Structure of a Skeletal Muscle - Video Tutorials & Practice Problems
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1
concept
Organization of Muscle Tissue
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We're now gonna talk about the structure of a skeletal muscle. And we really want to think about the organization of the skeletal muscle tissue. And here we're gonna talk about that organization starting at the whole muscle and getting down to the individual muscle cell or the muscle fiber. Now, later on, we're gonna take it further and get down to the individual proteins that are responsible for contraction. But again, here we're talking about it from the muscle down to the muscle cell or the muscle fiber. Now, before we go on, let's orient ourselves to the picture. Here, we see a bone that's the humerus and you see a tendon coming off the bone and it's pulling this muscle away and we see this sort of whole body of the muscle that we then sort of cut off in cross section. And in this cross section, you can see these sort of uh groupings within these sort of bundles within there and we've cut that off, but we've pulled one of those things out and then we do the same thing, we cut it off in cross section again and we pull one of those things out and then we're not gonna get to in this one, but we're sort of hinting at it. There's gonna be even more levels of organization down below. Now, you're very likely to see a picture like this in your textbook in a lecture, maybe even on the test. So, practice organizing your thoughts around it and think about what these different levels are. So, what's in here? Well, there's gonna be muscle fibers, muscle fibers are the individual muscle cells and really the bulk of the muscle is gonna be those muscle fibers, but also running out all throughout here are gonna be nerves. And in our image here, we have these yellow, it's sort of a nerve cut off in cross section. Uh these nerves need to run all through the muscle because they need to connect to every individual muscle fiber. The nerves of the nervous system are what tell the muscle fiber when to contract. Also needing to run through there though are the blood vessels. And we see those sort of cut off in cross section here in blue and red. And we also see two of them pulled out. Again, these blood vessels need to get down to every muscle fiber because those fibers use a lot of oxygen as they burn through a TP as they could track. Then finally, we have connective tissue and connective tissue. There's gonna be several different layers of connective tissue that you're gonna need to know about in this diagram. And so every time we talk about connective tissue, sort of organize our thoughts. We've put it in a gray box just like you see here. So let's talk about these bundles. The first thing that we're gonna talk about, the smallest level for this diagram is going to be the muscle fiber and the muscle fiber we've said is just another name for a muscle cell. These muscle fibers are gonna be really long, multi nucleated cells. And you can just sort of think about it. Basically, one muscle fiber is basically the length of a muscle cell. Importantly, in a skeletal muscle, muscle fiber never stops. And then another one starts sort of in series with each other. The muscle fiber runs the entire length of the contraction. And we've pulled one muscle fiber out here and you can see it and you can see these multiple nuclei along the edge there. Remember multiple nuclei, they're formed from smaller cells and you need lots of nuclei because they're really big, really big cells that need all these nuclei to support their functions. Now to surround the, the muscle fiber, we're gonna have a layer of connective tissue that we are gonna call endo mysia. And that Endomysium is gonna surround every individual muscle fiber. And here it's really important just to remember your prefixes, endo means within so deepest down within the muscle fiber at the lowest level surrounding every individual muscle fiber, we have the Endomysium. So it's surrounding this muscle fiber here. And then as we go up, this is gonna show muscle fibers in cross section all bundled together running in between each of these muscle fibers that all this white conti connective tissue here. That's all gonna be that Endomysium, Endomysium surrounding the individual muscle fibers. But you can see we have a bundle of muscle fibers together that are organized and that is what we're gonna call a fascicle. A fascicle is just a bundle of muscle fibers. We say a bundle, we're talking somewhere dozens of muscle fibers together. This fascicle is organized again by being surrounded by connective tissue. It has the Endomysium within the fascicle and then surrounding the fascicle is going to be the perry. My. That prefix, perry just means to surround something. What does the perimysium surround? It surrounds the fascicle? And again, if we look up a le level, we can see up here, each one of these things is a fascicle and you can see each one is surrounded by this connective tissue. So this connective tissue that you see at this level is the paras. Now, this level that we're looking at is the whole muscle. So this muscle is really a bundle of fascicles. And again, we're talking about a few to several dozen fascicles usually to make up a muscle. So what holds this whole muscle together? Well, around the outside of this muscle, we have another layer of connective tissue that's surrounding the entire thing that we are gonna call the epimysium. And that prefix epi remember that means on the surface of something around the outside. So the epimysium surrounds the outside of the entire muscle. Now that connective tissue is gonna be really important for contraction. Each muscle fiber is connected to the Endomysium that surrounds it. And that Endomysium just like the muscle fiber is running all the way down through the muscle and same thing with the paras and the omy and it all reaches the end of the muscle and it's sort of bound together and that's the connection. The connective tissue is the connection that connects the muscle to the bone. Now, if it comes together, like you see here in sort of like a rope, that is what we are gonna call a tendon. So a rope like connection where all those connective tissues sort of are bundled together like a rope, that's a tendon. Some muscles however, have more flat connections where the muscle is flat and it comes in more like a sheet like connection. And that sheet like connection, we call an aponeurosis or the plural aponeurosis, aponeurosis or aosis. You can think like your abdominal muscles, right? They don't come together to a point to, to connect to your muscle, to your bones like a rope, they come in flat with a sheet at the end. Now, aosis, you're probably familiar with tendons, but aponeuroses, you may need to know for this unit, you're definitely gonna wanna know it later on when we learn the individual muscles because we're gonna learn what bones they connect to and how they connect and we're gonna be using it a lot there. All right. So that's the structure of muscle from the muscle fiber up to the whole muscle. Later on, we're gonna go from the muscle fiber down. But first, like, always give an example and practice problems that follow. I'll see you there.
2
example
Structure of a Skeletal Muscle Example 1
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Our example tells me that marbling and beef is the presence of fat in connective tissue layers within the meat. This intramuscular fat makes the meat more tender and flavorful. And it says below are two cuts of meat. Displaying differences in marbling on the left is a typical steak. So we can look down and we have this picture on the left here. This is the steak that you just might buy at the grocery store. On the right is Kobe beef, a delicacy from Japan known for its extreme marbling. And we look at this picture on the right and we see that there's fat just all throughout this meat, just sort of clumps and specks of fat all throughout the state. Then it's gonna ask us because marbling is present in connective tissue in what structure or structures of the muscle would you predict to find the excess fat found in Kobe beef? All right. So this is really just coming down to what connective tissue layers are inside the muscle. So let's remember our connective tissue layers. We had three of them. First, we had the epimysium followed by the paise and then finally, the Endomysium the pi was that layer surrounding the outside of the entire muscle? So is that what you expect to see in marbling? Well, marbling we said is within the meat. So Kobe beef, that epimysium might be fattier, but that's gonna be fat on the outside of the muscle. It's not gonna contribute directly to the marbling. So I don't think that's gonna be part of my answer. So next we can talk about the param param. Remember surrounds the individual fascicles and you can see that you can see that on the grocery store meat here, right? You see these lines of of connective tissue and fat all running through the muscle. That's the paras, you can see the fascicles and a cut of steak. Now you can see that fat even just a little bit on a grocery store meat, which means it's definitely gonna be present in that Kobe beef mo probably most of what you're seeing here. These big clumps of fat is just a whole bunch of fat in that paralyze you. So I'm gonna start by writing Perry Miy. Now, as we go down from the param, we reach the Endomysium, remember the Endomysium surrounds each individual muscle fiber within the fascicle. Now, you can't see that because the muscle fibers are microscopic, but there could be fat there as well. And I would bet that some of these flecks of uh fat that we see in this Kobe beef is actually down in the Endomysium. It's just sort of big clumps of fat within those fascicles. So I'm gonna also write here and do my all right to recount which type of connective tissue layers are gonna be within the meat or within the muscle. The paray surrounds the fascicles, the Endomysium surrounds the individual muscle fibers. And now next time you're eating your steak, tartare Kobe beef, you know why it tastes so darn good. See you in the next video.
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Problem
Problem
What layer of connective tissue surrounds and separates muscle fibers?
A
Epimysium.
B
Perimysium.
C
Endomysium.
D
Fascicle.
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Problem
Problem
A fascicle is a bundle of:
A
Epimysium.
B
Muscle Fibers.
C
Myofilaments.
D
Muscles.
5
concept
The Muscle Fiber
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7m
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To continue talking about the structure of a skeletal muscle. We're now gonna talk about the muscle fiber. And from here, we're gonna go down down to some of the individual proteins that are gonna be involved. And I just wanna ground ourselves first though. What we're saying here is that for a muscle fiber to contract the tissue must shorten. And that's really sort of our fundamental question is how does a muscle fiber shorten? How does the muscle actually get shorter? So to answer that, we're gonna start by looking at this picture here and we have a picture of a muscle fiber, at least a small section of it. And remember the muscle fiber. That's just another name for the skeletal muscle cell. It's these giant cells, these big cylinder, extremely long cells, basically the same length as a muscle. And as we look here, we're gonna see that it's sort of, we, we see a section of it and we see it sort of cut away in cross section in pieces and we can see some of the structures inside and some of those are pulled out so that we can see what's going on in there just like we did previously with the whole muscle. Now, remember this muscle fiber is gonna be surrounded by the Endomysium, that small layer of connective tissue that surrounds and separates each individual muscle fiber. So to start, we'll just note that's sort of drawn in down here, but we're sort of pulling it away so that we can see what's inside. And the first thing that we note, well, it's also cut away, but this would be wrapping, the whole thing would be this sort of translucent section here that you see. And that's just gonna be the cell membrane. But the cell membrane of a muscle fiber is highly specialized. So we give it its own special name. We're gonna call it the sarcolemma. And sarco is just a prefix from Greek that means flesh. So it's used sometimes to refer to muscles and lemma is also from Greek. It means a covering or wrapper or a sheath around something. So we're gonna say that the sarcolemma is just this plasma membrane of the muscle fiber that is wrapping the myofibril and the myo fibers will get into more detail in a section in a second. But these are these long rod like organelles that are within the sarcolemma that make up the muscle fiber. And so here we can think about this sort of repeating structure of bundles that we started talking about. And previously we had the muscle was a bundle of fascicles, the fascicle was a bundle of muscle fibers. And now the muscle fiber is gonna be a bundle of these myofibril. But before we talk about the myofibril, I want to talk about the membrane a little bit more. So the sarcolemma is highly specialized to sort of send an electrochemical signal, telling the muscle fiber to contract. But you also need to get that electrochemical signal down deep within the muscle fiber so that the whole cell is contracting together to do that. We have the T tubules and the T tubules you can see here in pink, they're sort of coming down here and they are surrounding like a little ring each around these myofibril. Now, these are connected to and really, they're kind of an extension of the sarcolemma. So we're gonna say that the T tubules are these membrane canals or you could say tunnels or passageways that are extensions of the sarcolemma and reach down deep within and surround each one of these myofibril in a little ring. So as the electrochemical signal comes down the circle lemma, it's gonna then dive down in these t tubules and get deep within the muscle fiber. Now, what they're actually trying to stimulate is this next membrane system that we're gonna talk about and that's gonna be the sarcoplasmic reticulum that we're writing down here in blue. Now, sarcoplasmic reticulum, you might notice that sounds a lot like endoplasmic reticulum. There's a reason for that the sarcoplasmic reticulum is the highly specialized endoplasmic reticulum of muscle cells and the sarcoplasmic reticulum. You can see it surrounds these myo fibers. You can see it's really here in close connection with these tube tubules. And then it kind of spreads out in this sort of convoluted connected tunnel tube like system of membrane. And you can see it sort of cut off in cross section all along here and it's surrounding and separating these myofibril. Its job is to store and then release calcium ions. The signal for the muscle to actually contract down like at the molecular level is the release of calcium ions from the sarcoplasmic reticulum into the myofibril. Now, we're gonna get into that in a lot more detail going forward. But for now, let's now focus on the myofiber. So the myofibril we've already said is this myofibril, uh these are these long rod shaped organelles and you can sort of see a bunch in cross section here. We have a few here and then we've pulled one out so that you can really see it there. The myofibril is really just sort of this organizing structure and what it's organizing and uh is going to be the myo filaments. And you can see again, we've pulled out the maya filaments there. And if you look here at these in the cross section, uh it might be a little hard to see because they're very small, but the cross sections you might also notice have these really regular organized structure within them. And that's those maya filaments cut off in cross section, the myo filaments, that's what's actually gonna be responsible for getting this muscle to contract. So the mayo filaments, these are the proteins, the Acton and the Mycin. All right, there's gonna be other proteins going on in here. But Acton and Mycin, these are the major players that are actually gonna get this muscle to get shorter. This though is gonna be the first structure that isn't really long. Remember, our muscle fiber is basically the same length as the muscle, the muscle, the myofibril is the same length as the muscle fiber. The myo filaments are gonna be organized in these repeating structures and you can kind of see it in this myofiber that we've pulled out. You have sort of a darker section that is more purple in it, then a lighter section, a darker section, more purple in it. A lighter section that pattern sort of continues in these filaments that we've pulled out. That repeated structure is going to be the Sarco Mer and the Sarco mirror we are going to say is the contractile unit. We're sometimes gonna call this the fundamental unit of muscle contraction. What actually gets shorter in the muscle, the sarcomere gets shorter and the muscle fiber gets shorter because you can see this repeated structure is repeated over and over again. A lot one myofibril has thousands or tens of thousands of these in succession, I did a quick back of the envelope calculation. If you have a 10 centimeter long muscle, which is about, I don't know. Yay big. It's gonna have something like 40,000 Sarker end to end in every single one of these myofibril that repeated structure repeating over and over again, these acting and mycin filaments, the way they're organized is gonna create the striated appearance that you see. When you look at a skeletal muscle from that striated appearance, when you look at the cells, it looks straight, you're seeing the repeated structure of these proteins, the actinomycin. Now this is that structure. We're not gonna worry about it. Too much detail here, we're gonna talk about it in a lot more detail going forward. Now, we have like always example and practice problems to follow and then I look forward to get into the step by step process of how these signals are passed and how these proteins actually work to make the muscle contract. See you there.
6
example
Structure of a Skeletal Muscle Example 2
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2m
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Our example wants us to arrange the following from superficial to deep by placing the correct letter in each blank. So we have myo filaments, epimysium, paray and noises and sarcolemma. So when I think of a muscle and the structure of muscle, what's most superficial, what's surrounding the entire muscle is gonna be a layer of connective tissue. And we call that layer of connective tissue that surrounds the entire muscle, the epimysium. So I'm gonna put b first remember, epi means on the surface on the outside of something. So the epimysium is the connective tissue layer that surrounds the entire muscle. Now, within the epimysium, we're gonna have these bundles of fascicles. But when I look down fascicles, well, it's two arrows down, there's a space between it. So what surrounds the fascicle? We have another connective tissue layer and that's going to be the paralyzing. So see paray, the connective tissue layer that surrounds the fascicle is gonna come next. Well, the fascicle that's a bundle, it's a bundle of muscle fibers. But I don't see muscle fibers on my list here. So when I look and I think what's associated with the muscle fiber, what I see next is the sarcolemma. The sarcolemma is going to be the cell membrane of the muscle fiber. But there's another connective tissue layer that comes first, the connective tissue layer that surrounds each muscle fiber and therefore surrounds the circle lemma is going to be the Endomysium. Remember that prefix endo, endo means within it's deep within the fascicle. So the endomysium surrounds the sarcolemma. The sarcolemma is the uh the cell membrane of the muscle fiber. So, e comes next and now we made it to the myofibril myofibril. Remember those uh those long rod like organelles that make up the majority of the inside of a muscle fiber. Well, what are those myofibril is made of? They're made of the contractile proteins. We're gonna call here the myofilament. We only had one option left but good thing. It was the right answer. All right. So the myo filaments, the proteins that are actually gonna do the contracting deep deep within the muscle. So to recap superficial, we have the epimysium surrounds the entire muscle. The paras is within the epimysium and it surrounds the fascicle. The fascicle is gonna be a bundle of muscle fibers, but each muscle fiber is surrounded by the Endomysium. The Endomysium surrounds the cell membrane of the muscle fiber, which is the sarcolemma within the sarcolemma. We have the rod like organelles called the myofibril and the proteins that make those up are going to be the myo filaments. That's our answer more problems to follow. I'll see you there.
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Problem
Problem
The plasma membrane of a muscle fiber is called the ____________ and it wraps bundles of ____________.
A
Sarcolemma: myofibrils.
B
Sarcoplasmic reticulum: myofilaments.
C
Sarcomere: myofilaments.
D
Sarcoplasmic reticulum: myofibrils.
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Problem
Problem
True or False: if false, chose the answer that corrects the statement.
The smallest contractile unit of a muscle is the muscle cell.
A
True.
B
False, the smallest contractile unit is the fascicle.
C
False, the smallest contractile unit is the myofibril.
D
False, the smallest contractile unit is the sarcomere.
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