 Hey everybody, Dr. O here. In this video we're going to cover the sliding filament theory. We're going to actually talk about how muscles contract. We've covered all the key players. Now it's time to see them in action. So really big picture. What happens is myosin, the thick filaments inside our sarcomeres, when the time is right, when the situation is right, they can grab on to actin and pull it towards the center. So you see those purple myosins there in the middle? They can pull actin towards the center. I can show you a picture here. At the top it's relaxed and at the bottom it's contracted because as you can see those purple myosin heads are grabbing on to actin and pulling it towards the center. So it's moving in both directions. That's how a sarcomere shortens. Each myofibril is made of hundreds and thousands of these sarcomeres and each muscle is made of all of those. So as all these sarcomeres shorten, so will the muscle. So that's the real big picture of this sliding filament theory of contraction. How I like to think about it, I have a picture here because I like to think that this motion of the myosin head is very similar to the ores that someone's using when they row a boat. So think of the paddle, the head of that paddle in this image here as the myosin heads. They go up, they cock, they grab, they grab on to whatever they're grabbing to, which in this case would be the water, and they pull. And in order to work again, they have to detach, re-cock, reposition themselves and pull again. And if this is myosin, it's going to happen as long as there's ATP and as long as there's calcium. Let's go ahead and take a look here. I'm going to start at the right hand side. I've already told you that troponin and tropomyosin form what are called the shielding proteins. So this process cannot happen as long as tropomyosin is in the way. Troponin is what holds tropomyosin there, but troponin also has calcium receptors. So as you can see here on the left side, interstate, so calcium is going to be dumped out of the sarcoplasm reticulum there in this muscle fiber, and that calcium is going to bind on to troponin, which is going to move tropomyosin out of the way. So myosin can now grab onto actin. So I just, again, have this little, Oliver's little ball here. So right now actin cannot bind to myosin because tropomyosin's in the way. Calcium binds to troponin, it moves tropomyosin out of the way, and now the myosin head can attach to actin and pull it towards it. So that's going to be what's called cross bridge formation. Another one of those terms you're going to hear a lot. If a cross bridge between actin and myosin doesn't form, then you cannot have muscle contraction. Okay, the next step here is now you see the ax over the cross bridges formed, and now you see the sliding filament. The myosin is pulling actin over it. So this actin is sliding on top of myosin as it's being pulled toward the center of a sarcomere. Another term you might see here, this actual movement, is called the power stroke. So if you see that term, that's what it's talking about here. And as you can see, it has needed ATP. But here's where you really need it. And I think this is what's most interesting to me is that a single myosin head can only pull actin so far. It needs to, after it's pulled as far as it can, it has to detach from actin, re-cock, and then grab on and do it again. It cannot detach. So myosin cannot detach without ATP. So if you run out of ATP, myosin and actin are stuck together. So that's what rigor mortis is. So soon after someone dies, their muscles are going to get stiff and rigid because there isn't any ATP being produced. And we only store a few seconds' worth of it in our body. So since there's no ATP being produced, actin and myosin are now stuck together. Now rigor mortis goes away as the body actually starts to break down. So I think that's just kind of interesting. Without a constant supply of ATP, this attachment can't stop. So I think that's kind of interesting. All right, and then here we have, I've already said each head can only pull actin so far. So for this to keep happening, it needs to detach with using ATP. Then it has to re-cock, and now it's going to be ready to grab actin again. And that process is going to require ATP. So as you can see here, you need a constant supply of ATP for myosin to grab on to actin, to perform the power stroke to pull it towards it, and then to detach. Then lastly here, so this will continue to go on as long as you have a constant supply of ATP, but you also need a constant supply of calcium. So for muscles to relax, ATP needs to detach actin and myosin. All this calcium is going to be reabsorbed and gotten out of the way. And then that tropomyosin, troponin, tropomyosin complex, the shielding proteins will get back in the way, and then myosin will try to grab on to actin, but it can't because those shielding proteins are in the way. Okay, that's the basics of muscle contraction. I hope this helps. Have a wonderful day, be blessed.