عضلة القلب
Cardiomyocytes contain T-tubules, pouches of cell membrane that run from the cell surface to the cell's interior which help to improve the efficiency of contraction. The majority of these cells contain only one nucleus (although they may have as many as four), unlike skeletal muscle cells which contain many nuclei. Cardiac muscle cells contain many mitochondria which provide the energy needed for the cell in the form of adenosine triphosphate (ATP), making them highly resistant to fatigue.