As we start talking about muscle, it's really important to remember that there are actually 3 types of muscle tissue in the human body, and that's going to be skeletal muscle, cardiac muscle, and smooth muscle. And we're going to spend the most time talking about skeletal muscle, but you are likely going to need to know some details about cardiac muscle and smooth muscle as well. So here we just want to step back and think at the high level, how are these different muscle tissues similar and different? So first up, we can talk about where they're located. Skeletal muscle is located connected to bones, and that's right in the name. Skeletal muscle is connected to your skeleton. And when you think of the muscles of your body, you're thinking about your skeletal muscle. Cardiac muscle, that's in the heart, and again, that's right in the name. Cardiac means heart. It's the only place you find cardiac muscle. Smooth muscle, well, it's a little more complicated where that's located. It's in a number of places, but I usually think of it as really in those hollow organs of the body. And by that I mean the organs of your digestive tract, your bladder. It's also in blood vessels, and so when it squeezes down it's able to push food through your digestive tract. It's able to evacuate your bladder. It's able to control where blood goes through the body by squeezing down or relaxing in different blood vessels.

Next, we want to talk about voluntary or involuntary. And by this we mean, can you consciously think about it and make the muscle contract or relax? Well, skeletal muscle, that's definitely voluntary. If I think about it for, really, any skeletal muscle, if I want to move my arm, I did it. Right? Definitely voluntary. Cardiac muscle, that's going to be involuntary. I cannot think about it and make my heart beat. Likewise, I can't think about it and make my heart not beat. That's probably a good thing. You don't want to have to remember to have your heart beat. Cardiac muscle actually doesn't need any input from the nervous system to beat. It will just contract on its own. The nervous system does sort of turn that beating up or down, depending on factors in the body. But that's also something that you don't have direct control of. It's just sort of measuring states of your body, how much carbon dioxide is in your body, emotional state, etcetera. You can't think about it and make your heart beat. Likewise, you cannot think about it and make smooth muscle contract. Smooth muscle is also involuntary. I can't think about it and push food through my digestive tract. I can't think about it and make blood go one place in my body or somewhere else. Now you might think about it and say, well, I mentioned the bladder. Well, you actually don't have conscious control of the bladder muscle either. Now there is a skeletal muscle that is voluntary, that's a sphincter that controls whether urine can leave the body, and you have conscious control over that. But whether or not that bladder actually squeezes and pushes the urine out of your body, that's involuntary. You can't think about it and make the bladder do that. When you squeeze down to push urine out of the body consciously, you're actually contracting skeletal muscles of the abdominal wall to compress that pelvic cavity and push on the bladder. You're not contracting the bladder muscle consciously.

Next, we have striated, or whether the muscle has striations. Striations are sort of these crosswise stripes, this striping pattern that runs crosswise across the cell. Skeletal muscle is definitely striated. It's really clear to see this under a light microscope. So down here at the bottom we can see an image of some skeletal muscle, and we can see really clearly we have 3 cells here, and each one has these crosswise stripes running right across it. So sometimes I say it kinda looks almost like zebra striping, but sometimes I think more it looks like almost like there's a fingerprint on the muscle, or like you're looking maybe at the bottom of a snake. I don't know if you've seen the underside of the snake, it kind of looks like that. Alright. Cardiac muscle, it's also going to be striated, but it's harder to see. So this image of cardiac muscle here, you can see this branching pattern that is unique to cardiac muscle, and it's hard to see the striations here, but you can sort of make out that there's something going on up there. Maybe some striations going down there. Now there are these sort of darker single lines going across, kind of here, here, here. Those aren't the striations. Cardiac muscle also has something called called intercalated discs, and that's where one cardiac muscle is sort of linked to the next cardiac muscle in series, and you can see those as well. That's not what we're talking about. We're talking about the striations, and striations have to do with the protein structure, how the proteins are organized. And so the fact that cardiac muscle and skeletal muscle both have some striations, it tells you something that those proteins are organized in a similar way in these two muscle types. Now that's different for smooth muscle. Smooth muscle is going to be non-striated, and smooth muscle, remember, it's a sort of spindle shape, meaning it's sort of long and skinny, being a little fatter in the middle. And we can see that here, and there is just no crosswise striping pattern going across these. That tells you down at the protein level inside the cell, it's going to be organized a little bit differently than our muscle than our other two muscle types.

Finally, we want to talk about nuclei per cell. Skeletal muscle has many, and again we can see here in this image we just have 1, 2, 3 skeletal muscles, but look at all the nuclei we have. There's a whole bunch of them, and that's because skeletal muscles are gigantic. They're extremely long. They're formed by smaller cells coming together and fusing together to make these gigantic cells that are essentially the same length as the entire muscle. And because they're so big, they require these multiple nuclei to support the needs of the cell. Now cardiac muscle and smooth muscle both only have one nucleus per cell, and that's because these cells are much smaller. The cardiac muscle has this branching pattern, but again I said where you see these sort of horizontal dark lines, that's where those cells are joined together. The smooth muscle has these sort of spindle shaped, and it's sort of hard to see the individual cells here, but each one of these nuclei is going to sort of be in the middle of each cell. Alright, with that, we're now going to go into that skeletal muscle in a lot more detail. But before we do, like always, we have an example and practice problems to follow. Give them a try.