Sliding Filament Model - Pre-process of Movement

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.

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 causes a chain of events leading to muscle relaxation.
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 deeper into the groove of the actin dimer, 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.