Sliding Filament Theory and the Sacromere - Video Tutorials & Practice Problems

Sliding Filament Theory

Now that we understand the structure of muscles from the whole muscle all the way down to the Sarre that fundamental unit of muscle contraction. We can start to answer the question of how do muscles contract? How does that sarcoma actually get shorter to do that? We're gonna start by talking about the sliding filament theory of muscle contraction and sliding filament theory is a really excellent conceptual framework for how muscles contract. Eventually. We're gonna learn all the molecules involved in 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 now know those filaments are Mycin and Acton, but back then, they didn't know they were Mycin and Acton. So they called the Mycin the thick filament and for obvious reasons, it was thicker than the other filament. Now, Mycin and the thick filament, those can sometimes be used interchangeably. So, so you should be familiar with that term, the thick filament, the thick filament or the Mycin that's gonna be anchored to the center of the Sarker and sort of reaching out in both directions. And now we now know the Mycin is this big protein with all these sort of arms and hands that are reaching off of it. Each Mycin has 300 arms or hands reaching off though. Technically, they're called the Mio and heads. But think of them like arms and hands and they are reaching out because my ain is this protein and we're just gonna say that my has one love is a protein that wants to pull on a rope. If you give me the chance, it's gonna grab onto a rope and it's just gonna pull hand over, hand on a rope. That's our Mycin. Then our Acton. Well, our Acton is gonna be the thin filament. You can remember that because Ain is thin and well, if Mycin was anchored to the middle of the sark, the act, the Acton is anchored to the ends of the Sarkar. And if my just wants to pull on a rope while the Acton, that's the rope. So let's now just step back before we go on here. And we're gonna look at an animation of this. And so remember this is all happening in three dimensions in these structures called the myofibril. We have these repeated Sarco mares. And so we're gonna zoom in on one sarcoma here and here. You can see the thick filaments, those thick myo filaments, those are the Mycin and you can see all those little heads or little hands reaching out and they're gonna start pulling on the Acton, those thin myo filaments. And as they do that, you can see the Acton got pulled towards the center of the sarcomere and the sarcomere, our fundamental unit of muscle contraction, it got shorter. All right. So let's look at that again. Now, the thing that I want you to notice here is as this mia as these mias and these thick filaments, they're gonna reach out and they're gonna pull on the Acton hand over hand, the Acton sliding in the Myo and the Acton are not changing size. The Acton is sliding in towards the center of the SAR mirror. So the amount that they overlap is increasing and the sarcoma is what gets shorter, the Acton slides over the mycin, hence the name the sliding filament theory of muscle contraction. OK. So let's go back to our page here. So to sort of build this out this analogy out a little bit more we have drawn down here. These guys in purple, these are our Mias ands and they're anchored to the, to the center of the Sarka and they're facing out in both directions ready to pull on this rope, Miss Acton, this Acton is the rope. And importantly, you'll notice at the start, this Acton is just overlapping the mice in just a little bit. It sort of ends at each at the hands here and another one starts and then it ends here and it's just a little bit of overlap. Remember that Acton or the thin filament, it's anchored to the ends of the Sara to mark the ends of the sarcoma. We have these knots and like you might see in a tug of war. So these are the ends of the Sar Comar. So here see here, this would be 123 sarcomas that we're looking at. Now as the Mio as these mice and heads or these arms and hands reach out and they start pulling hand over hand. Now, you can see what happens here. The Acton does not change size, what happens the rope gets pulled past and now there is more overlap. The mice and the Acton are the same size, there's more overlap. And when you look at it, we draw in the ends of the sarco mis here, you can see each sarcoma gets a little bit shorter, but because each guy is linked back to back, they gotta stay back to back because one Acton is linked to the Acton in the next sarcoma. When each sarcoma gets a little bit short, shorter, the entire myofibril, it's a lot shorter. So if we look over here, we have our sort of more formal drawing of a sarcoma here. And you can see in purple, we have the thick filaments out reaching out from the middle of the sarcoma. We have the thin filaments in yellow. These are the acting filaments attached to the ends of the Sarcoma there and we can draw in the ends of the SARC mirror just like we did before. And here it's sort of this zigzagging line and we're gonna name that zigzagging line a little bit more formally later on. But just for now, that's the end of the Sarre. And importantly, you can see that when we start here, there's just not a lot of overlap between the Mycin and the acting a little bit, but not a lot. Now, Myson pulls hand over hand, everything slides in. I'm gonna draw in my new or my edges of my sarcoma here down here as well. And you can see each sarcoma it's 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 Acton and the Mycin. That's what increased. The filaments did not change size. The sarcoma 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 gonna say that during contraction, the filaments, the Actinomycin, they stay the same size, they slide past each other and when they slide past each other, that overlap of the Acton and Mycin increases, and I'm gonna 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 gonna go into all those step by step, details and molecules involved in a lot more detail coming up. Well, first, like always we have an example, practice problems will follow. I'll see you there.

Sliding Filament Theory and the Sacromere Example 1

Our example tells us that the image below shows multiple Sarker. And then it asks us to draw arrows to label at least two myo filaments, draw arrows to label at least two acting filaments. And then asks us the question when the muscle contracts, which filament gets closer to the center of the sarcoma. So as I look at this image, I see this sort of long uh drawing here showing all these protein filaments. And I see this repeating pattern and that repeating pattern is the sarcoma. So the first thing I wanna do just to orient myself to this picture, I'm gonna draw the ends of the sarcoma so I can see the individual sarcoma. So the ends of the sarcoma are these sort of zigzagging lines here. I'm just gonna draw a straight line over it right. Here's another zigzagging line, those zigzagging lines we haven't defined yet, but those are gonna be called the Z disks and those mark the ends of the Sarker. So as I look here, I now have 1234 Sark mis 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 in on this sarcomere number three 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 uh sort of like many arms that are kind of reaching out or the, the heads that are reaching off of them. And then I have these yellow filaments that are attached to the ends of the Sark mire. All right. So one of those, the Mycin one is the acting. Do you remember? Which is which? All right. Well, the Mycin was the thick filament and we said in our story, right, that the Mycin 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 and keep going. There's another one, there's another one. All right, those horizontal purple filaments there. That's the mio that means the Acton is those yellow filaments. Remember the yellow filaments? The Acton, that's like the rope that the my is gonna pull on. So there is one, there is one, there's one said just label two. We got three. We're doing great. So that's the acting. All right, Myelin, that's uh located at the center of the Sarcomere. The Acton is attached to the ends of the sarcoma. So that means when he asks this question down here, which filament gets closer to the center of the sarcoma. Well, the center of the sarcoma is right here. The ends are out here that my is gonna grab on that act and it's gonna pull it in and that Acton is gonna slide closer to the center of the sarcomere. So my answer is the Acton. All right. Like always we have more problems to follow and then we're gonna go into the proteins in the sarco me 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 Sarker 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 mio pulls on the acting and we can look at our image of a sarcomere here. We, we can see it in more detail than we've seen before. These zigzagging lines. These represent the end of the sar mere. We're gonna define those a little bit more coming up later. Uh But we can see one sarcomere here and we see all the filaments and the proteins we're gonna talk about inside there. And remember this sarcoma is this repeating structure where to make up one myofiber, you have tens of thousands of these in a row and all of these are just gonna get a little bit shorter when the muscle contracts. Now to do that, the first proteins we're gonna talk about is the mycin and the Acton. And remember the Mycin, this is the thick filament. And we can see that here reaching out from the center of the Sar comme these purple filaments and they have all these myo heads sticking off of them and those myo heads, remember we said those are like little arms that are reaching out to grab onto the Acton to pull on it now because they're called mias and heads. One sort of memory tool that I've heard before that I really like is that Meac is this many headed medusa? Myson has all these heads just like the snakes off medusa's heads reaching out and they're reaching out to grab onto the Acton. Now, remember the Acton, that's the thin filament and our memory tool for that ain is thin. And we can see reaching out from the ends of the sarcoma. We have the Acton in this sort of yellowish orange here. And it's the important thing to note here is that it's overlapping the Mycin but not by a lot. It's just overlapping just a little bit here. The Mycin heads so those Mycin heads can pull on the act. Now, we've said before that Mycin just wants to pull on act and 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 mycin pulls? Well, to see that we're gonna zoom in on an acting filament here. And you'll notice that there's some regulatory proteins on here. There's this green thread like protein that's wrapped in around the Acton and then there's this sort of blue globular protein there. So the first one that green protein we're gonna call Tropomyosin and Tropomyosin, we're gonna call a thread like protein. And you can see it wraps around the Acton and you can see the acting here. It's actually Acton is made of these smaller subunits. So, Acton is sort of almost like these beads on a string. It's really sort of like these two 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 at on that are the Acton looks like they have little holes in the beads, those drawn in, those are the myo binding sites on the Acton. So that's the place that the myo can bind to pull on the Acton. But you'll also notice that this Troppo Mycin, this thread like protein, it's in front of all those little binding sites. So the Troppo Mycin is really there because it blocks the Mycin binding sites on the act. As long as that Tropomyosin is there that Mycin 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 I by that, I just mean that, well, all our other proteins here are like threads or filaments. Uh but this is just a smaller protein and it's gonna be bound to both the acting and to the Tropomyosin and it's also gonna 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 gonna release calcium into the myofibril into the sarcoma. And that calcium is going to bind to the troponin. And when that happens, that troponin is just gonna sort of change its shape just a little bit and it's going to open the binding sites on the Acton by moving the Troppo Mycin. It's just gonna sort of pull that Troppo Mycin just a little bit out of the way as long as it's bound to calcium. Now, we're gonna go through the step by steps of how all of this works in more detail going on. Right now. We really just wanna remember what the proteins are where they are and their basic rules. Finally, for these, we have a little bit of a memory tool. I say that Troppo Mycin says no to the Myo. So the Tropomyosin is that thread like protein blocking the binding sites. The Mycin wants to bind the Tropomyosin says no to the Mycin. In contrast, we have, the troponin is going to open the binding site and the calcium binds to it. It's gonna pull that Tropomyosin out of the way and open the binding site for the Mycin. So the muscle and the Sarre can contract. All right, 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 Sarre and comes out lined up with the myo filament, actually, sorry interacts with the myo filament and goes all the way to the middle of the SARC mere just like that on, on both sides as well. So this elastic filament is made of the protein titan and it is there to help the Sarker retain its shape. It's an elastic protein and the sarcoma is always, you know, con 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 rather because it's stretched or whether it's becau because it contracted. So the Titan keeping the shape, helping it retain its shape, return to its original size. But you'll also notice this word Titan that sounds like it's a really big protein. It is fun fact about pro uh titan that you may need to know because it's such a fun fact. It is the world's largest known protein. Now, you may look at this picture and say, hey, that mycin filament looks a lot bigger than the titan. Well, all together the Mycin filament is bigger. But remember, the Mycin filament is made up of 300 plus smaller Mycin proteins that come together to make this filament, the titan, that's a single amino acid sequence and it is something like 30,000 amino acids make up the titan protein. Now, for comparison, hemoglobin, the protein in your blood that carries oxygen around it has four subunits and all four subunits put together. It's a little under 600 amino acids. So, 600 amino acids in hemoglobin, 30,000 in Titan. It's just, it's, it's a big protein. All right. Again, we're gonna learn all the step by step mechanism of contraction. Coming up before we get there though. We're gonna look at the sarco me one more time. We're gonna look at what it looks like through an electron microscope and we're gonna line these proteins up to that picture. I'm looking forward to it and I'll see you there.

Sliding Filament Theory and the Sacromere Example 2

Our example asks us to imagine whether the mycin binding sites on the Acton will be more likely to be exposed or blocked in the following two scenarios. Likewise, we want to predict whether you would expect overall mycin binding to increase or decrease in each scenario. And in images shown for reference, right, we've seen this image before in orange, this is the Acton and in green, we see the troppo mycin wrapping around the Acton and then in blue, we see these troponin molecules. All right. So our first scenario is going to be imagine that there is no troppo mycin present on the act in filament, right? If there's no troppo mycin, would you expect the Mycin 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 Mycin because it's blocking the binding site. So if you take it away, well, those binding sites are just, they're just gonna be exposed, there's nothing left to block them all right. If you remove the troppo Mycin, if you expose the binding sites is binding, going to increase or decrease. Well, mycin, if it gets the chance is gonna bind. So if the binding sites are exposed, that is going to increase the amount of binding, there's nothing there to stop it. All right. Our next scenario is to imagine that the calcium binding sites on the troponin molecules are nonfunctional. All right, if the calcium binding sites on the troponin molecules are nonfunctional, do you think mycin binding sites would be more likely to be exposed or more likely to be blocked? All right. 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 trop mycin. So if the calcium can't bind the troponin, can't move the trom mycins, that means that those mycin binding sites are going to remain blocked. And if the mycin binding sites are blocked, well, can the mein bind? Heck no. So binding is going to decrease. All right. So that's the roles of troponin and tram mycin. 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.

Sliding Filament Theory and the Sacromere Example 3

Our example, gives us an image of a sark mirror 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 brackets and then label them. We need to mark the Z disc and the M line with arrows and to label them. And then we need to draw a line to represent the length of one act in filament. And finally, we need to describe if and how each component changes in size during a muscle contraction. All right, filling out this table is to, to talk about the size. I'm actually gonna do that as we go. So let's go ahead and get started. We have that a band. All right. 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 Sar Mi. So I can draw the edges of where it's dark, you know, sort of roughly there to about here and I'll draw my back bracket there. That's gonna be my A band. And I remember that because the vowel in dark is a So the dark band is the A band. All right. 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 Acton overlap. But really, it's, it's defined in size by where the MYO is. So the MYO is attached to the middle of Sark Mare 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 act in the Mycin don't change size, they slide past each other. So if our A band is the region where the Mycin is and that Mycin doesn't change size, that means when the muscle contracts, then in the A band, we expect no change in size. All right. Next, let's look at the I band. All right, if the A band is the dark band, well, the I band, I'm gonna 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 IBA. And remember I remember that because I is the vowel in light. So the I band is the light band. So does the eye band change size when the muscle contracts? Well, the I band we said is the region where there's only Acton and the Acton when the muscle contracts. Remember that my is gonna pull on the Acton and it's gonna slide towards the middle of the Sark mirror. And more and more of the Acton is going to be overlapping the Myo. So if more and more of the My Acton is overlapping the Mycin, well, then less and less of it isn't overlapping the Mycin. That means that I band that region where there's only Acton is going to get shorter. So, so I'm gonna write here gets shorter. All right, that brings us to the H zone. The H zone is this region in the middle of the Sar Kamir, 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 Acton comes in, it overlaps the Mycin but doesn't overlap all of the Mycin. Usually there's gonna be a region between those Acton filaments and it's a little bit brighter because that's where there's only MYO. So I will label my H zone sort of right here to here and think what happens when it contracts. All right. Well, when the muscle contracts, that Acton gets pulled towards the center of the Sar Mare and as it gets pulled towards the center of the Sark mare, that space between it is gonna get smaller and smaller and smaller that space between it's the H zone. And eventually the Acton is gonna actually reach the middle of the Sara, that H zone will even disappear. So I'm gonna write here. What happens? The H zone, it gets shorter and I'll write, eventually it disappears, it gets shorter and eventually disappears as that acting reaches the middle of the Sara. All right. Next, we need to mark the Z dis and the M line with arrows. So we'll start with the Z dis, the Z disk. Remember Z is at the end of the alphabet and our Z diss mark the ends of the Sarker. So that's these lines right here. Here's one of them. Here's my other one. And I'm gonna, right here. This is the Z disk, here is my other Z desk. And now does the Z disk change size when the Sarker tracks? Well, the Z disc is this structural component that's really there to anchor the Acton to hold the ends of the Acton. And when the Acton gets pulled in that Z disk gets pulled closer to the middle of the Sarcoma, and that's how that Sara gets shorter, but the zisk doesn't change size, it does move but it doesn't change size. So here does it change size? I'm gonna write no change. All right. And that brings us to the M line. All right. The M line is this line right down the middle of the Sara. Mi M I remember stands for middle. And so I'm gonna 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 Mycin. So if it's anchoring the Mycin, it doesn't change size, it just stays right in the same place as that Mycin pulls on the act and, and the acting gets closer and closer to the M line. But the M line just like the Z disk on. Right. No change. All right. The last thing that we want to do here is that we want to draw a line to represent the length of one Acton filament. Well, remember the Acton filament is gonna go from the Z disk. So right here, let's change our color, the Z disk right here. It's gonna go over the I band cross into the A band where it's overlapping the myo and it's gonna reach as far as the edge of the H zone. All right, the eight zone is the region between the act and filaments. So that is my line. This is the length of one act in show. All right, I can draw it on the other side too. Just for fun. Here we go. There we go. That's my acting. All right, 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 gonna 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.