 but not all myofibers are the same. We have basically two different flavors of myofibers, and I'm just going to make some lists here. We have slow twitch fibers, and we have quick twitch fibers. Now, two categories, awesome, black or white, right? No, of course not. In Mesio, what we often find is that nothing is ever black and white, and there's always exceptions. And the fact is that there aren't just two kinds of myofibers. There's a range of myofibers. Your textbook or some textbook that I was referring to was talking about a total of three different kinds. I'm imagining that eventually we're going to find all sorts of different kinds of myofiber flavors. In this class, we're just going to look at the two extremes, because they're different enough and interesting enough it gives you a sense of the kinds of things that can be variable between the two types of fibers. We'll start with the quick twitch fibers. First of all, they're called quick twitch dogs, and hopefully, I don't know if I said this or not, but a twitch is one action potential for the somatic motor neuron and the resulting shortening of the myofiber that occurs. That's one twitch. You can imagine that if I want to contract my massive biceps, I'm going to have to do a lot of stimulation and do a whole bunch of twitches in order to make that entire muscle shorten over time. But one twitch is just one message followed by the shortening that happens. So quick twitch fibers, they shorten, they contract two to three times faster than the slow twitch fibers, and you should probably be wondering, dude, how do they do that? Well, let me tell you about it. First of all, they have fast myosin. What? True story. The myosin molecules in quick twitch muscle fibers are faster, for real, like they can basically take the ATP and split it faster than the myosin that's found in slow twitch muscles. It's just a different chemical form, but it does its job faster. The other thing that is faster in quick twitch muscles is that they can remove, they can take the calcium from the cytoplasm. Once calcium has been dumped out to stimulate a twitch, they can take the calcium back and put it back into the sarcoplasmic reticulum faster than a slow twitch muscle. So calcium's out there, and we can really fast put all the calcium's back in the sarcoplasmic reticulum jail for calcium. And then now you've got this huge concentration gradient again, so you can fire another twitch faster than the slow twitch. Now, slow twitch, the calcium comes out, it does its thing, we get a slow contraction, and then we have to run around, we're just way less efficient at cleaning up. It's like your kids cleaning up their room or you cleaning up your kids' room, and yeah, you know that I'm the quick twitch fiber in that little analogy. Here's another one. Quick twitch fibers are, are you ready for this? Anaerobic, anaerobic, more anaerobic than slow twitch fibers. And what does that mean to you? Well, draw your brain back into the land of general bio when you study the process of cellular respiration, because cellular respiration, if you remember, there were all those stages, and glycolysis was an oxygen-free breakdown of glucose that resulted in what, two ATPs or something ridiculous. And you might be thinking, awesome, we scored two ATPs out of that, that's fantastic. But remember that if we do have oxygen present to be the final electron acceptor to be the electron transport chain in the mitochondria, remember that? Of course you do. Then if they're there to catch those electrons, if oxygen is there to catch the electrons, you get like 38 ATPs out of a single molecule of glucose. So the fact that the quick twitch muscles are, muscle fibers are anaerobic, that means that they fatigue fast. Why? Dogs of a feather. They fatigue fast because they run out of energy because they don't go through the whole process. Quick twitch fibers in a second, I'm going to tell you about, well, okay, maybe I won't. They're light, they're whitish fibers. They actually are bigger as well. This is a cross section of a muscle fiber. Look, I'm circling all my myofibers. These are myofibers. You can see the nuclei squished around the edges of the myofibers. These are also myofibers. These are my nuclei squished around the edges of the myofibers. But look, these are actually the same critter. And look at how much smaller the fibers are in this slow twitch muscle. These are my slow twitches and these are my quick twitches. Quick twitch fibers are bigger. And the bottom line there is that they're going to fatigue faster. They're going to run out of energy faster. So there's no need to be able to diffuse oxygen into the muscle fiber because they're going to be too tired by the time the oxygen gets in there. So they can get bigger. Whereas slow twitch fibers utilize aerobic cellular respiration. Aerobic cellular respiration. So they're going to go through the whole cellular respiration process. They need oxygen to be that final electron acceptor. They want oxygen in those cells. So they're smaller diameter so that they can diffuse more oxygen in. Look at this. That's a slow twitch fiber. And this is a quick twitch fiber. I think that's a really interesting quality. Slow twitch fibers, because they're anaerobic, I mean, excuse me, because they're aerobic, they need oxygen. So they have a high concentration of a molecule called myoglobin. And this myoglobin is like hemoglobin in a red blood cell. And you know red blood cells carry oxygen around and deliver it to your cells. Well, the myoglobin is like an oxygen refrigerator. It's like an oxygen storage zone. And myoglobin, just like hemoglobin, makes your red blood cells red. Myoglobin is red. And so the reason why your slow twitch fibers are darker is because they're more full than this myoglobin substance. They're also darker because they have a lot more of my good buddy required to go through aerobic cellular, I'm going to humor her, aerobic cellular respiration. Who? Pictionary. Who's this guy? My favorite organelle. That's a mitochondria. That's a mitochondrion. And slow twitch fibers have lots and lots of mitochondria because they have so many energy needs. Now they fatigue a lot slower. They contract slower, but they can go for longer periods of time. And that comes in handy. I think that's it. All right. So the next thing we're going to talk about is how muscles can work together, this really interesting characteristic of a group of muscles contracting together. And that's called summation.