 We're diving in. We're diving into specifically the myofibril. And this is actually going to answer for us why are muscle fibers themselves, the myofibrils, the muscle cells, why they have a striated look about them. It has to do with the way thick and thin filaments are organized in the myofibril. Do you remember that inside a myofiber, inside the muscle cell, we had bundles of myofibrils? Let's go look at what a myofibril is. Ready, my freddies. But of course you are. Oops, not white. White won't show up. Myofibril. And there's many of them inside a single muscle cell. In fact, there's so many of them in there that the nuclei can't even fit in the middle of the cell. Like these myofibrils are like spaghetti noodles. Squished inside nylons. If you had a nylon full of spaghetti noodles, that's like a muscle cell. Well, that was really cool. Okay, so what is this myofibril we speak of? It's a specific organization of thick and thin filaments. Take a deep breath. What I'm going to do is I'm going to show you how we're going to make our thick filaments blue and we're going to make our thin filaments green. And I'm going to tell you some specific parts of what we're building. But keep in mind, thick and thin filaments, what are these? These are just long, thin protein molecules. Thick filaments are made of a protein called myosin. And they go like, like a, like a this. I'm making it look thick. And they actually have, dude, these things are so awesome. They have these little heads that attach to the thin filaments. They have like little arrows. And, I can't help it, I just have to draw the whole thing for you. They attach to the, oh crikey's, there's a line. I think it's the M line. This is one myofilament. And it attaches at the M line to another myofilament. And these are all my thick myofilaments. And can you just accept that we're going to have the same structures attached? Okay, fine, I'll draw them for you. Look, I'm drawing them. Oh, there we go. Okay, they all got drawn. Now, I actually have another M line and another set of thick filaments attached to it. And what are these? They're just proteins. And look, I've even got another one with another set of thick filaments attached to it. You're cool, right? And just go ahead and accept they all have the little heads sticking on them. Now, are thick filaments the only filaments in a myofibril? No, we also have thin filaments. Okay, for this, this is what gets super cool. Thin filaments span the middle. They actually attach at a line called the Z line. These are my thin filaments. Thin filaments are mostly actin protein. Now, I'm not going to go into crazy details for how the muscle contraction itself works, but what I want you to do is imagine a whole bunch of thick and thin filament bundles. And they go like, I mean, I want you to know that this is going to continue on that way. This one is going to continue on that way. They're long. They're like the whole length of a muscle cell. And muscle cells are really long. So we have these overlapping like, whoa, it's just like that. Like these are thick filaments and these are thin filaments and they overlap right here. And then can you visualize, like, dude, what's going to happen with those little heads? Well, what actually happens is that those heads grab the actin, which is the thin filaments. The little heads grab the thin filaments and pull on them. And the pulling moves the thin filaments in this direction. Seriously, it's phenomenal. The thin filament is going to move in this direction. We had one here, which we do have one there. You can totally draw one in. Can you imagine that if we do this, if the thick filaments grab the thin filaments, we're actually going to shorten this thing. Are you awesome with that idea? This whole thing from z-line to z-line, that whole thing is one sarcomere. Like yourself on out, like, get up and walk away from your computer and look at that picture right there, push pause first. Look at that picture right there. And do you agree that we have stripes? We've got these big blue stripes and we've got these big green stripes and then another blue stripe and then another green stripe. And holy, all the way down now. I'm just for our purposes here. I'm going to take this whole thing, okay, this whole thing right here. That is a myofibril. And then, huh, I made you one. I made you a myofibril. This is a spaghetti noodle because I'm obsessed with this idea of these bundles of bundles. My spaghetti noodle, if I were amazing, I could break the spaghetti noodle up into thick filaments and thin filaments. I'm not that amazing, so I zoomed out, but what I'm telling you is that this zone right here is a lot of thick filaments, that black. This white stripe is a lot of thin filaments. And in fact, we can actually label the parts of this thing. They have names, but of course they do. So look, the place that is all actin, that's all thin filament, it's all the light band. It's the white part of my spaghetti noodle, myofibril. That's called the eye band. The eye band contains only thin filament. Guess what? It contains only thick filament. Of course, it's the A band. Now, thick filament, thick, dark, black in my spaghetti noodle, rather A dark. The word dark has an A in it. Eye light, the word light has an eye in it. So the light eye band is made mostly of actin, all of actin, and the dark A band is made all of myosin, we'll say myosin. And from z-line to z-line, we have one sarcomere. How many sarcomeres did I draw here? How many complete sarcomeres did I draw? One, one complete sarcomere, because I've got one z-line to z-line. I've got a little bit of an extra one here and a little bit of an extra one here. And remember that the myofibril itself is like tons of sarcomeres end on end, attached and attached and attached forever. For the whole length of a skeletal muscle organ, which is kind of mind-blowing if you think about that, myofibrils are made of myofilaments that are organized in a structure that creates a striated look and we can nail it down and name it. These are these strings of sarcomeres. The striation, the striping, a sarcomere isn't like a thing. A sarcomere is an appearance that happens because of how the myofilaments are stacked together. I understand that that is a really odd, it's really conceptual, I understand that it is challenging to visualize that, because not only is it super conceptual, it's also happening at a cellular level. So can we actually go in and see our rock and sarcomere? Uh-uh, you can't see the sarcomere at all. That's okay. So let's look really fast in no detail at all. I want to look at how this functions. When I look at this thing, I do totally, the entire sarcomere shortens as the myosin fibers pull the actin filaments together. I can see how that whole thing shortens and then the whole thing relaxes again. But I think it's more helpful to look at it in an animation, so let's go do that.