Ct fibers around the muscle fibers + ct rings between atria & ventricles at the coronary sulcus . Allows openings to remain open at all times (valves open + close) . Provides electrical insulation prevents simultaneous contraction of chambers : valves, atrioventricular (av) valves, bicuspid (mitral) valve. Left side 2 cusps : tricuspid valve. Chordae tendineae (connective tissue) attach av valve cusps to papillary muscles (projections of myocardium) . O prevent eversion of cusps: semilunar valves. Separates left ventricle + aorta : pulmonary. Separates right ventricle + pulmonary trunk : cardiac muscle cells form both contractile myocardium + conduction system . Similarities to skeletal muscle: : striated (myofibrils with sarcomeres, has sarcoplasmic reticulum & t-tubules. Differences: : branched (myofibrils with sarcomeres, uninucleated, intercalated discs = region where two fibers meet. Contain anchoring and gap junctions : conduction system. Cardiac muscle cells modified to produce and conduct electrical impulses .