Definition: the involuntary muscle comprising the myocardium and walls of the pulmonary veins and superior vena cava, consisting of anastomosing transversely striated muscle fibers formed of cells united at intercalated discs; the one or two nuclei of each cell are centrally located and the longitudinally arranged myofibrils have considerable sarcoplasm around them; connective tissue is limited to reticular and fine collagenous fibers; contraction is rhythmic and intrinsically stimulated.
Synonym(s): muscle of heart
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