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Anatomy of a skeletal muscle fiber

Understanding the structure of a muscle fiber. Created by Sal Khan.
Video transcript
I think we have a respectable sense of how muscles contract on the molecular level. Let's take a step back now and just understand how muscles look, at least structurally, or how they relate to things that we normally associate with muscles. So let me draw a flexing bicep right here. That's their elbow and let's say that's their hand right there. So this is their bicep and it's flexing. I think we've all seen diagrams of what muscles look, at least on kind of a macro level and it's connected to the bones at either end. Let me draw the bones. I'm not going to detail where-- so it's connected to the bones at either end by tendons. So this right here would be some bone. Right there would be another bone that it's connected to. And then this is tendons, which connects the bones to the muscles. We have the general sense-- connected to two bones, when it contracts it moves some part of our skeletal system. So we're actually focused on skeletal muscles. The other types are smooth muscles and cardiac muscles. Cardiac muscles are those, as you can imagine, in our heart. And smooth muscles are-- these are more involuntary, slow moving muscles and things like our digestive tract. And I'll do video on that in the future, but most of the time when people say muscles, we associate them with skeletal muscles that move our skeletal system around, allow us to run and lift and talk and do and bite things. So this is what we normally associate-- let's dig in a little bit deeper here. So if I were to take a cross section of this bicep right there-- if I were to take a cross section of that muscle right there-- so let me do it big. And then it looks something like this. This is the inside of this muscle over here. Now I said back here, we had our tendon. And then there's actually a covering; there's no strict demarcation or dividing line between the tendon and the covering around this muscle, but that covering is called the epimysium and it's really just connective tissue that covers the muscle, kind of protects it, reduces friction between the muscle and the surrounding bone and other tissue that might be in this person's arm right there. And then within this muscle, you have connective tissue on the inside. Let me do it in another color. I'll do it in orange. This is called a perimyseum, and that's also just connective tissue inside of the actual muscle. And then each of these things that the perimysium is dividing off-- let me say if we were to take one of these things and allow it to go a little bit further-- so if we were to take this thing right here-- what this perimysium is dividing off-- and if we were to pull it out-- actually, let me do this one right here. If we were to pull this one out just like that-- so you have the perimysium surrounding it, right? This is all perimysium, and it's just a fancy word for connective tissue. There's other stuff in there. You could have nerves and you could have capillaries, all sorts of stuff because you have to get blood and neuronal signals to your muscles of entry so it's not just connective tissue. It's other things that have to be able to eventually get to your muscle cells. So each of these-- I guess you'd call it subfibers, but these are pretty big subfibers of the muscle. This is called a fascicle. The connective tissue inside of the fascicle is called the endomysium. So once again, more connective tissues, has capillaries in it, has nerves in it, all of the things that have to eventually come in contact with muscle cells. We're inside of a single muscle. All this green connective tissue is endomysium. And each of these things that are in the endomysium are an actual muscle cell. This is an actual muscle cell. I'll do it in purple. So this thing right here-- I can pull it out a little bit. If I pull this out, this is an actual muscle cell. This is what we wanted to get to, but we're going to go even within the muscle cell to see, understand how all the myosin and the actin filaments fit into that muscle cell. So this right here is a muscle cell or a myofiber. The two prefixes you'll see a lot when dealing with muscles-- you're going to see myo, which you can imagine refers to muscle. And you're also going to see the word sarco, like sarcolemma, or sarcoplasmic reticulum. So you're also go see the prefix sarco and that's flesh-- so sarcophagus-- or you can think of other things that start with sarco. So sarco is flesh. Muscle is flesh and myo is muscle. So this is myofiber. This is an actual muscle cell and so let's zoom in on the actual muscle. So let me actually draw it really a lot bigger here. So an actual muscle cell is called a myofiber. It's called a fiber because it's longer than it is wide and they come in various-- let me draw the myofiber like this. I'll take a cross section of the muscle cell as well. And these can be relatively short-- several hundred micrometers-- or it could be quite long-- at least quite long by cellular standards. We're talking several centimeters. Think of it as a cell. That's quite a long cell. Because it's so long, it actually has to have multiple nucleuses. Actually, to draw the nucleuses, let me do a better job drawing the myofiber. I'm going to make little lumps in the outside membranes where the nucleuses can fit on this myofiber. Remember, this is just one of these individual muscle cells and they're really long so they have multiple nucleuses. Let me take its cross section because we're going to go inside of this muscle cell. So I said it's multinucleated. So if we kind of imagine its membrane being transparent, there'd be one nucleus over here, another nucleus over here, another nucleus over here, another nucleus over there. And the reason why it's multinucleated is so that over large distances, you don't have to wait for proteins to get all the way from this nucleus all the way over to this part of the muscle cell. You can actually have the DNA information close to where it needs to be. So it's multinucleated. I read one-- I think it was 30 or so nucleuses per millimeter of muscle tissue is what the average is. I don't know if that's actually the case, but the nucleuses are kind of right under the membrane of the muscle cell-- and you remember what that's called from the last video. The membrane of the muscle cell is the sarcolemma. These are the nucleuses. And then if you take the cross section of that, there are tubes within that called myofibrils. So here there's a bunch of tubes inside of the actual cell. Let me pull one of them out. So I've pulled out one of these tubes. This is a myofibril. And if you were to look at this under a light microscope, you'll see it has little striations on it. the striations will look something like that, like that, like that, and there'll be little thin ones like that, like that. And inside of these myofibrils is where we'll find our myosin and actin filaments. So let's zoom in over here on this myofibril. We'll just keep zooming until we get to the molecular level. So this myofibril, which is-- remember, it's inside of the muscle cell, inside of the myofiber. The myofiber is a muscle cell. Myofibral is a-- you can view it as a tube inside of the muscle cell. These are the things that are actually doing the contraction. So if I were to zoom in on a myofibril, you're going to see it-- it's going to look something like that and it's going to have those bands in it. So the bands are going to look something like this. You're going to have these short bands like that. Then you're going to have wider bands like that, like these little dark-- trying my best to draw them relatively neatly and there could be a little line right there. Then the same thing repeats over here. So each of these units of repetition is called a sarcomere. And these units of repetition go from one-- this is called a Z-line to another Z-line. And all of this terminology comes out of when people just looked under a microscope and they saw these lines, they started attaching names to it. And just so you have the other terminology-- we'll talk about how this relates to the myosin and the actin in a second. This right here is the A-band. And then this distance right here or these parts right here, these are called the I-bands. And we'll talk about really in a few seconds how that relates to the mechanisms or the units that we talked-- or the molecules that we talked about in the last video. So if you were to zoom in here, if you were to go into these myofibrils, if you were to take a cross section of these myofibrils, what you'll find is-- if you were to cut it up, maybe slice it-- if you were slice it parallel to the actual screen that you're looking at, you're going to see something like this. So this is going to be your Z-band. This is your next Z-band. So I'm zooming in on sarcomere now. This is another Z-band. Then you have your actin filaments. Now we're getting to that molecular level that I talked about. And then in between the actin filaments, you have your myosin filaments. Remember, the myosin filaments had those two heads on them. They each have two heads like that, that crawl along the actin filaments. I'm just drawing a couple of them and then they're attached at the middle just like that. We'll talk about in a second what happens when the muscle actually contracts. And I could draw it again over here. So it has many more heads than what I'm drawing, but this just gives you an idea of what's happening. These are the myosin, I guess, proteins and they all intertwined like we saw in the previous video and then there'll be another one over here. I don't have to draw in detail. So you can see immediately that the A-band corresponds to where we have our myosin. So this is our A-band right here. And there is an overlap. They do overlap each other, even in the resting state, but the I-band is where you only have actin filaments, no myosin. And then the myosin filaments are held in place by titin, which you can kind of imagine as a springy protein. I want to do it in a different color than that. So the myosin is held in place by titin. It's attached to the Z-band by titin. So what happened? So we have all of these-- when a neuron excites-- so let me draw an endpoint of a neuron right here, the endpoint of an axon of a neuron right there. It's a motor neuron. It's telling this guy to contract. You have the action potential. The action potential travels along the membrane, really in all directions. And then it eventually, if we look at it from this view, they have those little transverse or T-tubules. They essentially go into the cell and continue to propagate the action potential. Those trigger the sarcoplasmic reticulum to release calcium. The calcium attaches to the troponin that's attached to these actin filaments that moves the tropomyosin out of the way, and then the crawling can occur. The myosin can start using ATP to crawl along these actin filaments. And so as you can imagine, as they crawl along, their power stroke is going to push-- you can either view it as the actin filaments in that way or you can say that the myosin is going to want to move in that direction, but you're pulling on both sides of a rope, right? So the myosin is going to stay in one place and the actin filaments are going to be pulled together. And that's essentially how the muscle is contracting. So we've, hopefully, in this video, connected the big picture from the flexing muscle all the way over here to exactly what's happening at the molecular level that we learned in the last few videos. And you can imagine, when this happens to all of the myofibrils inside of the muscle, right, because the sarcoplasmic reticulum's releasing calcium generally into the cytoplasm of-- which is also called myoplasm, because we're dealing with muscle cells-- the cytoplasm of this muscle cell. The calcium floods all of these myofibrils. It's able to attach to all of the troponin-- or at least a lot of the troponin on top of these actin filaments and then the whole muscle contracts. And then when that's done, each muscle fiber, myofiber, or each muscle cell will not have that much contracting power. But when you couple it with all of them that are around it-- if you just have one, actually, working, or a few of them, you'll just have a twitch. But if you have all of them contracting together, then that's actually going to create the force to actually do some work, or actually pull your bones together, or lift some weights. So hopefully you found that mildly useful.