Troponin is an accessory protein found in thin filaments (also known as actin filaments). Thin filaments are structural components of a repeating unit called the sarcomere. These thin filaments play an important role in muscle contraction. Troponin consists of three sub-units; troponin T, I and C. When the muscle is at rest the T and I sub-units bind to tropomyosin and actin resulting in an inhibitory process that prevents myosin heads from binding to actin. When the concentration of calcium ions raises in the cytoplasm, due to the influx from the sarcoplasmic reticulum, the calcium ions bind to troponin C. This binding causes troponin I to release its hold on actin, which allows tropomyosin to move back to its normal position. Consequently, the myosin binding site is uncovered, allowing myosin heads to bind to actin and form a cross bridge, triggering contraction.