Sliding Filament Theory and the Sacromere - Online Tutor, Practice Problems & Exam Prep

The sliding filament theory explains muscle contraction through the interaction of myosin (thick filament) and actin (thin filament). Myosin heads pull on actin, increasing their overlap without changing size, resulting in a shorter sarcomere. Key proteins include tropomyosin, which blocks myosin binding sites, and troponin, which, when bound to calcium, moves tropomyosin to allow contraction. The sarcomere's structure features z discs, m lines, and distinct bands: I bands (light, actin only) and A bands (dark, overlapping actin and myosin), with an H zone (myosin only) that diminishes during contraction.

Sliding Filament Theory

Now that we understand the structure of muscles from the whole muscle all the way down to the sarcomere, the fundamental unit of muscle contraction, we can start to answer the question of how muscles contract. How does that sarcomere actually get shorter? To do that, we're going to start by talking about the sliding filament theory of muscle contraction, and the sliding filament theory is a really excellent conceptual framework for how muscles contract. Eventually, we're going to learn all the molecules involved and the step-by-step processes. But when people started asking this question, how do muscles get shorter? They didn't know what molecules were involved. They knew there were two filaments. Now, we know those filaments are myosin and actin, but back then they didn't know they were myosin and actin, so they called the myosin the thick filament. And for obvious reasons, it was thicker than the other filament. Now, myosin and the thick filament, those can sometimes be used interchangeably, so you should be familiar with that term, the thick filament. The thick filament or the myosin, that's going to be anchored to the center of the sarcomere and sort of reaching out in both directions. And now, we know the myosin is this big protein with all these sort of arms and hands that are reaching off of it. Each myosin has 300 arms or hands reaching off. Though technically they're called the myosin heads, but think of them like arms and hands. And they are reaching out because myosin is this protein, and myosin has one love, it's a protein that wants to pull on a rope. If you give myosin the chance, it's going to grab onto a rope and it's just going to pull hand over hand on a rope. That's our myosin. Then our actin well, our actin is going to be the thin filament. You can remember that because actin is thin. And, well, if myosin was anchored to the middle of the sarcomere, the actin is anchored to the ends of the sarcomere. And if myosin just wants to pull on a rope, pull on the actin, that's the rope. So let's now just step back before we go on here, and we're going to look at an animation of this. And so remember this is all happening in three dimensions. In these structures called the myofibrils, we have these repeated sarcomeres. And so we're going to zoom in on one sarcomere here. And here you can see the thick filaments, those thick myofilaments, those are the myosin, and you can see all those little heads or little hands reaching out, and they're going to start pulling on the actin, those thin myofilaments. And as they do that, you can see the actin got pulled towards the center of the sarcomere, and the sarcomere, our fundamental unit of muscle contraction, it got shorter. Alright. So let's look at that again. Now, the thing that I want you to notice here is as these myosin, these thick filaments, they're going to reach out and they're going to pull on the actin hand over hand. The actin's sliding in. The myosin and the actin are not changing size. The actin is sliding in towards the center of the sarcomere, so the amount that they overlap is increasing, and the sarcomere is what gets shorter. The actin slides over the myosin, hence the name the sliding filament theory of muscle contraction.

Okay. So let's go back to our page here. So to sort of build this analogy out a little bit more, we have drawn down here these guys in purple. These are our myosins, and they're anchored to the center of the sarcomere, and they're facing out in both directions ready to pull on this rope. This actin is the rope. And importantly, you'll notice at the start, this actin is just overlapping the myosin just a little bit. It sort of ends at each hand here, and another one starts, and then it ends here. And it's just a little bit of overlap. Remember that actin or the thin filament, it's anchored to the ends of the sarcomere. To mark the ends of the sarcomere we have these knots, kind of like you might see in a tug of war. So these are the ends of the sarcomeres. So here see here this would be one, two, three sarcomeres that we're looking at. Now as the myosin heads or these arms and hands reach out and they start pulling hand over hand, now you can see what happens here. The actin does not change size. What happens? The rope gets pulled past and now there is more overlap. The myosin and the actin are the same size. There's more overlap. And when you look at it, we draw in the ends of the sarcomeres here, you can see each sarcomere gets a little bit shorter, but because each guy is linked back to back, they gotta stay back to back because one actin is linked to the actin in the next sarcomere. When each sarcomere gets a little bit shorter, the entire myofibril gets a lot shorter. So if we look over here, we have our, sort of, more formal drawing of a sarcomere here. And you can see in purple, we have the thick filaments reaching out from the middle of the sarcomere. We have the thin filaments in yellow. These are the actin filaments attached the ends of the sarcomere there, and we can draw in the ends of the sarcomere just like we did before. In here, it's sort of this zigzaggy line, and we're going to name that zigzaggy line a little bit more formally later on, but just for now that's the end of the sarcomere. And importantly, you can see that when we start here, there's just not a lot of overlap between the myosin and the actin. A little bit, but not a lot. Now myosin pulls hand over hand. Everything slides in. I'm going to draw in my new or my edges of my sarcomere here, down here as well, and you can see each sarcomere gets a little bit shorter. The whole thing, because everything's linked, gets a lot shorter. And what you'll notice what happened? The amount of overlap between the actin and the myosin, that's what increased. The filaments did not change size. The sarcomere got shorter because the overlap increased. All right. Just to finish this off, to sort of give ourselves a formal note here at the end, we're going to say that during contraction the filaments, the actin and myosin, they stay the same size. They slide past each other, and when they slide past each other that overlap of the actin and myosin increases. And I'm going to just draw an up arrow to represent increases in sort of shorthand there. So they slide past each other, increasing the overlap. Hence the name the sliding filament theory of muscle contraction. Now we're going to go into all those step-by-step details and molecules involved in a lot more detail coming up. But first, like always, we have an example, practice problems to follow. I'll see you there.

Sliding Filament Theory and the Sacromere Example 1

Our example tells us that the image below shows multiple sarcomeres, and then it asks us to draw arrows to label at least 2 myosin filaments, draw arrows to label at least 2 actin filaments. And then it asks us the question when the muscle contracts, which filament gets closer to the center of the sarcomere? So as I look at this image, I see this sort of long, drawing here showing all these protein filaments, and I see this repeating pattern, and that repeating pattern is the sarcomere. So the first thing I want to do, just to orient myself to this picture, I'm gonna draw the ends of the sarcomere in, so I can see the individual sarcomeres. So the ends of the sarcomere are these sort of zigzaggy lines here. I'm just gonna draw a straight line over it. Right? Here's another zigzaggy line. Those zigzaggy lines we haven't defined yet, but those are gonna be called the z discs, and those mark the ends of the sarcomere. So as I look here, I now have one, two, three, four sarcomeres, four spaces between those blue lines that I drew in. Now, again, just to simplify things, I think I'm just gonna try and look at one of them. So I'm gonna shade in and just sort of ignore the parts that I'm shading in, and let's just focus on this sarcomere number 3 right here. As I look at that and I look at the filaments I see, I see two major types of filaments here. I see the purple filaments right there, sort of in the center of the sarcomere, and those have those, sort of like many arms that are kind of reaching out or the heads that are reaching off of them. And then I have these yellow filaments that are attached to the ends of the sarcomere. Alright, so one of those the myosin, one is the actin. Do you remember which is which? All right. Well, the myosin was the thick filament. And we said in our story, right, that the myosin just wants to pull on a rope. And so it has all these hands to just sort of pull hand over hand on that rope. So when you look at this, the thicker filament with many sort of hands, they're called heads, but there are many hands coming off of it. That's that purple filament. So there's one. There's one. Said two, but I can keep going. There's another one. There's another one. Alright? Those horizontal purple filaments there, that's the myosin. That means the actin is those yellow filaments. Remember the yellow filaments? The actin? That's like the rope that the myosin is gonna pull on. So there's one. There's one. There's one. Said just label two. We got three. We're doing great. So that's the actin. Alright. Myosin, that's, located at the center of the sarcomere. The actin is attached to the ends of the sarcomere. So that means when we ask this question down here, which filament gets closer to the center of the sarcomere? Well, the center of the sarcomere is right here. The ends are out here. That myosin is gonna grab on that actin, it's gonna pull it in, and that actin is gonna slide closer to the center of the sarcomere. So my answer is the actin. Alright. Like always, we have more problems to follow, and then we're gonna go into the proteins in the sarcomere in a lot more detail coming up. And I'll see you there.

During a muscle contraction, the ___________ pulls on the ___________, shortening the muscle.

• A. Actin: Myosin.
• B. Myosin: Sarcomere.
• C. Sarcomere: Actin.
• D. Myosin: Actin.

During a muscle contraction, what gets shorter?

• A. Actin.
• B. Sarcomere.
• C. The thick and thin filaments.
• D. Myosin

Proteins of the Sarcomere

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...

Sliding Filament Theory and the Sacromere Example 2

Our example asks us to imagine whether the myosin binding sites on the actin will be more likely to be exposed or blocked in the following two scenarios. Likewise, we want to predict whether you would expect overall myosin binding to increase or decrease in each scenario and an image is shown for reference. Right? We've seen this image before. In orange, this is the actin, and in green, we see the tropomyosin wrapping around the actin, and then in blue, we see these troponin molecules.

Alright. So our first scenario is going to be imagine that there is no tropomyosin present on the actin filament. If there's no tropomyosin, would you expect the myosin binding sites to be more likely to be exposed or more likely to be blocked? Well, remember the tropomyosin, that's this green filament drawn on here, and it is there blocking the binding sites. That's what it is there for. The tropomyosin is there to say no to the myosin because it's blocking the binding site. So if you take it away, well, those binding sites are just going to be exposed. There's nothing left to block them. If you remove the tropomyosin, if you expose the binding sites, is binding going to increase or decrease? Well, myosin, if it gets the chance, is going to bind. So if the binding sites are exposed, that is going to increase the amount of binding. There's nothing there to stop it.

Our next scenario is to imagine that the calcium binding sites on the troponin molecules are nonfunctional. If the calcium binding sites on the troponin molecules are nonfunctional, do you think the myosin binding sites would be more likely to be exposed or more likely to be blocked? So remember, this is the troponin molecules here, and when calcium binds to the troponin molecules, the troponin opens the binding sites by moving the tropomyosin. So if the calcium can't bind, the troponin can't move the tropomyosin. That means that those myosin binding sites are going to remain blocked. And if the myosin binding sites are blocked, well, can the myosin bind? Heck no. So binding is going to decrease.

Alright, so that's the roles of troponin and tropomyosin. Like always, we have more practice problems to follow. Give them a try.

Myosin storage myopathy is a rare congenital condition where some of the myosin folds incorrectly in certain skeletal muscles creating clumps. Individuals with this condition exhibit muscle weakness and may be delayed in learning to walk as infants. What changes would you expect to see if you were to examine the sarcomere of an individual with this condition?

• A. The thick filament would form protein clumps.
• B. The thin filament would form protein clumps.
• C. The titin would form protein clumps.
• D. The fascicle proteins would form clumps.

The contractile proteins of the sarcomere include which of the following?

• A. Troponin.
• B. Tropomyosin.
• C. Titin.
• D. None of the above.

Which protein contributes to the structural integrity of the sarcomere?

• A. Troponin.
• B. Tropomyosin.
• C. Titin.
• D. Actin.

Structure of the Sarcomere: Bands, Zones, Discs & Lines

If we finish up talking about the structure of the sarcomere, we're going to talk about these discs, lines, bands, and zones. These are structures and regions of the sarcomere that are named for how they look on an electron microscope or a transmission electron microscope, or TEM. We have an image like this that we're going to look at here, and you've probably seen an image like this in your textbook or in lecture. This is an electron microscope image of 1 sarcomere, and below it, we've drawn this illustration of the proteins involved that we went through previously. And what we're going to do here is we're going to name all these bands and zones and we're going to see how they relate to the actual proteins that make up the sarcomere. So the first thing that I notice when I look at this image are these dark lines here going up and down marking the end of the sarcomere, marking the two edges of the sarcomere. Those lines are what we're going to call the Z disk or the Z disks, and the Z disk is the end of the sarcomere. One way you can remember that, well, Z is at the end of the alphabet and the Z disk is at the end of the sarcomere. As I look at my illustration here, again we see these up and down lines here. You can see how that lines up with sort of these zigzaggy lines in our illustration of the proteins here. Another way I remember it when I'm looking at this sort of protein drawing: this zigzaggy line is the Z disk. You may be wondering why it is called the disk when we're drawing it as a line or a zigzaggy line. Remember, this is all in the myofibril, and the myofibril is this big long cylinder organelle. So the sarcomere is actually sort of a small cylinder. It's kind of the shape of a soup can or something like that. So the ends of the cylinder, of that short cylinder, the sarcomere, is going to be a circle shape or a disk in 3 dimensions. And when you look at a cross-section of it in 2 dimensions, it's going to end up looking like a line. The Z disk is there at the end of the sarcomere, and remember that the Z disk is anchoring the actin. That's what the actin is going to be attached to. Now, in the middle of the sarcomere, we have this other line right there and that is going to be our M line. The M line has the myomesin protein that anchors the myosin. Now, myomesin protein, you probably don't need to know the name of that protein but like always, check your own notes, know what your professor wants you to know. But we can look at our image here. This dark region here, that's that protein and in our image, we can see it running up and down and it is anchoring that myosin that's reaching out in both directions. This line in the middle is the M line and that's the easier way to remember it. M stands for the middle. As we look at this, there are also going to be these sort of bands and zones, and these bands and zones refer to the sort of bigger, lighter, and dark regions that we see. First off, when I look at it, obviously there are some light regions on each end, and there's a big dark region in the middle. So the I band is going to be the light bands. I always write it with a capital I. I write light with a capital I because the I band is the light band. That vowel helps me remember that the light band is the I band. This is the area with just actin. So again, if we look over at our illustration here, we have this light band here. If we look down at our drawing, this sort of loosely lines up with the light band. The myosin ends and where there is no myosin, where there's just these actin filaments, that is the light band with that Z disk right in the middle of it. And it's the light band because the actin is the thin filament, and the thin filament is going to look lighter when looked at through an electron microscope. So we have the light band. We also have the A band. The A band, therefore, is going to be the dark band, and again, I write it with that capital A to remind myself that the A band is the dark band, remind myself of that vowel. If the I band has the actin in it, well the A band has the myosin in it but it's actually the area with both actin and myosin because remember that actin and myosin need to overlap so that myosin can pull on the actin hand over hand so that the sarcomere can contract. So if we look at our illustration over here, we have the dark band, this dark region here, and that loosely lines up with the edges of the myosin, and the myosin is the thick filament so it looks darker through the microscope, but it's also overlapping this actin here. The dark band is the same size as the myosin, but it has the actin overlapping it. But the actin doesn't overlap all of the myosin. And in our image here, you can see this section in the middle with the M line in the middle that looks a little bit brighter. That section that looks a little bit brighter is the H zone, and the H zone is that center region where there is just myosin, where the actin doesn't overlap. And we can see that again here, that lighter region that loosely matches up to our illustration here, that section where the actin is not overlapping. Now that H actually stands for heli, and heli is Greek for bright. And because it's the region where the actin isn't overlapping, when the muscle contracts, the actin gets pulled in and it's going to overlap that myosin more and more, so you can imagine this H zone is going to get smaller and smaller. It's eventually going to even disappear when the muscle is fully contracted because that actin will be completely overlapping the myosin. Understanding these zones and discs and proteins and how they all match up, in my experience, is something that professors love to ask about. We're going to practice some more in the example to follow. I'll see you there.

Sliding Filament Theory and the Sacromere Example 3

Our example gives us an image of a sarcomere from an electron microscope, and we need to do a number of things with it. We need to mark the A band, the I band, and the H zone with brackets, and then label them. We need to mark the Z disk and the M line with arrows and label them, and then we need to draw a line to represent the length of 1 actin filament. And finally, we need to describe if and how each component changes in size during a muscle contraction. Alright. Filling out this table to talk about the size, I'm actually going to do that as we go. So let's go ahead and get started. We have the A band. Alright. So where is the A band? Well, remember the A band is the same thing as the dark band when you're looking at a sarcomere. So I can draw the edges of where it's dark, you know, sort of roughly there to about here. I'll draw my back bracket there. That's going to be my A band. And I remember that because the vowel in dark is A, so the dark band is the A band. Alright. Now, let's think: does this A band change size when the muscle contracts? Well, the A band is the region where the myosin and the actin overlap, but really it's defined in size by where the myosin is. So the myosin is attached to the middle of the sarcomere, and it reaches out sort of right to here to the edge of the A band. So, in our sliding filament theory of muscle contraction, we say that those filaments, the actin, the myosin, don't change size. They slide past each other. So if our A band is the region where the myosin is and that myosin doesn't change size, that means when the muscle contracts, then in the A band we expect no change in size. Alright. Next, let's look at the I band. Alright. If the A band is the dark band, well, the I band I'm going to sort of start right next to the A band where it's a little bit lighter, and I'll draw my bracket around sort of roughly there. And that is going to be our I band. And remember, I remember that because I is the vowel in light. So the I band is the light band. So, does the I band change size when the muscle contracts? Well, the I band we said is the region where there's only actin, and the actin when the muscle contracts, remember that myosin is going to pull on the actin and it's going to slide towards the middle of the sarcomere, and more and more of the actin is going to be overlapping the myosin. So if more and more of the actin is overlapping the myosin, well, then less and less of it isn't overlapping the myosin. That means that I band, that region where there's only actin, is going to get shorter. So I'm going to write here, gets shorter. Alright. That brings us to the H zone. The H zone is this region in the middle of the sarcomere that's just a little bit lighter than the rest of the A band. And remember, we said the H zone is a little bit lighter because the actin comes in, it overlaps the myosin, but doesn't overlap all the myosin usually. There's going to be a region between those actin filaments, and it's a little bit brighter because that's where there's only myosin. So I will label my H zone, sort of right here to here, and think what happens when it contracts. Alright. Well, when the muscle contracts, that actin gets pulled towards the center of the sarcomere, and as it gets pulled towards the center of the sarcomere, that space between it is going to get smaller and smaller and smaller. That space between it's the H zone, and eventually the actin's going to actually reach the middle of the sarcomere. That H zone will even disappear. So I'm going to write here what happens to the H zone. It gets shorter and I'll write, eventually, it disappears as that actin reaches the middle of the sarcomere. Alright. Next, we need to mark the Z disk and the M line with arrows. So we'll start with the Z disk. The Z disk, remember, Z is at the end of the alphabet, and our Z disks mark the ends of the sarcomere. So that's these lines right here. Here's one of them. Here's my other one, and I'm going to write here, this is the Z disk. Here is my other Z disk. And now, does the Z disk change size when the sarcomere contracts? Well, the Z disk is this structural component that's really there to anchor the actin, to hold the ends of the actin. And when the actin gets pulled in, that Z disk gets pulled closer to the middle of the sarcomere. That's how that sarcomere gets shorter. But the Z disk doesn't change size. It does move, but it doesn't change size. So here, does it change size? I'm going to write no change. Alright. And that brings us to the M line. Alright. The M line is this line right down the middle of the sarcomere. M, I remember, stands for middle, and so I'm going to draw an arrow right here. This is my M line. I'll just draw it out so I can write. This is my M line. The M line right down the middle, and it is anchoring the myosin. So if it's anchoring the myosin, it doesn't change size. It just stays right in the same place as that myosin pulls on the actin, and the actin gets closer and closer to the M line, but the M line, just like the Z disk on the right, no change. Alright. The last thing that we want to do here is that we want to draw a line to represent the length of 1 actin filament. Well, remember, the actin filament is going to go from the Z disk so right here let's change our color the Z disk right here is going to go over the I band, cross into the A band where it's overlapping the myosin, and it's going to reach as far as the edge of the H zone. Alright. The H zone is the region between the actin filaments. So that is my line. This is the length of 1 actin filament. I can draw it on the other side too just for fun. There we go. There we go. That's my actin. Alright. Understanding all these parts, how they relate to the proteins, and what happens to them when they contract, is often a fun test question that you're going to see. So I encourage you to practice it. We have more practice problems to follow. Give them a try.

The box in the image below surrounds what structure?

• A. A Band.
• B. I Band.
• C. Z Disc.
• D. M Line.

Which region is the same length as the actin filament? An image is shown for reference.

• A. A Band.
• B. I Band.
• C. From the edge of the H zone to the Z disc.
• D. From the Z disc to the M line.

What of the following is NOT found in the A band?

• A. M Line.
• B. H Zone.
• C. Z Disc.
• D. Areas with both actin and myosin.

Here’s what students ask on this topic:

What is the sliding filament theory of muscle contraction?

The sliding filament theory explains how muscles contract by describing the interaction between myosin (thick filament) and actin (thin filament). Myosin heads attach to actin filaments and pull them towards the center of the sarcomere, increasing the overlap between the filaments without changing their size. This process shortens the sarcomere, the fundamental unit of muscle contraction. Key proteins involved include tropomyosin, which blocks myosin binding sites on actin, and troponin, which, when bound to calcium, moves tropomyosin to allow myosin to bind to actin and initiate contraction.

What are the roles of tropomyosin and troponin in muscle contraction?

Tropomyosin and troponin are regulatory proteins that control muscle contraction. Tropomyosin is a thread-like protein that wraps around actin filaments, blocking myosin binding sites and preventing contraction. Troponin is a globular protein that binds to both actin and tropomyosin. When calcium ions are released from the sarcoplasmic reticulum, they bind to troponin, causing it to change shape. This shape change moves tropomyosin away from the myosin binding sites on actin, allowing myosin heads to attach to actin and initiate muscle contraction.

What is the structure and function of a sarcomere?

A sarcomere is the fundamental unit of muscle contraction, found within myofibrils in muscle fibers. It is defined by Z discs at each end, which anchor the actin (thin) filaments. The central region contains the M line, which anchors the myosin (thick) filaments. The sarcomere also features distinct bands: the I band (light, actin only), the A band (dark, overlapping actin and myosin), and the H zone (myosin only). During contraction, myosin heads pull actin filaments towards the center, increasing overlap and shortening the sarcomere.

How do the I band, A band, and H zone change during muscle contraction?

During muscle contraction, the I band and H zone decrease in size, while the A band remains the same. The I band, which contains only actin filaments, shortens as actin is pulled towards the center of the sarcomere. The H zone, which contains only myosin filaments, also shortens and can disappear entirely as actin filaments overlap more with myosin. The A band, which includes the entire length of the myosin filaments and the overlapping actin, does not change in length because the filaments themselves do not change size.

What is the role of calcium in muscle contraction?

Calcium plays a crucial role in muscle contraction by regulating the interaction between actin and myosin. When a muscle receives a signal to contract, calcium ions are released from the sarcoplasmic reticulum into the sarcomere. These calcium ions bind to troponin, causing it to change shape. This shape change moves tropomyosin away from the myosin binding sites on actin, allowing myosin heads to attach to actin and initiate the contraction process. Without calcium, the binding sites remain blocked, and muscle contraction cannot occur.