K. So in this video, we're going to be talking about actin and skeletal muscle contractions. Skeletal muscle contractions depend on interactions between two proteins, actin and myosin. We talked about actin, so what is myosin? Myosin are dimers. They have two heads, which have ATPase capabilities, and a tail that is the coiled coil, which just pretty much means it's wrapped around each other and extends outward from the heads. They also form filaments, myosin filaments, and they are clusters of this myosin 2. They kind of look like double-headed arrows, some people say. I think they look like other things. You can kind of figure out what they look like by yourself. But anyway, that's what people say. There are two myosin filaments that bind to each actin filament. Each one of these myosins binds in opposite orientations, allowing them to move the actin in opposite directions. This will make more sense when I show you the picture, but you'll have to bear with me on some of these pictures. Right now, this is what myosin looks like. You have your two heads, these have ATPase capabilities, and you have this long coiled coil tail. This is going to be a myosin filament; there are lots of them here you can see, and they're bound in opposite directions to the actin. Skeletal muscle is made up of these actinomyosins, and their distinct organization, which you actually need to know what all of these features are called. This organization and these features allow for muscles to contract. Bundles of actinomyosin are called myofibrils. Inside the myofibrils, oh, goodness. I can't say that. And then you have sarcomeres inside of them. The sarcomeres are the tiny units that actually contract. There are many of them in these fibrils. Inside a sarcomere, there are a lot of different structures you need to know about. There is an A band and this is composed of myosin. You may also see this as a thick filament. It is the same exact thing. Then you have the H zone, which is in the A band and this is a lighter region without actin, and then you have the M line, which is the disc in the middle. I think that might be the easiest one to remember because it's the M line in the middle. But then you also have the I band; you have the A and I band. The I band is called the light band and it is composed of actin; you may see it as thin filaments and no myosin. The very end of both ends are called the Z line or the disc. So, let's look at this, what this actually looks like. Here you have a sarcomere, and it goes from here to here, and the lines are here in blue, that's the Z disc. So then, you have the M line that is easily in the middle. You have the H zone, and so this is going to be myosin that's not overlapping with actin. Here you have your myosin in red. So you can see there is overlapping of actin. Then you have your A band, which has actin and myosin. And then you have your I band, which is much smaller and contains only this thin filament, which is actin. Those are the structures that make up the sarcomere, and you're going to have to know them, it's going to just be crucial. So those are the structures. So now let's move on.
Muscle Contractions - Online Tutor, Practice Problems & Exam Prep
Created using AISkeletal muscle contractions occur through the interaction of actin and myosin proteins within sarcomeres. Myosin, a dimer with ATPase capabilities, binds to actin, initiating contraction through ATP hydrolysis, which causes conformational changes and cross-bridge formation. The I band and H zone shorten as the Z lines come closer together, while the lengths of actin and myosin remain unchanged. Calcium ions play a crucial role by binding to troponin, allowing tropomyosin to expose actin binding sites for myosin. This sliding filament model is essential for understanding muscle contraction dynamics.
Skeletal Muscle Contractions
Video transcript
Contraction Steps
Video transcript
Okay. So now we're going to talk about the steps to a muscle contraction. And so if I'm just going to give a one thing overview of how muscle contraction happens, well, it happens because through the shortening of the sarcomeres that we talked about. So that's just the overview. Skeletal muscle contraction occurs through shortening of sarcomeres. But, of course, it's not that simple, and there are 4 steps that we're going to go over, and then each or a couple of the steps have even more, like, intricate things we're going to talk about. But let's just go through them.
So the first one is that the myosin binds to actin, so it's a fairly easy step to remember. Then the second step is an ATP hydrolysis step, and so, ATP hydrolysis, of course, results in some type of change, and that change is a conformational change. And that allows myosin to bind very tightly to actin, and this begins to stimulate the movement. So, I'm going to talk about this, but I'm going to come back to this in just one second.
For the 3rd step, once the ATP has been hydrolyzed, what happens are these things called cross bridges, which form. And this is going to be an overlap between the thin and thick filaments, which remember the thin are actin, actin and myosin. So there begins to be an overlap in the sarcomere, and it overlaps because there's it's shortening. So things are having to be condensed together, and there's overlap. And I'll I'll come back to this.
And then, the final thing that happens is that ATP binds again. This disassociates the cross bridge, and so it relaxes and elongates out again and returns to its relaxed stage.
So let's go back and talk about these intricate things. So the first thing, myosin binds to actin that's easier to remember. The second thing is the ATP hydrolysis step, and that allows myosin to bind tightly. So what happens is there's this other protein called tropomyosin, and this is a protein that normally, in its relaxed state, is going to bind to the place, the actin binding site, preventing myosin from binding there. But, when, ATP hydrolysis occurs, what happens is that, there's also another thing that happens, which is calcium comes in and binds to this other protein called troponin, and when calcium is bound to troponin, that alters tropomyosin, so you have calcium binding to troponin, alters tropomyosins, and then tropomyosins, sort of moves a little and releases that binding site, so myosin, can bind to actin or more tightly than it does. Then, once it's bound really tightly, you have a cross bridge form.
Remember, this is an overlap. So what happens is the I band and the H zone shorten so that the Z line which is the ends come closer together. So you'll have to go back and relook at the I band and the H zone, remember what these are, so that you know what thing is exactly shortening, so the Z lines come together. But the important thing here is that nothing is actually the length is not changing at all. Like, the length of the myosin, the length of the actin, they're staying exactly the same. The difference is that the actin is sort of just sliding past. So my hands right now, they're the same length. So, if they come together, they're still gonna be the same length, but the distance between, you know, this wrist and this wrist is now shorter because they slid past each other and that's resulted in the sarcomere shortening. So that's the same thing.
So, just like my hands don't change length when the when the wrist get closer together and shorten the distance, same here. All the bands, all the actin, all the myosin stays the same length, it's just they come across each other until they get shorter. And then finally, you get ATP binding, which dissociates the cross bridge and returns to its relaxed state.
So here, you have a relaxed state, and you can see there are, you know, big portions here that aren't overlapping. You have actin filament. You have your Z disc, which is the very end here and here. This is its relaxed state. Then, all the steps happen, and you have a contracted state. And you can see that the bands are exactly the same length, but everything is now overlapped more. So, that these Z discs are closer together and it's more contracted.
So let me also go over troponin and tropomyosin, which remember this is for step 2. So here you have a filament here, and an actin filament, which has troponin bound to here. And you have myosin, which wants to bind so that it can contract. So what happens is calcium comes in and, binds to troponin, which then binds to tropomyosin, which you can see is this green thing here, this like, thin protein that's overlapping this thin filament. So when calcium comes in binds to troponin, troponin will change tropomyosin's conformation, and that will allow for myosin to come in and bind the actin filament. So that is how that step 2 works. So with that, let's now move on.
Which of the following is not a structure of the sarcomere?
Which of the following structures is composed of actin, but no myosin?
When calcium binds to troponin, what happens to tropomyosin?
When a cross-bridge structure is formed during a muscle contraction, the band lengths shorten and contract.
Here’s what students ask on this topic:
What are the main proteins involved in skeletal muscle contraction?
The main proteins involved in skeletal muscle contraction are actin and myosin. Actin is a thin filament, while myosin is a thick filament with ATPase capabilities. Myosin binds to actin to initiate muscle contraction. This interaction is regulated by the proteins troponin and tropomyosin. Troponin binds to calcium ions, which causes a conformational change in tropomyosin, exposing the binding sites on actin for myosin. This process is essential for the sliding filament model of muscle contraction, where the sarcomere shortens without changing the lengths of actin and myosin filaments.
Created using AIHow does ATP hydrolysis contribute to muscle contraction?
ATP hydrolysis plays a crucial role in muscle contraction by providing the energy needed for myosin to bind tightly to actin. When ATP is hydrolyzed to ADP and inorganic phosphate, it causes a conformational change in the myosin head, allowing it to bind more tightly to actin. This binding forms a cross-bridge, leading to the sliding of actin filaments past myosin filaments, which shortens the sarcomere and results in muscle contraction. The cycle repeats as new ATP binds to myosin, causing it to release actin and return to its relaxed state.
Created using AIWhat is the role of calcium ions in muscle contraction?
Calcium ions play a pivotal role in muscle contraction by regulating the interaction between actin and myosin. When a muscle cell is stimulated, calcium ions are released from the sarcoplasmic reticulum into the cytoplasm. These calcium ions bind to troponin, causing a conformational change that moves tropomyosin away from the actin binding sites. This exposure allows myosin heads to bind to actin, forming cross-bridges and initiating muscle contraction. The removal of calcium ions from the cytoplasm leads to the re-blocking of the binding sites by tropomyosin, causing muscle relaxation.
Created using AIWhat are the steps involved in the sliding filament model of muscle contraction?
The sliding filament model of muscle contraction involves several key steps: 1) Myosin binds to actin, forming a cross-bridge. 2) ATP hydrolysis occurs, causing a conformational change in the myosin head, which binds tightly to actin. 3) The myosin head pivots, pulling the actin filament toward the center of the sarcomere, shortening the I band and H zone. 4) A new ATP molecule binds to myosin, causing it to release actin and return to its relaxed state. This cycle repeats, leading to the sliding of actin filaments past myosin filaments, resulting in muscle contraction.
Created using AIWhat is the structure and function of a sarcomere in muscle contraction?
A sarcomere is the basic contractile unit of skeletal muscle, composed of actin (thin) and myosin (thick) filaments. It is delineated by Z lines, with the A band containing myosin and overlapping actin, the I band containing only actin, and the H zone containing only myosin. During muscle contraction, the sarcomere shortens as the Z lines come closer together, the I band and H zone decrease in size, and the actin filaments slide past the myosin filaments. This sliding mechanism, driven by ATP hydrolysis and regulated by calcium ions, is essential for muscle contraction.
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