is a specific kind of: muscle, muscular tissue.

is a broader term that covers: cardiac pacemaker, pacemaker, SA node, sinoatrial node | papillary muscle | atrioventricular bundle, atrioventricular trunk, bundle of His, truncus atrioventricularis | atrioventricular node | myocardium | Purkinje fiber | Purkinje's system, Purkinje's tissue, Purkinje network.

is a part of: heart, pump, ticker.