Describe the structure of muscle and how it contracts

Muscle is made of many myofibrils. These contain many myofilaments. Within each myofilaments is a string of sarcomeres. Each sarcomere contains myosin (thick filaments) and actin (thin filaments). Myosin has myocin heads which bind to actin and pull actin along, reducing the length of the sarcomeres which globally results in muscle contraction.
Actin has tropomyocin coiled around it which prevents direct binding of myocin heads during muscle relaxation. On activation of the motor neuron, Ca2+ is released from the synapse and is free to bind to the troponin complex (troponin binding actin to tropomyocin). This causes the troponin to change shape, exposing the actin binding sites to myocin heads, allowing for muscle contraction.