 we have two kinds of cardiac muscle cells. We have contractile cells and auto-rhythmic cells. 99% of all cardiac muscle cells are contractile. And here's the deal with contractile cells. They receive action potentials from someone else. So the action potential actually passes into the contractile cell, enters via... Does anybody want to guess? I bet I can hear you if you yell really loud. The action potential enters via the gap junctions in the intercalated discs. Yeah. So there is no threshold when you think about a contractile, a contractile cardiac muscle cell. I am going to draw you the action potential graph. But what I want you to remember is that down here, while we're waiting for an action potential to arrive or the message to let's go, we're hanging out at resting membrane potential, which in a contractile cell is negative 90 millivolts. That's different than my chilling somatic motor neuron, which is, you know, negative 70. So there are some things that are different, but there's no stimulus. The action potential is already happening and it enters through the gap junctions in the intercalated discs. Okay. As soon as the action potential enters the cytoplasm in my contractile cell, automatically, we'll see what I can do down here, action potential enters and who's going to open makes perfect sense, voltage-gated sodium channels open. Voltage-gated sodium channels open, sodium rushes out, zoom orama, we bust a move and depolarize to about 20 millivolts. I'm doing an action potential graph. Don't be disturbed or confused by the fact that we did, we just did graphs of skeletal muscle contraction. We did tension graphs, we did some action potential graphs of our skeletal muscle. This is different. What would you expect to happen next? I mean, everything that we've dealt with so far says that as soon as the action potential, as soon as the voltage-gated sodium channels open, then we're going to get voltage-gated potassium channels opening. And that's actually true. Voltage-gated potassium channels open. And what are we going to expect to happen when voltage-gated potassium channels open? We're totally going to expect a repolarization as potassium rushes out of the cell. But this is where the plot gets even crazier because guess what? Something happens in the contractile muscle cell that is unusual cardiac muscle cell. Stage three, we have calcium channels that open. What? Seriously? Now think about this. Calcium channels open. Calcium rushes in. Positive charges are rushing in. We would have thought, here, we would have thought potassium is going to leave, but now we have, sure, potassium is leaving, but calcium is also coming in. And what's that going to do to my action potential? It's going to create a plateau because the potassium that's leaving is being balanced by the calcium that's coming in. The calcium coming in plays a role in maintaining this plateau. And think about this. What does this plateau do? My friend, the contractile cell cannot be re-stimulated during this time. This is a refractory period. Why? Because we're already fully depolarized. How can we depolarize more? It doesn't matter if another action potential comes in. I am not going to contract during this time. Eventually, the calcium came in. Eventually, now this is really cool. We have fast potassium channels. These guys opened slower than the sodium channels, but still faster than these guys. Slow potassium channels open. Now our calcium has equilibrated, our calcium channels closed, and these slow potassium channels open. Now we are going to repolarize and go back to where we started. Once we've repolarized, that means that we're ready to fire again. That means that the message can come in through the gap junction and say, okay, you can go ahead and do it again now. During this refractory period, this is 0.25 to 0.3 seconds long. Think about that. So how long was the... I mean, even our skeletal muscle contraction was 100 milliseconds. This is 300 milliseconds. So it was 0.1 seconds for skeletal muscle. And it's 0.3 seconds. Like, from start to finish, you can't beat your heart faster than one contraction every 0.3 seconds. And if you do the math, that if we go with this one right here, we could get our heart rate up to 240 beats per minute. And I went online to see, okay, what is the highest heart rate that you could possibly have? And everything that I saw went to 220. That if you go higher than 220, now think about this. Then go back to your little heart, the pump. If you're pumping faster than 240 beats per minute or 220 beats per minute, you're pumping so fast that is there any room for blood to come in, so you're pumping nothing. Your heart is beating so fast that it can no longer do its job. It's almost like it's in a giant cramp, which is, you can imagine, terribly unproductive. And any critter that had a heart that beat like that is probably going to be a done deal, sorry for your luck, man, and not pass on their genes, so it makes sense that we wouldn't have that phenomenon happening. Okay, so contractile cells are awesome. You can think about this. If we increase the permeability to calcium, we make more calcium come in. We can actually change the force that we can generate with this muscle tissue. This is just the action potential. This is just the action potential that sends the message to the rest of the muscle cell to go ahead and activate our sarcomeres and shorten myosin heads, grab and are acting and shortening the muscle tissue. So that whole process is happening during this period of time, during the refractory period, that whole process is happening. What doesn't happen, and this is wild, too, what doesn't happen is our contractile cells can't say, hey, fire again, folks. We must relax the cardiac muscle tissue before firing another action potential. What happens in skeletal muscle when we fire a whole bunch of times? We get tetanus. This is sustained, constant, smooth contraction. And just go ahead and think about that for a second. What would happen if you ended up with tetanus in your heart muscle? Yeah, that's not going to be evolutionarily sound. How does the contractile muscle get the action potential in the first place? Because we've already established that it doesn't come from the nervous system. So where does it come from? Where is this right here? Where is this? Oh, I'm going to mark it in orange. Where does that come from? Where does the action potential come from? Seriously? Could it possibly be the amazing auto-rhythmic cells? Stay tuned.