Heart muscle cell (human); nucleus, mitochondria, actin and myosin fibres, A bands, I bands and Z lines, coloured transmission electron micrograph (TEM). Cardiac muscle, unlike skeletal muscle, is composed of separate cellular elements. A cardiac muscle cell has a large nucleus (blue) and numerous mitochondria (red). The high concentration of mitochondria reflects the large energy demands of predominant the heart. Contractile proteins of actin and myosin myofilaments are in the cytoplasm. They form bands (sarcomeres) of varying density: A bands - yellow, I bands - grey, Z lines - orange. The Z lines are regions with dense protein disks. The heart produces regular electrical impulses causing the muscle myofibrils to slide over one another and contract the cardiac muscle. Magnification: x2, 025 when shortest axis printed at 25 millimetres.
