Explain the sliding filament theory of muscle contraction

Muscle contraction begins after an action potential reaches the required muscle and causes the calcium ion protein channels on the sarcoplasmic reticulum to open. This allows the calcium ions to diffuse down their concentration gradient into the muscle cytoplasm. At rest the protein tropomyosin blocks the myosin binding sites on the actin molecule, which means that the myosin heads attached to the myosin filament (running adjacent to the actin filament) cannot attach to the actin molecule to form a cross bridge between the two molecules. The release of calcium ions causes the change in shape of the tropomyosin molecule, unblocking the myosin binding sites. This allows the myosin heads on the myosin filament to attach to the myosin binding sites on the actin and form a cross bridge, which in enabled due to an ADP molecule being attached to the myosin head. The myosin heads then change their angle, allowing the actin filament to be pulled along which is known as a power stroke, and in doing so a molecule of ADP is released. This allows an ATP molecule to then attach to the myosin head. The calcium ions can then activate the enzyme ATPase, which will hydrolyse the ATP to ADP and a phosphate, and in doing so will provide the energy to allow the myosin head to return to its original position, to enable another power stroke to take place where the myosin head attaches to another myosin binding site further along, allowing the actin filament to move further. This cycle will repeat as long as the calcium ion concentration in the muscle cytoplasm remains high, allowing muscle contraction.