Sliding filament mechanism

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, it bends.
3. Once it 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 "cases" 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. In order to signal when to contract, calcium is released, and interacts.

1. When the muscle does not need to contract (is in resting state), thin strands called tropomyosin , which wrap around the actin filaments, block the myosin binding sites. This inhibits the myosin from binding to actin, and therefore causing a chain of events leading to muscle contraction.
2. Structures called troponin are attached to the tropomyosin.
3. When calcium is introduced into the muscle fiber, they bond to troponin.
4. Calcium then pulls troponin, causing tropomyosin to be pulled as well, and therefore causing the myosin binding sites to be exposed.
5. Myosin binds to the now-exposed binding sites, and muscles contract.