Our example gives us an image of a sarcomere from an electron microscope, and we need to do a number of things with it. We need to mark the A band, the I band, and the H zone with brackets, and then label them. We need to mark the Z disk and the M line with arrows and label them, and then we need to draw a line to represent the length of 1 actin filament. And finally, we need to describe if and how each component changes in size during a muscle contraction. Alright. Filling out this table to talk about the size, I'm actually going to do that as we go. So let's go ahead and get started. We have the A band. Alright. So where is the A band? Well, remember the A band is the same thing as the dark band when you're looking at a sarcomere. So I can draw the edges of where it's dark, you know, sort of roughly there to about here. I'll draw my back bracket there. That's going to be my A band. And I remember that because the vowel in dark is A, so the dark band is the A band. Alright. Now, let's think: does this A band change size when the muscle contracts? Well, the A band is the region where the myosin and the actin overlap, but really it's defined in size by where the myosin is. So the myosin is attached to the middle of the sarcomere, and it reaches out sort of right to here to the edge of the A band. So, in our sliding filament theory of muscle contraction, we say that those filaments, the actin, the myosin, don't change size. They slide past each other. So if our A band is the region where the myosin is and that myosin doesn't change size, that means when the muscle contracts, then in the A band we expect no change in size. Alright. Next, let's look at the I band. Alright. If the A band is the dark band, well, the I band I'm going to sort of start right next to the A band where it's a little bit lighter, and I'll draw my bracket around sort of roughly there. And that is going to be our I band. And remember, I remember that because I is the vowel in light. So the I band is the light band. So, does the I band change size when the muscle contracts? Well, the I band we said is the region where there's only actin, and the actin when the muscle contracts, remember that myosin is going to pull on the actin and it's going to slide towards the middle of the sarcomere, and more and more of the actin is going to be overlapping the myosin. So if more and more of the actin is overlapping the myosin, well, then less and less of it isn't overlapping the myosin. That means that I band, that region where there's only actin, is going to get shorter. So I'm going to write here, gets shorter. Alright. That brings us to the H zone. The H zone is this region in the middle of the sarcomere that's just a little bit lighter than the rest of the A band. And remember, we said the H zone is a little bit lighter because the actin comes in, it overlaps the myosin, but doesn't overlap all the myosin usually. There's going to be a region between those actin filaments, and it's a little bit brighter because that's where there's only myosin. So I will label my H zone, sort of right here to here, and think what happens when it contracts. Alright. Well, when the muscle contracts, that actin gets pulled towards the center of the sarcomere, and as it gets pulled towards the center of the sarcomere, that space between it is going to get smaller and smaller and smaller. That space between it's the H zone, and eventually the actin's going to actually reach the middle of the sarcomere. That H zone will even disappear. So I'm going to write here what happens to the H zone. It gets shorter and I'll write, eventually, it disappears as that actin reaches the middle of the sarcomere. Alright. Next, we need to mark the Z disk and the M line with arrows. So we'll start with the Z disk. The Z disk, remember, Z is at the end of the alphabet, and our Z disks mark the ends of the sarcomere. So that's these lines right here. Here's one of them. Here's my other one, and I'm going to write here, this is the Z disk. Here is my other Z disk. And now, does the Z disk change size when the sarcomere contracts? Well, the Z disk is this structural component that's really there to anchor the actin, to hold the ends of the actin. And when the actin gets pulled in, that Z disk gets pulled closer to the middle of the sarcomere. That's how that sarcomere gets shorter. But the Z disk doesn't change size. It does move, but it doesn't change size. So here, does it change size? I'm going to write no change. Alright. And that brings us to the M line. Alright. The M line is this line right down the middle of the sarcomere. M, I remember, stands for middle, and so I'm going to draw an arrow right here. This is my M line. I'll just draw it out so I can write. This is my M line. The M line right down the middle, and it is anchoring the myosin. So if it's anchoring the myosin, it doesn't change size. It just stays right in the same place as that myosin pulls on the actin, and the actin gets closer and closer to the M line, but the M line, just like the Z disk on the right, no change. Alright. The last thing that we want to do here is that we want to draw a line to represent the length of 1 actin filament. Well, remember, the actin filament is going to go from the Z disk so right here let's change our color the Z disk right here is going to go over the I band, cross into the A band where it's overlapping the myosin, and it's going to reach as far as the edge of the H zone. Alright. The H zone is the region between the actin filaments. So that is my line. This is the length of 1 actin filament. I can draw it on the other side too just for fun. There we go. There we go. That's my actin. Alright. Understanding all these parts, how they relate to the proteins, and what happens to them when they contract, is often a fun test question that you're going to see. So I encourage you to practice it. We have more practice problems to follow. Give them a try.
Table of contents
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9. Muscle Tissue
Sliding Filament Theory and the Sacromere
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