myocardium

The myocardium is the heart's engine — the muscle that actually does the squeezing. If the heart is a pump, the myocardium is the motor inside the wall that drives each contraction.

It is the thick middle layer of the heart wall, sandwiched between the inner lining (endocardium) and the outer covering (epicardium). It is built from specialized muscle cells called cardiomyocytes that contract in a coordinated wave triggered by the heart's electrical system, ejecting blood with every beat.

Unlike skeletal muscle, the myocardium works tirelessly and never takes a real rest, so it has a huge appetite for oxygen delivered by the coronary arteries. When that supply is cut off, myocardial cells begin to die within minutes — this is what happens in a myocardial infarction, or heart attack. The thickness of the myocardium varies by chamber, being greatest in the left ventricle.