Cardiac muscle, unlike skeletal muscle, is composed of separate cellular elements. A cardiac muscle cell has a large nucleus (light blue) and numerous mitochondria (purple). The high concentration of mitochondria reflects the large energy demands of predominant the heart. Contractile proteins of actin and myosin myofilaments are in the cytoplasm. They form bands of varying density: A band - orange, I band - yellow, Z line - red. The Z lines are regions with dense arrangements of actin. The heart produces regular electrical impulses causing the muscle myofibrils to slide over one another and contract the cardiac muscle.

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