 Does this look familiar? We were just here. This is a muscle cell, and the muscle cell is filled with these weird myofibrils. And myofibrils are bundles of proteins called myofilaments. And we're going to spend this section talking about the thick filaments. So just keep it in mind that we're going to put together a bunch of thick filaments and that's how we're going to build our myofibril. And how we build our myofibril is going to enable it to do its job. But before we do that, we need to figure out, dude, what is a thick filament made out of? So I'm going to show you, except not on that page and not on that page. What? How about on this page? Yes. I'm going to draw you a picture of the primary protein in a protein in a thick filament. It's the contractile protein myosin. And it's important to recognize that it's contractile because that's what, I mean, that's the whole point, right? The muscle cell contracts. And so if you don't have a contractile protein, then the muscle cell isn't going to contract. Shorten. Okay, watch. This is a myosin head. This is a myosin molecule. It looks just like this. I think I want to draw it like this first. This myosin molecule is like cocked. It's not cocked. It's contracted. It's bent. And this is myosin. And here's the deal. There are a couple of binding sites on myosin. First of all, it has a binding site for actin. So I'm going to make a little key down here. This is an actin binding site. And hopefully by this point in the semester, you're like, oh, binding site, actin binding site. It must attach to actin. It must have a sticky spot for actin at this point, which makes you think there probably is a similar sticky spot on actin that would bind with myosin. Actin is not found in the thick filament. There's another binding site. And I'm going to give you a hint with the color. The color might actually psych you out, because that's the color that I've been making. The nicotinic acetylcholine receptor is, in fact, that looks just like one. It isn't. That is an ATP binding site. Ah, ATP. What's that? Energy. And does it make sense to you that we're going to need some ATP to contract a muscle? Totally. Okay. Guess what? Myosin, the thick filament. I'm going to show you how we put myosin molecules together to make a thick filament. Myosin can exist in two states, two forms. It can be folded like that, or it can be, and this is where the only word that I can think of to describe this is that it's cocked. And look, it still has its two binding sites. But guess what has to happen? We still have this little actin binding site, and we have an ATP binding site, and it's like pulling back a catapult. If you cock, I mean, can't you like pull back a catapult and stick it there and like hold it, and then all you have to do is push a button and pobling, you're going to fling like water balloons at someone. That's what it's like. And guess how you cock the myosin head? When I think of cocking the myosin head, it's like cocking a gun, you are pulling back the hammer so that all you have to do is pull the trigger, you don't have to do anything else, and then you're going to get an action. The only way to cock the myosin head is to have ATP bind to its binding site. And in fact, what happens, what happens is that ATP binds and the energy that's in ATP, we know that ATP is amazing. It's like this energy-rich molecule. The energy that was in ATP is used to pull back the myosin head, and now it's ready to go. But we also know that when we use ATP, what does it turn into? It turns into ADP plus P. So that ATP molecule is going to turn into ADP plus P, adenosine diphosphate plus this extra phosphate molecule, and that actually stays inside the binding site. And only if you have ADP plus P in that binding site, that's the only way you can have the myosin head cocked. Okay. Don't freak out. I'm going to take you back to the big picture. The only protein that we're going to be concerned about in the thick filament is myosin. So now I'm going to show you how the myosin molecules are arranged. Remember, look, there it is, thick filament. Okay? And we're just going to move this. Don't freak out, please. I beg of you. This is just a myofibril. Do you agree? Can you see how? Oh, there's the myofibril. Oh, there's the myofibril. Same thing. Awesome. This is how my thick filament is built out of myosin molecules. Do you agree? Look, there's my little myosin. Here's my little myosin heads. This one looks cocked to me. And then look at how we arrange them. We make a bundle of them. I think I read somewhere that there are, like, 200 myosin molecules in one thick filament. And look at how they're not just arranged haphazardly. They're organized, and we're going to look at how they're organized and name all the parts of the organization. But can you kind of visualize that if we stuck a whole bunch of cocked myosin molecules, so they're all ready to go, if we can get them all to cock, or to contract, to flex, we can actually, kind of imagine maybe shortening something like a myofibril. Ooh, the plot thick ends. Let's go look at what a thin filament is going to look like.