To finish up the steps of muscle contraction, we're going to talk about the cross bridge cycle, and because this is our 3rd major stage of muscle contraction, we have it labeled here as C. So the cross bridge cycle, we're going to say, is the interaction of the myosin head and the actin in a way that leads to the sarcomere shortening. And that's our goal. If we want a muscle to contract, we need that fundamental unit of muscle contraction, the sarcomere, to get shorter. So we have this cross bridge cycle broken up into 4 steps here. But before we dive into this, let's just remind ourselves where we are in this process. At the end of the excitation-contraction coupling, we've dumped this calcium into the sarcomere. The calcium binds to the troponin. Troponin moves the tropomyosin out of the way. That exposes the binding site, and we said that that myosin head was just waiting there ready to go. As soon as that binding site was open, that myosin head is going to bind to the actin. So, that's where we're at. The first step is that the myosin head is going to bind to the exposed actin. And we have that shown in our illustration here. We see the actin here in sort of gold. We have the tropomyosin that's now moved out of the way. We can see those exposed binding sites and the myosin head comes up and it binds to the actin.

Now, one thing you might notice here that we haven't put in an illustration before is that bound to this myosin head we have an ADP and an inorganic phosphate. ADP and an inorganic phosphate are produced when you split or hydrolyze an ATP molecule, and ATP is that unit of chemical energy in the cell. So for this myosin head to have been ready to go, to be cocked and ready to go, ready to bind to that binding site as soon as it was exposed, it has to have already sort of loaded itself up with that chemical energy from the ATP. So before this started, this myosin head is already bound to an ATP and it's already split or hydrolyzed that ATP into the ADP and phosphate to capture some of that chemical energy, so it was waiting there ready to go. Well, now it's gone. Now it's come up. It's bound to that actin, so the next thing it's going to do, it's going to pull on the actin. We said that's all myosin really wants to do. Myosin wants to pull on that actin? Well, now it gets to. Myosin performs the power stroke. Power stroke, it pulls on the actin and that's going to move the actin and it's also gonna result in the release of the ADP and that inorganic phosphate. And so we can see that in our illustration here. Here we have the myosin head coming up, it's pulling on the actin in that way, and you can see it's releasing that ADP and it's releasing the phosphate. Now technically the phosphate and the ADP are released at different times in this process, but that's a level of detail you usually don't need to know. Usually, you just need to know that it's released as part of the powering of this power stroke.

Alright, so it's now pulled on the actin, it's moved the actin, but remember I've sort of always described it as almost like a hand over hand, again and again motion. So for this myosin head to be able to pull on the actin again, it needs to release from the actin. It needs to go back, get cocked again, and do the whole process over. So to release, it's actually going to need a new molecule of ATP. So ATP, a new molecule of ATP, we're going to say binds to the myosin head and that releases it from the actin. So we can see here we have the myosin head coming down, it's releasing, and that's because this new molecule of ATP has come in and bound to that myosin head. But now it needs to get back cocked, ready to go, it needs to transfer some of that chemical energy from the ATP into the myosin head. To do that, it needs to split the ATP or hydrolyze it into that ADP or inorganic phosphate. So we're going to say here, the ATP is hydrolyzed and that myosin head now moves into that cocked position and it is ready to go again. And if that binding site is open, it's going to go up again, bind to the actin, pull on the actin, a new ATP molecule will come in, bind, it will release, it will hydrolyze it, it will get cocked, ready to go again, and if that binding site is still open, it's going to do this cross bridge cycle over and over and over again. Alright. Let's look at an animation of this so that we can sort of see this whole process through. So we have the myosin there on the bottom, the actin on the top, and you can see that the binding sites on the actin are exposed, and this myosin head is ready to go. So it's going to come up and it's going to bind to the actin, and here it's going to already release that phosphate. And now it is gonna do this power stroke. It is ready to move, and as part of that power stroke, it's gonna release this ADP, and now it pulls on the actin. And that's our movement. That's been our goal. Now though, we need to release from the actin so that we can do it again. To do that, we're going to have this new molecule of ATP come in. It's gonna bind to the myosin head. That allows the myosin head to release. But to get cocked again ready to go, it needs to hydrolyze this ATP. It needs to split it into the ADP and the phosphate. That's gonna transfer that chemical energy from the ATP into the myosin head, and it can move back. Now it's cocked, ready to go, ready to do it all again. Alright. So, let's move back to our page here and just to finish things up, I just want to remind ourselves, well here we've been looking at one myosin head. But, of course, it's not just one myosin head doing this. During contraction there are going to be thousands of cross bridges that are going to contribute to the sarcomere, to the shortening of just one sarcomere. Right? When we think of the myosomere, this myosin filament, every filament had over 300 of these myosin heads. These hands reaching out ready to grab onto that actin. The myosin filament reached out in both directions from the middle of the sarcomere. To remind ourselves of our analogy at the beginning. We had these guys pulling on a rope. But it's not just one guy facing in each direction. You have to think of thousands of hands pulling in each direction, and this sarcomere had many myosin filaments. And remember, the sarcomere then is repeated over and over again. We said in a 10 centimeter muscle, you'd have something like 40,000 sarcomeres repeated end to end. And that's in just one myofibril. In a muscle fiber, you have a whole bundle of myofibrils. The muscle fiber then is bundled to make a fascicle and that fascicle, a whole bundle of fascicles makes up a muscle. So when you think about that, how many individual myosin heads? When you think about a muscle like your bicep or your quadricep contracting, you do all that math, all that multiplication, I have no idea how many myosin heads are contributing to the contraction of your biceps muscle. But it is a heck of a lot. Sort of an unfathomable number. Each individual myosin head is just pulling on this actin just a little bit over and over again, and when you multiply that all out you end up with these very large and powerful muscle contractions. Alright. We've now talked about the steps of muscle contractions starting with the action potential in the neuron and now talking about how this sarcomere actually gets shorter with the actin sliding over the myosin. That was our goal, so good job, everybody.