Protein Angst

Thursday, March 17, 2005

Hearts and Minds

I want you to take a look at the cardiac action potential. That right there is why your heart keeps on beating properly.

The action potential is an electrical pulse that stimulates the heart cells to beat. It travels from cell to cell like a Girl Scout selling cookies. When Little Miss Action Potential rings a heart cell's doorbell, though, she really upsets the natural order of things.

You see, a heart cell has a resting membrane potential. If you measured it with an electrode, it'd be negative. That's what you see in Phase 4 on that plot, which is graphing voltage over time.

The action potential makes it positive, and chaos ensues.

First of all, sodium channels open because they're designed to open at a positive voltage, and now sodium ions flow into the cell. Sodium ions, as you may know, are positive. So now the cell is becoming even more positive, as you can see in Phase 0.

"This is crap!" says the cell. Well, not really, because cells can't talk. But what I love about nature is that everything loves balance, everything loves the status quo, and when that balance is upset, it will do its damndest to set things straight.

So before things get too ridiculously positive, the potassium channels open to let potassium ions flow out of the cell. Potassium ions, as you may know, are also positive. Too much positive charge coming in? Simple, shuttle out some positive charge of your own. It's beautiful. It's Phase 1.

Then you have Phase 2, also known as the plateau phase, cause...look. It's a plateau. During Phase 2, sodium, calcium, and potassium ions are all in balance. There's very little inward or outward current here. It's like the channels are all waiting to see who'll make the first move. It's high noon at the Cardiac Corral.

And then Phase 3 hits. A couple very important potassium channels open, like, "The hell with it, the voltage is going back to normal RIGHT NOW." Except they're right on cue. If they were too early or too late, your heart would beat improperly and you would die, and dying is no fun.

All that happens during one action potential. One heart beat. Ion channels opening, closing. Ions going in and out of the cell. All day, every day, 24/7. It's a good thing ions don't get tired. Right now, they just did all that. Oh, look, they just did it again. And again. And again. And—hey, they took a second off. Dude, are you all right?