Events involved in the contraction of a muscle
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
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.