 So the question you should be asking now is where does the action potential come from? We're talking about these contractile cells which receive an action potential message directly through the gap junctions in their intercalated discs, causes them to fire. We've got this awesome mechanism for how contractile muscle cells contract with this magical action potential that appeared out of nowhere. We know that it didn't come from the nervous system as heart muscle can beat without nervous input. So it's not like the nervous system is dumping neurotransmitter on contractile cells and telling them to go. So who's doing the job? That would be your friendly auto-rhythmic cells. So auto-rhythmic cells are so cool. Here's the deal. They do not have a resting potential. Contractile cells did not have threshold. Auto-rhythmic cells have no resting potential. However, they begin the cycle at about negative 60 millivolts. And here's the thing. They have special channels. They do have threshold. Negative 40 is threshold. They have special channels. Oops, that's not what I meant to do. I meant to go like that. Auto-rhythmic cells have special channels called IF channels. And the F actually comes from the word funny. These are funny channels. The researchers thought they were super funny because they're leaky. They're leaky for sodium and potassium. Tell me, true dog pounds, who do you think they're more leaky for? Look what's happening to my membrane potential as these funny channels are doing their thing. I'm slowly depolarizing, which means that we're probably a little more leaky to sodium than to potassium. And this is totally true. Now, these funny channels create what's called the pacemaker potential. Pacemaker potential. It's not a resting potential because it doesn't stay at the same level. It's a pacemaker potential because can you imagine if... Actually, I'm not going to tell you how to imagine like making this fire faster or slower. I'll let you think about that. Eventually, the leaky channels allow enough sodium into the cell that we reach threshold. And once we reach threshold, funny channels close. And guess what kind of channels open? Calcium channels open and calcium rushes in. Calcium rushes in until we reach about 20 millivolts. Calcium comes in, we become more positive. This makes perfect sense. And then, okay, so let's make a note of that. Calcium comes in when these calcium channels open. And then, who's going to open next? Look what's going to happen. We're going to repolarize. How about opening up some potassium channels? Potassium goes out. We repolarize the cell. This is the pattern that is maintained by our friend, the auto-rhythmic cells. As soon as we reach threshold, we generate an action potential. And then, we head back down to where you start at negative 60, it isn't the resting membrane potential. It's just the starting point. And then, my funny channels are leaky. So, we can't hang out at a certain place. Eventually, we reach threshold because of those leaky channels and we fire an action potential. Now, this, my friends, this right here, we got to have a better color than that. Boy. Okay, I'll just show you how awful this color is. It's terrible. That, what was my point? We say, oh, this, we've got a positive 20 membrane potential inside our cytoplasm now. And that cytoplasm, that positive 20 passes through the gap junction and activates the voltage-gated sodium channels in the contractile cells. Only 1% of all of your heart muscle cells are auto-rhythmic cells. However, because of this phenomenon right here, they generate all the action potentials to keep your heart going. There's some anatomy here. We actually need to take a look at how auto-rhythmic cells are distributed through the heart itself so that we can see if they're firing at a certain level, how does the message get all the way through the entire heart and initiate a contraction of your heart, a beat? How do the auto-rhythmic cells coordinate heart beats? Well, we have to look at how they're anatomically arranged in the heart, and that's through the electrical conduction system.