Source: Video and Images Created by Amanda Soderlind
Welcome to this lesson today on muscle contractions, micro level. Today, we will be taking a look at the micro level of muscle contractions, so discussing how a muscle contraction happens on a small scale level. So muscle contractions basically are just the shortening of muscles which acts to generate force. So when you contract a muscle, that muscle is able to generate a force. And fibers within muscles shorten causing contractions. So contractions happen because these fibers within the muscles are shortening.
And the sarcomere is the basic unit of contraction. So in this lesson, we're going to be taking a look specifically at sarcomeres and how they act in order to produce a muscle contraction. So muscle contractions rely on calcium. So a muscle contraction can occur when there's an increase in calcium ions. And calcium is either stored or released by the sarcoplasmic reticulum. So the sarcoplasmic reticulum is like the endoplasmic reticulum of muscle fibers. It stores and releases calcium necessary for these muscle contractions.
So let's take a look at this diagram here, just to give you a general idea, so you have a picture of what we're talking about when we talk about a sarcomere, or a muscle fiber. So this right here, the big part of this, we're going to pretend is a bicep muscle. And then with that bicep muscle, we have all of these different bundles of fibers. And if we break this down, we can take a look at one fiber. And that fiber is actually made of a myofibril. So fibers are made up of something called myofibrils. And within myofibrils, we find sarcomeres. So as I mentioned, sarcomeres are the basic unit of contraction.
So myofibrils and fibers will line up parallel to each other in a muscle tissue. And these myofibrils have this banded appearance, so when they're bundled together, it's what gives muscles their striated, or striped look, is because of the lining up of these banded myofibrils. So if we take a closer up look of this myofibril, we'll notice that the myofibril is made up of sarcomeres. So sarcomeres are portions of myofibrils. So we're going to label some other things on this diagram here as well. This line right here is our Z-band.
So Z-bands mark the ends of sarcomeres. So when a sarcomere shortens, or when a contraction happens, it's because the sarcomere is shortening and new Z-bands are moving closer together. And then these up here are called I-bands. And then we have our A-band. So you'll notice an A-band composes part of the sarcomere. So when a contraction happens, we have something within our sarcomeres we'll discuss in a second called thick and thin filaments. So when a contraction happens, the thick and thin filaments are overlapping each other within our A-band region.
So let's take a look at some more diagrams to help explain this a little bit more. I'm going to zoom out just a little bit here. So as I mentioned, within our sarcomere, we have something called thick and thin filaments. And those thick and thin filaments work together in order to produce a contraction. So if we take a look here, we have a protein called actin and a protein called myosin. Actin is a protein that's referred to as the thin filament in a contraction, whereas myosin is the protein referred to as a thick filament. And as I mentioned, these work in order to produce a contraction.
So actin is a protein. It kind of looks like a strand of beads, whereas myosin, if we're talking about just one molecule of myosin, it's composed of a head and a tail. But when we take a look at myosin, this is actually composed of several molecules of myosin. And you can see the heads on the diagram right here. And those heads play an important role in a contraction. So let's take a look here at this diagram, just to kind of visualize what's happening during a contraction. OK. So we're going to label these here as actin, our thin filaments, and then these here as myosin, our thick filaments. And then we have our Z-bands on each end.
So as I mentioned, the Z-bands mark the end of a sarcomere. So we're pretending like this here is one sarcomere. When a contraction happens, actin and myosin, as I mentioned, overlap with each other and the Z-bands will move closer together. So if you take a look at this diagram right here, so this would be a relaxed muscle fiber. And this would be one that has contracted. So you'll notice that those Z-bands have moved closer together, and the actin and myosin are now overlapped. So this is kind of a general idea of what's happening in a muscle contraction.
Now let's take a look at this diagram over here. So we're going to explain this in a little bit more detail, exactly what's happening with actin and myosin in a contraction. So myosin again is our thick filament. So this is going to be myosin. And this is actin. OK, we have our actin and our myosin. And these are our myosin heads. So this is an example of a resting sarcomere. It's at rest.
But when calcium is released from the sarcoplasmic reticulum, as I mentioned, calcium plays an important role in muscle contractions. So when the sarcoplasmic reticulum releases calcium, myosin is then allowed to bind to actin. So when that calcium is released, we'll have the myosin heads now bound to actin. So you'll notice up here, they're not bound to each other. But here they are. So that calcium was released and myosin can now bind to actin. Once myosin has bound to actin, the myosin heads will pull the actin filaments toward the center of the sarcomere. So these heads are now bound to actin. And they're going to pull them towards the center of the sarcomere.
And in order for this to happen, it requires the ATP energy. And as this happens, the Z-bands are also moving towards the center of the sarcomere. So these heads are attached and they're pulling them towards the center, just like when we saw in this diagram here, where they're being pulled towards the center and overlapping. So that's what's going to happen in a muscle contraction. That's how actin and myosin will interact. When ATP will then bind to myosin, myosin will detach from actin. And then it will return to its resting state.
So there are kind of two types of contractions that can occur. We have tetanus and twitch. So a twitch is a type of contraction where just one contraction will happen in response to the firing of a motor neuron. So we're talking about just one contraction. But tetanus is a sustained contraction that's caused by repeated muscle twitches. So if a muscle twitches over and over and over and over again, it can sustain that contraction.
So this lesson has been an overview on muscle contractions at the micro level.