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Let's talk about muscles. And I've drawn the human body on the right, kind of a figure of it. And I want to talk about the three major types of muscles. And I thought it would be helpful to have a picture, because then we can actually draw on there and show where the different types of muscles might be. So when I mention muscles, the word I want you to start thinking about in your head is movement. So think about all the different types of movements that might happen in your body. Just be really creative and start thinking of all the different movements. You might have, for example-- a really easy one would be, maybe, let's say your leg is moving. I'm going to just draw on our picture as we talk. But let's say your leg is moving because you're playing soccer. And so you've got this giant muscle in here, and this muscle is attached to a bone. Right? There's a little bone here. I guess not so little, right? This as the largest bone in the body. It's called the femur. And so this muscle is attached to the femur. And this muscle is going to be attached by way of tendon. It's going to have tendons on both sides. And so this tendon is attaching it to the bone and allowing it to act on the bones. So this is an example of skeletal muscle. Right? So this skeletal muscle is going to be attached to a tendon and bone. Now, that brings up the question-- does every skeletal muscle have to be attached to a tendon and bone? Well, the answer is no, actually. There are some muscles that really aren't attached to tendons at all. In fact, right above the muscle we just drew is a muscle called the external oblique muscle. And don't worry so much about the names. But the idea here is that this muscle is actually not attached to a tendon. Well, in a sense, I guess, you could think of it as a tendon, but it's like a flat tendon. Basically a giant kind of sheet of fibrous tissue. And this fibrous tissue, is it floating in midair? No. It's going to be connected to fibrous tissue on the other side, because, of course, your body is symmetric and so you've got fibrous tissue on the other side. And you guessed it, on the other side of that you've got another external oblique. So you've got these muscles that are kind of coming in to not really a tendon but really a flat tendon, or something that looks like a flat tendon, and we call that an aponeurosis. You might hear these words. I just want you to be familiar with them. And now if someone asks you, is every muscle in the body attached to a tendon and bone? You can say no. Some are attached to a flat tendon called an aponeurosis. The idea here is that you can kind of start identifying skeletal muscles. They're usually the muscles that you can see on your body. Actually, I don't even need to put quotes. That's the actual name for it. No need for quotes there. So you can identify skeletal muscles pretty easily. But what about the other two? What about the cardiac and smooth muscle? I mean, you might wonder, does cardiac mean heart? And is that the only type of cardiac muscle out there? And the answer is yes. This is your heart muscle right here. And the only type of cardiac muscle that we have in our body would be related to the heart. So in the heart, you can find specialized cells that were so interesting and different from skeletal and smooth muscles, they got their own name and category. These are the cardiac cells. And you can only find them in the heart. I guess we're making a column of where you can find these cells. So what about smooth muscle? Where can you find smooth muscle? Well, for smooth muscle, think about any hollow organ. Any organ that's got space on the inside and blood vessels. Those are the two major categories. Those aren't the only ones, but those are the major ones. That'll get you about 95% of the way there. So blood vessels and hollow organs are what you should think about. And hollow organs could be anything from-- let's say, your stomach would be a hollow organ. Let me just put these examples here. Or your bowels would be a hollow organ, anything like that. So I'm just going to write stomach here just to jog your memory. Where there's basically some empty cavity on the inside. Right? And then as for blood vessels, just remember one of the largest blood vessels, for example, is the aorta. And the aorta kind of comes up and over like that. And it's kind of like a hollow organ, as well. Right? I mean, there's a space on the inside of that blood vessel. And blood is usually flowing through that space, but at least it's hollow. So it's really not that different conceptually from the hollow organ. And just like in the hollow organ, the smooth muscle is in the walls of these things. So think about where the smooth muscle would be. It would be in the walls of the hollow organ or in the walls of the blood vessel. So that tells you where to find these different muscle types. Right? And thinking about movement, smooth muscle can help the stomach, for example, move food forward. Cardiac muscle is going to help your heart beat. That's a pretty important movement. And skeletal muscle, I mean, we use that every single day. Every time you give your friend a high five or give your mom a hug, those are skeletal muscles that are helping your body move around. Right? So let's move on. Let's think about some other differences between these categories. Let's talk about now the movement control. So who controls the movement? Do you control it, or is it automatically done? So smooth muscle is what I would consider automatic, or I'm going to call it involuntary because you'll probably see that word more often. Involuntary just means that your body is automatically taking care of it. And the same is true for your cardiac muscle-- involuntary. Meaning, you don't have to actually think about the next heartbeat. It just happens automatically. Right? And skeletal muscle is the opposite-- there, it's voluntary. Meaning if I didn't want to get up, then I would not get up. Or if I didn't want to go running, then I wouldn't go running. All of those movements in my body are under my control. I can decide when to do those things. Right? Actually, maybe I'll draw little arrows here-- what about speed? Which ones are fast, and which ones are slow? So up here, the smooth muscle is the slowest and the skeletal muscle would be the fastest, which is pretty cool because the voluntary stuff-- the stuff you control yourself-- is the fastest. And the stuff that's happening automatically is pretty slow. And actually it's nice, because cardiac muscle is somewhere in between the two. Somewhere in the middle. So when your blood vessels get tinier or they get big and vasodilate, all that stuff is happening on a pretty slow time scale as compared to, let's say, I jump and try to catch a ball. That's all happening really, really quickly. Thousands of little muscle movements are happening really lightning quick. And so those would be the fastest. Now the final thing I'm going to draw is what these things look like. So how do they look? If you actually take a look at these cells-- let's actually look at each of these one by one and figure out what they would look like. So the smooth muscle actually looks like a little eye, or like an almond-- sometimes it's described that way. But I think of it as an eye. One single eye. And you can see that the edges, or the ends, are kind of tapered like that. And so sometimes you'll see that these are described as spindle shaped. I think that's kind of a holdover from a time period long ago when people thought about spindles more than they do now. And the other thing, it's got one nuclei. Drew that right in the middle. One nuclei. And it's in the middle of the cell. So that's basically what a smooth muscle cell looks like. What about a cardiac cell? Well, this cell is branched. That's actually one of the most interesting hallmark features of it. Now, not every single cardiac cell is branched. Some are actually just kind of humdrum-looking, normal, maybe like this. But the fact that you can find branched ones is what really makes these so easy to recognize. If you look at a whole bunch-- I'm going to erase this guy now that you know he exists, but I'm going to focus on the branched one because these are the ones that make them very easy to spot. And they also have nuclei. Sometimes one, but sometimes two, which is interesting because, you know, usually you think, one cell, one nuclei. But the reason I had to point that out for the smooth muscle cell, that there's only one, is that sometimes these cardiac cells have more than one. So the two features-- I'm going to just write out here-- branched and one or two nuclei. Not always two, but they can have two. And they're also located kind of in the middle of this cell. And I'll show you what I mean by middle when I draw the skeletal muscle. I'll do that now. This is the skeletal muscle, and it's got something like this. It's got these little outpouchings I'm trying to draw for you. And you'll see in just a second what I'm drawing. These are little spots on the edge, or on the periphery, for nuclei. And notice that there's not one nuclei, not two nuclei, but bunches of nuclei. So these cells are actually working as a giant cell, in a sense. So these are actually, first of all, they're straight. They're not branched. So straight. And they've got many nuclei. This is actually really, really important, and you can see how it would be easy to spot these guys, right? Because they've got many nuclei, and the nuclei themselves are in the periphery, kind of on the edges. That's why I wanted to point out that the other two are in the middle. Now, kind of a final point is that if you were to look at these under a microscope-- and actually, this is something that was noticed a long time ago-- they would look something like this. And this is called striated. So they basically have these striations. But notice that the smooth muscle cells don't have this. It's really just the skeletal muscle and the cardiac muscle that has these striations. Sometimes you'll hear about striated muscle, and they could be talking about either of the two. Right? They could be talking about cardiac or skeletal, but you know that they're not talking about the smooth muscle. So this is striated. And striated just refers to those stripes. And that's what it looks like under a microscope. And we'll talk about exactly why they're striated what that would imply about the cell in another video. But I just want you to get a kind of a rough lay of the land. And now you can see there's actually some interesting stuff here. You have some similarities between the cardiac and the smooth muscle. They're both involuntary. You've got some similarities between the skeletal and the cardiac. They're both striated. And so you can see how all three are somehow similar, but also somehow different from one another.