How does muscle contraction work?

So the muscle is made up of 2 muscle filaments, a thick filament called myosin and a thin one actin. This can be remembered by actin having 2 syllables which is less than myosin which has 3 so actin is thinner. The filaments overlap normally and during contraction, the filaments slide further over each other creating a greater overlap which shortens the length of the muscle. This is called the sliding filament theory and works by forming cross-bridges between the actin and myosin filaments via myosin heads. Muscles are made of long muscle fibres which are then made up of many long myofibrils. Transversely, the myofibrils are split into sarcomeres. A sarcomere is the smallest functional unit of a muscle. In the sliding filament theory, an impulse stimulates the contraction by arriving at the neuromuscular junction and passing the impulses through the T tubules to spread the impulse as quickly as possible. The impulses cause the Calcium channels in the sarcoplasmic reticulum (store) to open and release calcium ions. These calcium ions bind to troponin in presence of a nerve impulse. Troponin is another protein molecule which is associated with a different protein, tropomyosin. At rest, tropomyosin covers the myosin binding sites on the actin molecule and so no cross-bridges are formed. This can be remembered by thinking of tropomyosin as a door, which can't open due to the lock (troponin). BUT when calcium binds to troponin (during a contraction) there is a structural change, which then also changes the structure of tropomyosin to uncover the myosin binding sites on the actin molecule so myosin can bind. The calcium is like a key which fits into the lock of a door (troponin) and causes a change, that opens the door. Energy from ATP is used to swing the myosin heads, once attached to the actin, to create a greater overlap and shorten the length of the muscle. Once the myosin head is moved, it is released from the actin molecule and 'cocks' back in order to bind again, further along the actin.