Active contraction vs. passive recoil

So ever since I first learned about myosin and actin, there was always this thought that popped in my head, a kind of an analogy, if you will. And for a long time, I thought this analogy was pretty much spot-on. But then, I gave it some more thought. And I realized that I was wrong. And so I'm going to share with you what my analogy had always been. And you'll see, I'm sure pretty quickly, why I was mistaken. So let me start out by just drawing out the actin and myosin. This is, of course, in red, the actin. And in purple, I've got my myosin. This will be my myosin here. And this is, of course, three different myosin and actins I'm going to draw for you. I'm going to show different stages of how they could look. This one is a little bit more stretched out here, and I'll draw it like that. And the final one will be very, very stretched out. We'll actually just get almost off this screen, something like this. So these are my three actins and myosins. And we know, if we were to actually take a step back, maybe I could even label them A, B, C. Let's call this one A. Let's call this one B. And the third one will be C. So we've got our A, B, and C. And there's this helpful way of looking at this stuff. We call it the tension-length curve. So I'll put tension over here. And this is, of course, a unit of force. Thinking about how forcefully something is contracting. And then, over here-- let me actually erase that-- over here, we have sarcomere length. So these are our two axes. And on the graph, we can quickly just put where A, B, and C would lie. So you can see that based on the way that I've drawn this stretch-- I'm just going to divide this in half-- based on the way I've drawn it out, A is actually going to have almost no force. That's going to be the conclusion we can reach. It's going to be something like this. And then B will be somewhere up here. Let's draw B right here, because there's going to be a lot of force there. And C will be-- I'll draw it right here at the edge. Also, almost no force. And you remember, that this actually falls on a curve that we drew out before. Something like this. Where actually, I didn't draw all the points here, but it kind of goes like that. So this is our tension-length curve. And you can see where A, B, and C fit on that curve. Now on the side, what I want to draw on the other side-- I'm going to draw out what my analogy used to be. The way I used to think about it. And it also breaks down into an A, B, C. And I'll just write it out here. And it's something that I always used to play with as a kid. I always used to love slingshots. And so I'm going to draw three slingshots, one, two, three. And each one will actually have a rubber band attached to it. And I'm going to stretch it out to different lengths. So let's say, this first one, I don't stretch much at all. Then, this second one, I stretch really far, as far as I can. And then, this third one, I stretch it so far that it kind of snaps. And, of course, if I have a slingshot, I need a stone. So I'm going to put my purple stone right there. And I'll put my purple stone right there, at the tip. And then, this purple stone, I guess I have to hold it because otherwise it would just fall down, right? So what would happen if I actually now try to plot out, on this side, on the right side of your screen, if I plot it out. Similar to the tension length, but in this case, instead of tension, let's put distance. And this would be the distance traveled of my stone. So maybe I can rewrite this and make it a little bit more roomy. So distance of my stone traveled, and that will be here. And then, I can also, on the x-axis I can put something like, how much I stretched my rubber band. I'll just put, let's say, stretch. And then you'll know that means how much I stretched out the rubber band on my slingshot. Now, if I actually let go of all three, the stone would probably fall right there, on A. And it would fall right there on C. But for B, it would launch. It would launch away. And so in terms of distance, I could actually plot that out. I could say, well, for A, I had almost no distance. I would say zero distance. And for C, kind of the same thing. I'd say really no distance. But for B, I had a lot of distance. So I actually did really well with B. And this is how I always thought about the heart. I always thought, well, it's very similar in some ways to a slingshot. You have an up and a down, right? And so I always walked around with that idea. But I gave it some more thought, recently. I was thinking, is this really accurate? And I think the answer is no. And let me show you why. So on the slingshot side-- let's do this side first-- what do we have exactly? We have elastic energy. And that's just the elastic in the band. But there is energy stored up there because it's a potential energy. It's actually very similar to what happens in our arteries, where you store up energy in our elastic large arteries, like the aorta. But you have this elastic energy and when you let go of the stone, what basically happens is that you convert all that to kinetic energy. Right? So you're converting it all to kinetic energy energy of movement. And when you let go of that stone, it happens automatically. So you really don't have to put energy into it because you already had elastic energy, it was already stored up. So in that sense, we often think of this process-- and this is actually the important part-- we often think of this process as being passive. So you'll often see the word passive. Put that down here, passive. And that simply means that we didn't have to add any energy. But specifically, the kind of energy we're talking about is chemical energy. So when people say there's a passive process, usually in biology, what we're talking about is not having to use chemical energy. And, of course, in the slingshot example, there was no chemical energy used. But in my heart example, in my sarcomere, there was chemical energy. In fact, what we're really doing is we're converting chemical energy. And specifically, the type of chemical energy we're talking about, if you remember, is ATP. Remember, all those myosin heads are working and grinding through ATP. So this is really ATP energy that we're burning through. And we're creating, again, kinetic energy. Sometimes I call it mechanical energy. But both times, what I mean, with kinetic or mechanical energy, is to say that you basically have the heart pumping. You actually have movement of the heart. And the way that you're getting it is by burning through all this ATP. So in that sense, because we're burning ATP, oftentimes in biology, we call this an active process. Now in both cases, you're just changing one form of energy to another. So it's not like I was completely wrong with my thought process. I mean, there are some strong similarities. And at the end of the day, both of them are creating movement. So there is a similarity there. You're changing energy forms and you're creating movement. But the key difference is in what type of energy we're starting with. And I want to make sure it's very, very clear that with the heart, it often looks like a rubber band. It even sometimes feels like it could be like a rubber band, where you're stretching out. But really, never forget that the myosins are grinding through ATP. And that is the way that you're actually able to create the kinetic energy. Whereas in the elastic band, you're actually using elastic energy. So that's the key difference.