Video - Cardiac excitation-contraction coupling

Cardiac excitation-contraction coupling is the process by which an electrical signal generated by the sinoatrial node (the heart's natural pacemaker) is converted into a mechanical force that makes the heart contract. It occurs in a series of events that include:

Excitation: The process by which an electrical signal causes calcium ions to enter the cell.

Calcium is released from the sarcoplasmic reticulum.

Myofilament activation: The released calcium ions bind to troponin C, which causes tropomyosin to move out of the way and expose binding sites on actin for myosin heads. With the help of ATP, the myosin heads then attach to the actin filaments and slide them past each other, shortening the sarcomere (the basic unit of muscle contraction) and leading to cardiac muscle contraction.

Relaxation: The calcium ions are removed from the cytosol by the sarcoplasmic reticulum, and tropomyosin returns to its position blocking the binding sites on actin. This prevents further interaction between myosin and actin, and the muscle relaxes.