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How does calcium entering cardiac muscle cells trigger contraction?

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Diagram showing the structure of human striated or skeletal muscle, with labeled cross-sections from whole muscle to fascicle, muscle fiber, myofibril, and sarcomere, highlighting actin and myosin filaments. — muscle The structure of striated, or skeletal, muscle. Striated muscle tissue, such as the tissue of the human biceps muscle, consists of long fine fibers, each of which is in effect a bundle of finer myofibrils. Within each myofibril are filaments of the proteins myosin and actin; these filaments slide past one another as the muscle contracts and expands. On each myofibril, regularly occurring dark bands, called Z lines, can be seen where actin and myosin filaments overlap. The region between two Z lines is called a sarcomere; sarcomeres can be considered the primary structural and functional unit of muscle tissue.

In cardiac muscle, the process of excitation-contraction coupling begins when an action potential travels along the plasma membrane and into the cell via the transverse tubular system. During the plateau phase of this action potential, calcium ions enter the cell through voltage-sensitive channels.123

This initial influx of calcium acts as a trigger, stimulating the release of a much larger quantity of calcium from the sarcoplasmic reticulum. This release causes the calcium concentration within the cytosol to rise by nearly 100-fold.123

The increased calcium binds to the troponin-tropomyosin system associated with actin filaments. This binding induces a conformational change that shifts the position of these proteins, removing the inhibition that normally prevents actin from interacting with myosin. Once this interaction is enabled, cross-bridge cycling occurs in the presence of ATP, resulting in muscle contraction. Relaxation follows when the sarcoplasmic reticulum reabsorbs the calcium, allowing the troponin-tropomyosin system to return to its original, inhibitory state.123