Muscle Fiber Structure

Muscle Fiber Structure


In this video we’re going to be talking about the microanatomy of a muscle fiber. The term musclefiber basically means a muscle cell. They’re called muscle fibers because they are such highly specialized cells and because they have a long cylindrical shape like a fiber. Now, because the structures in a muscle fiber are so highly specialized a lot of the structures that we’re used to thinking of from cell anatomy, they have special terms. So you will encounter the term sarcolemma. Sarcolemma is basically the plasma membrane of a muscle fiber. You will also encounter the term sarcoplasm, cytoplasm of a muscle fiber. Lastly you’ll see the sarcoplasmic reticulum, and that is basically the endoplasmic reticulum of the muscle fiber. You’ll see it abbreviated “SR”. I may abbreviate it [SR] during this video. And what it is is a highly specialized smooth endoplasmic reticulum in muscle fibers. So what we’re looking at here is a cartoon version of a muscle fiber. You can see the sarcolemma wrapped around the muscle fiber. You can see the nuclei of the muscle fiber and the nuclei are shoved out to the edges of the muscle fiber because the sarcoplasm the muscle fiber interior is packed full of these cylindrical structures called myofibrils. We’ll talk about the structure of a myofibril in great detail in this video because myofibrils are really – The myofibril is really where the magic happens in a muscle fiber. Myofibrils are the contractile elements when a muscle fiber contracts. It shortens in length because its myofibrils shorten in length. Right. The myofibrils are protein rods, highly highly organized protein complexes, and as a result of the arrangement of proteins in the myofibril you see striations of muscle fiber. Remember the striations are one of the distinctive ways to recognize skeletal muscle under the microscope and the reason why skeletal muscle has the striated appearance, or striped appearance striated is kind of a more technical way of saying striped. But these striations come from alternating dark bands and light bands. So you have the dark bands which are called the A bands and the light bands which are called the I bands and to keep that straight notice that dArk has an A in it and lIght has an I in it. The sarcomere is the smallest contractile unit of a myofibril. So if you take a bunch of sarcomeres and you line them up end-to-end, you have a myofibril. Each sarcomere is made up of a regular arrangement of protein. The primary proteins we’re going to be concerned with are myosin, represented here in red, and actin, represented here in blue. Now, sarcomere extends from basically one I band, from basically the middle of an I band, to the middle of the next I band. In the middle of the I band is a line of accessory proteins called the Z disc or the Z line. It’s the Z discs that mark the ends of each sarcomere. This is a more detailed view of the sarcomere. Again we’re looking at the cartoon version. This isn’t what you’d actually see unless you had an incredibly powerful microscope. In a few slides, we’ll see what the sarcomere actually looks like under the microscope. So here we have a sarcomere. It’s extending from Z-disc to Z-disc. You can see there are some accessory proteins forming kind of an anchoring point at each z-disc. You have the A bands there and you have I bands at either end of the Sarcomere. In the center of the A band you have a line called the M line which is again made up of accessory proteins. The main molecules that we’re interested in, the main proteins that we’re interested in when it comes to understanding how muscles contract are this: the actin filaments, referred to as the thin filaments, and the myosin filaments, referred to as the thick filaments. Thin filaments are polymers of the protein actin and they’re thin because – they’re referred to as thin filaments because they’re narrower in diameter. The thick filaments are polymers of the protein myosin and they’re referred to as thick filaments because they’re actually thicker in diameter. You also have what are called elastic filaments, and that’s what these yellow spring-like looking things are. And elastic filaments are polymers of a protein called titin. The elastic filaments run through the center of each thick filament and they anchor these thick myosin filaments at the Z-disc. So they kind of hold the myosin filaments attached to the Z-disc. These titin filaments are important because they help a muscle return to its resting length after contraction. When the muscle contracts, these titin filaments get squished together and just like how if you squish a spring together it gets harder – you can only push it so far together and as soon as you release it returns back to its resting length. Similarly, once the muscle’s relaxed, the titin filaments kind of push the muscle back to its resting length. Looking at the thick filaments in a little bit more detail you can see that they are made up of a number of copies of myosin protein. Myosin is kind of a funny-looking protein. It’s a little bit like a single Q-Tip. It has a long tail and then a head that has two important sites to it. It has a place where these myosin heads can actually bind to the actin on thin filaments and it has a site where the myosin head can actually break down ATP. In between the head and the tail there’s the hinge region and this is an area where the myosin filaments, where the myosin is flexible so the heads can actually move along, move at an angle at that hinge region. The thin filaments are made up of two spirals of actin subunits. Actin is actually a globular protein so each individual actin protein is shaped kind of like a little ball with just an active site where myosin can actually bind. So this is the site where the myosin heads attach. But when you string these actin subunits together, like this, you get a thin filament, an extended polymer of actin protein. So the actin forms the base of the thin filament, but attached to the thin filament you also have two regulatory molecules, troponin and tropomyosin. Tropomyosin and troponin together they basically control whether a myosin head can bind to the actin thin filament or not. So tropomyosin – Notice how tropomyosin, this long filamentous protein, notice how it’s covering up the active sites of the actin molecules. So tropomyosin, when it’s on the actin, when a muscle is at rest, when a muscle is inactive, tropomyosin covers the actin binding sites. Troponin holds tropomyosin on, so troponin basically makes sure that tropomyosin stays covering those actin binding sites. But when that muscle is stimulated to contract, troponin changes shape, rolls tropomyosin off of those actin binding sites, and now those active sites are exposed and the myosin heads can actually bind. So it’s the activation of the muscle fiber which causes troponin to change shape, which causes tropomyosin to move. That’s what allows myosin heads to grab on to actin and begin the process of actin and myosin sliding along each other, which is what causes the contraction. It’s the arrangements of the thin and thick filaments that leads to the banding that you can see on myofibrils and consequently on skeletal muscle underneath the microscope. The A band corresponds to the central region of the sarcomere and the darkest parts of the A band are the regions where you have both thin and thick filaments. It looks darker under the microscope because it’s very densely packed with protein, having both thin and thick filaments. The light I band only contains thin filaments. And so it’s much lighter under the microscope because the thin filaments are much thinner and this region is much less protein-dense. The very center of the A band it gets a little bit lighter again, because you have just the thick filaments. But again, they’re thicker than thin filaments so it still looks darker than in the I band. And then you have a line at the very center which represents the M Line, because you have accessory proteins. And you also have darker at the ends of each sarcomere, where the accessory proteins of the Z-line are located. This region, corresponding to this region, is very important for skeletal muscle contraction. [This] is called the zone of overlap. This is where you have both thin and thick filaments overlapping each other, so this is the only region where myosin can actually grab on to the thin filaments. When the muscle actually contracts, what happens is that those heads on myosin grab onto actin and kind of pull actin in towards the center of the sarcomere, and this happens at all the sarcomeres along the myofibril, on all the myofibrils of a muscle fiber at the same time. So in order for the myosin heads to actually grab on to actin, there has to be actin in the same area as the myosin heads, so this zone of overlap, this is the only place where that grabbing on, that attachment between myosin and actin can happen. This slide and the next slide illustrate what actually happens in the muscle fiber when a muscle contracts. It illustrates the sliding filament model of contraction. Now the filaments that are sliding, of course, are actin. Actin filaments slide along the myosin filaments. When the muscle fiber is active, when it’s actually contracting, you get attachments forming between the myosin Heads and the actin Filaments on both sides. And as the myosin heads start burning ATP, remember they have that ATP burning site in their myosin heads, these heads flex up and pull the actin filaments in towards the M-line. Now they can only go so far. The actin filaments can’t really cross each other and they can’t cross that M-line because there’s a whole bunch of accessory proteins there. But that pulling of the actin filaments in, that’s the sliding filament model, that’s what shortens the myofibrils, shortens the muscle fiber, and causes your muscles to contract. That is the contraction of your muscles. Here we’re seeing what a sarcomere looks like in a relaxed muscle fiber, so contraction hasn’t happened yet. You have zone of overlap, but you also have a zone where there are actin filaments without any myosin thick filaments. Let’s see how that changes after contraction. Here we’re looking at a fully contracted sarcomere. Notice the myosin thick filaments haven’t changed in length. They’re not actually moving. But what has happened is that the actin thin filaments have been pulled into the middle. Also notice the shortening of the I band. The I band is much shorter now because the actin filaments have been pulled in in the zone of overlap. Well, the sarcomere is almost all, almost completely zone of overlap, it’s almost completely A band now. I encourage you to take a moment or two flipping back and forth between these two pictures and comparing. Look in particular at the I bands, see how the the I band changes and gets smaller, and look at how the zone of overlap expands. And what’s happening is that the entire sarcomere is shortening. It’s the burning of ATP, ATP hydrolysis, that allows myosin to walk along the actin filaments. So although the myosin itself isn’t actually moving it pulls the actin filaments in and because the actin filaments are attached at the two accessory proteins at the Z line, the whole sarcomere contracts and all the Sarcomeres contract. This video gives you a general overview of the structure of a muscle fiber and in particular the structure of sarcomeres, the smallest contractile units in a muscle fiber. I strongly strongly recommend that you watch animations contracting muscle fibers and animations of the sliding filament theory in order to really visualize what’s happening at the sarcomere once the muscle is stimulated to contract. For now, after studying this video you should be able to describe the structure of a muscle fiber using the appropriate terminology. Use the terms sarcolemma, sarcoplasm, sarcoplasmic reticulum sarcomere, myofibril, thin filament, Tthick filaments. You should be able to explain what myofibrils and sarcomeres are, you should be able to distinguish between the thin and thick filaments, think structure, what they’re made of, and what they do during muscle fiber contraction. You should also be able to explain, generally, the sliding filament theory of muscle contraction.