A Closer Look at Cardiac Muscle Cells
Cardiac muscle cells are rectangular shaped cells connected by regions called intercalated discs. Intercalated discs contain gap junctions and desmosomes. The gap junctions, which are protein-lined tunnels, allow direct transmission of the depolarizing current from cell to cell, across the chambers of the heart, so that the cells contract in unison. Because of the way these gap junctions function, the cardiac muscle cells are said to be electrically coupled. The desmosomes hold the cardiac muscle cells together during contraction, induced by the sliding of the cardiac myofibrils. Sliding is regulated by the intracellular concentration of calcium ions released by the sarcoplasmic reticulum.

The Intrinsic Conduction System of the Heart