If we finish up talking about the structure of the sarcomere, we're going to talk about these discs, lines, bands, and zones. These are structures and regions of the sarcomere that are named for how they look on an electron microscope or a transmission electron microscope, or TEM. We have an image like this that we're going to look at here, and you've probably seen an image like this in your textbook or in lecture. This is an electron microscope image of 1 sarcomere, and below it, we've drawn this illustration of the proteins involved that we went through previously. And what we're going to do here is we're going to name all these bands and zones and we're going to see how they relate to the actual proteins that make up the sarcomere. So the first thing that I notice when I look at this image are these dark lines here going up and down marking the end of the sarcomere, marking the two edges of the sarcomere. Those lines are what we're going to call the Z disk or the Z disks, and the Z disk is the end of the sarcomere. One way you can remember that, well, Z is at the end of the alphabet and the Z disk is at the end of the sarcomere. As I look at my illustration here, again we see these up and down lines here. You can see how that lines up with sort of these zigzaggy lines in our illustration of the proteins here. Another way I remember it when I'm looking at this sort of protein drawing: this zigzaggy line is the Z disk. You may be wondering why it is called the disk when we're drawing it as a line or a zigzaggy line. Remember, this is all in the myofibril, and the myofibril is this big long cylinder organelle. So the sarcomere is actually sort of a small cylinder. It's kind of the shape of a soup can or something like that. So the ends of the cylinder, of that short cylinder, the sarcomere, is going to be a circle shape or a disk in 3 dimensions. And when you look at a cross-section of it in 2 dimensions, it's going to end up looking like a line. The Z disk is there at the end of the sarcomere, and remember that the Z disk is anchoring the actin. That's what the actin is going to be attached to. Now, in the middle of the sarcomere, we have this other line right there and that is going to be our M line. The M line has the myomesin protein that anchors the myosin. Now, myomesin protein, you probably don't need to know the name of that protein but like always, check your own notes, know what your professor wants you to know. But we can look at our image here. This dark region here, that's that protein and in our image, we can see it running up and down and it is anchoring that myosin that's reaching out in both directions. This line in the middle is the M line and that's the easier way to remember it. M stands for the middle. As we look at this, there are also going to be these sort of bands and zones, and these bands and zones refer to the sort of bigger, lighter, and dark regions that we see. First off, when I look at it, obviously there are some light regions on each end, and there's a big dark region in the middle. So the I band is going to be the light bands. I always write it with a capital I. I write light with a capital I because the I band is the light band. That vowel helps me remember that the light band is the I band. This is the area with just actin. So again, if we look over at our illustration here, we have this light band here. If we look down at our drawing, this sort of loosely lines up with the light band. The myosin ends and where there is no myosin, where there's just these actin filaments, that is the light band with that Z disk right in the middle of it. And it's the light band because the actin is the thin filament, and the thin filament is going to look lighter when looked at through an electron microscope. So we have the light band. We also have the A band. The A band, therefore, is going to be the dark band, and again, I write it with that capital A to remind myself that the A band is the dark band, remind myself of that vowel. If the I band has the actin in it, well the A band has the myosin in it but it's actually the area with both actin and myosin because remember that actin and myosin need to overlap so that myosin can pull on the actin hand over hand so that the sarcomere can contract. So if we look at our illustration over here, we have the dark band, this dark region here, and that loosely lines up with the edges of the myosin, and the myosin is the thick filament so it looks darker through the microscope, but it's also overlapping this actin here. The dark band is the same size as the myosin, but it has the actin overlapping it. But the actin doesn't overlap all of the myosin. And in our image here, you can see this section in the middle with the M line in the middle that looks a little bit brighter. That section that looks a little bit brighter is the H zone, and the H zone is that center region where there is just myosin, where the actin doesn't overlap. And we can see that again here, that lighter region that loosely matches up to our illustration here, that section where the actin is not overlapping. Now that H actually stands for heli, and heli is Greek for bright. And because it's the region where the actin isn't overlapping, when the muscle contracts, the actin gets pulled in and it's going to overlap that myosin more and more, so you can imagine this H zone is going to get smaller and smaller. It's eventually going to even disappear when the muscle is fully contracted because that actin will be completely overlapping the myosin. Understanding these zones and discs and proteins and how they all match up, in my experience, is something that professors love to ask about. We're going to practice some more in the example to follow. I'll see you there.
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9. Muscle Tissue
Sliding Filament Theory and the Sacromere
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