To continue talking about the structure of a skeletal muscle, we're now going to talk about the muscle fiber, and from here, we're going to go down to some of the individual proteins that are going to be involved. And I just want to ground ourselves first though. What we're saying here is that for a muscle fiber to contract, the tissue must shorten. And that's really sort of our fundamental question is how does a muscle fiber shorten? How does the muscle actually get shorter? So to answer that, we're going to start by looking at this picture here, and we have a picture of a muscle fiber, at least a small section of it. And remember, the muscle fiber, that's just another name for the skeletal muscle cell. That's these giant cells, these big cylinders, extremely long cells, basically the same length as a muscle. And as we look here, we're going to see that it's sort of we see a section of it and we see it sort of cut away in cross-section in pieces, and we can see some of the structures inside, and some of those are pulled out so that we can see what's going on in there just like we did previously with the whole muscle. Now remember, this muscle fiber is going to be surrounded by the endomysium, that smallest layer of connective tissue that surrounds and separates each individual muscle fiber. So to start, we'll just note that's sort of drawn in down here, but we're sort of pulling it away so that we can see what's inside. And the first thing that we know well, it's also cut away, but this would be wrapping the whole thing, would be this sort of translucent section here that you see, and that's just going to be the cell membrane. But the cell membrane of a muscle fiber is highly specialized, so we give it its own special name. We're going to call it the sarcolemma. And sarco is just a prefix from Greek that means flesh, so it's used sometimes to refer to muscles. And lemma is also from Greek. It means a covering or a wrapping or a sheath around something. So we're going to say that the sarcolemma is just this plasma membrane of the muscle fiber that is wrapping the myofibrils. And the myofibrils, we'll get into more detail in a second, but these are these long, rod-like organelles that are within the sarcolemma that make up the muscle fiber. And so here, we can think about this sort of repeating structure of bundles that we started talking about previously. We had the muscle was a bundle of fascicles. The fascicle was a bundle of muscle fibers. And now the muscle fiber is going to be a bundle of these myofibrils. Alright. But before we talk about the myofibrils, I want to talk about the membrane a little bit more. So the sarcolemma is highly specialized to sort of send an electrochemical signal telling the muscle fiber to contract. But you also need to get that electrochemical signal down deep within the muscle fiber, so that the whole cell is contracting together. To do that, we have the T tubules. And the T tubules, you can see here in pink, they're sort of coming down here and they are surrounding like a little ring each around these myofibrils. Now these are connected to, and really they're kind of an extension of the sarcolemma. So we're going to say that the T tubules are these membrane canals, or you could say tunnels or passageways, that are extensions of the sarcolemma and reach down deep within and surround each one of these myofibrils in a little ring. So as the electrochemical signal comes down the sarcolemma, it's going to then dive down in these T tubules and get deep within the muscle fiber. Now, what they're actually trying to stimulate is this next membrane system that we're going to talk about, and that's going to be the sarcoplasmic reticulum that we're writing down here in blue. Now, the sarcoplasmic reticulum, you might notice that sounds a lot like endoplasmic reticulum. There's a reason for that. The sarcoplasmic reticulum is the highly specialized endoplasmic reticulum of muscle cells. And the sarcoplasmic reticulum, you can see it surrounds these myofibrils. You can see it's really here in close connection with these T tubules, and then it kind of spreads out in this sort of convoluted connected tunnel tube-like system of membrane, and you can see it sort cut off in cross-section all along here, and it's surrounding and separating these myofibrils. Its job is to store and then release calcium ions. The signal for the muscle to actually contract down, like, at the molecular level is the release of calcium ions from the sarcoplasmic reticulum into the myofibrils. Now we're going to get into that in a lot more detail going forward. But for now, let's now focus on the myofibril. So the myofibril, we've already said, is this myofibrils. These are these long rod-shaped organelles, and you can sort of see a bunch in cross-section here. We have a few here, and then we've pulled one out so that you can really see it there. The myofibril is really just sort of this organizing structure, and what it's organizing, is going to be the myofilaments. And you can see again, we've pulled out the myofilaments there. And if you look here at these in the cross-section, it might be a little hard to see because they're very small, but the cross-sections you might also notice have these really regular organized structures within them and that's those myofilaments cut off in cross-section. The myofilaments, that's what's actually going to be responsible for getting this muscle to contract. So the myofilaments, these are the proteins the actin and the myosin. Alright? There's going to be other proteins going on in here, but actin and myosin, these are the major players that are actually going to get this muscle to get shorter. This, though, is going to be the first structure that isn't really long. Remember, our muscle fiber is basically the same length as the muscle. The muscle the myofibrils the same length as the muscle fiber. The myofilaments are going to be organized in these repeating structures, and you can kinda see it in this myofibril that we pulled out. You have sort of a darker section that has more purple in it, and then a lighter section, a darker section more purple in it, a lighter section that pattern sort of continues in these filaments that we pulled out. That repeated structure is going to be the sarcomere, And the sarcomere, we are going to say, is the contractile unit. We're sometimes going to call this the fundamental unit of muscle contraction. What actually gets shorter in the muscle? The sarcomere gets shorter. And the muscle fiber gets shorter because you can see this repeated structure is repeated over and over again a lot. One myofibril has 1,000 or tens of thousands of these in succession. I did a quick back of the envelope calculation. If you have a 10 centimeter long muscle, which is about, I don't know, yay big, it's going to have something like 40,000 sarcomeres, end to end in every single one of these myofibrils. That repeated structure, repeating over and over again, these actin and myosin filaments, the way they're organized, is going to create the striated appearance that you see when you look at a skeletal muscle. From that striated appearance, when you look at the cells, it looks striped. You're seeing the repeated structure of these proteins, the actin and myosin. Now, this is that structure. We're not going to worry about it too much detail here. We're going to talk about it in a lot more detail going forward. For now, we have, like always, example and practice problems to follow, and then I look forward to getting to the step-by-step process of how these signals are passed and how these proteins actually work to make the muscle contract. See you there.