CV Physiology | Frank-Starling Mechanism
There is no single Frank-Starling curve on which the ventricle operates. Instead, there is a family of curves, each of which is defined by the afterload and. This length-tension relationship, also known as the Frank-Starling law, is due to as Starling's law of the heart, and is defined as the relationship between stroke . Looking for online definition of Frank-Starling relationship in the Medical Dictionary? Frank-Starling relationship explanation free. What is Frank-Starling.
And I want you to keep an eye on how many myosin heads are actually working, almost as if you're the taskmaster and you've got to make sure that the myosin heads are all working. Make sure you keep an eye on exactly how many are doing what we want them to do, which is contract or pull in the actin. So let me actually just take a little shortcut here so I don't have to keep redrawing this.
I'm going to move this down here, and I'll do it again. And I'll move it even lower. So we have our myosin there. Now, around the myosin-- in fact, let me label it while I still can.
This is our myosin. Around our myosin, we have, of course, actin.
I'll write it bigger just so you can see it very clearly. We have actin, and actin is we'll do in red. But because we have a very low preload-- or almost no preload-- I'm going to show you what that means for our molecules.
You're going to have something like this where you have everything kind of crowded together. And that's kind of the core issue I want to point out to you. You have lots of crowding problems. And of course, the myosin-- on the ends of here-- this is our Z-Disk. I'm going to write "Z-Disk.
What I'm showing you is kind of a part of the sarcomere. Remember, the sarcomere is kind of the basic unit of contraction, and it usually goes from Z-Disk to Z-Disk.
So this is just a part of it because you'd have many, many more actins and myosins stacked up and below it.
But this is just to kind of give you a sense for what we're looking at. And this is, of course, our actin. The question is-- and I guess I should-- sorry. Before the question, let me throw in titin.
This little green molecule is titin. So the question is-- how would contraction occur? If you were to look at this scenario and you're kind of an inspector, you're just kind of assessing for problems, would you expect that there would be any problems? Would you expect any problems here? And afterwards, I also want you to think about force. What kind of force do you expect to get out of this sort of arrangement?
A lot of force, or a little force? What do you think? Well, immediately, I can see some problems. I mean, you know that the whole goal is to pull the Z-Disks in closer to each other. That's the whole point. The myosin is going to yank on the actin ropes-- you could think of it as a rope-- and yank the Z-Disks in.
And if there's really almost no space here-- see this right here, there's almost no space here. And this myosin is basically almost touching the Z-Disk. This guy right here is almost touching the Z-Disk already. Well, then, what do I really expect to happen? There's going to be almost no force because the problem-- and I'm going to write it very clearly-- is that the myosin is crowded.
Meaning it's right up against the Z-Disk right from the beginning. And that's a problem. Because that means-- what can you really hope to achieve if you've already gotten the myosin already against the Z-Disk? There's really no space for you to yank the actin in to bring the Z-Disk in closer. There's no space there. So I would say that's the biggest problem. And secondly, there's actually another problem here.
And that's around actin. Because the actin has polarity, and this is an important issue. These two actin molecules that I've drawn arrows around are fundamentally different because there's a directionality to the way those proteins are laid out. And we call that "polarity. And what that means is that then myosin can't simply reach up and grab the nearest actin. It has to grab the correct actin.
So for example, these four right here-- I'm going to draw a little circle around them in yellow-- these four really want this actin on this side. And these four down here, they really want the actin on this side. But both of those groups of myosins are blocked by the other actin. So for example, these four at the top are blocked by this segment right here, and these bottom four are being blocked by-- I could actually change it.
I could say these. Or this segment right there. So there's actually some actin-blocking going on. So I call that "actin overlap" or "actin blocking. Let's call it "overlap" because I think that makes a little bit more sense. So you've got some actin overlap, but that's kind of a secondary point here because the main issue is that myosin, frankly, is just crowded. So in terms of force, would I expect any? I would say no. I wouldn't really expect any because there's really nothing for the myosin to really get done.
There's just no space. Now, let's say we stretch things out.
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- Frank-Starling Mechanism
This is scenario two. So things are a little bit stretched out now. You're looking at our graph up above. Now, things are stretched out-- meaning that here, instead of the way it was drawn before-- let me, actually, kind of correct it and draw it like this.
You still have to consider the polarity issue, but things are a little bit more spaced out now. You've got something like that. And going on the other side, you've got something like, let's say, that. So look at this, and now tell me what you think. You've got a couple of myosins that are still blocked. You still have a little bit of blockage here.
These ones are blocked, and these ones are blocked. The main reason, again, for the blockage is that there's a polarity issue here and here, meaning that those myosins cannot simply bind whatever is closer.
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And they're really not able to get over to the side where the actin is, where they need to bind. So those 4 out of 20 myosin heads are not going to be able to work. But the rest of it is actually looking a lot better than before. We have some improvement. So here, you've got some actin overlap issues. So in terms of problems, I would say actin overlap is still kind of an issue. In terms of force, I wouldn't say no anymore because now, at least, the "myosin is crowded" problem has gone away.
It's not as crowded, and there is room to move. So I would say I would expect some force. So when there's contraction, I would expect some force here. So things are definitely getting better. The stretching is helping things out because it's basically moving the actin so that it's not congesting the area. And the myosin is similarly moving away from the Z-Disk. Let me make a few more of these. I'll make one more, and we'll keep going. It was initially thought that changing the length of sarcomeres influences the overlap between thick and thin filaments causing different amounts of cross-bridge cycling.
Further studies debate these findings by showing that the amount of active tension generated could be changed while the number of cross-bridges remained constant.
Decades of research have led to evidence supporting several mechanisms operating simultaneously. Increasing sarcomere length leads to increased affinity of troponin for calcium which facilitates the interaction of actin and myosin.
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Stretching the myocytes leads to decreased interfilament lattice spacing which brings actin and myosin closer together.
Stretch causes titin to reduce lattice space and change cross-bridge orientation. Calcium binding to troponin induces conformational changes in the troponin-tropomyosin complex allowing stretch-dependent activation. Stretching induces cooperativity in which initial cross-bridge formation potentiates further binding leading to increased active tension for any given calcium concentration.
Clinical Significance The Frank-Starling relationship describes how the left ventricle responds to increased preload under normal conditions. Figure 2 demonstrates this in a graphical representation. This is represented by moving along various points on the green curve and appears similar to the length-tension relationship image 1.
Changes in contractility cause the normal curve to shift up or down meaning there is a difference in contractility for any given preload. Of note, there is a plateau of the Frank-Starling relationship at higher amounts of preload.
This might be due to increases in sarcomere length that exceed the optimal value leading to decreases in affinity of calcium for troponin. As a result, there is very little stretching to produce the length dependent activation observed in the Frank-Starling relationship.
The Frank-Starling relationship is essential in understanding the underlying pathophysiology of heart failure. Heart failure is divided into diastolic and systolic dysfunction. Diastolic heart failure frequently occurs when the left ventricle has difficulty filling due to changes induced by chronic pressure overload as in hypertension or aortic stenosis. Concentric hypertrophy is an adaptation to the increased wall tension leading to the formation of new sarcomeres in parallel. This causes an increase in wall thickness and a decrease in internal chamber size.
Under these conditions, there is a decrease in ventricular compliance which impairs filling of the left ventricle. Ultimately, the reduction in preload results in a decreased stroke volume according to the principles established by the Frank-Starling relationship which correlates with a movement towards the left along the green curve in image 2.