Muscle Contraction When a action potential arrives via a motor neurone to the muscle - it depolarizes the transverse tubules, which triggers calcium to be released from the sarcoplasmic reticulum by the opening of calcium channels in the sarcoplasmic reticulum membrane.
The calcium ions bind with the troponin molecules on the actin filaments - causing them to change shape, this causes the tropomyosin to move, exposing a site that the myosin can bind to.
The myosin then quickly tilts through 45 degrees - so the actin gets pulled towards the centre of the sacromere. As teh head tilts the ADP and Pi are released, and an ATP molecule takes their place.
The myosin head then hydrolyses the ATP to ADP and Pi - the energy generated from this is used to detach the myosin head from the actin filament. The cycle then starts all over again!