Cardiac muscle
Cardiac muscle cells also called cardiomyocytes are the contractile cells of the cardiac muscle. The cells are surrounded by an extracellular matrix produced by supporting fibroblast cells. Specialised modified cardiomyocytes known as pacemaker cells, set the rhythm of the heart contractions. The pacemaker cells are only weakly contractile without sarcomeres, and are connected to neighboring contractile cells via gap junctions. They are located in the sinoatrial node positioned on the wall of the right atrium, near the entrance of the superior vena cava.