Events involved in the contraction of a muscle fiber

The focus here will be on one sarcomere - but the same events take place in every sarcomere within a muscle cell.

1. Calcium diffuses into the cell from the sarcoplasmic reticulum (SR) and transverse tubules. This is a diffusion event and is very quick.
2. Calcium binds with the troponin and tropomyosin system and changes its shape - opening up myosin cross bridge binding sites on the actin molecules.
3. Myosin crossbridges attach to actin and undergo a power stroke. This event requires ATP. Power strokes will continue until the calcium is moved from the T and T system.
4. During contraction, Z -lines move closer together, H-zones get smaller, but the protein filaments do not get shorter, but rather "slide" over each other. This is the sliding filament theory of muscle contraction.