Muscular system introduction
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Type 1 and type 2 muscle fibers
What's the difference between type 1 muscle fibers and the equally descript type 2 muscle fibers? And I hate it when they give things names like type 1 and type 2. That's not really descriptive. That doesn't tell me a lot of information about these types of muscle fibers. So what I do is I get back at them. I come up with one golden rule. And this one golden rule will help me go through a table, like we're about to do right here, to differentiate between type 1 and type 2 muscle fibers. So the golden rule I'm going to have for this table here is that mitochondria-- whoops, look how I wrote that "i" there. Mitochondria are present in greater quantities in your type 1 muscle fibers, type 1. So mitochondria are more prevalent in type 1 muscle fibers than in type 2. And just based on that knowledge alone, we should be able to go through and fill out this table. All right, so let's start from the top. I may have alluded to it here through the way I wrote this out, but the color type 1 muscle fibers are often noted as? Red. And why do you think that is? Well, what are mitochondria used for? Mitochondria are used in biochemical processes that help us make energy. And the main process they function in that I'm going to reference a couple of times is called oxidative phosphorylation. Now, what is that term mean to you? What does that suggest? What are the two things that are probably involved in oxidative phosphorylation? So just as the name suggests, oxidative means oxygen is going to be involved here. And then phosphorylation means that something is going to receive a phosphate group. Phosphoryl- -ation, something will receive a phosphate group. So the oxidative part applies here to red, the same way that the color red shows up in our arteries. Think about it. Why are arteries red, and why are veins blue? Well, arteries have more oxygen than veins do. And because of that, the color of our type 1 muscle fibers will be red because they produce more energy from oxygen than type 2 muscle fibers do. More oxygen is present in type 1, so they're red. And so we could say then that our type 2 muscle fibers will be white. All right, what about the speed of contraction that we see here? How fast do type 1 muscle fibers contract? Well, let's think about the process of making energy with mitochondria. If type 1 muscle fibers rely on mitochondria for energy, think about all the processes that have to go into making energy through oxidative phosphorylation. You can't just do this right away. You have to have glycolysis happen. You have to have the Krebs cycle occur. You need to make NADH and FADH2, all of these cofactors that have to go into this electron transport chain. There's a lot of things that have to be done. So mitochondria take a while to make energy. So that means that the contraction speed here is also going to be slow because it takes a while to make that energy. And so on the flip side, that means that the contraction speed of type 2, or white muscle fibers, is going to be fast. What about the conduction velocity? How quickly are we going to be able to receive a neuronal impulse or a nerve signal to type 1 muscle fibers to contract? This is also going to be slow. And the term here is called "slow twitch." And I remember that the same way I remember that the contraction speed is slow in type 1 muscle fibers. On the flip side, we call type 2 muscle fibers fast twitch muscle fibers. All right, now, what types of activities do you think type 1 muscle fibers are going to be involved in? Well, oxidative phosphorylation has another name. The mitochondria is used for a specific type of respiration. That's a cue term right there. That's something that should cue another word in your mind. If mitochondria is involved for a type of cellular respiration, that type of [INAUDIBLE]-- aerobic. Aerobic respiration requires mitochondria to be present. And so because there's more mitochondria in type 1 muscle fibers, type 1 muscle fibers will undergo aerobic respiration. What about type 2? Well, because they don't have as much mitochondria, they're going to have to undergo anaerobic respiration, or respiration in the absence of oxygen. That's what a aerobic means-- "by using oxygen." Anaerobic means "without oxygen." OK, how long will these muscle fibers be contracting? Well, let's think about how much energy we're making if we have mitochondria versus if we don't have mitochondria. We make a lot more energy if we have mitochondria. And so the duration of contraction then will be long. You're going to have longer contractions when you're able to make a lot more energy or ATP, because you've got mitochondria present in greater quantities. If you lack mitochondria or you undergo anaerobic respiration because you can't undergo oxidative phosphorylation as much, you're going to have a lot of short-duration contractions. What's the difference between these two? Well, if I'm having a long-duration contraction, that's something like the muscles in my back that I use for standing or in my legs so that way I can walk at a certain pace. Short-duration muscle fibers, those that contract really quickly and that's it, are things like the ones in our arms when we shake somebody's hand or if we flick something with our finger. If we do that for a long period of time, we're going to get really tired. So these muscle fibers are not meant to be working for the entire day. If we're in line at Disneyland, we want to be using our long-duration muscle fibers, the ones in our back, in our glutes, in our legs. So that way, we can stand for a long period of time. We don't want to be standing on our hands or our fingers because we won't be able to last by using type 2 muscle fibers. All right. So which of these types of muscle fibers are most likely to fatigue? Well, if type 1 muscle fibers are able to contract for a long period of time, we say then that they are fatigue resistant. They are fatigue resistant because we have enough energy for type 1 muscle fibers to contract for a long period of time. But type 2 muscle fibers do not have as much energy, because they don't have as much mitochondria. And so as such, they easily fatigue. So I'll write easily right here. They are not resistant. What about the power of contraction that's associated with type 1 muscle fibers? Well, I talked about some pretty heavy muscles just now, the ones in your glutes, your back. And if we're having long durations of contraction, these are going to be some pretty big, powerful muscle groups. And so I'll write strong here. Because again, noting back to the mitochondria analogy, if we have a lot more mitochondria present in type 1 muscle fibers, they're going to have a lot more energy to contract and more muscles contracting. More muscle cells contracting at the same time elicits a stronger, more powerful contraction overall. On the flip side, type 2 muscle fibers-- not enough mitochondria, not enough energy, not enough power-- so these are going to be weak contractions. And lastly, how do we store our energy in type 1 versus type 2? This might be a little bit of a trick, but you can figure this out. With mitochondria, we make a lot of energy. And if we have a lot of energy, are we going to leave that sitting around as ATP? No, ATP is a really labile, or a really reactive molecule. If we have it there, we're anticipating it to be used right away. And so what type 1 muscle fibers do is they store their energy in triglycerides, these fatty substances. And so that's why we put energy into fat. So that way, we can use it later on for the stronger, longer-duration contractions. If we want it to contract right away we'll just use raw ATP. And so the ATP that's just sitting around is used in type 2 muscle fibers. Also, we'll use something called creatine phosphate, which is very similar to ATP in the sense that it has a phosphate group that it'll be able to donate to produce energy. And so this phosphate group is very reactive, very labile. If it's sitting around, that means it's going to be used any second now for a quick, short-duration contraction. So by using this one golden rule here that mitochondria are more prevalent in type 1 muscle fibers, we can figure out this whole table here. And that's the difference between type 1 and type 2 muscle fibers.