Before we get into our discussion of the specific muscles that are affecting the joints that we just discussed, let's talk a little bit more about the specifics of what muscle actually is. In order to understand that, we need to understand the properties of muscular tissue. So I'm going to discuss four specific properties. These are not just unique to skeletal muscle which will be our focus but of the three main categories, skeletal, smooth, as well as cardiac muscle. So when you're talking muscular tissue, one thing that you need to understand is that it's electrically excitable. What in the heck does that mean? This means that it can change due to some type of external stimuli, be it something electrical or in terms of hormones. And we'll talk about that when we get into smooth muscle. So you can have a stimuli, so an ion change in terms of the muscle tissue allowing for an action potential to allow for a contraction. So you have the specific two muscles, as well as nerve cells. And you'll get into that quite a bit more with Dr. Sullivan in subsequent videos. Now, all muscle tissue is capable of contracting. So with the action potential that allows for those sarcomeres within the skeletal muscle to contract allowing for the muscles to move, and move the bone in turn. So contractility is a property of muscular tissue. Extensibility is a big one, the ability to stretch. Now, depending on what type of muscle tissue you're talking about, you can have quite a bit of extensibility or less so. So skeletal muscle is kind of between cardiac and smooth muscle, it's kind of one of the in between. And the connective tissue that's associated with the muscle tissue will really play a role in terms of only allowing it to stretch so much. And lastly, elasticity. So you have the extensibility which is the ability to stretch. Elasticity is the ability for it to go back to its original shape without damage. So elasticity plays as bigger role in terms as extensibility. All right, functions of muscular tissue. Obviously, the big one, producing body movements. When you have the contraction of the muscle, it's going to pull the bone allowing for movements to occur at the joints. So without muscles, you're not going to have body movements. In addition, many of the muscles of your body are continuously contracting. Not really forceful contractions, but smaller contractions allowing for the stabilization of body position. Allowing for me to sit upright, or if I were to stand up, you would have those muscles of the lower limb as well as the pelvis, stabilizing your body positions. Additionally, muscles can store and move substances within the body. Calcium's a big one. Calcium keeps coming back particularly when we're talking about musculoskeletal system. So you have the storage of calcium in some. You have blood as well, that is going to play a big role in terms of being stored within the body for the muscles. And lastly, producing heat. Think about it, when your muscles contract, this is going to release some heat. One of the best examples or way to really visualize this, is thinking about when you're really, really cold, what's one of the first things you're going to do? Your teeth are going to kind of chatter, or you're going to shiver throughout the body. That is allowing for muscles to contract and allow for heat to be released. Now, we are going to focus mainly in terms of skeletal muscle cells. When we're talking about the musculoskeletal system, we're talking skeletal muscle cells. Now, both skeletal muscles as well as cardiac muscles are referred to as striated muscles. And all this means, is that the two main contractile protein filaments which are myosin and actin. These two particular proteins are going to be very well-organized into what's referred to as a sarcomere. So, this organization, when you're looking histologically at these particular cells, will allow for this very organized appearance that you see in skeletal muscle cells and cardiac muscle cells. Now skeletal muscle cells, for the most part, are voluntarily controlled. Okay, so you think about it, I'm thinking about moving my elbow. I'm contracting the muscles that are going to allow that to happen. Now it's important to note, that not all skeletal muscle cells have a purely voluntary component. There is going to be some involuntary control for some specific muscles. So we'll talk about that in more detail later. Now the major difference between the skeletal muscle cell and the cardiac muscle cell, well obviously the main one, is that cardiac muscle cells are solely located within the heart. So they're making up the walls of the heart. Another important part is that this is going to be involuntarily controlled. And what's very unique, and you'll go into quite a bit more depth with Dr. Fox in subsequent lectures, is that cardiac muscle cells have their own pacemaker within. So whereas, you're going to have nerves that are going to allow for skeletal muscle cells to send that signal to allow for an action potential. That you're going to have components within the heart, totally within the heart, not part of the central nervous system, that are going to allow for the heart to actually, those muscle cells to contract. So they are unique in terms of having its own pacemaker within the heart. Now the last, and kind of the more unique of the muscle cells is the smooth muscle cells. And you're going to have these, obviously as the image shows associated with your gastrointestinal portions forming the walls of your GI system. But also importantly, you're going to have smooth muscle cells associated with blood vessels. So particularly, in terms of arteries, you're going to have quite a bit of smooth muscle cells associated with that. Now you're looking at the schematic here, and you can see that you don't have that typical striation that you have in the skeletal muscle cells as well as the cardiac muscle cells. You do not have the organization of those filaments, the myosin and the actin, into the sarcomere. So histologically, you don't have that presentation. Now you very much obviously have myosin and actin because there's contraction, it's just not organized in the same manner. Now one of the more unique things about the smooth muscle cells is the amount of extensibility that's allowed in these. So the ability to stretch the smooth muscle cells is quite a bit more than you have in the skeletal and cardiac muscle cells. Additionally, as similar to the cardiac muscle cells, this is going to be involuntarily controlled. So, that's kind of an intro in terms of the types of muscle cells that you're going to encounter throughout your time in this anatomy MOOC. But we're going to focus on the skeletal muscle cells as we discuss the MSK portion.
