Sliding filament model

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The sliding filament mechanism is a process used by muscles to contract.

[edit] 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. So each myosin 'walks' along an actin filament repeatedly binding, ratcheting and letting go. By this means the thick filament slides over thin filament.

1. Myosin heads bind to the passive actin filaments at the myosin binding sites.
2. As soon as the myosin head binds to actin, the head bends at its hinge.
3. Once the head bends, the myosin loses energy, and remains attached to the actin.
4. When re-energized by ATP, the myosin head detaches from the actin filament, and is ready to attach and bend 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.

All real muscle cells are composed of a number of actin and myosin filaments in series. The basic unit 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 together by so-called 'Z lines', which anchor the ends of actin filaments in such away that the filaments on each side of the Z line point in opposite directions. 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.

[edit] 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 the actin. This is accomplished by the introduction of calcium into the cytoplasm of the muscle cell.

1. 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 causing a chain of events leading to muscle contraction.
2. Molecules called troponin are attached to the tropomyosin.
3. When calcium is introduced into the muscle cell (fiber), calcium ions bind to troponin molecules.
4. Calcium binding changes the shape of troponin, causing tropomyosin to be moved as well, therefore causing the myosin binding sites on the actin to be exposed.
5. 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.