thaler chapter 1

The Cells of the Heart

17

Myocardial Cells The myocardial cells constitute by far the largest part of the heart tissue. They are responsible for the heavy labor of repeatedly contracting and relaxing, thereby delivering blood to the rest of the body. These cells are about 50 to 100 μm in length and contain an abundance of the contractile proteins actin and myosin. When a wave of depolarization reaches a myocardial cell, calcium is released within the cell, causing the cell to contract. This process, in which calcium plays the key intermediary role, is called excitation– contraction coupling .

A

Actin Myosin

Ca

Ca

Ca

Ca

B

Myocardial cells can transmit an electrical current just like electrical conducting cells, but they do so far less efficiently. Thus, a wave of depolarization, upon reaching the myocardial cells, will spread slowly across the entire myocardium. Depolarization causes calcium to be released within a myocardial cell. This influx of calcium allows actin and myosin, the contractile proteins, to interact, causing the cell to contract. ( A ) A resting myocardial cell. ( B ) A depolarized, contracted myocardial cell.