GLOSSARY

Troponin is a complex of three small proteins located at regular intervals along the thin actin filaments and, together with tropomyosin, it controls the start and end of contraction in skeletal and cardiac muscle.

Components and Function

• Troponin C: binds calcium and is in effect the calcium sensor of the muscle.
• Troponin I: inhibits the binding of myosin to actin at rest.
• Troponin T: anchors the whole troponin complex to tropomyosin.

When a nerve signal triggers the release of calcium from the sarcoplasmic reticulum, calcium binds to troponin C, the troponin complex changes shape, and tropomyosin moves away from the myosin-binding sites; as a result, cross-bridges form and the muscle contracts.