When Ions Rush In.
Heart cell rhythm
depends on the opening and closing of a complex series of valves on the cell membrane,
called ion channels. Some valves let certain ions ike
potassium (K+) flow out, others let different ions like sodium (Na+) flow in. There are
also pumps that actively move ions one direction or another.
The number of K+ ions is greater inside a resting cell than outside.
But the number of Na+ ions is greater outside. When a heart cell beats, sodium channels
open allowing a rapid, transient in-rush of Na+ ions, then close within about two
one-thousandth's (2/1000) of a second. This depolarizes the membrane with the positive
ions moving in. Then there is then a slower, but prolonged (1/2 second), release of
potassium to the outside of the cell which repolarizes the cell membrane.
The beat is actually much more complex and involves other ions. But the
end result of this depolarization-repolarization is that the contractile filaments in the
cell engage, and the cell contracts.