 There everybody, Dr. O here. There's going to be several videos where we talk about sarcomeres. Sarcomeres are the functional unit of a muscle. Now this is where contraction actually takes place, but in this video I just want to talk about the parts of the sarcomere. I know students get very confused when they look at this jumbled picture here at the top. So I just want to walk through the individual parts. What does z-line mean, H-zone, that kind of thing for just this video. Let's start with the z-line actually. You can see the z-lines there. They're in the area where you only see greens. That's going to be your thin filaments active. The z-lines are the end of each sarcomere. So they define where one sarcomere begins and then where it ends and I guess where the next one begins. So the z-line only has the thin filaments which are called actin. They're actually connected or anchored there to the z-line. So a sarcomere is going to run from one z-line to the next. Remember that. That's a sarcomere, z-line to z-line. The whole sarcomere is either at the top. Only has thin filaments at the z-line here because that's where they're actually anchored. So that is the z-lines and they're very, very important. Next we have, we have the m-lines. So m-lines, like the name implies, they're going to be found in the middle and they're going to be in the middle of the myosin, thick head. So I think it's easier to remember, m-middle, m-myosin. So the m-line, the middle of each sarcomere and it's going to be where the thick filament myosin is going to be. It's going to be right there in the middle. This part, so the m-line only contains thick filaments. So the z-line only contains the thin filaments actin. The m-line only contains the thick filaments myosin. Then we have the eye bands. You see at the bottom there, lighter eye band, darker a-band. That's because the thick filaments are thicker and darker. So the eye band is going to be this area around a z-line. So it's going to actually be part of two separate sarcomeres, the one on the left and then the one that we're actually looking at here. The eye band only has thin filaments and the eye band is going to get smaller when muscles contract. So as muscles contract, the myosin is actually pulling and tugging on actin. So as muscles contract, this sarcomere is going to shrink down. I'll show you that in a video on muscle contraction. So the eye band is going to be one of the parts that gets smaller during contraction. So that's the eye band. It only has the thin filaments actin and it gets smaller during contraction. Next we have the H-zone or H-band. I usually call it the H-band, but you see H-zone here on this picture. So this is going to be a zone right there around that m-line that only contains the thick filaments myosin. So the H-zone or H-band is also going to get smaller during contraction. That's because myosin is not moving, but it's pulling actin toward it so that H-zone is going to be squeezed in as actin is pulled towards the center of that m-line. So the H-band or H-zone is the area with only myosin and it also will get smaller during contraction. Then lastly we have the A-band. So the A-band is going to be, this is going to be the entire area in a sarcomere where there will be thick filaments, the myosin. And this will not change in size because during contraction, yes, actin is going to come and lay over top of myosin, the thick filament, but the myosin isn't changing length. So the A-band will not change size. All right, so the Z-lines are going to define, so a sarcomere runs from Z-line to Z-line. That's where actin is anchored. The m-line in the middle is where myosin, right in the middle of myosin, and that's going to be where we have our thick filaments. The A-band is going to be the length of myosin and will not change with contraction. But the H-zone, the area where there's only myosin, will get smaller with contraction and the I-bands will also get smaller with contraction as actin is pulled towards the center of the sarcomere. All right, so those are the parts of the sarcomere. We'll talk about the function of them in a separate video. I hope this helps. Have a wonderful day. Be blessed.