 Hey everybody, Dr. O here. In this video I'm going to cover the basics of a muscle fiber or a skeletal muscle cell and cover the terminology so that the next few videos make more sense as far as muscle contraction, parts of a sarcomere, etc. So here we see at the top we see a skeletal muscle fiber also known as a skeletal muscle cell. But muscle cells are actually very unique. First thing you'll notice here is there's multiple nuclei. That's because a single muscle cell is actually formed by the fusion of hundreds or thousands of muscle stem cells called myoblasts. So a single muscle cell, if you want to call it that, or a muscle fiber can have hundreds of nuclei. And that's because muscle cells are so metabolically active they need these nuclei churning out RNA to make all the proteins associated with muscle metabolism. That's one thing that's kind of unique about muscles. And then also because a muscle cell or muscle fiber is made of really hundreds of cells or more, they can be huge. A muscle fiber might be 100 micrometers across, but they can be as long like in the sartorius, the longest muscle in your body. They can be up to 12 inches. So here you see if you want to call a muscle fiber an actual cell, then we have cells that are up to a foot long. So I think it's, there's a little asterisk there though because I don't really think of them as just a single cell. Those are the first two key things to note here. Then you can see that muscle fibers or muscle cells are made of myofibrils. So the, you see here at the bottom in myofibril, myofibrils are going to be made of your thin filaments called actin and thick filaments called myosin. I'll go into a little more detail there in just a moment, but let's cover a little bit more of the terminology you see here. And you notice this term sarco, sarco a lot. So sarco is Greek for flesh. So the sarco lemma is going to be the name for the plasma membrane or cell membrane around a muscle cell. So that's the sarco lemma. Then we see the, the juice inside the cell, what's normally called the cytoplasm is called the sarcoplasm. And then there on the left-hand side, you see the sarcoplasmic reticulum. That's a fancy name for the smooth endoplasmic reticulum of a muscle cell. And its job is to store, release, and then retrieve calcium. And you'll see that calcium plays critical roles in muscle contractions. And then you see there the sarcomere. So I'll cover the parts of the sarcomere in a separate video, but for now just know the sarcomere is the functional unit of muscle. So the more sarcomeres you have, the obviously the larger and more powerful the muscle can be. Let's take a closer look here at a sarcomere, which is made of these thin and thick filaments, and I'll cover how they work later. But I just want to cover a couple of the key things here. So on the right-hand side, you see the thin filament actin. So the key with actin is actin does have those binding sites for myosin. So, so imagine this is actin here, the thin filament. Myosin has these heads and myosin wants to grab onto actin and pull it. But it normally can't because of those proteins you see there. What's called the troponin tropomyosin complex. These are known as shielding proteins. So you see tropomyosin, that long orange structure, its job is to stay in the way. So I just have, I had something sitting here, a ball me and my son play with. So as long as this ball is in my hand, then myosin can't grab this actin. So tropomyosin's job is to stay in the way. And then troponin is attached to it, but troponin has calcium receptors. So when muscles contract, and I'll cover this later, calcium binds to troponin, which pulls these shielding proteins out of the way. And now the binding sites for myosin on actin are exposed. And the myosin heads are able to grab, and that's how you get the sliding filament theory or the muscle contraction. So that's the key features with actin, the tropomyosin, the shielding protein there, and then troponin. Then on the myosin, the main thing there are those heads. So a single thick filament will have hundreds of these heads. And these heads are designed to when they're in the presence of ATP. And as long as calcium has got these shielding proteins out of the way, the job of these heads is to grab onto actin and pull it. And that pulling is how our muscles contract. Okay, so for now, that's all the key terminology. We'll see these things in action in a later video. But videos would hopefully at least now know what actin and myosin are. All these sarcos, sarcomere, cycloplasm, etc. And you understand what makes muscle cells special. I hope this helps. Have a wonderful day. Be blessed.