles to contract. It is a cycle of repetitive events that cause a thin filament to slide over a thick filament and generate tension in the muscle. It was independently developed by Andrew F. Huxley and Rolf Niedergerke and by Hugh Huxley and Jean Hanson in 1954.
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. Calcium ions are released. This calcium bonds to troponin, allowing the myosin head to bind with the binding site.
- 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 high-energy, extended position.
- The cocked myosin head binds to a newly exposed active site on the thin filament, generating a cross-bridge between actin and myosin.
- Myosin releases the ADP and phosphate and returns to a low-energy position, pulling the thin filament along, and 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.
- The binding of ATP destabilizes the myosin-actin bond, allowing myosin to detach from actin. While detached, ATP hydrolysis occurs "recharging" the myosin head. If the actin binding sites are still available, myosin can bind actin again.
- 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.
All muscle cells are composed of a number of actin and myosin filaments in series. The basic unit of organisation of these contractile proteins in striated muscle cells (i.e., the cells that compose cardiac and skeletal muscle, but not in smooth muscle tissue) is called the sarcomere. It consists of a central bidirectional thick filament flanked by two actin filaments, orientated 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. Thus, the ends of the sarcomere are drawn in and the sarcomere shortens. Sarcomeres are connected by so-called 'Z lines', which anchor the ends of actin filaments in such a way that the filaments on each side of the Z line point in opposite directions (with reversed polarity). By this means, sarcomeres are arranged in series. When a muscle fiber contracts, all sarcomeres contract simultaneously so that force is transmitted to the fiber ends.
Physiologically, this contraction is not uniform across the sarcomere; the central position of the thick filaments becomes unstable and can shift during contraction. However the actions of elastic proteins such as Titin are hypothesised to maintain uniform tension across the sarcomere and pull the thick filament into a central position.
Pre-process of movement
If the process of movement were to continue constantly, all muscles would constantly be contracted. Therefore, the body needs a way to control the ability of myosin to bind to actin. This is accomplished by the introduction of calcium into the cytoplasm of the muscle cell.
- When the muscle does not need to contract (is in a resting state), thin strands of a protein called tropomyosin are wrapped around the actin filaments, blocking the myosin binding sites. This inhibits the myosin from binding to actin, and therefore causes a chain of events leading to muscle relaxation.
- Molecules called troponin are attached to the tropomyosin.
- When calcium is introduced into the muscle cell (fiber), calcium ions bind to troponin molecules.
- Calcium binding changes the shape of troponin, causing tropomyosin to be moved deeper into the groove of the actin dimer, therefore causing the myosin binding sites on the actin to be exposed.
- Myosin binds to the now-exposed binding sites, and muscles contract via the sliding-filament mechanism.
Nerve impulses affect the way in which calcium bonds to the troponin.
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