March 8, 2011

041 An Introduction to Skeletal Muscle Contraction

Ever wonder how our muscles contract and what makes them do so? In this video, Leslie gives a clear overview of how muscle contraction works.


Transcript of Today’s Episode

{Video begins with Leslie in a gym doing some sets of bicep curls} Oh, hey! Hello and welcome to another episode of Interactive Biology TV where we’re making Biology fun. My name is Leslie Samuel and in this episode, Episode 41, I’m going to give you an introduction to skeletal muscle contraction. But, one second. Let me finish my set. {Leslie goes back to finish a few more sets of his bicep curls.}

So, here I am working out in the gym and I am doing some bicep curls. I’m lifting the weight and there’s muscle contraction happening. Now, when I’m talking about skeletal muscle, I’m talking about this voluntary muscle. The muscle that I have control over and it’s the ones that I use to move my bones, to move my body, to walk, to lift weights, and as you can see here, I am doing some bicep curls. I am using my biceps.

Now, let’s look over here to the right. You will see that the biceps are these muscles over here. And, what I’m actually doing is I’m contracting these muscles, making these muscle fibers shorter, and as they become shorter, they are actually pulling my arm up in that direction. So, I’m making these muscles shorter by contracting them and that is pulling on the bones in my arms, my lower arms and that is raising my lower arms.

Now, you can see here that there are a lot of stripes. We call these stripes, striations. And, each individual fiber that you see here is one muscle cell. So, these cells are narrow but, they are also very long. So, these are the muscle cells and you can see they are many muscle cells that make up each muscle and each muscle group.

Now, what we are going to do is we are going to take one of these muscle fibers and look at what’s happening there. So, let’s go to the next slide.

Here we have one muscle fiber and, you can see we have an axon coming in. So, the axon here is number one and that axon makes a synaptic connection with this muscle cell or another thing you can call it is a muscle fiber. And the interesting thing about this muscle fiber is that it’s made up of these tubes that kind of go all the way along, kind of like little fibers inside the muscle fiber and those are called myofibrils. So, we have the muscle fiber or the muscle cell which can be very long and very slender and narrow. We have the axon that is coming in making a synaptic connection with the muscle cell and these little fibrils, myofibrils inside of the muscle cell. This entire thing with the neuron, the motor neuron and the muscle cell, this is called a neuromuscular junction. In other words, it’s the junction or the connection between the nervous system and the muscle cells.

Now, what I’m going to do is I’m going to take one of these myofibrils, these guys in here and I am going to look at that down here. So, let’s go ahead and do that. And, this is going to look a little strange but, what I’m basically doing, this is one fiber. All of these here, it actually extends much longer than that. This is looking at one of these fibers. And you’re going to see that there are these different sections, and it goes from here to here, and then, from here to here. We’re going to talk about what these different components are and what role they play in muscle contraction.

Now, right here, each one of these units, I call a sarcomere. The sarcomere, this important unit here is the functional unit of contraction. This is really where the contraction happens. The sarcomere is made up of two main fibers. The main fibers are actin, and this narrow one here, I’m going to call actin; and the thicker fiber is called myosin. So, we have actin filaments and myosin filaments. So, we have actin and we have myosin.

And what’s happening as I contract my muscles is that on the myosin we actually have these little heads that extend and, you know what I’m just going to zoom in on one of those heads and you can see that over here. We have the myosin head here and it’s connecting to the actin and what that does when contraction is supposed to happen, this actually moves and pulls against the actin. So, you can imagine here. You have these little heads that are pulling, pulling in this direction, and what ends up happening is, this distance here shortens as this moves in and this moves in because of the heads that are pulling on it.

So, once again up here you have the myosin head that’s moving in this direction. As it moves in that direction, it pulls the actin along, and that pulling shortens the sarcomere so that you might have maybe the sarcomere being this long instead of that long. So, it’s shortening it. And, what that’s going to do, it doesn’t only happen here, it happens here, it happens here, it happens here all along the muscle fiber or the muscle cell, that shortens the muscle and that causes contraction.

All right so, we’re just kind of going over the major details that are happening. In future episodes, we are actually going to look at these individual steps and break them down a little more. But, for right now, that’s just an introduction into muscle contraction, specifically, skeletal muscle contraction.

That’s it for this video. If you have any questions, you can leave them in the comments and you can always visit the website at for more Biology videos and other resources. That’s it for now, and I’ll see you on the next one.


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