The cardiac muscle cell

The cardiac myocyte is the most physically energetic cell in the body, cont racting constantly, without tiring, 3 billion times or more in an average h uman lifespan. By coordinating its beating activity with that of its 3 bill ion neighbours in the main pump of the human heart, over 7,000 litres of bl ood are pumped per day, without conscious effort, along 100,000 miles of bl ood vessels. A detailed picture of the membrane organisation of the cardiac muscle cell underpins our understanding of how the electrical impulse, gen erated within the heart, stimulates coordinated contraction of the cardiac chambers. This article highlights, with the aid of modern cellular imaging methods, key components of the membrane machinery responsible for coupling electrical excitation and contraction in the cardiomyocyte, focusing on pla sma membrane/sarcoplasmic reticulum and plasma membrane/plasma membrane jun ctions. (C) 2000 John Wiley & Sons, Inc.