Cardiac muscle tissue is involuntary and is heavily infused with mitochondria. Cardiac muscle (heart muscle or myocardium) is an involuntary, striated muscle found in the walls of the heart. It consists of cardiomyocytes (the muscle cells) which contain myofibrils. Myofibrils are specialized organelles which consist of sarcomeres (the fundamental contractile units of muscle cells). Cardiomyocytes have a high amount of mitochondria important for the production of ATP, which is necessary for the contractile function of muscle.