Skeletal Muscle Contraction: Study with Video Lessons, Practice Problems & Examples

Skeletal muscle contraction occurs through the actomyosin cycle, involving myosin and actin interactions. The cycle consists of five steps: myosin binds to actin without ATP, ATP binding releases myosin, ATP hydrolysis cocks the myosin head, inorganic phosphate release strengthens binding, and the power stroke shifts actin towards the M disc. Regulation is controlled by troponin and tropomyosin, which block myosin binding sites until calcium is released from the sarcoplasmic reticulum, allowing contraction. Understanding this cycle is crucial for grasping muscle function and relaxation mechanisms.

Skeletal Muscle Contraction

In this video, we're going to begin our discussion on skeletal muscle contraction. So the sliding filament model is what describes the nature of a contracting sarcomere. If we take a look at our image down below, notice on the top here what we have is a relaxed muscle. We have this extended arm and the biceps and the triceps are in a relaxed position. The relaxed muscle that we see here is going to correspond with a relaxed sarcomere. Notice up above what we have is a relaxed sarcomere, the same image of the sarcomere that we had in our last lesson contracted here with this guy that's waving high to us and of course the contracted muscle is going to correspond with a contracted sarcomere and that's exactly what we have right here. Notice we have a contracted sarcomere which is smaller in length than the relaxed sarcomere which is longer in length. If we closely compare the relaxed sarcomere with the contracted sarcomere down below, you'll notice there are few regions that are getting smaller in size. The first is going to be the H zone, which recall H is a thick letter. The H zone only contains the thick myosin filament without any thin actin filaments whatsoever. The H zone here in the relaxed muscle is quite large, in comparison to the H zone down below, which is much, much smaller in the contracted muscle. The H zone definitely reduces in size upon contraction. But notice also the I bands, which because I is a thin letter, it only contains the thin actin filaments and no thick myosin filaments. There's an I band on either side. They are also much larger in the relaxed muscle in comparison to the I bands in the contracted muscle, which are much smaller. You can see these dotted lines here are showing the narrowing of the I bands, as well as the narrowing of the H zone. Up above, we can say that it's the H zone and the I bands that are going to reduce in size when it comes to a contracting sarcomere. What helps me remember that it's the H zone and the I bands that reduce in size is that HI says, hi. Whenever you go to wave hi to someone like this, your muscles contract and, of course, the H zone and the I bands are also going to contract and reduce in size. Hopefully, that'll help you guys remember, that it's the H zone and I bands that reduce in size.

Now, down below in this text, we're describing the nature of the sliding filament model further, which is that the thick myosin filaments that we see in red up above are going to pull the thin actin microfilaments which are the structures in green that we see. The thin actin filaments are going to get pulled towards the M disc which is this blue disc that we see here in the center. We can see that these green arrows are showing that the actin filaments on either side here are getting pulled towards, the M disc here in the middle. During a muscle contraction in order to shorten the muscle and so you can see that's why we have this shorter sarcomere here upon contraction. What's also very important to note is that the A band which recall is the length of the entire thick myosin filament, actually does not reduce in size and it does not change during a muscle contraction. Notice that the length of the thick myosin filament indicated by the A band here does not change when it comes to a muscle contraction. That's very important to note. But what you should also note is that the Z discs are going to get pulled closer to the M disc. Just like the actin gets pulled closer to the M disc. Recall that the Z discs are these purple structures that we see on the end. Notice that the purple structures, in comparison to being up above, they're getting pulled in the direction closer to the M disc. The reason for that is because, recall from our previous lesson videos that the actin filaments here in green are actually anchored to the Z discs. If the actin is getting pulled closer to the M disc, then that means that by default, the Z disc is also going to get pulled closer to the M disc. That's exactly what we're describing here in these two lines. It's also important to note that the volume of the muscle is actually unchanged, during a muscle contraction. There's no change in the volume, but the muscle does become shorter upon contraction. Really, the main takeaways here about the sliding filament model that describes the nature of the contracting sarcomere is that the H zone and the I bands are going to reduce in size, when it comes to a contracting muscle. And then the A bands, the A band, does not reduce in size. Now that we've covered the sliding filament model, we'll be able to talk more about the biochemical mechanism of a contracting sarcomere in our next lesson video. I'll see you guys there.

Which statement best describes the sliding filament model of a sarcomere contraction?

• A. The A band and the H zone both become smaller.
• B. The I bands gets smaller but the H zone remains the same.
• C. The I bands gets smaller while the A band gets larger.
• D. The I bands and H zone get smaller while the A band remains the same size.

Skeletal Muscle Contraction

Alright. So in this video, we're finally going to talk about the biochemical events that lead to muscle contraction. These biochemical events occur in a cycle known as the actomyosin cycle. Actomyosin is a word that seems to be a merge of actin and myosin. It describes how actin and myosin interact with each other to create muscle contraction. The actomyosin cycle can really be described in 5 general steps, and it is a cycle, so it's going to begin and end at the same place. It's a 5-step cycle of biochemical events that results in only the contraction of a sarcomere. We've got these 5 steps numbered down below in our text, and notice that the numbers in the text correspond with the numbers that we see down below in our image. Now, I'll admit, at first glance, this image looks pretty complicated, but I can tell you it's definitely not as complicated as it looks. Let's go ahead and get started here with step number 1, which is really for us to just realize that without any ATP, the myosin head is going to bind tightly to the actin microfilaments. Notice at the top, we have a relaxed sarcomere and notice that, we're zooming into this specific region of the sarcomere. All of the images of the actomyosin cycle are just a zoom-in of this right-hand side of the sarcomere. It's important to keep in mind because in all of these images down below, to the right of the image, we're going to find the Z disk and to the left of the image, we're going to find the M disk. Again, because we're zoomed into this little area right here in all of these images. And so, of course, with step number 1 here in this first box, let's remind ourselves that without ATP, myosin heads are going to bind tightly to the actin microfilaments. So zoomed in, what you can see is we have the myosin filament up above in red, and you can see the myosin head is right here. And then down below, we have the actin microfilaments. And so notice that there's no indication of ATP whatsoever in this box. Without ATP, the myosin head here is going to bind very tightly to the actin microfilament and that's exactly what we see in this image. So really that's it for step number 1. So we can move on to step 2, which is of course going to include ATP. In the presence of ATP, it will bind to the myosin head and disrupt the actin-myosin interaction. If the actin-myosin interaction is disrupted, that's going to cause the myosin head to release the actin. So, let's take a look down below at our image to visualize this. In step number 2, ATP is coming into play. The ATP is going to bind to the myosin head like we see here and disrupt the interaction between the myosin head and the actin microfilaments. You can see that causes the myosin head to release the actin upon ATP binding and that's exactly what we see here. The ATP, the myosin head has released the actin And really that's it for step number 2. So we can move on to step number 3. In step number 3, what we need to realize is that the myosin head is capable of hydrolyzing the ATP molecule that it bound into ADP and inorganic phosphate. The ATP hydrolysis is actually going to cause the myosin head to change confirmations to a high-energy state. Really, this ATP hydrolyzation is going to cause the myosin head to cock backwards into this high-energy state and weakly interact with the actin closer to the Z disk. And so, if we take a look at step number 3 down below, let's remind ourselves, in step number 2, the myosin head is bound to ATP. So in step number 3, the myosin head that was bound to ATP is going to hydrolyze the ATP into ADP and inorganic phosphate. And so when it does that, it cocks back the myosin head. ATP hydrolysis cocks back the myosin head into a high-energy state, where the myosin head is going to be cocked back towards the Z disk. That's because recall that in all of these images, we are zoomed into this little region right here. So if we're getting cocked back to the right, here, like we are here in this image, the right is going to be towards this Z disk over here. And so, that's why we're saying it's getting cocked back towards the Z disk. Really this cocked back position, you can think of it like a slingshot. So the myosin head is getting cocked back and pulled back just like this slingshot here is getting cocked back. And so it's in a high-energy state, just like the myosin head. And so you can see that the myosin head here is going to weakly interact with the actin, and so they're not quite strongly bound. You can see we have a dotted line here to represent this weak interaction and so really that's it for step number 3, the cocking back of the myosin head. So in step number 4, what happens is the myosin head releases the inorganic phosphate that it hydrolyzed, that was the result of the hydrolysis of ATP. So, the release of the inorganic phosphate causes the myosin head to strengthen its binding to actin. So if we take a look at step number 4, which is right here, we can see that the inorganic phosphate is getting released. So, in step number 3, the ATP hydrolyzation led to ADP and inorganic phosphate. And so the inorganic phosphate, not the ADP, is released first. So the inorganic phosphate is released. So the myosin head releases the inorganic phosphate, and that allows the myosin head to bind to the actin much stronger and so you can see that the myosin head here is bound strongly to the actin. And really that's it for step number 4. And so in step number 5, which is really our last step here, what happens is the myosin power stroke is going to follow the release of inorganic phosphate. And so the power stroke is literally just a movement of the myosin head. So this power stroke is going to pull the actin that it's strongly bound to at this point. It's going to pull the actin towards the M disk and return the myosin back to the original state that it was in back in step number 1. And so if we go down to step number 5 down below, what you can see is that the myosin power stroke is going to follow the release of the inorganic phosphate, and the power stroke is just this movement of the myosin head from the cocked back position, forwards, towards the M disk. And so because the myosin head moves towards the M disk and it's strongly bound to the actin, it's also going to shift the actin in this direction to the left, which recall zoomed into this area right here and moving to the left is going to bring us closer to the M disk. And so the myosin power stroke will shift the actin towards the M disk. And so, that is going to allow, up above in our image here, the actin is going to be moved in this direction and that is going to allow for the muscle contracting, contraction, the contracted muscle. And so, what follows this power stroke are that ADP is going to be released, and the cycle is going to be repeated until the myosin binding sites on actin are blocked. And so, down below in our image, notice that the power stroke leads to the ADP molecule being released, which we can see here and then, of course, that brings us back to the beginning of the cycle where this actin can be fur...

Skeletal Muscle Contraction

Alright, so in this video, we're going to talk about a summary of the actomyosin cycle, and here in this table, we have that summary. Notice that in the first column, we have the steps 1, 2, 3, 4, and 5 and then in the second column, what we have are the details of each step that you need to know. Of course, in the first step, what we need to know is that if there's no ATP, then you are going to bind to me. The "you" is really just going to be the myosin head, and of course, the "me" is going to be actin. If there's no ATP, then the myosin is going to bind to the actin. We can see that when there is no ATP, the myosin head is bound to the actin.

Now, in step number 2, of course, ATP is going to come into play. The ATP binds, and the myosin is released. That's exactly what happens in step number 2: ATP binds to the myosin head, and the myosin releases the actin. In step number 3, the myosin is going to hydrolyze the ATP. ATP hydrolysis cocks back the myosin head into a high-energy state. That's exactly what we see in step number 3: the myosin head is cocking back into this high-energy state. When it gets cocked back, you can think of it like a slingshot getting cocked back into its high-energy state.

Then, of course, step number 4 is going to be the inorganic phosphate is released, and that's exactly what we see here in step number 4. The inorganic phosphate is released, and that allows the myosin head to bind tightly to the actin filament. In step number 5, of course, the power stroke is going to follow, which is going to shift the actin, and then ADP is released to regenerate the original myosin. From step number 4, the power stroke occurs, and the power stroke shifts the actin, and then ADP is released so that the cycle can restart back at step number 1.

By using this table here to condense the steps, you can begin to familiarize yourself with the actomyosin cycle. That concludes this video, and I'll see you guys in our next one.

Fill-in the blanks with numbers (1-7) to put the events of the actomyosin cycle in order from beginning to end:

a) _____: Myosin releases P_i.

b) 1 : Myosin binds ATP.

c) _____: Myosin head bonds tightly to thin actin filament.

d) _____: Myosin power stroke occurs.

e) _____: Myosin-actin interaction is broken.

f) _____: Myosin head pivots to a high-energy state.

g) _____: Myosin hydrolyzes ATP to ADP and P_i.

So this practice problem wants us to fill in the blanks that we see down below with the numbers 1 through 7 in order to put the events of the actomyosin cycle in order from beginning to end. Notice that the first event is given to us as the myosin binds ATP. We know from our last lesson video that when there's no ATP, then you bind to me. This implies that the myosin is going to bind to the actin when there's no ATP. But if there is ATP, then it's going to bind to the myosin head and break the myosin-actin interaction. This is going to be step number 2. After the myosin-actin interaction is broken, the myosin head is released from the actin. The next step is for the myosin head to hydrolyze the ATP that it just bound into ADP and inorganic phosphate. The hydrolysis of the ATP leads to the myosin head pivoting or cocking back into a high-energy state. After the myosin head cocks back into a high-energy state, the next step is to release the inorganic phosphate. The myosin head then releases the inorganic phosphate that resulted from ATP hydrolysis. Releasing the inorganic phosphate actually strengthens the myosin head's bond to the actin filament. After the inorganic phosphate is released, myosin binds tightly to the actin filament. Shortly after, the myosin power stroke follows, pulling the actin filaments towards the M disc and allowing for a muscle contraction to occur. These are the correct order of the events in the actomyosin cycle from beginning to end. That concludes this practice problem, and I'll see you guys in our next video.

Skeletal Muscle Contraction

So now that we know that muscle contractions occur via the actomyosin cycle, in this video, we're going to talk about how muscle contraction is regulated by 2 proteins, troponin and tropomyosin. What's interesting to note is that myosin and actin together make up about 80% of the protein mass in a muscle fiber cell, but the remaining 20% consists mainly of 2 other types of thin filaments, which are troponin and tropomyosin. Troponin and tropomyosin actually form a complex with each other, and the role of this troponin-tropomyosin complex is to bind to actin and block the myosin binding sites to allow for muscle relaxation. What helps me remember that the troponin-tropomyosin complex binds to actin and not to myosin is that the 't's here in troponin and tropomyosin match with the 't' in actin. Notice myosin does not have any 't's. This is how I know that troponin and tropomyosin complex binds to actin.

Let's take a look down below at our image and notice over here on the left-hand side, the green actin microfilaments are represented by these green balls, and the myosin heads are represented by these structures. Notice that troponin is this small light purple protein, and tropomyosin is this much longer protein that we see extended right in these positions. What helps me remember that troponin is this small guy and tropomyosin is the longer guy is that troponin is a much smaller word than tropomyosin, which is much longer. What's important to know is that this long tropomyosin is actually blocking the myosin binding sites on actin. If myosin is not able to bind to actin, we know from our previous lesson videos that the actomyosin cycle is not going to be able to occur. If the actomyosin cycle cannot occur because these myosin heads are being blocked from binding, then that's going to prevent muscle contraction and allow for muscle relaxation.

This entire side over here represents muscle relaxation. The troponin-tropomyosin complex will actually regulate muscle contractions in a way that the muscle contractions will only occur with nervous system signals from our brains and spinal cords. Upon receiving a nervous system signal, a myofibril's sarcoplasmic reticulum is stimulated to release calcium. Recall from our previous lesson videos, we mentioned that the sarcoplasmic reticulum was going to be important for a muscle contraction. This is exactly why it's because the sarcoplasmic reticulum is stimulated to release calcium upon receiving a signal from the nervous system. Troponin's role is really to bind to the released calcium from the sarcoplasmic reticulum. When the calcium binds to the troponin, it causes a conformational change in the entire troponin-tropomyosin complex. Tropomyosin's change is going to expose the myosin binding sites that it was once blocking. Once the myosin binding sites are exposed, that is going to allow for the actomyosin cycle to initiate and lead to muscle contraction.

Let's take a look down below at our image to tie together everything that we just talked about. Of course, this whole troponin-tropomyosin complex here is going to help us regulate our muscle contractions so that they only occur with our nervous system signals. So here we have the brain and spinal cord to represent the nervous system. When a myofibril receives a signal from the nervous system, the sarcoplasmic reticulum, which is this yellow membranous structure here, is going to be stimulated to release calcium, which are these yellow circles that we see here in this image. The release of calcium is going to lead to calcium binding to the troponin. You can see the little yellow dot here is bound to the troponin, and that causes a conformational change in the entire troponin-tropomyosin complex. Notice that the tropomyosin is no longer blocking the myosin binding sites, and instead, the myosin binding sites indicated by these little positions on the actin are exposed. What we can say is that calcium, these little yellow circles here, are going to bind to troponin and allow for muscle contraction to occur because now notice that the myosin heads are bound to the actin and of course, if the myosin heads can bind to the actin, then that's going to allow for the actomyosin cycle to occur, leading to muscle contraction. The complex is only going to allow for muscle contraction upon receiving a signal from our nervous systems. Over time, the calcium that was released by the sarcoplasmic reticulum is going to be returned to the sarcoplasmic reticulum, therefore decreasing the concentration of calcium. Low calcium concentration means that calcium is going to dissociate from the troponin and, of course, go back into the sarcoplasmic reticulum. This means that the troponin-tropomyosin complex is going to revert back to its position where it was blocking the myosin binding sites, and this is what allows for muscle relaxation to occur. Overall, now that we understand the actomyosin cycle and troponin-tropomyosin regulation, we understand muscle contraction and muscle relaxation.

That concludes this video, and we'll be able to get practice applying the concepts that we've learned in our next couple of videos. So, I'll see you guys there.

Muscle contraction is directly caused by:

• A. Conformational changes in actin.
• B. Conformational changes in myosin.
• C. Conformational changes in the A band.
• D. Conformational changes in the Z disk.

Which of the following does not occur during a muscle contraction?

• A. 'Power stroke'; the thick filament pulls the actin thin filament towards the M line.
• B. ATP is hydrolyzed; the heads of myosin shift into a high energy state.
• C. ATP binds to myosin heads; increases the affinity of the myosin head for actin.
• D. The myosin head rebinds to the actin closer to the Z disk prior to the power-stroke.
• E. All the above occur.

Which of the following is false concerning the sliding-filament model of muscle contraction?

• A. The myosin head hydrolyzes ATP causing a conformational shift of the myosin head.
• B. When a muscle shortens or lengthens, the H zones and I bands of sarcomeres change in size.
• C. Neither the thick or thin filaments change in length during a muscle contraction.
• D. After ATP hydrolyzation, the myosin head first releases P_i before it releases the ADP.
• E. Actin detaches from the myosin head with energy from ATP hydrolysis.

Which of the following statements correctly describes the relationship between cytosolic [Ca2+] and the corresponding sarcomere response?

• A. Increasing [Ca²⁺] causes troponin to bind to tropomyosin.
• B. Increasing [Ca²⁺] causes movement of tropomyosin, exposing myosin-binding-sites on actin.
• C. Decreasing [Ca²⁺] promotes interactions between actin and myosin.
• D. Increasing [Ca²⁺] causes troponin and tropomyosin to bind to actin.
• E. Increasing [Ca²⁺] causes dissociation of Ca²⁺ from troponin.

Here’s what students ask on this topic:

What is the sliding filament model of muscle contraction?

The sliding filament model describes how muscles contract by the sliding of actin (thin) filaments over myosin (thick) filaments within the sarcomere, the basic unit of a muscle fiber. During contraction, the H zone and I bands decrease in size, while the A band remains constant. Myosin heads bind to actin, forming cross-bridges, and pull the actin filaments towards the M line, shortening the sarcomere. This process is powered by ATP and results in muscle contraction. The Z discs, which anchor the actin filaments, are pulled closer together, leading to the overall shortening of the muscle fiber.

What are the steps of the actomyosin cycle in muscle contraction?

The actomyosin cycle consists of five steps: 1) Myosin binds tightly to actin in the absence of ATP. 2) ATP binds to the myosin head, causing it to release actin. 3) Myosin hydrolyzes ATP to ADP and inorganic phosphate (Pi), cocking the myosin head into a high-energy state. 4) The release of Pi strengthens the myosin-actin binding. 5) The power stroke occurs, moving the actin filament towards the M line, and ADP is released, returning myosin to its original state. This cycle repeats, leading to muscle contraction.

How do troponin and tropomyosin regulate muscle contraction?

Troponin and tropomyosin regulate muscle contraction by controlling the access of myosin to actin binding sites. In a relaxed muscle, tropomyosin blocks these sites on actin. When a muscle receives a signal from the nervous system, calcium ions are released from the sarcoplasmic reticulum and bind to troponin. This binding causes a conformational change in the troponin-tropomyosin complex, moving tropomyosin away from the binding sites on actin. This exposure allows myosin heads to bind to actin, initiating the actomyosin cycle and leading to muscle contraction.

What role does ATP play in muscle contraction?

ATP plays a crucial role in muscle contraction by providing the energy needed for the actomyosin cycle. Initially, ATP binds to the myosin head, causing it to release actin. The myosin head then hydrolyzes ATP into ADP and inorganic phosphate (Pi), which cocks the myosin head into a high-energy state. The release of Pi strengthens the myosin-actin binding, and the subsequent power stroke moves the actin filament towards the M line. Finally, ADP is released, and the cycle can repeat as long as ATP is available, enabling continuous muscle contraction.

What happens to the sarcomere during muscle contraction?

During muscle contraction, the sarcomere shortens as the actin filaments slide over the myosin filaments. The H zone and I bands decrease in size, while the A band remains constant. The Z discs, which anchor the actin filaments, are pulled closer together, reducing the overall length of the sarcomere. This shortening of multiple sarcomeres along a muscle fiber leads to the contraction of the entire muscle. The sliding filament model explains this process, where myosin heads form cross-bridges with actin and pull the filaments towards the M line, powered by ATP.