Outline the "Sliding Filament Theory" of muscle contraction.

In the resting state, the myosin heads have a partially hydrolysed ATP molecule (ADP + Pi) giving them potential energy. When there is an influx of calcium ions as a result of sarcolemma depolarisation, the calcium ions bind to troponin, and this causes a conformational change that allows myosin binding sites to be revealed on the actin. The myosin heads then bind to these myosin binding sites forming a "cross-bridge", ATP is then completely hydrolysed and the change in conformation causes the heads to move in the "Power Stroke" that causes the actin to move towards the centre of the sarcomere, the cross bridge is broken by partial hydrolysis of another fresh molecule of ATP, "re-cocking" the myosin heads so that they can continue the cycle to cause shortening of the sarcomere (contraction of the muscle)