Transmission electron micrograph (TEM) of the ultrastructure of a cardiac muscle cell (cardiomyocyte) showing an elongated nucleus, many mitochondria and striations of contractile filaments organised into sarcomeres. Cardiac muscle cells are branched and connected end-to-end by special junctions called intercalated discs which also function to provide electrical coupling. This allows for rapid synchronization of heart muscle contractions essential for normal heart function. Magnification: x2,000 when width printed at 10cm.