We now want to talk about the structure of the sarcomere down to the protein level in a lot more detail. And remember, the sarcomere is the contractile unit or what I've called before the fundamental unit of muscle contraction, and the sarcomere contracts, we've said, when the myosin pulls on the actin, and we can look at our image of a sarcomere here. We can see it in more detail than we've seen before. These zigzaggy lines, these represent the end of the sarcomere. We're going to define those a little bit more coming up later, but we can see one sarcomere here and we see all the filaments and the proteins we're going to talk about inside there. And remember, this sarcomere is this repeating structure where to make up one myofibril you have tens of thousands of these in a row, and all these are just going to get a little bit shorter when the muscle contracts. Now to do that, the first proteins we're going to talk about are the myosin and the actin, and remember the myosin, this is the thick filament, and we can see that here reaching out from the center of the sarcomere, these purple filaments, and they have all these myosin heads sticking off of them. And those myosin heads, remember, we said those are like little arms that are reaching out to grab onto the actin to pull on it. Now because they're called myosin heads, one sort of memory tool that I've heard before that I really like is that myosin is this many headed Medusa. Myosin has all these heads just like the snakes off Medusa's heads reaching out, and they're reaching out to grab onto the actin. Now remember the actin, that's the thin filament, and our memory tool for that actin is thin, and we can see reaching out from the ends of the sarcomere. We have the actin in this sort of yellowish orange here, and it's the important thing to note here is that it's overlapping the myosin, but not by a lot. It's just overlapping just a little bit here the myosin heads, so those myosin heads can pull on the actin. Now, we've said before that myosin just wants to pull on actin, that's all it wants to do, but of course your muscles aren't contracting all the time. Most of the time they're relaxed. So how do we control when myosin pulls? Well, to see that we're going to zoom in on an actin filament here, and you'll notice that there's some regulatory proteins on here. There's this green thread like protein that's wrapped around the actin, and then there's this sort of blue globular protein there. So the first one, that green protein, we're going to call tropomyosin. And tropomyosin we're going to call a thread like protein, and you can see it wraps around the actin. And you can see the actin here, it's actually actin is made of these smaller subunits. So actin is sort of almost like these beads on a string. It's really sort of like these 2 beads, sets of beads that are kind of twisted around each other a little bit. And you'll notice that on these sort of beads that are on that are the actin, it looks like they have little holes in the beads. Those drawn in, those are the myosin binding sites on the actin. So that's the place that the myosin can bind to pull on the actin. But you'll also notice that this tropomyosin, this thread like protein, it's in front of all those little binding sites. So the tropomyosin is really there because it blocks the myosin binding sites on the actin. As long as that tropomyosin is there, that myosin is blocked. It cannot bind. So how does it move? Well, we have this globular protein, this sort of smaller protein here, the troponin. So I'm calling that a globular protein, and by that I just mean that all our other proteins here are like threads or filaments. But this is just a smaller protein, and it's going to be bound to both the actin and to the tropomyosin. And it's also going to be able to bind to calcium. Now remember, we talked about the sarcoplasmic reticulum. And when the sarcoplasmic reticulum gets a signal to contract, it's going to release calcium into the myofibril, into the sarcomere, and that calcium is going to bind to the troponin. And when that happens, that troponin is just going to sort of change its shape just a little bit, and it's going to open the binding sites on the actin by moving the tropomyosin. It's just going to sort of pull that tropomyosin just a little bit out of the way as long as it's bound to calcium. Now, we're going to go through the step by steps of how all of this works in more detail going on. Right now, we really just want to remember what the proteins are, where they are, and their basic rules. Finally, for these, we have a little memory tool. I say that tropomyosin says no to the myosin. So the tropomyosin is that thread like protein blocking the binding sites. The myosin wants to bind. The tropomyosin says no to the myosin. In contrast, we have the troponin is going to open the binding site, and the calcium binds to it. It's going to pull that tropomyosin out of the way and open the binding site for the myosin so the muscle and the sarcomere can contract. Alright. Our final protein that we're you're likely to need to know about is a structural protein, and this is an elastic filament. The elastic filament, you can see here it attaches to the end of the sarcomere and comes out lined up with the myosin filament. Actually, sort of interacts with the myosin filament and goes all the way to the middle of the sarcomere, just like that, on both sides as well. So this elastic filament is made of the protein titin, and it is there to help the sarcomere retain its shape. It's an elastic protein, and the sarcomere is always, you know, contracting and relaxing, and that elastic protein is there to help it get back to its original size whenever it strays from that original size, whether because it stretched or whether it's because it contracted. So the titin keeping the shape, helping it retain...
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9. Muscle Tissue
Sliding Filament Theory and the Sacromere
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