What does C mean in UNCLASSIFIED

C (Ca2 Release Activated Ca2) is a mechanism by which the release of calcium ions (Ca2+) from the sarcoplasmic reticulum (SR) triggers further release of Ca2+ ions through ryanodine receptors (RyRs). This process is known as calcium-induced calcium release (CICR) and plays a crucial role in muscle contraction.

When an action potential reaches the muscle membrane, it initiates the opening of voltage-gated calcium channels (VGCCs). The influx of Ca2+ ions through VGCCs binds to specific receptors on the SR, called ryanodine receptors (RyRs). This binding triggers a conformational change in RyRs, causing them to open and release a large amount of Ca2+ ions into the cytosol. The increase in cytosolic Ca2+ concentration activates other RyRs, leading to a further release of Ca2+ ions from the SR. This process continues in a self-amplifying manner, resulting in a massive release of Ca2+ ions, which initiates muscle contraction.

C plays a vital role in muscle contraction. It ensures that the SR releases sufficient Ca2+ ions to trigger the sliding of actin and myosin filaments, which generates muscle force. Additionally, C is involved in other physiological processes, such as regulating gene expression, cell proliferation, and apoptosis.

Dysregulation of C can contribute to several muscle disorders, including malignant hyperthermia, central core disease, and ryanodine receptor disorders. Malignant hyperthermia is a potentially life-threatening condition triggered by certain anesthetics, which leads to uncontrolled muscle contraction and a rapid increase in body temperature. Central core disease is a genetic disorder characterized by muscle weakness and stiffness. Ryanodine receptor disorders are a group of rare genetic conditions that affect the function of RyRs, resulting in muscle weakness and impaired muscle coordination.