 In order to make a myofibril, we have to have thick filaments and thin filaments. And thin filaments have some more weird stuff going on. So we're not just dealing with a single protein in thin filaments, but I just want to remind you that we're still just stacking up some proteins and the whole purpose is to put them together into a myofibril. And remember, myofibril is filled up a myofiber, the muscle cell itself. So let's look at the parts of the actin molecule, I mean the thin filament. And really, the thin filament is mostly actin. And so if we were to look at actin, it actually kind of looks like this and it makes, oh, this is going to be a sad effort. It winds, there's actually like two strands of actin that they wind around each other. Okay, that's not too bad. Can you see how I'm trying to make them form like a double helix? Mm, fancy. Maybe I better make it go there so that you can see that it's happening all the way. Whoa, like that. Those are actin molecules. And they're just proteins. And this whole thing that I'm about to build is a thin filament. Actin, what do you think? What color did I make my myosin? I think blue. Guess what actins have. True story, they all have a myosin binding site. Okay, so I could draw this on every single one of these guys. And do you agree that, dude, if we're going to have filaments, we better have them be able to stick to each other. And it's true. Actin and myosin, when the moon and the stars align, and you send the message, jump, they can attach to each other. All right, there's something else. There is another molecule called tropomyosin. And I'm afraid I'm going to have to make tropomyosin green. Tropomyosin is another friendly. What have I drawn tropomyosin doing? Oh, if you were... Whoa. Oh, geez. I'm not sure what just happened in this area, but the bottom line is what I'm going to tell you. Tropomyosin blocks the myosin binding sites on actin. Did I draw that for you well enough? That would be cool with that. The tropomyosin blocks those binding sites. So when tropomyosin is in place, can you have a muscle contraction? No, no, no. You cannot because the binding sites are blocked. So guess what, we have to bring in somebody else. We have to bring in a gatekeeper. And that would be this guy. And this guy is called troponin. And troponin, just another protein. The troponin's job, this is very cool, so cool. Troponin holds tropomyosin in place, right? What's in place? Blocking the myosin binding sites, okay? Are you seeing, are you getting kind of like, ooh, I bet I can predict what's going to happen. I'm going to make the plot get thicker because, look, troponin has its own binding site. What? Yeah, it has, I mean, troponin, did I say troponin? I hope I did. Troponin has a calcium binding site. Did I already tell you? Or am I just imagining it? The calcium is stored in the sarcoplasmic reticulum of the muscle cell? Oh, so here's the deal. If calcium binds to troponin at the calcium binding site, troponin will change shape. And if troponin changes shape, it can no longer hold tropomyosin in position to cover the myosin binding sites. And now, actin's myosin binding sites are revealed. The truth is revealed and action can happen. Okay, should we go? I think you're good. I think what we need to do now is look at, oh, I think we do need to go back here, because our next job, what you can see, this is an excellent view. What you can see is that it's not like the caphazard tiddlywink pile of myofilaments. Thick and thin. They're super organized. They're so organized that through a microscope, you see stripes. That's how organized they are. They're like toe in that line. It's really important for their function. So they toe the line, and that's what we're going to talk about next. How are they organized into this myofibril? And the fact is they're organized into something called a sarcomere. Let's talk about sarcomere's next K.