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 Sark Amir. And so if we take a look at our image down below notice on the top here, what we have is a relaxed muscle. So we have this extended arm and the biceps and the triceps are in a relaxed position. And so the relaxed muscle that we see here is going to correspond with a relaxed Sark Amir. So notice up above. Over here, what we have is a relaxed Sark Amir, which is the same image of the SAARC Amir that we had in our last lesson video. Now notice down below. What we have is a contracted muscle. And so the bicep and the triceps are contracted here with this guy that's waving hi to us. And so, of course, the contracted muscle is going to correspond with a contracted Sark Amir. And so that's exactly what we have right here. So notice we have a contracted Sark Amir which is smaller in length than the relaxed Sark Amir, which is longer and length. And so if we closely compare the relaxed Sark Amir with the contracted Sark Amir down below, Which will notice is that there are few regions that are getting smaller in size, and the first is going to be the H Zone, which recall H is a thick letters. So the H Zone Onley contains the thick Maya's and film it without any thin acting filaments whatsoever. And so the thick 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. So the H zone definitely reduces in size upon contraction. But notice that also the I bands, which, because I is a thin letter it on Lee contains the thin acting filaments and no thick myosin filaments. So there's the eye band on either side. They are also much, much larger in the relaxed muscle in comparison to the eye bands and the contracted muscle, which are much, much smaller. And so you can see that these dotted lines here are showing the narrowing of the eye bands, a zwelling as the narrowing of the H zone and so up above. What we can say is that it's the H Zone and the I bands that are going to reduce in size when it comes to a contracting Sark Amir. And so what helps me remember that it's the H zone and the I bands that reduce in size is that H I says hi. And so whenever you go toe 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. And so hopefully that will help you guys remember that it's the H zone and I've bands that reduce in size now down below in this text were 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 acting. Micro filaments, which are the structures and green that we see on the thin acting filaments, are gonna get pulled towards the M disc which is again this blue disk that we see here in the center and so we can see that thes green arrows are showing that the acting filaments on either side here are getting pulled towards theme 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, uh, sarcoma here upon contraction. Now what's also very important to note is that the A band, which recall is the length of the entire thick myosin filament, uh, actually does not reduce in size, and it does not change during a muscle contraction. So notice that the length of the thick Meyssan film it indicated by the A band here, uh, it does not change when it comes to a muscle contraction, so that's very important to note. But what you should also note is that the Z discs, um, are going to get pulled closer to the M disc, just like the acting gets pulled closer to the M disc. And so recall that the Z discs are these purple structures that we see on the end and notice that the purple structures, uh in comparison to being up above their getting pulled in the direction closer to the M disc. And the reason for that is because recall from our previous lesson videos that the acting filaments here in green are actually anchored to the Z discs. And so if the acting is getting pulled closer to the M disc, then that means that by default, the Z disk is also going to get pulled closer to the M disc. And so that's exactly what we're describing here in these two lines. Now, it's also important to note that the volume of the muscle is actually unchanged during a muscle contraction, and so there's no change in the volume, but the muscle does become shorter upon contraction. And so, really, the main takeaways here about the sliding filament model that describes the nature of the contracting Sark Amir is that the high zone or theme H zone and the 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. And so now that we've covered, the sliding filament model will be able to talk Maura about the biochemical mechanism off a contracting Sark Amir in our next less video. So 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.

all right, so in this video, we're finally going to talk about the biochemical events that lead to a muscle contraction. And so these biochemical events occur in a cycle that's known as the Acto myosin cycle. And so Acto Myson is a word that seems to be emerge of acting and Maya Seuin and so really it describes how acting and Maya's and interact with each other to create the muscle contraction. And so the Acto myosin cycle can really be described in five general steps, and it is a cycle, so it's going to begin and end at the same place. And so it's a five step cycle of biochemical events that results in on Lee the contraction of a Sark amir. And so we've got these five steps number down below in our text and noticed that the numbers in the text correspond with numbers that we see down below in our image. Now I'll admit, at first glance, this image looks pretty darn complicated. But I can tell you it's definitely not as complicated as it looks, and so let's go ahead and get started here with step number one, which is really for us to just realize that without any eight eep, the Miocene head is going to bind tightly to the acting micro filaments. And so let's take a look down below at our image and notice at the top. What we have is a relaxed muscle. And so here we have a relaxed Sark amir and notice that we're zooming into this specific region that we see here of the SAARC Amir. And so all of these images that we see down below of the Acto myosin cycle, or just zoom in of this right hand side of the SAARC Amir And so that's important to keep in mind because we need to understand that 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 disc again because we're zoomed into this little area right here in all of these images. And so, of course, with step number one here in this first box, uh, let's remind ourselves that without a teepee, mice and heads are gonna bind tightly to the act in micro filaments so zoomed in what you can see is we have the Miocene filament up above and red and you can see the Miocene head is right here and then down below. We have the acting micro filaments and so notice that there's no indication of a teepee whatsoever in this box. And so, without a teepee, the mice in head here is going to bind very tightly to the act in micro filament. And that's exactly what we see in this image. So really, that's it for Step number one so we can move on to Step two, which is of course, going to include a teepee. And so, in the presence of a teepee, it will bind to the Miocene head and disrupt the acting myson interaction. And so if the act in Miocene interaction is disrupted, that's going to cause the Miocene head to release the acting. So let's take a look down below it, our image to visualize this and so you can see that in step number two right here, 80 p is coming into play, and so the A T P is going to bind to the Mayas and head like we see here and disrupt the interaction between the Mayas and Head and the acting Micro filaments. And so you can see that causes the Mayas and head to release uh, the acting upon a teepee binding. And that's exactly what we see here. The 80 p the medicine had has released the acting and really, that's it. First step number two so we can move on to step number three. And so in step number three, what we need to realize is that the Miocene head is capable of hydra allies ing the ATP molecule that it bound into a deep and inorganic phosphate. And so the 80 p hydrolyzed ation is actually going to cause the Miocene head to change confirmations to a high energy state. And so, really, this 80 p hydrolyzed ation is going to cause the Miocene head to cock backwards into this high energy, state and weekly interact with the act in closer to the Z disk. And so again, if we take a look at step number three down below, let's remind ourselves in step number two, the mice and head is bound to 80 p. So in step number three, the mice and head that was bound 80 p is going toe hydrolyzed the 80 p into ADP and inorganic phosphate. And so when it does that, it cocks back. Uhh the mice and head. And so ATP, hydraulics, ISS cocks back the mice and head into a high energy state where the Mayas and head eyes going to be cocked back towards the Z disk. And 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, uh, here, like we are here in this image, the right is gonna be towards this Z disc over here. And so that's why we're saying it's getting cocked back towards the Z disk. And so really, this cocked back position, you can think of it like a slingshot. So the Miocene 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 Miocene head. And so you can see that the mice in head here is going toe weekly, interact with the acting and so they're not quite strongly bound. You can see we have a dotted line here to represent this week interaction. And so really, that's it for step number three is the cocking back of the Mayas and head. And so in step number four, what happens is the Miocene head releases the inorganic phosphate that it hydrolyzed. Uh uh, That was the result of the hide realization of ATP. So the release of the inorganic phosphate causes the mice and head to strengthen. It's binding to acting. And so if we take a look at step number four, which is right here, we can see that the inorganic phosphate is getting released. So, uh, in step number 3 80 p hydrolyzed ation led to ADP and inorganic phosphate. And so the inorganic phosphate, not the A d p. Is released first, so the inorganic phosphate is released. So the Meyssan had releases the inorganic phosphate and that allows the Meyssan had to bind to the acting much stronger. And so you can see that the Miocene head here is bound strongly to the acting And really that's it for step number four. And so in step number five, which is really our last step here, what happens is the mice and power stroke is going to follow the release of inorganic phosphate. And so the power stroke is literally just, ah, movement of the Mayas and head. And so this power stroke is going toe pull the acting that it's strong, strongly bound to. At this point, it's gonna pull the act in towards the M disc and return the mice and back to the original state that it was in back in step number one. And so, if we go down to step number five down below, what you can see is that the mice and power stroke is going to follow the release of the inorganic phosphate and the power stroke is just this movement of the mice in head from the cocked back position forwards towards the M disc on DSO because the Meyssan head moves towards the MDs and it's strongly bound to the acting, it's also going to shift the acting in this direction to the left, which recall zoomed into the left, uh, zoomed into this area right here and moving to the left is gonna bring us closer to the M disk. And so the mice and power shirt will shift the acting towards the M disc And so, uh, that is going to allow up above in our image here. The acting 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 is, um, a d p. I's going to be released, and the cycle is going to be repeated until the mice and binding sites on acting are blocked and so down below in our image, notice that the power stroke leads to, uh, the A d. P molecule being released, which we can see here. And then. Of course, that brings us back to the beginning of the cycle where, UM this act and can be further pulled in even closer to the M disc in the next cycle. So each cycle Onley moves the acting just a small bit. So in order to get a full contraction, this cycle needs to occur multiple times so that it can get shifted closer and closer to the M disc. And so again, this cycle is going to continuously happen. The atomized and cycle on Lea allows Azzawi mentioned up above it on Lee results in the contraction of a Sark amir. And so the relax ation of a Sark amir is not going to occur until the mice and binding sites on acting are blocked. And so we can prevent the act of mice and cycle from occurring if we put up a blocker here to prevent the acting to prevent the medicine from binding to the acting, So this cycle again is going to continue to happen over and over and over until this position here is being blocked. And so we'll be able to talk about in our next lesson video exactly what blocks these mice and binding sites and allows for, um, muscle relaxation. And so this is quite a lot of information to remember about the atomized and cycle. So in our next video, we'll be able to talk about, um, or condensed way toe, memorize the steps of the atomized and cycle. So I'll see you guys in our next video

All right, So in this video, we're going to talk about a summary of the Acto myosin cycle on here in this table we have that summary. And so notice that in the first column we have the steps 1234 and five And then in the second column, what we have is the details of each steps that you guys should need to know. And so, of course, in the first step, what we need to know is that if there is no a teepee, then you are gonna buy him to May. And so the U is really just going to be the My Ozon head. And of course, the May is going to be acted. And so if there's no 80 P, then the Miocene is going to bind to the acting. And so, of course, if we go up to number one in our text, in our image from above, we can see that when there is no 80 p, the mice and head is bound to the acting now in step number two. Of course, a teepee is gonna come into play and so the 80 p binds and the myson is released And so that's exactly what happens in Step number two. Is that a teepee? Bonds of the Miocene head and the Myson releases Theatric ting. Now in step number three, the Miocene is going to hide relies the a t p So 80 p high hydraulic sis cocks back the Miocene head into ah, high energy state. And so in step number three, that's exactly what we see is the Meyssan head is cocking back into this high energy state and when it gets cock back, you can think of it like a slingshot getting cocked back into its high energy state. That, of course, step number four is going to be the inorganic phosphate is released. And so that's exactly what we see here. In step number four, Thean organic phosphate is released and that allows the mice and head to bind tightly to the acting filament and then in step number five. Of course, the power stroke is going to follow, which is going to shift the acting, and then a d. P. Is released to regenerate the original myson. And so here what you can see is from step number four, the power stroke is going to occur and the power stroke shifts the acting on. Then a D. P. Is released so that the cycle can restart back out. Step number one. And so, by using this table here to condense the steps, you can begin to familiarize yourself with, uh, the act of mice and cycle. So that concludes this video, and I'll see you guys in our next one.

So now that we know that muscle contractions occur via the Acto myosin cycle in this video, we're going to talk about how muscle relaxation occurs via regulation from two proteins, troponin and troppo Miocene. And so what's interesting to note is that Maya Seuin and acting together make up about 80% of the protein mass in a muscle fiber cells. But the remaining 20% consists mainly of two other types of thin filaments which are troponin and troppo. Miocene, now troponin and troppo Miocene, actually form a complex with each other. And so the role of this troponin, troppo, Mayas and complex is to bind to act in and block the Miocene binding sites to allow for muscle relaxation. And so what helps me remember that the troponin trope, um, eyes and complex bind to acting and not to my Yasin is that the teas here in troponin and trope, um, eyes and match with the tea and acting and notice Maya's and does not have any ts. And so this is how I know that troponin and triple mice and complex bind to act in. So let's take a look down below at our image and notice Over here on the left hand side, the green acting micro filaments are represented by these green balls that we see here. And the mice and heads are represented by these structures that we see up above. And so notice that the two new proteins that we're introducing troponin and troppo Matthiasson are bound to the acting micro filaments and they're not bound to Liasson. Now notice that troponin is this small, light purple protein that we see here and here and then troppo Miocene is this much longer protein that we see extended, uh, right in these positions. And so what helps me remember that troponin is this small guy and troppo Matthiasson is the longer guy is that troponin is a much smaller word than trope Oh Miocene which is much longer. And so what's important to know is that this long troppo Meyssan is actually blocking, so it blocks the Miocene binding sites on acting. And of course, if myson is not able to bind to acting, we know from our previous lesson videos that the acting Acto myosin cycle is not going to be able to occur. And if the Acto myosin cycle can occur because these mice and heads are being blocked from binding, UH, then that's going to prevent a muscle contraction. And if a muscle contraction is prevented, that's going to allow for muscle relaxation, a relaxed muscle. So this entire side over here represents muscle relaxation. And so what's important to know is that the troponin troppo, Mayas and complex will actually regulate muscle contractions in a way that the muscle contractions will Onley occur with nervous system signals from our brains and spinal cords. And so upon receiving a nervous system signal from our brains and spinal cords, a Maya fiber als Sarko plasmid ridiculous um, is stimulated to release calcium and so recall from our previous lesson videos, we mentioned that the Circle plasmid ridiculous was going to be important to allow for a muscle contraction. And this is exactly why it's because the sarka plastic ridiculous is stimulated to release calcium upon receiving a signal from the nervous system. Now what you'll note here is that troponin its role is really to bind to the released calcium from the Sarko Plasma ridiculous, and when the calcium binds to the troponin it causes a confirmation will change in the entire troponin, troppo, Mayas and complex and so troppo Maya Seasons Change is going to expose the mice and binding sites that it was once blocking. And so, once, uh, the mice and binding sites are exposed, that is going to allow for the optimized and cycle to initiate and lead to muscle contraction. So let's take a look down below at our image to tie together everything that we just talked about. And so, of course, this whole troponin troppo mice and complex here is going to help us regulate our muscle contractions so that they Onley occur with our nervous system signals. So here we have the brain and spinal cord to represent the nervous system. And so when a myo fiber all receives a signal from the nervous system, the Sarko plasmid ridiculous which is this yellow membrane is structure here, eyes going to be stimulated to release calcium which are these yellow circles that we see here in this image. And so the release of calcium eyes going toe lead to calcium binding to the troponin, and so you can see the little yellow dot here is bound to the troponin, and that causes a confirmation I'll change in the entire Troponin troppo, Mayas and complex, and so notice that the troppo myson is no longer blocking the Mayas and binding sites. And instead the Mayas and binding sites indicated by these little positions on the acting are exposed. So we have exposed mice and binding sites. And so what we can say is that calcium thes little yellow circles here are going thio bind to troponin and allow for a muscle contraction to occur because now notice that the Mayas and heads are bound to the acting. And of course, if the mice and heads combined to the act and then that's going to allow for the Acto myosin cycle to occur on, it's going to lead to a muscle contraction and so you can see how the troponin trope, um eyes and complex is only going to allow for a muscle contraction upon receiving a signal from our nervous systems. And so it's important to know is that over time the calcium that was released by the Sarko Plasmid ridiculous um, is going to be returned to the Sarko Plasmid Ridiculous, um therefore decreasing the concentration of calcium and so low calcium concentration means that calcium eyes going to disassociate from the troponin on. Of course, it's going to go back into the Sarko plasma ridiculous, and that means that the troponin trope, um eyes and complex is going to go back. It's going thio again, block acting's mice and binding sites and allow for muscle relaxation to occur. So over time the calcium concentrations get decreased and the troponin, troppo, Mayas and complex goes backwards to the original position where it was blocking the Mayas and binding sites. And again, this is what allows for the muscle relaxation to occur. And so now that we understand, uh, the Acto myosin cycle and troponin and trope, um, eyes and regulation, we understand muscle contraction and muscle relaxation. So 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.