A calcium spark is the microscopic release of calcium (Ca2+) from a store known as the sarcoplasmic reticulum (SR), located within muscle cells.[1] This release occurs through an ion channel within the membrane of the SR, known as a ryanodine receptor (RyR), which opens upon activation.[2] This process is important as it helps to maintain Ca2+ concentration within the cell. It also initiates muscle contraction in skeletal and cardiac muscles and muscle relaxation in smooth muscles. Ca2+ sparks are important in physiology as they show how Ca2+ can be used at a subcellular level, to signal both local changes, known as local control,[3] as well as whole cell changes.