How do muscles contract? Describe the sliding filament theory.

When an action potential from a motor neurone stimulates a muscle cell, a number of things happen to cause sarcomeres (the smallest contracting unit of a muscle cell) to shorten and contract very rapidly.
Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm. The ions bind to a protein called troponin which is attached to another protein called tropomyosin. This causes the tropomyosin to move out of the actin-mysoin binding site allowing the myosin to bind to the actin: an actinomyosin cross bridge is formed. The calcium ions also activate ATP hydrolase: energy released from the breakdown of ATP is used to change the shape of the myosin head, break the actinomyosin cross bridge and return the myosin head to its starting position so the process can repeat. You can remember this process as 'move, detach, reattach'.