Steps of Muscle Contraction - Video Tutorials & Practice Problems
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1
concept
Overview of Muscle Contraction
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4m
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We now want to talk about the steps of muscle contraction down with the cellular and molecular level in a lot more detail. But before we look at these step by step processes, we just wanna step back and look at an overview of what's going on here. So we're gonna start out by saying that muscle contraction involves two major things that need to happen. First up is this transmission of a nervous signal, skeletal muscle contracts when it gets a signal from the nervous system. So how is that signal received? How is that signal spread throughout the entire muscle fiber? And then we also have to think about the contraction of the sarcoma. Now, we said the Sarker is the fundamental unit of muscle contraction. And so far, we said the mio and pulls on the acting to make that happen. But we wanna talk about at the molecular level in a lot more detail, what's going on in the sarcomere. Now, as you look here, you'll notice there's three boxes we're about to fill in and we've just talked about two major processes. That third box is gonna be talking about. How do we link these two things. How do we get from an electrochemical signal to actually the mechanics of contraction. All right. So to start though, we're talking about this transmission of a nervous signal. And we're gonna start talking about the neuromuscular junction. The neuromuscular junction is just where the nervous system meets a muscle fiber. So we're gonna say at the neuromuscular junction, a muscle cell is stimulated by the nervous system and that's gonna result in the initiation of an action potential. And action potential is this wave of electrochemical signal that's gonna spread down the muscle fiber through the sarcolemma, the muscle fiber cell membrane. Now, to illustrate this, we can see here, we have the axon terminal, the end of this nervous system cell and it's releasing neurotransmitters into the synapse, this small space between the two cells, they're binding to receptors on the cell membrane. And that's gonna start this action potential that's gonna flow down the sarcolemma. Now, I just went through all those steps really quickly. Don't worry, we're gonna go through those steps later on in a lot more detail, a lot more slowly. All right. So now we have this action potential and it's spreading down the sarcolemma, but we need to get it down into the sarcomere and get to that mechanics of contraction. That link we're gonna talk about as the excitation contraction coupling. So we're gonna say that that starts with the action potential spreads or in more technical terms, it propagates along the sarcolemma and enters the T tubules. And we can look here at our image, we can see the edge of the muscle fiber. So we have the sarcolemma here and it's showing this action potential spreading down and it's gonna dive down into this pink T tubule which surrounds like a ring around this myofiber, right, that gets that electrochemical signal down deep into the muscle fiber. Now, you also notice right up in close connection with that T tubule is this blue structure that is the sarcoplasmic reticulum that highly specialized endoplasmic reticulum, the muscle fiber, when that signal goes down through the T tubule, that's gonna signal the sarcoplasmic reticulum to release calcium ions. And I'm gonna write that in shorthand here as C A two plus. All right. So it's gonna release the calcium ions and those are gonna go into the myofibril into the sarco mares and that is our link. Those calcium ions are going to result in the mycin binding sites becoming exposed. And here we can again, look at our image. We're zoomed way in here we see in pink. These are these calcium ions coming in, they're going to bind to the troponin. The troponin is gonna open the binding site by moving this Tropomyosin this green filament surrounding the Acton. We can see the mycin binding sites there on the Acton, those are gonna become exposed. All right. Now, we can talk about the mechanics of contraction once those Mycin binding sites are exposed, the Mycin is going to bind to the Acton. We're gonna call that binding a cross bridge. And once bound, that Mycin is gonna pull on the Acton and we call that pulling motion, the power stroke, right? So we can see that here. We see the Mycin is bound to the Acton and it's pulling the Acton that way. OK. Again, for all of these, we just talked a lot of steps. We're gonna go through all those steps much more slowly and in more detail coming up. I'm looking forward to it and I hope you are too.
2
example
Steps of Muscle Contraction Example 1
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Our example tells us that below is a list of structures that are found in the muscle fiber. And we need to mark the structure of the T if its primary role is involved in transferring a signal through the cell, mark the structure with an R if it's directly involved in regulating whether the sarcomere contracts and mark the structure with AC if it's directly involved in the mechanics of contraction. All right. So we have this big long process and we started by breaking it up into these groups, transferring a signal regulating contraction and then actually contracting. So let's look at our structures here. We're gonna start off with the acting. Which group would you put that in the, transferring a signal regulating contraction or actually the mechanics of contraction? Well, Acton, I'm gonna mark it with ac that Acton gets pulled on by the Mio and that's what causes the sarcoma to contract that is directly involved in the mechanics of contraction. All right, let's look at the calcium ions. Which group would you put the calcium ions in? Oh I'm gonna mark the calcium ions with an R calcium ions get dumped into the myofibril into the sar comme and that results in the binding sites on the Acton opening. So, if the calcium is in the Sarker, there can be contraction. If the calcium is not in the sarcomere, there can't be contraction that's directly involved in regulating. Next, we have the mias, what do you think about the mycin? Well, if I marked Acton with AC, then my is getting ac as well my holes on the Acton And that's how the sarcomere contracts, that's directly involved in the mechanics of contraction. Next, we have the sarcolemma. Which group would you put the sarcolemma in? Well, I'm marking the sarcolemma with A T the sarcolemma is the cell membrane of the muscle fiber and it spreads that action potential throughout the muscle fiber, spreading that signal. So the entire muscle fiber knows to contract at the same time. All right. Next up, we have the sarcoplasmic reticulum. Which group would you put that in? Well, this is the one that gives me a little trouble. I can see arguments for two groups, but I'm definitely marking it with an R for regulating contraction. When the sarcoplasmic reticulum gets the signal, it's going to release the calcium ions. The releasing of the calcium ions is what signals the muscle to contract. That's what opens the binding site and allows the mycin to pull on the Acton. Now, I can see an argument for putting it in the transferring a signal uh section because transferring a signal. The sarcoplasmic reign reticulum gets the that action potential and that action potential signals the sarcoplasmic reticulum to release the calcium ions. But again, because it's releasing the calcium ions, I think it's better placed in the regulation group. All right, that brings me to the troponin. Which group would you put the troponin in? Now, I'm marking troponin with an R troponin. We said troponin opens the binding site. So it's involved in opening the binding site that's allowing whether the contraction can occur or not. When the calcium binds to the troponin, that troponin kind of changes shape and that opens the binding site. Next, we have the tram mycin. Well, if the troponin is a regulating group, then the trop amycin is in that regulating group as well. The troponin moves the tropomyosin because remember the Tropomyosin is there saying no to the Mycin. It's this filament that's there blocking the binding site. So if the Tropomyosin is blocking the binding binding site, the mycin can't bind the cell can't contract once it moves, contraction can occur. And then finally, we have the T tubule, all right, the T tubule, I'm marking that with a t the action potential moves down the sarcolemma and then it goes into those tubes, the T tubules which are these extensions of the sarcolemma which dive deep into the cell and surround in a little bit of a ring around the mayo fibrils that action potential down through the T tubule then tells the sarcoplasmic reticulum to release the calcium ion. So it is definitely there transferring that signal deep within the cell. All right, understanding these roles is gonna be really important going forward. We have more practice problems to follow. I'll see you there.
3
Problem
Problem
True or False: if false, choose the answer that best corrects the statement.
The events of excitation-contraction coupling involve converting the electrochemical signal to the mechanical movement of contraction.
A
True.
B
False, excitation-contraction coupling involves the reception of the nerve signal at the neuromuscular junction.
C
False, excitation-contraction coupling involves the movement of actin by the myosin power stroke.
D
False, excitation-contraction coupling involves propagation of the signal through the sarcolemma and T tubules.
4
concept
Neurotransmitters & Action Potentials
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6m
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We said, the first steps of muscle contraction are gonna be all about how the muscle fiber receives a signal from the nervous system and then how it spreads that signal throughout the entire cell. So to understand how that works, we need to talk about neurotransmitters and action potentials. Now, neurotransmitters and action potentials, we're gonna talk about in a lot more detail when we get to the nervous system. But to understand how the muscular system works and to be able to answer some questions about it, you need to know the basics. So neurotransmitters, there's gonna be these chemical messengers that are used at a synapse and a synapse, we're just gonna say is a really small space between the axon and the muscle. And so if we look over here, we have this small image here. This is the axon that end of a neuron, that highly specialized nervous tissue cell and it comes down and this is gonna be our membrane of the muscle fiber there. And you can see that there's a small space between the two, they don't actually quite touch. So to guess it get acro a message across that synapse, the neuron dumps these neurotransmitters into the synapse. You can see all these little dots in here. It's dumping those neurotransmitters into the snaps. They're gonna diffuse across the synapse and they're going to bind to receptors in the membrane of the muscle fiber. Now, when enough of them bind, that'll stimulate an action potential. Now, before we get to the action potential, though, we just need to know that acetylcholine is the neurotransmitter of the neuromuscular junction in the nervous system, there's all sorts of different neurotransmitters that are used, but there's only one neurotransmitter that is used to stimulate muscle fibers and that is acetylcholine. All right. So we passed the message on to the muscle fiber. Now it's spreading, it's gonna spread through an action potential. An action potential is going to be this wave of electric signal that moves along a membrane. And in this case, our membrane is the sarcolemma or the membrane of the muscle fiber. And to understand how this works, we need to know that muscle fibers are polarized. And by that, we mean that they have a negative charge, I'm just gonna write ne with a minus sign here, negative charge inside and they have a positive charge which I'm gonna write as a pause plus positive charge on the outside polarized means they have a separation of charge and an action potential is just gonna be this really brief. And by brief, I mean millisecond change of polarization and it's gonna be caused by the movement of two ions, sodium and potassium N A and K plus ions. And I really suggest that you just remember that sodium is N A and K plus is potassium. If you don't know that yet is really important. Ion snow for anatomy and physiology. OK. So let's see how this works. We have some illustrations here. We're gonna start with our polarized cell here and this is our muscle fiber rest, we have all these sodium ions N A plus ions on the outside of the cell. And we have a lot of potassium ions in high concentration on the inside of the cell. And that results in the cell being positively charged on the outside and negatively charged on the inside. Now, one way to remember this, we can say that the cell is swimming in a salty sodium C, that's something that we're gonna say again in the nervous system, your cells are swimming in a salty sodium C and the sodium is positively charged. So you have a positive charge on the outside of the cell. Now what confuses people sometimes a little bit. This potassium you'll note also has a positive charge. Don't worry about that too much. We're just talking about net charges. There's a lot of other stuff going on here. And so we really just want to remember the sodiums on the outside potassiums on the inside, positively charged outside, negatively charged inside. So we said we want to flip that charge. So when we flip it, we're gonna look at this image here we are gonna call that depolarization. So the first to depolarize the cell, we said we have a really high concentration of sodium on the outside. It's swimming in a salty sodium sea. So we open the sodium channel and this high concentration of sodium causes the sodium ions to diffuse into the cell. As the sodium or as All right here N A plus moves inside the cell. Well, it's gonna bring along with it, its positive charge and that's gonna cause the inside of the cell to become more positive. So we've now flipped the charge. It's become just a little bit more positive on the inside and negatively charged on the outside. But the action potential we're gonna flip it and then we're gonna flip it back. So to flip it back, we're gonna talk about repolarization. Well, the sodium came in to make the inside of the cell positive. So now we're gonna open the potassium channels, we have a really high con concentration of potassium inside the cell. So when you open that potassium channel, that high concentration is gonna cause them to diffuse out of the cell. So we're gonna write here, the K plus moves outside the cell. And with that, it's gonna bring its positive charge with it. It's gonna turn the outside of the cell positive the inside of the negative and the charge is restored, right. So that flip and flip back of the charge, that's the action potential and it's gonna be passed in this wave down the cell membrane. It's gonna go really fast, fast enough that essentially your entire muscle fiber, which can be inches or in some cases longer than a foot long is gonna basically all start contracting at the same time. All right. Again, we're gonna go into all of this in a lot more detail in the nervous system. But for the muscular system, you should be familiar with it at about this level. We'll practice it some more in the example to follow. I'll see you there.
5
example
Steps of Muscle Contraction Example 2
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3m
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Our example here tells us that the membrane potential inside a cell during an action potential is graph below. So before we go on, I just want to orient myself to the graph. All right, on the X axis, we have time and this is in milliseconds. So this is really fast and in the y axis, we have membrane potential and it goes from negative 90 millivolts up to zero and then on to positive 30. Now, we don't really need to probably worry about those values or worry about the units. We just know that we're starting negative, we're gonna cross zero and we're gonna end up slightly positively charged again inside the cell. Now, what it's asking us to do is to identify which part of the graph refers to depolarization and which refers to rep polarization. And we have this image here showing our potassium ions and our sodium ions and our positive charge outside the cell and negative charge inside the cell that we've talked about already just to use as a reference. All right. So as we go along here, we're gonna follow this line, we see that it starts at negative 90 millivolts So we're negatively charged inside the cell. And then it's gonna say that the sodium channels are gonna open. So remember our cell is swimming in a salty sodium C so those sodium channels open and the sodium flows inside the cell because it's in high concentration outside of the cell. So it's gonna diffuse to the inside and with it, it's gonna bring its positive charge. So we can see as this sodium is flowing into the cell inside of the cell is getting more and more and more positive. It comes up to zero and it's even gonna pass zero until it's even a little bit positively charged inside the cell. Well, if we started out polarized this, getting rid of that charge or bringing the charges on the other side, we're gonna call that depolarization. So, depolarization is when this curve increases up to the zero mark and then even overshoots it just a little bit. Now, now that we've overshot it, we can see that the sodium channels are gonna close. So no more sodium can come in the cell and the potassium channels are gonna open. Well, remember the potassium is in high concentration inside the cell. So the potassium is now gonna flow out of the cell and as it flows out of the cell, it's gonna bring with it, it's positive charge, it's taking positive charges out of the cell. So the charge on the inside of the cell is gonna start falling it's gonna fall and fall. And then somewhere down here, the sodium channels are gonna close until it, well, it's just gonna level out right where it started again. So we depolarized and now we're getting this charge to fall back to where it started at. So we're gonna call that rep polarization. OK. Again, depolarization, repolarization curves like that. This you're gonna see in a lot more detail when you talk about the nervous system. But following the these ions, sodium coming into the cell, potassium moving out of the cell and how they bring their charges with it is gonna be really important for understanding how an action potential spreads this message through the sarcolemma. We're gonna talk all about this and how it specifically refers to the muscle fiber coming up next.
6
Problem
Problem
During an action potential, the phase where ___________ moves into the cell results in depolarization while the phase where ______________ exits the cell results in repolarization.
A
Na+: K+
B
Ca2+: Na+
C
Na+: Ca2+
D
K+: Ca2+
7
concept
A. Events at the Neuromuscular Junction
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5m
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We said that the first major step of muscle contraction is that that skeletal muscle fiber needs to receive a signal from the nervous system. And then it has to spread that signal throughout the muscle fiber, receiving that signal from the nervous system. We're gonna call the events at the neuromuscular junction. And to remind ourselves that this is the first of three major steps, we have it labeled here as a. So the neuromuscular junction, that's the connection between the nervous system and the motor end plate of the muscle fiber. Now remember the cell membrane of the muscle fiber, we call the sarcolemma and there's gonna be one small region of the sarcolemma that's gonna be specialized for receiving this signal from the neuron. That specialized region is gonna be the motor and play. All right. Now, remember these don't actually connect to each other. They have this real small space between each other that we call the synapse. So to get this signal across the synapse, we need to use a neurotransmitter. And the neurotransmitter used for muscles is acetylcholine or as we're gonna write here, ach. So I'm gonna write ach going forward just because it's easier. But whenever you see, ach, just know that means acetylcholine, it's the neurotransmitter used at the neuromuscular junction. All right. So let's go through this step by step. First, up the action potential is gonna arrive at the axon terminal. All right, the axon is the extension of the neuron, highly specialized nervous tissue cell and neurons send these electrical messages using action potentials that flipping of the charge using the sodium and the potassium ions. So this action potential is gonna arrive at the terminal. And we can see here in yellow is our axon terminal. And we see this uh act uh action potential coming in with those arrows there. Now, the word terminal just means the end of something. But the way I remember that this yellow structure here is called the terminal is that if I get off a train, I get off at a train terminal, how do we get the message off of the axon? The message gets off the axon at the axon terminal. All right. So we've gotten this electrical signal down into the terminal. What that's gonna do is cause voltage gated calcium channels to open. And we can see that here in our image, we have the calcium on the outside here and we can see these channels here and when that action potential comes in, that's gonna open the channels and the calcium is gonna flow in to the axon terminal. Now, you may remember in the sarcomere, calcium entering the Sarker is gonna cause the Sarker to start contracting. So in both cases, an action potential is gonna stimulate the release of calcium and the release of these calcium ions is gonna start the process we're talking about. In this case, the release of the calcium ions as they enter the axon is going to release that acetylcholine ach it's gonna release it into the synapse. So you can see here in our illustration, we have all these vesicles and in these vesicles, you see these little sort of blue dots, that's the acetylcholine, this calcium enters it the axon terminal and these vesicles sort of dump via exocytosis, this acetylcholine into the synapse there, that acetylcholine is just gonna sort of diffuse across the synapse. It's gonna diffuse across the synaptic cleft. That's just another word for the synapse. And it's gonna bind to the receptors in the sarcolemma. And we can see that here, this uh sort of pinkish membrane here is our sarcolemma. It's sort of this wavy membrane at this neuromuscular junction. And you can see here we have these receptors there that are gonna bind to the acetylcholine. Now, when the acetylcholine binds to the receptors, the sodium ion channels are gonna open in the circle lemma, right. So that binding of the neurotransmitter acetylcholine causes sodium ion channels to open. Now, you remember opening sodium ion channels, that's how we start an action potential. So the action potential is gonna start those sodium ions are gonna enter, enter the sarcolemma. They're gonna bring their positive charge with it. That's gonna depolarize the membrane. We're then gonna c have the potassium ions leave, that'll rep polarize. And that's just gonna happen like a switch flipping back and forth, going down like a wave down the membrane. All right. So I'll just show here that's gonna cause this action potential to go out and spread in both directions down the muscle fiber. And that's gonna be the signal in the muscle fiber to start contracting. OK. But we have this Ceo choline now in the membrane and we gotta get rid of it because if the CTO Choline just keeps binding to these receptors here. Well, if it keeps binding, then the muscle is just gonna keep getting action potentials and it's just gonna keep contracting. So there's two ways that we get rid of the acetylcholine. First off, the acetylcholine can just sort of diffuse out of the synapse and some acetylcholine will do that. But more importantly, the acetylcholine is gonna be broken down by an enzyme and that enzyme is acetylcholine esterase. Acetylcholinesterase is gonna be in the synapse and it's gonna just be breaking down acetylcholine into acetic acid and cline. And when that happens, when that acetylcholine is gone, the signal stops. Now that cline actually gets taken back up by the axon terminal, it gets recycled into more acetylcholine so that we can do this whole process again. But for our pur purposes, we started an action potential. That was our goal, our membranes now excited. And now we have to figure out how do we couple that excitation to the contraction? That's what we'll be talking about next. But first, we have an example in practice problems. Give him a try.
8
example
Steps of Muscle Contraction Example 3
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3m
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9
Problem
Problem
What is the role of the calcium ion in the signaling of an action potential at the neuromuscular junction?
A
Calcium moves across the synaptic cleft to relay the signal to the muscle tissue.
B
Calcium causes the muscle cell to depolarize propagating the action potential.
C
Calcium is important for contraction in the sarcomere; it does not play a role at the axon terminal.
D
Calcium entering the axon terminal triggers the release of Acetylcholine into the synaptic cleft.
10
Problem
Problem
True or false: if false, choose the answer that best corrects the statement:
The motor neuron is in contact with the sarcolemma in order to efficiently pass the electrical signal to the muscle fiber.
A
True.
B
False: the axon terminal touches the endomysium.
C
False: the motor neuron forms a synapse with the muscle fiber at the neuromuscular junction.
D
False: the axon terminal touches the sarcolemma passing on a chemical signal.
11
concept
B. Excitation-Contraction Coupling
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5m
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12
example
Steps of Muscle Contraction Example 4
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2m
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Our example says that the term excitation contraction coupling refers to the events that turn an action potential into a muscle contraction. And we want to put the steps in order. And so we see five steps here. And these steps though, start with an action potential propagates through the muscle fiber and they end with the cross bridges form and the muscle contracts. All right. So the action potential propagates through the muscle fiber, what's gonna come after that? Well, as I look here and I think about the excitation contraction coupling, I'm definitely looking for something that talks about excitation. And as I look here, there's one thing that talks about an action potential that's C it says the action potential travels down the sarcolemma and the T tubules, right? That's that wave of depolarization, spreading through the cell spreading that signal. So I'm almost certain that that comes next and put ac there and I'm also gonna just cross it out up here for a little bookkeeping. All right. So we have the action potential now going down into the T tubules. What's gonna come after that? Well, as I think, what's gonna come after that, I'm gonna look for something about the sarcoplasmic reticulum because the action potential goes down through the t tubules and it stimulates the sarcoplasmic reticulum. So look, do you see something about the sarcoplasmic reticulum? I do D here voltage gated channels open, releasing calcium into the sara. Now, it doesn't say sarcoplasmic reticulum, but this is what it's talking about. That's what the sarcoplasmic reticulum do does when it gets that signal from the t tubules, it releases the calcium. So D is gonna come next. Now, where does the calcium go after that? Well, the calcium binds to troponin. So I'm almost certain A is gonna come next, gonna write that down. And what happens when the calcium binds to the troponin? Remember the troponins ha kind of changes its shape and when it does that, it pulls on the tram mycin and pulls it out of the way. There we go. E the troponin changes confirmation moving the trop amycin. So that's coming next. And that leaves me with one option. Let's make sure it comes last. So the last option is Mycin binding sites on Acton are exposed. All right, is that the last step? Absolutely that Tropomyosin moves, that moves out of the way of the binding sites. So the mycin can bind to the Acton. So B comes last and once those Mycin binding sites on the axon are exposed, well, then the cross bridges can form and the muscle can contract. All right, we're gonna go into those in a step by step fashion coming up. But first we got some more practice problems tomorrow.
13
Problem
Problem
Voltage gated channels respond to the depolarization of an action potential by releasing Ca2+. Where are these channels located?
A
Sarcolemma.
B
Sarcoplasmic Reticulum.
C
Sarcomere.
D
T-Tubule.
14
Problem
Problem
How does tropomyosin regulate muscle contraction?
A
Tropomyosin binds calcium, changing the confirmation of troponin.
B
Tropomyosin prevents myosin heads from binding to actin in the absence of calcium.
C
Tropomyosin wraps myosin preventing actin from binding in the absence of calcium.
D
Tropomyosin releases calcium during an action potential.
15
Problem
Problem
In a skeletal muscle fiber, which structure would you expect to have the greatest total surface area?
A
Sarcolemma.
B
Sarcoplasmic Reticulum.
C
Sarcomere.
D
T-Tubule.
16
concept
C. Cross Bridge Cycle
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7m
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17
example
Steps of Muscle Contraction Example 5
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3m
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Our example says that the events of the crossbridge cycle as they relate to Acton and Mycin are numbered in order below, separately, events as they relate to A TP are labeled ABC. But they're not necessarily in the correct order. We need to match the steps as they relate to Actinomycin to the steps as they relate to A TP by matching each letter to the correct numbered step. And we also note not all numbered steps correspond to a step as it relates to A TP. All right. So here at number 1234, we have those steps of the cross bridge cycle. But you'll notice this is just talking about the Mycin and the acting over here on the right ABC, we have what's happening with the chemical energy that's powering these steps, the A TP and the AD P in the inorganic phosphate. And so we have to match that sort of chemical energy that's powering the step with the step that's happening with the Mycin and the active. So let's take a look first up, we have one, we said that the Mycin head binds to the Acton. So does that correspond to the A TP binding to the Mycin head, the A TP being hydrolyzed, the AD P and the inorganic phosphate being released from the Mycin or does it correspond to none of them? What do you think? All right. Remember the mycin head is going to bind to the Acton as soon as that Acton is exposed, as soon as the Tropomyosin moves out of the way. And that's because that Mycin head was cocked and ready to go. It already had chemical energy transferred into the Mycin head. It's ready to go and it's gonna bind right away. That means it does not correspond directly to one of these steps over here. So I'm just gonna put a little dash, it doesn't correspond directly to one of those steps. All right. Next, we have the power stroke, power stroke when that Mycin actually pulls on the acting and we get the movement. So which of these steps does that correspond to? Well, we said that's when the AD P and the inorganic phosphate are released from the Mycin. And remember when we watched the animation, we saw, they're not actually just released at the exact same time during the power stroke. It's a little more complex than that, but we don't really need to worry about in that much detail. Usually you just need to know that they're released as p part of the power stroke as part of powering that power stroke. So I'm gonna put that C during the power stroke, ad P and inorganic phosphate are released from the mycin. All right, next, we say the mice and head releases from the acting, right? So remember it pulls on the Acton now it's got to let go do it again. Which step of the A TP cycle here helps it do that. We said that that happens when the A TP binds to the bi mycin head. The binding of the A TP to Mycin allows it to release from the acting so it can start this process again. So I'm gonna put a on this line here next to the number three. And then finally, we have the mice and head moves into the cocked position. Well, we only have one option left here but remember to move into the cock position. It needs to transfer some of that chemical energy from the A TP into the Mycin head. It does that by hydrolyzing the A TP splitting that A TP into the AD P in inorganic phosphate. So that is letter B here. I am gonna write B on this fourth line here, right? Knowing those steps, knowing how they're powered by the chemical energy. So if it can convert that chemical energy into mechanical energy is really important. So get familiar with it, more problems to file. Give them a try.
18
Problem
Problem
Which part of the cross-bridge cycle is called the power stroke?
A
Cocking of the myosin head.
B
ATP hydrolysis.
C
Myosin pulling the actin.
D
Binding of myosin heads to actin.
19
Problem
Problem
What would happen if a muscle completely ran out of ATP during a muscle contraction.
A
The myosin head would not move into the cocked position.
B
After the power stroke, the myosin would remain bound to the actin.
C
The myosin would bind to the actin, but the power stroke would not occur.
D
The sarcoplasmic reticulum would be unable to release calcium.
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