Action potentials in pacemaker cells

Definitions & Key takeaways

Action potentials are voltage changes that propagate along the surface of cells. In the heart, they are generated by specialized cell structures called pacemaker cells, which use them to control the rhythmic contraction of muscles.

In cardiac pacemaker cells, action potentials occur when specialized channels in the cell membrane open and allow ions to flow into or out of the cell. This change in electric charge makes the cell more positive on the inside, which attracts more ions from neighboring cells and triggers a chain reaction that propagates the action potential along the heart muscle. This eventually leads to the contraction of the heart and pumps blood around our bodies.

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Introductions0:00–1:06

Action potentials are the really rapid electrical changes that occur across the membrane of certain cells, and often propagates from one cell to an adjacent cell.
Cells in the heart communicate this way. That signal’s gotta start somewhere, so some of these cells, called pacemaker cells, have the responsibility of setting the rhythm and the pace of the heartbeat.
They’ve got this really important job, but they’re a relatively tiny group -- only about 1% of the heart cells -- and they’re able to continually generate new action potentials that get conducted to the rest of the heart -- the other 99% -- and that’s what tells the heart pump.
Now, pacemaker cells also listen to which usually come from neighboring pacemaker cells. But if those don’t come, then a pacemaker cell will simply launch its own and that action potential will then spread around.
This is called automaticity, and that’s easy to remember because it’s got “automatic” right in it. So let’s start by mapping out those pacemaker cells.

Cardiac conduction system1:06–2:17

The first clump of pacemaker cells is tucked up here into the corner of the right atria, and that’s the sinoatrial node, which sometimes gets called the SA node.
We’ve also got pacemaker cells in internodal tracts between nodes, in the atrioventricular, or AV node, the Bundle of His, and the Purkinje fibers, and that’s our electrical conduction system.
And all around these pacemaker cells are heart muscle cells or cardiomyocytes and they pick up the action potential too, but that happens just a tiny bit more slowly -- we can think of these bands of pacemaker cells as highways that carry the action potential to its destination super fast, and then the muscle cells are like little side roads where it’s slower.
That’s important because we want all of the myocytes to pick up that action potential and contract at the same time. We call this whole system a functional syncytium, which means that the mechanical, chemical, and electrical connections between these cells allow them to act as one unit in some ways, and it’s the pacemaker cells that make that happen.
Okay, now let’s take a closer look at the chemistry that gets that action potential moving. Action potentials are initiated by depolarization, which is the opposite of polarization.

Membrane potential2:17–3:21

Polarization is when there’s a higher negative charge inside the cell relative to outside the cell, and that difference in charge is called the membrane potential.
So if the membrane potential is negative the inside of the cell is more negative than the outside, if it’s positive the inside is more positive than the outside, and if it’s 0mV, then the inside and outside have the same charge - there’s 0mV of difference.
Ok -- so, the key here is understanding how the membrane potential changes, and it all comes down to the movement of ions.
Specifically, two factors -- which ion wants to move across the membrane, and how permeable the membrane is to that ion.
So, depolarization is when ions move across the membrane and the membrane potential becomes less negative or even slightly positive.

Depolarization wave3:21–4:10

Think of a really pessimistic negative cell throwing his hands up and enjoying a moment of joy. When one cell depolarizes enough - it can cause some ions to flow into neighboring cells and trigger them to depolarize as well.
If one cell after another depolarizes, then there’s a depolarization wave which you can imagine would look like a wave moving through a crowd at a football stadium.
Each depolarization wave causes heart muscle contraction, so the rate at which depolarization waves ripple through the heart actually sets the heart rate.
So if depolarization waves are going through about once per second, that means that your heart beats once per second, or sixty times in a minute.

Action potential4:10–4:28

Now let’s focus on a single pacemaker cell going through a single action potential. The action potential of both pacemaker cells and myocytes are broken into five phases, often shown on a graph of membrane potential vs.
time. We’re going to start at Phase 4, because why not.

Phase 44:28–5:27

Phase 4 is the pacemaker potential phase, and it starts when the pacemaker cell is just sort of hanging out with an overall charge or membrane potential of -65mV.
Pacemaker cells have ion channels called hyperpolarization-activated cyclic nucleotide-gated channels, or HCN channels on their surface.
That’s a lot to take in, but basically these little HCN channels are like gates that open when the membrane potential gets really negative, like -65mV, and they specifically let in just positively charged sodium ions.
An electric current is a flow of electric charge, so when these sodium ions rush into the cell, it’s a type of current that scientists have named the funny current, which is a super weird name.
There’s nothing funny about it, it should be called the salty current. So as the sodium flows in through the HCN channels, in other words, as the cell becomes more permeable to sodium, the membrane potential starts to depolarize slowly, or drift upwards all the way to about -50mV.

Phase 05:27–6:13

After phase 4 is phase 0, this phase is also known as the depolarization phase, even though the cell was slowly depolarizing in phase 4.
Once the cell’s membrane potential reaches about -50mV, voltage-gated calcium channels start to open up, allowing calcium to flow into the cell.
With sodium and calcium flowing into the cell, the membrane potential rises all the way up past 0mV and into positive territory - peaking out at about +10mV.
This happens super-fast, lasting only about 0.5 milliseconds. The calcium channels remain open until it hits about the +10mV mark, at which point they start to close.

Phase 36:13–7:05

Once the cell hits +10mV, we’re in Phase 3. That’s because pacemaker cells don’t have a Phase 1 or Phase 2 -- they just go right from depolarizing to repolarizing again.
At this point the potassium channels open up and the calcium channels close up, so the only ion channels that remains open are the potassium channels which allow potassium to leave the cell, and the HCN channels which allow sodium to enter the cell.
Because there are so many potassium ion channels relative to HCN channels, there’s a net outward positive current, meaning that positively charged ions are leaving the cell faster than they’re coming in.
So, the membrane potential goes down again, all the way back to about -65 mV, which means that one full heartbeat has taken place, and then the whole process starts over with phase 4 again.

Review7:05–7:32

Okay, to recap: pacemaker cells generate action potentials automatically, and this sets the heart rate. Each action potential spreads through the heart muscle cells in a process called a depolarization wave; each depolarization wave leading to a heart beat.
There are three phases of a pacemaker action potential, phase 4 which is the slow depolarization, phase 0 which is the rapid depolarization, and phase 3 which is the repolarization.