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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
.
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.
Ca
Ca
A
B
Ca
Ca
Actin Myosin
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.