Anatomy of a skeletal muscle cell

Now, why don't we take a step back and take a look at how muscles work on more of a macro level? So why don't we start out by just drawing a little muscle right here. Let's say this is just the bicep, and on either side, there's a little bit of tendon that's kind of attached to it right here. So that's some tendon, so I'll label that right there. And that's just some tendon that's on either side of our muscle, our bicep right here. And the tendon attaches our muscle over to some bone, so there's a little bone right there. There's a little bone right there. Label these guys. So that's one bone end, and then here's another one. So if it's our bicep, it's anchored onto our humerus. This is our bicep, right? So I'll just draw it in the arm. Here, this guy's sort of flexing right there, coming down to the elbow. There. And if we want a better idea of what's going on here in this bicep, this muscle, why don't we just take a cut. So let me just cut right about there. We've got a cross-sectional look at what's going on in our muscle. So here's a little cut we just did right about here. Our muscle just hanging out in the middle, sort of that same shape that comes down about here. And we've still got our tendon, of course, that's anchoring our muscle over to our bone. Don't forget the tendon that's still right here. And the tendon is just a type of connective tissue, and this is somewhat continuous with the connective tissue that covers the outermost layer of our muscle right here. So I'm just drawing that in. This outer layer of muscle we've got here is called the epimysium, and that's continuous with the tendon, and it's supposed to help protect our muscle here so it doesn't shear against the bone or all the other things that are in the compartment of our arm, if we're talking about the bicep. But as you know, this can apply to the muscles in our leg or in our jaw when we're chewing-- anything that we control. So in addition to this connective tissue layer, there's another connective tissue layer that's on the inside right here, underneath the epimysium. And so this connective tissue layer I'll just draw as a circle or as a sheath that's kind of sitting around here. This guy is called the perimysium, and this perimysium covers subunits of muscle that sit right here. And there's a bunch of them, and they all have their own names. So I'll take this one right here and just kind of draw it out a little more so we can take a closer look inside. So this little dude right here, this muscle subunit that's covered by this perimysium that I'm shading in right here. This is called a fascicle. It's got two names, actually. So it can be called a fascicle-- fas-kick-el-- C-I-C-L-E. It's also known as a fasciculus. So it depends on whether you're talking about fascicles or a single fasciculus, whatever term you want to use. And then within each of the fascicles, there's another connective tissue layer. This is called the endomysium. Now, this covers individual muscle cells. So finally, we've reached the individual muscle cells. I'll draw one of these dudes coming out right here. This is an individual muscle cell that's covered by the endomysium. And so the muscle cell that I'm writing out over here-- it has a special name, as well. So we can call it a muscle cell, but we can also call it a myo-- myo meaning muscle-- fiber. So this is shaped like a fiber, because it is longer than it is wide. And again, this endomysium, just like the perimysium, contains nerves and blood vessels that can help conduct neuronal signals and blood towards the individual myofiber and the connective tissue that sits around here. OK, so now that we've gotten to the muscle cell, why don't we just scroll down a little bit and just focus in on this guy. Now, while we might be tempted to draw the muscle cell just kind of like that fiber that I just drew over there, like this sort of rectangle, we remember in fact that it's shaped a little differently-- like a pipe that has a couple of bumps outside. Do you guys remember? Why do we have bumps on the outside of our muscle cells or the outside of our myofibers? I think I heard one of you said, because there are nuclei that sit on the outskirts of our muscle cells. And that is absolutely correct. This is a single nucleus that I'm drawing right here. Here's one nucleus. And this is sort of the storage unit of DNA that can help us replicate or make more of our myofibers or our muscle cells, and they sit on the outskirts of our myofiber, towards sort of the edge of our plasma membrane. This plasma membrane has a special name in muscle. It's called the sarcolemma. We've got a couple of important prefixes that we mentioned here. Remember I mentioned myo from above? Myo just means muscle. Just keep that in the back of your mind. And then sarco-- whenever you see sarco, that refers to just flesh. And we often see this in the context of muscle because the covering of our muscle cell right here, that membrane, we call it the sarcolemma. The cytoplasm right here that's within the muscle cell-- we call that the sarcoplasm. And as we get further in, we're also going to talk about the sarcomere. And so we're talking about our myofiber right here-- let me just make it look a little more tubular. So the myofiber itself, this muscle cell, has a bunch of smaller units within it, too. And these smaller units are where we have our main contractions occurring. So I'm going to draw one of these guys out right here, and this is just called the myo-- like muscle-- fibril. Not fiber, but myofibril. Now we've come to where we're storing our myosin and our actin that's sitting inside here. This is where the actual contraction will occur. So if we look at our muscle cells under a microscope, we'll see that they've got these striations on them, these bands, because remember, another name for skeletal muscle is striated skeletal muscle. So they have these lines that are here that you'll see under a microscope. So if we blow that up-- let me just get some space down here to talk about it. So if I were to draw just a blown-up version of it right here, we'd have our striation line right there. I'll draw another one right here and right here. Just put it in this line in this box right here. And we have all these bands that we would see under the microscope, right? So we have the striations that are on the sides, and there are these bands that are kind of going across this unit right here that we're looking at. Now, what are these striations right here? Well, we talked about these before. Sal mentioned these are the z-lines. So the z-lines are these striations we see under the microscope, and so I've drawn three of them here for you guys. Z-lines, then we'll just connect that one back here. And remember, the space between two z-lines, going from here all the way to here-- that's the sarcomere, and this is our most basic unit of contraction. This is where we're going to have our actin and our myosin fibers interact and have us flex, at our most macro level we'll get back to in a second. And there are different parts of the sarcomere, right? There is the part that's designated the A-band that's in the middle. There's this other part up here called the I-band. All right. So let's focus in on a single sarcomere right here. So I'm drawing the outskirts of our sarcomere, of course, so that's going to be our z-line. That's hanging out on either side. I've drawn two of them here. Anchored to our Z-line is going to be our actin filaments. Here are the actin filaments that we've heard about before. I'll just label this. This is our actin filament. Remember, sitting inside is going to be our myosin, and our myosin filament-- remember, it's got two heads and it's associated right here with the actin. It wants to kind of pull on the actin and just crawl along the actin. I'll draw one here, as well. Myosin heads-- two myosin heads right there. And they're attached top and bottom like that. And they just want to walk across, all right? I want to make sure that I draw that here, too. And you get the picture down here. And so anchoring our myosin filaments in the sarcomere is going to be titin. We'll just draw the titin here. It's not attached to the ends of the myosin, but you can kind of see that it's holding it in place from somewhere deeper in right there. So that's our titin. And again, this is our myosin, this guy. Myosin filaments with the two heads that come up. And at this point, we can appreciate some of the bands we talked about over here. The part that's both myosin and actin is called the A-band. That's the A-band that we drew on the left side over here. And the part that's only actin that doesn't involve any of the myosin-- that's this point right here, and it continues on into the other sarcomere. This is the I-band. The I-band. And the way I think about it, I kind of looks like a one, right? So it's got one of our two major filaments. And then A is the alternative one, the other one, that's got myosin and actin in it. So that's the A-band and the I-band. So now when you recall-- so there's this axon fiber that's going to come in and release a message, an action potential that's going to come here and depolarize our sarcolemma. It's going to sort of spread everywhere. It's not just going to go in one direction. And one of the things that we have in our sarcolemma are t-tubules that can allow the depolarization, or this action potential, to go deep within our muscle cell or our myofiber to cause the sarcoplasmic reticulum to release calcium. Calcium then, as you remember, goes on to bind troponin. Troponin that's sitting on our actin will then tell tropomyosin, get the heck out of the way. And then our myosin filaments that we drew right here can go ahead and use ATP to sort of walk along our actin filaments. And so they would sort of walk along this way, and they would, relatively to the actin filament, stand still. They would be crawling this way, but wouldn't really do any of the moving. They'd be anchored down. It's the actin filaments, actually, that move. The actin filaments are going to be moving closer in to the center, and that effectively causes our I-band to get smaller. The I-band is going to get smaller when we have our sarcomere contract. And because the A-band involves however far the myosin spreads, the A-band does not change. Only the I-band changes here, as we effectively bring the two z-lines closer to each other and shorten the length of our sarcomere. And that's what's happening on our most micro-molecular level right here, with the sarcomere contracting. And all of that began with this axon fiber spreading the signal. So I hope you can appreciate, just kind of going from the top right here, when we're contracting our skeletal muscle and we go through all these smaller layers, what's happening at this molecular level right here. What's allowing us to contract, to flex our arm or kick a ball or do something of that nature. And I hope that you found all this to be somewhat useful to you.