Cardiac muscle. Transmission electron micrograph (TEM) of a longitudinal section through cardiac (heart) muscle cells. Heart muscle cells are branched and joined end-to-end by special tight junctions known as intercalated discs. The junctions serve to physically and electrically connect successive muscle cells. Also known as cardiomyocytes, these cells show prominent and orderly arrangements of their contractile proteins into sarcomeres (light grey rectangles) that serve as the smallest units of muscle contraction. The huge number of mitochondria (dark grey, oval) are necessary for the continuous energy supply required by cardiac muscle. Magnification: x5600, when printed 10 centimetres wide.