What are cardiac muscles composed of?

Cardiac muscles are composed of specialized cells known as cardiomyocytes. These cells are semi-spindle shaped, branched, and surrounded by the sarcolemma, a type of plasma membrane that acts as a barrier between intracellular and extracellular contents. Within the sarcolemma is one centrally-located nucleus and an abundance of mitochondria. The mitochondria play a key role in generating the large amounts of ATP and myoglobin to meet the demands of muscle contraction. 

Each cardiomyocyte is composed of chains of rod-like units called myofibrils, which are made up of repeating sections of sarcomeres. Sarcomeres, which are the contractile units of cardiomyocytes, are composed of thin myofilaments that contain the protein actin and thick myofilaments that contain the protein myosin. 

Cardiomyocytes are connected to each other end to end by intercalated disks.