 This is so cool. Okay. In this section, we're going to talk about all the different ways that the central nervous system can basically control, modify, fine tune messages of movement. Okay, so watch. I'm going to draw you several, I think we'll just do four. Okay, I'm going to draw you four somatic motor neurons coming out of the central nervous system. And I'm not going to actually draw all of my myofibers, but each one of these neurons is in charge of a team of myofibers, skeletal muscle cells. So I'm going to represent all of them with this box. And I'm going to tell you that, okay, this one has, let's just say, 10 cells. So 10 myofibers are in this particular motor unit. And my second, you know what, I think I have to give these guys names. This one's going to be in charge of 40 cells. This is going to be Ralph. And this is going to be Bob. And then we're going to have Fred. Fred is going to be in charge of six cells. So Fred has a relatively small motor unit. And then we're going to have Alf. And Alf is going to be in charge of 70 cells. Okay, the central nervous system can dictate which motor units are going to be fired. So the central nervous system can say Bob, go. Fred, go. And what do you know about motor units? Some of them are quick twitch, and some of them are not. I'm drawing a slow twitch. These guys are all slow twitches, and Fred's are all slow twitches. So basically, the central nervous system can say, okay, we're going to fire only the motor neurons that are in charge of slow twitch motor units. And the message then will end up being a slower response. Like the contraction overall is going to be slower, because it's only the slow twitch fibers that are being told to fire. The central nervous system can also say, you know what, we're going to go ahead and activate three of us. Not all of us, just three of us. The message is going to go out to three of us, and by controlling the number of motor units that are activated, the central nervous system can control the end result, the end contraction, the gross motor movement that takes place. The central nervous system can control how quickly the message is sent. Can say, okay, Bob, you're going to fire at this rate. Okay, I'm just drawing this on here, because what we know is that that will allow, at that pace, we probably won't reach tetanus with my 10 cells at the rate that Bob is firing. But you know what, Ralph, you are going to fire quicker, and therefore, all of your 40 cells over there are going to reach sustained, smooth, steady tetanus. By changing the, what, the message, the speed of the message, we can change the kind of contraction that we get. And then this part is the part that just blows my mind apart into a million pieces. The central nervous system will do something called asynchronous firing. Synchronous, asynchronous, it wasn't firing, recruitment. Okay, asynchronous recruitment means that all the motor units that are affecting a certain muscle that the central nervous system has decided needs to contract, all of them are going to fire at different times. So Bob, you go first. And everybody is probably in this scenario. Let's have everybody fire at the same pace. Let's have everybody fire, like, we're going to let them all go. And we're going to create tetanus in everybody. But asynchronous recruitment means that Bob is going to go, and then Fred's going to go, and then Ralph is going to go, and Bob is finished now, and then we're going to throw Alf in there, and then we're going to get some Ralph, and then some more Alf, and then a little bit more Fred, and now we'll let Bob come back, and Fred is relaxed by this point, and Ralph is going, why, why would we do that? Seriously, like, who are these people and why are they contracting like this? Because when Alf is not firing, he's resting. And then when Bob is firing for him, Alf gets to rest. And when Bob's turned to rest, this asynchronous recruitment allows or reduces the chances of fatigue in the muscle. Now, think about this for a second. Imagine that Bob Ralph, Fred and Alf, have to lift a maximal, like, something that is crazy, it's so heavy. And the central nervous system knows, you know, we're going to lift 1,000 pounds, fellas. So what you're going to do about it? Do you think any of them are going to be able to not be firing if we're going to try and lift 1,000 pounds? Well, my muscles could do it, but the rest of you guys, probably not. And Bob, Ralph, Fred, and Alf probably are going to have to fire maximally all of them together in order to lift 1,000 pounds. What does that mean? Like, when you carry out a mat, like you've got the heaviest load that you can possibly carry, how long can you carry that thing for your muscle literally fatigues and you drop your load? It doesn't take very long. If the load isn't maximal, so that you don't have to fire everybody at the same time to keep carrying it, you can avoid fatigue. And that's why your motor units work together. All right, we've got one more concept of different kinds of movement. So now we're going really big, and then we're finished.