Medical Definition of cardiac muscle
: the principal muscle tissue of the vertebrate heart that is made up of elongated striated muscle fibers each of which consists of a single cell that has an intrinsic rhythm of contraction and relaxation even when isolated, is joined physically at its often branched ends to other such cells by intercalated disks, and in intact myocardial tissue is synchronized to function in contraction especially by electrical signals of extrinsic origin passing through gap junctions in the intercalated disks—compare smooth muscle, striated muscle
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