Heart muscle, light micrograph. Cardiac muscle cells are characterised by central nuclei, cross-striations of contractile filaments, branching extensions linking cells and intercellular junctions. These are noted as thin dense lines physically joining cells end-to-end and are called intercalated discs. Discs also provide for rapid electrical coupling between the cells essential for coordination contraction of heart muscle. Acrylic section. Haematoxylin-eosin stain. Magnification: x550 when printed at 10cm.