Also known as myocardiocyte, the cardiomyocyte is the muscular fiber of the myocardium, which is the heart muscle. A cardiomyocyte is a cell that contains myofibrils, which are long chains of sarcomeres, which in turn is the contractile units of the cell. Cardiomyocytes show similar patterns to skeletal muscle cells, but unlike multinucleated skeletal cells, myocardiocytes contain only one or two nuclei. During fetal and postnatal development cardiomyocytes become terminally differentiated muscular cells that are connected end to end by gap junctions, allowing concerted contractile activity. The contraction-relaxation cycle of cardiomyocytes is orchestrated by cyclic increases and decreases in intracellular Ca(2+) initiated by depolarization of the sarcolemma and sustained by calcium (Ca(2+)) release and re-uptake by the sarcoplasmic reticulum. When stressed, cardiomyocytes undergo hypertrophic growth and apoptotic responses in vivo as well as in cell culture models. Such changes predispose to heart failure in the longer term.
The functional intracellular contractile unit of cardiomyocytes is the sarcomere, an orderly arrangement of thick filaments composed principally of myosin, and thin filaments containing actin. Sarcomeres also contain the regulatory proteins troponin and tropomyosin. Cardiomyocytes are composed of many parallel myofilaments (arrays of sarcomeres in series), which are responsible for the striated appearance of these cells.