 There is no part of tidally-wink madness in the muscle cell. It is in the skeletal muscle cell. Now, if we think about smooth muscle, it does not have this kind of organization at all. Cardiac muscle does, and you'll see in a second that you could predict that. So if you can imagine stacking together or grouping or bundling your thick filaments of thin filaments in a way that they actually, they don't like go end to end, and they don't like go on top of each other. They actually like go in between each other. They like overlap like bristle blocks. Oh my gosh. Bristle blocks, you know those blocks that had all those pokies on them and then you stick them together, and it's like myosin thick filaments and actin thin filaments, and they go together like this. The way that they're organized, let's create stripes, like an illusion of stripes, and so they're organized into these structures called sarcomeres. So now that you have this vision for perspective, let's draw a picture of this whole thing there. Fresh page. Okay. Here's what it looks like. We have to have a start and a stop in order to keep perspective. And my start and my stop, I'm going to, they're called z-lines. And I'm making them in the same color that I made my thin filaments. And these are actual, like they're mostly made of actin, and they connect to thin filaments. Okay, so I'm drawing the thin filaments that connect to these things. And they also connect this way. Thin filaments do. Okay. Oh, dear. Sure. A little odd, but we'll go with it. Oh, geez. This is called a z-line, and this is going to be an important landmark when we talk about the sarcomere itself. In fact, a sarcomere is measured from z-line to z-line and the thin filaments attach to it. All right? So maybe I should make a little note down. Oops. Let's make it pink, like my thin filaments. Thin filaments, just so you don't forget. Now, my thick filaments were blue. Again, it's just like spaghetti noodles. Bristle blocks. My thick filaments are actually going to fit in between my thin filaments. And now, not only do you have to visualize this on a two-dimensional surface that I'm drawing right now, but you have to visualize this imagining that it's a whole, like, cylinder of these fibers. And look, there's some on this side, too. Because we could just, we have sarcomere after sarcomere sarcomere is end to end, make up your myofibril. And remember, let's draw these guys in, too. These are my little myosin heads. And they are getting ready to attack, attack, attach to the actin, the thin filaments. And we're going to look, I think, next. No, we're going to get a big overview. And then we're going to look at the mechanism of muscle contraction. So we could totally draw this on every single one of our muscle. I mean, every single one of our thick filaments. We could draw our little myosin heads. And right now, you can imagine, are you cool with this? Like, let's just do this for the fun of it. You can imagine that there's a little piece of ATP attached to every single one of them. ATP that turned into ADP plus P. And so all those myosin heads are ready to contract. They're all cocked. They've got ATP on them. And now, like, all they're waiting for is some binding sites to be exposed. And then they're going to contract. Dude, just try and imagine this for a second. I'm giving away the fun story, but can you imagine if these things contract and they end up like this? I mean, that's a whole, that's a, that's motion. That's movement, and if all of them do it, can you see how we're just going to shorten the whole thing? So amazing. What else do you need to know? Okay, now we need to label parts of our, the, this organizational structure. You've got the Z line, which is where the actin molecules attach, the thin filaments attach. You also have an M line. And this actually is a structure that goes down the middle. This is the M line. The Z, I feel like line is the wrong word for the Z, but I can't remember. Okay, so the M line goes right down the middle. All the places where, let's do it like this. Do you see how now I'm getting like stripes going here? There's a whole zone, like this green zone right here. And it's like the full length of the myosin heads, the thick filaments, the full length of them is like a darker spot. You can kind of see it if you like step away, you can be like, oh yeah, that's a dark spot. That's called the A band. The light spot that you see right next to it, from here to here, that's all actin. This is the I band. And there's another band. There's got to be another band, but of course. There is the H band. And my drawing is not perfect, which is why I'm having to show you the H band. This right here is my H band. The H band, dude, that might be the H zone. Band zone, I'm cool. The H zone, I think it is a zone. The H zone is only myosin, only thick filaments, no thin filaments. So the minute, like, just pretend like that's not there, and that's not there, and that's not there. Only thin filaments, thick filaments are found in the H band. I think there were five things we had to label on our sarcomere. Remember that one sarcomere was from Z line to Z line. That's one sarcomere. Now I'm going to take you back to the official picture and you can see that here's a Z, it's a Z disk, but of course it is. You can see the Z disk. Here's another Z disk, so that whole thing is a sarcomere. You can see my I band, which is light. The M line, which is in the middle of all the myosins. The A band, which is every piece of myosin. The H zone, which is only myosin. Wow, are you just like, you got this, makes perfect sense. Now, the next thing I want to do is just do a quick, like, back on up and make sure we're cool with what we're talking about in the big picture so we keep perspective. And then we're going to do the muscle contraction at this level of organization.