Action potentials in myocytes

Definitions & Key takeaways

An action potential (AP) is a voltage change that propagates along the membrane of a myocyte (muscle cell) or other cells such as a nerve cell. The AP is generated by the movement of positively charged ions, mainly Na+ and K+, across the plasma membrane. This generates an electrical current that travels down the length of the myocyte.

The AP triggers the release of Ca2+ from intracellular stores, which in turn activates contractile proteins within the myocyte. This ultimately leads to muscle contraction.

Chapters:

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Introduction0:00–1:07

Action potentials are the really fast electrical changes that happen across the membrane of certain cells and often propagates from one cell to an adjacent cell.
And cells in the heart communicate this way. Now that signals gotta start somewhere.
So some of these cells called pacemaker cells have the responsibility of setting the rhythm and pace of the heartbeat. So they got this really important job.
But they're a relatively tiny group and make up only about 1% of the heart cells. But they're able to continually generate new action potentials that get conducted to the rest of the heart or the other 99%.
And so these are what tell the heart to pump the cells that receive that signal are called myocytes because they make up the myocardium, which is the muscular middle layer of the heart myocytes are also called contractile cells because they contract to allow the heart to pump blood myocytes are different from skeletal muscle cells though which get their action potential signals directly from neurons.
Now let's focus on a single myocyte cell going through a single action potential. The action potential of a myocyte is broken into five phases.

Action Potential1:07–1:24

Often they're shown on a graph of membrane potential versus time we're gonna start with phase four because why not? In phase four or the resting phase, our little myocyte friend is at rest hanging out with an overall charge or membrane potential of negative 90 millivolts.

Phase 41:24–1:59

Now, the interesting thing is that it has gap junctions which are openings between two myocytes. So when the myocytes neighbor depolarizes some ions, mainly calcium ions start leaking through the gap junctions.
And that makes the membrane potential go up to about a negative 70 millivolts, negative 70 millivolts is called the threshold potential.
And it marks the start of phase zero. Phase zero is known as the depolarization phase.

Phase 01:59–2:45

Basically, some voltage gated sodium channels open up when they sense that the membrane potential is negative 70 millivolts.
And they allow sodium to rush into the cell creating an inward current. This rapid influx of sodium causes the myocytes membrane potential to go all the way up to plus 20 millivolts.
Now, if only a few ions had leaked through from the neighboring cell and the membrane potential didn't get to the threshold potential of negative 70 millivolts, then those voltage gated channels wouldn't open.
And there'd be no depolarization. Essentially, there's nothing in between, which is why that we say an action potential is an all or none process.

Phase 12:45–3:30

All right. So if the membrane potential rises above negative 70 millivolts and keeps going all the way to plus 20 millivolts, the myosides depolarized.
And we're in phase one, which is called initial repolarization. At this point, the sodium channels close.
And the voltage gated potassium channels sense that it's at about plus 20 millivolts and they open allowing potassium ions to leave the cell.
Those potassium ions have a positive charge. So the cell's membrane potential drops as this positive charge moves out, this is called an outward current because it's literally a current moving out of the cell.
So the membrane potential starts to fall. And we can see that this creates a little notch on our graph.
Shortly after this. Though the voltage gated calcium channels open up and that allows calcium ions into the cell as calcium flows in it brings with it a positive charge that positive charge from the calcium ions flowing in counterbalances the positive charge from the potassium ions that are flowing out.

Phase 23:30–4:11

So the membrane potential actually stays pretty stable and this is called phase two or the plateau phase. This calcium is super important because it's this influx of calcium that ultimately gets the myocyte to contract, which is how the heart contracts.
So phase two is what's responsible for the length of the action potential as well as the heartbeat itself during phase three or repolarization, the calcium channels close.

Phase 34:11–4:35

But the potassium channels stay open, resulting in a net outward positive current. At the same time, ion pumps start to move calcium ions out of the cell as well.
And that causes the heart to relax. Eventually the membrane potential gets back to negative 90 millivolts and we start over with phase four again.

Review4:35–5:25

All right. As a quick recap, cardiac myocytes receive action potentials or rapid voltage changes from pacemaker cells.
In phase four, the myocytes are at rest in phase zero sodium channels open up and there's an influx of sodium ions that makes the myocyte depolarize in phase one potassium channels open up and there's an outflux of potassium ions that brings down the charge a little bit in phase two calcium channels open up.
And there's an influx of calcium ions and that counterbalances the potassium ion outflux. So it's called the plateau phase in phase three calcium channels close, but the potassium channels stay open.
So there's an overall outflux of potassium ions that brings down the charge that repolarise the myocyte and then it enters the resting state again.