Heart muscle. Light micrograph of a section through heart (cardiac) muscle tissue, showing cardiomyocyte muscle cells. These cells have a centrally located nucleus and are branched and joined end-to-end by special intercellular connections called intercalated discs. These discs contain gap junctions allowing conduction of electrical impulses between very many cells enabling rapid and coordinated muscle contraction. The energy needed to maintain continuous heart contraction is provided by the thousands of mitochondria in each cell, seen as dark purple streaks throughout the cytoplasm. Connective tissue between cardiomyocytes has a rich supply of blood vessels. Magnification: x556, when printed 10 centimetres high.