Heart muscle calcium sparks. Fluorescence deconvolution light micrograph of a section through an adult cardiomyocyte (heart muscle cell), showing calcium sparks (green). Calcium sparks are the release of calcium ions (Ca2+) within muscle cells that activate muscle contraction. In cardiac muscle cells a calcium ion spark arises from the activation of a cluster of ryanodine receptors, causing calcium ions to be released from the sarcoplastic reticulum within the cell. Magnification: x400, when printed 10 cm wide.