 Hi, guys. Today we get to start our conversation about muscles. We're going to spend most of the next three lectures talking about skeletal muscle tissue, but there actually are two other kinds of muscle tissue, and I want to start us off talking about muscle tissue in general. For perspective, I want to remind you that muscle tissue is one of our four main tissue types that are found in the body. Remember that they were epithelial tissue, been there, done that, connective tissue, been there, done that many times, muscle tissue, doing that right now, and nervous tissue, and nervous tissue gets to wait until the next section. It's going to start us off when we go into our sixth lecture conversation about the nervous system. So it makes sense that we'll start this discussion by looking at what muscle tissue is overall. First of all, muscle tissue has a very unique structure that enables its function, which is shortening. Muscle tissue shortens, and usually muscle tissue shortens, muscle cells shorten along their long axis. If you imagine a cell that muscle cells actually are either sort of oblong or really, really, really, really oblong and kind of just long, and they shorten along their long axis, we're going to look at skeletal muscle cells in detail and the anatomy and how a skeletal muscle cell is really unique when compared to our normal vision of what a cell actually is. Because muscle tissue shortens like this, then we can have many different functions, and we should put muscle tissue inside structures that we want to change shape. Some of the functions of muscle tissue, skeletal muscle tissue like this bad boy, her, skeletal muscle tissue often will span a joint and when it shortens, it actually causes movement. You guys are totally good with that because we just did that. And we talked about joints and movements. It's skeletal muscles that power that. Smooth muscle, okay, we better come up with our different kinds of muscle tissue. Skeletal muscle, smooth muscle, and cardiac muscle. Now, in talking about what these are, keep in mind skeletal muscle, we're going to talk about it a ton. Smooth muscle, we've already seen a little bit, and cardiac muscle we're only going to find in the heart. I'm going to talk about these muscle tissues really fast to give you some histological perspective because today in lab, we are going to look at these different kinds of tissues. Aw, does it look kind of like, oh, I know what that is. That's actually smooth muscle tissue. This is a long section of smooth muscle cells. This is like a cross section where we took them right down the middle and sliced them up and now they look like little dots. Most of our slides don't stain that awesome where you can actually see the difference, but hopefully you're looking at this thing right here going, oh, boom, I know exactly what that is. And it's true, you should know exactly what this is because look, this right here, do you see some epithelium? Do you see some villi? Do you see some, you can't see the microvilli, but I'm telling you that they're there and a whole bunch of holy layer madness. Where are we, doggies? We're in a intestinal part of the digestive system. We're actually in the jejunum. And if you look at where this slide is zoomed in, we're zoomed into muscularis externa. What kind of tissue do we have in muscularis externa? We have smooth muscle. Histologically smooth muscle, the cells themselves look almost like little spindles and they have a nucleus. You can see that they are kind of oblong. They're kind of like spindle shaped or slug shaped. They look like slugs to me. The fact is that they shorten, but they do not have striations. And that is an important distinction between smooth muscle, skeletal muscle, and cardiac muscle. So smooth muscle has no striations and smooth muscle is involuntary. In other words, you can't be like, dude, contract my muscularis externa and let's do a little jig with the jejunum. That's not going to happen. No matter how much you want that to happen, your jejunum is not going to do a jig just because you said it should. So we put smooth muscle in places where we want movement or we want to be able to change the diameter of a tube, bronchioles. We don't really deal with that consciously. All right, another kind of muscle. Oh, this is cardiac muscle. Now, take a deep breath. I mean it. Do it. I'm going to write up here that we're in cardiac muscle land so you don't forget it. And the deep breath that you're taking is I'm just telling you, the cardiac muscle is striated. When I look at this slide, I'm like, oh, of course it's striated. I can totally see the striations. But sometimes I think that in histology we can see the things that we know are already there. So let's talk about some of the characteristics of cardiac muscle. There's one nucleus per cell. Okay, it is striated. They branch. The cells branch. Think about your heart. Why would it be important for cells to branch? And then they connect. The cells connect to each other at structures called. I'm going to write it in here. Intercalated disks. Now, take a deep breath and I'm going to draw over one. See that little line right there? That is not a striation. That's actually an intercalated disk where this cell, this cardiac muscle cell, is connecting to this cardiac muscle cell. Now, I don't know if I can see a place that illustrates super off here. Oh my gosh, look, here's an intercalated disk. I just drew over the top of it. And here's another characteristic of cardiac muscle that is really cool. Oh, I already wrote that down. It branches and it has intercalated disks, but of course, but that's a great example of branching. Now, branching and intercalated disks are really important structures, especially for physiology, because the heart never stops beating ever until you are dead. And then that is sad. We're sorry that your heart stopped beating. So the intercalated disks help hold the heart together. They also help the heart muscle contract all together without actually needing nervous input. That's a cool, fast fact that we'll deal with when we get to the heart. Okay, and this, my friends, this is where we're spending most of our time. In the lab today, we are going to have slides of smooth muscle and cardiac muscle, and you're going to have to identify them on the practical and on all your quizzes and whatever. But skeletal muscle is where we're focusing from here on out. So here's what I want to tell you. Skeletal muscle, oh, good lord. I kind of want to just undo that. Skeletal muscle is a single cell. It's this long, crazy thing right there. And what do you notice? Look, what's that thing? And that thing, and that thing, and that thing. What? This is one cell, doggies. There's like a million nuclei in skeletal muscle. Skeletal muscle cells have many nuclei. That's one of the characteristics. Skeletal muscle is nuclei. Do you see striations in my skeletal muscle? Dude, you can totally see the striations here, right? You don't even have to imagine it. See all those stripes? It's striated. Skeletal muscle is voluntary. I'm using skeletal muscle to move my lips. I'm using skeletal muscle to move my head and make noises with my larynx and flex my bicep to show you how massive I am. And what else do you want to know? Skeletal muscle is involved in temperature regulation when you shiver. If we did a cross-section of skeletal muscle, what do you notice? What do you think those little dots are along the edges? Those are nuclei. And each one of these circles that I'm drawing is one skeletal muscle cell. So we're actually going to spend some time looking at the skeletal muscle cells, why they're striated, where the striations come from, and a little bit about how they function. And then we're going gross anatomy and we're going to start looking at skeletal muscle organs. Let's start by getting big to make sure we know where we are and then we'll look at our tissue in more detail. Why are these striations here and why are the nuclei all around the edges? Okay, I'll be right back.