Sliding filament mechanism

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

Process of Movement

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 molecules closer together. This results in a muscle contraction.

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 then pulls 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 above mechanism.

Nerve impulses affect the way in which calcium bonds to the troponin.