Introduction to Muscles and Muscle Tissue - Online Tutor, Practice Problems & Exam Prep

Properties of Muscle Tissue

Hello, and welcome to the muscular system. Alright. As part of the muscular system, eventually, you are going to learn many of the individual muscles of the body, but we're not there yet. First, we just want to talk about muscle more generally. What is muscle? What does it do? And really importantly, how does it do it? So to kick that off, we are going to be talking about the properties of muscle tissue. And you'll remember when we talked about tissue, we said that muscle tissue is specialized for contraction, and it contracts in order to create some form of movement. At the tissue level, muscle is the only tissue in the body that can move. Now there's other things in the body at the cellular level that can move, but at the tissue level, it's just muscle. And muscle only moves in one way. It shortens or contracts with force. So by stating that, we can then sort of restate this function of muscle. We can say it a little bit more formally. What muscle is doing is it's converting chemical energy to mechanical energy. And mechanical energy is really just move. Now chemical energy in the cell, it's going to be largely stored in the form of ATP. So you can think of muscle as a tissue that's specialized to convert the chemical energy stored in ATP into mechanical energy or movement. Now another function that it does, sort of as a byproduct of this, is that it is going to generate heat. And we're not going to talk about that in much detail here. We did mention it very early on in the course when we were talking about homeostasis and maintaining body temperature. Generating heat though is something that you should definitely know is a function of muscle. Alright. So all muscle though also shares some properties, and so we're going to go through 4 properties that all muscle tissue shares. First up, and this is what we've sort of been talking about so far already, muscles have contractility, and that just means that muscles are able to forcibly shorten. Muscles get shorter with force. That's what they do. And to illustrate that we have, an arm here and we can see the bicep. Well, that bicep gets shorter with force. Well, the arm is going to bend up. When the tricep gets shorter with force that arm is going to straighten out again. Muscles are able to contract, they shorten with force, that's what they do. Another property though, muscles are also extensible. They have extensibility, and that just means that muscles are able to stretch. They have the ability to stretch. And to illustrate that, we have someone in a yoga pose or something like that. So muscles get shorter with force, but if you pull on a muscle, it will get longer. It's able to stretch. Importantly, it can't get longer with force, but if you pull on it, it will stretch. And I like to stretch in the morning. Feels good. We also are going to say that muscles are elastic. They have elasticity. And this just means when a muscle is stretched, or when it contracts, it's going to return to its original size. And we are going to illustrate that here with someone playing with a rubber band on their fingers. A muscle will contract. It can be stretched, but left to its own devices, it's going to go back to its sort of starting size. Now, finally, we have this, 4th property, excitability. And excitability is the one that you probably least sort of naturally associate with muscle. And that, property of excitability means that muscle is able to transmit stimuli. That's something that you probably more naturally associate with the nervous system. The nervous system passes stimuli using action potentials. And here as a To illustrate this we have the nervous system passing a signal to the muscular system. Well, muscle cells are also able to pass, stimuli with action potentials. The membranes of muscle cells are excitable, much like the nervous system, and that's going to be really important for the muscle cells knowing when they are supposed to contract. So those two properties, excitability and contractility, that's what we're going to probably spend the most amount of time talking about. How do muscle cells get the signal to contract? How do they spread that signal? And then most importantly, how do they actually contract or shorten with force? Alright. I'm looking forward to it. So let's get to it.

A main function of muscle is to convert chemical energy to mechanical energy. Which property of muscles relates most directly to this function?

• A. Contractility.
• B. Extensibility.
• C. Elasticity.
• D. Excitability.

Types of Muscle Tissue

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.

Introduction to Muscles and Muscle Tissue Example 1

Our example says that Amal claims he can lower his heart rate by thinking about it, just like how he can flex his bicep by thinking about it. Does this seem possible, given the type of muscle tissue in the heart? And explain your reasoning. Then we have an image here. Just as a reminder, we see some skeletal muscle, we see some cardiac muscle, and we see some smooth muscle. Alright. So can you think about it and lower your heart rate? Well, in the heart, we have that cardiac muscle. So knowing that, what do you think the answer is? I'm going to say no, you cannot. And the reason is that cardiac muscle is involuntary. Involuntary meaning you cannot directly think about it consciously and make that muscle contract. So your bicep, well, that is skeletal muscle. Your bicep, this sort of major muscle here in the arm, if I want to flex it, I just say, hey, muscle flex, and there you go. It does. Not hard to do. Smooth muscle, on the other hand, the muscle of, like, your internal organs, no conscious control over it. I cannot think, hey, push my food through my intestines more. Do that. I can't do it. Now, cardiac muscle beats on its own. It does not actually need an input from the brain to beat. What the brain tells it to do is when to speed up and slow down. But you do not have direct conscious control over that. What some people can do is they are able to calm themselves down and relax their body very efficiently, very quickly, or increase their emotional state in a way that their heart rate might go up. But that is always going to be a secondary effect. Those people are affecting their emotional state or their physical state in a way that the brain secondarily sends a message to the heart. If they had just sprinted a 100 meters and are out of breath, they are not going to be able to change their heart rate by doing that. You do not have that kind of conscious control. Alright. Problems to follow. I'll see you there.

You are looking through a microscope at muscle tissue and you do not see any striations. What type or types of muscle could you be looking at?

• A. Cardiac.
• B. Skeletal.
• C. Smooth.
• D. A & C are both correct.

What type of muscle tissue is shown in the slide below?

• A. Cardiac.
• B. Skeletal.
• C. Smooth.
• D. It is impossible to tell from the image.

Which type of muscle cell is typically the largest?

• A. Cardiac.
• B. Skeletal.
• C. Smooth.
• D. Different types of muscle cells are roughly the same size.