 Okay, this is really, really cool. So we already know that basically in order to get any kind of muscle contraction at all, we need to deliver a message from a somatic motor neuron. And you also already know that my somatic motor neuron, in order to get that thing to contract, it's chilling at its resting membrane potential of negative 70, the stimulus happens, the message, the action potential begins, we get the quick message. Does anybody remember how long that takes? Due to something like, you know, maybe two milliseconds, maybe. And then at the end of the action potential, at the end of my neuron, we're going to dump acetylcholine into the synapse. And then what does that cause? So acetylcholine is going to open sodium channels when it binds to the nicotinic receptors on the skeletal muscle fiber. That's going to initiate an action potential in the skeletal muscle. So shortly after the action potential ends, watch and be amazed, you're going to get an action potential in the skeletal muscle. And this also takes, you know, two to four milliseconds. So this is an action potential in skeletal muscle. And this one was my action potential in my somatic motor neuron. Now, now we have an action potential that has passed, traveled down the skeletal muscle fiber and it's gone down into my T-tubules. And now it's opening up the voltage-gated sodium-calcium channels in the sarcoplasmic reticulum. Are you following this? How long do you think this takes? Now we dump calcium into the sarcoplasm. Now calcium diffuses over to the myofibrils and attaches to the myofilaments and causes the contraction to begin. How long did that take? So let me tell you, the whole thing, it takes a while. It takes a while and then it causes a contraction of the skeletal muscle. This is no longer millivolts. I am now measuring tension. So on the same graph, I put them on the same graph, so take a deep breath and make sure you hear me when I say we have millivolts to measure our two action potentials, and now I'm showing you a graph of tension. There is this latent period, and that's the time that calcium is diffusing and calcium is getting over there and finally causing the myosin binding site on actin to be exposed so that we can actually begin the contraction and now we have contraction beginning. So headed up, watch this amazing feat of writing. This is contraction, increasing the tension in the muscle fiber, and this is relaxation, right? Releasing the tension in the myofiber. That right there is one twitch. Now, here's the thing that I want to tell you that is going to make you go, oh, well that's an interesting phenomenon. If you allow the muscle to fully relax and get back to this point, it takes, oh, let's just say, 100 milliseconds. So from start to finish, 100 milliseconds later, we will have one complete twitch done and we're back to our relaxed muscle where we started. Can you imagine that? What happens if we send a twitch more often? Well, since you asked, let's go over here and I'm going to draw you a new picture. This time, all I'm drawing is tension. It's all tension, dogs, nothing but tension. But this time, and we're going to do time down at the bottom, again, but this time, I'm going to make a little note, watch. This is going to indicate a stimulus. I'm not going to draw my action potentials, but do you agree that if we fire a stimulus that we're going to get all our action potentials, like that's what a stimulus is. It's the action potentials that are delivering the message. You deliver the stimulus and watch my tension. My tension increases. This is a stimulus that started the whole thing. If I don't let that muscle fiber relax all the way, if I'm like, nice try, doggy pound, you're almost like part way relaxed and you're thinking, oh yeah, I'm ready to just chill on the couch, I'm ready to relax, but your central nervous system says, I don't think so, pal. Here comes another stimulus. Guess what happens? You never relax. You actually increase the tension. What? Seriously? Yeah. Let's not let ourselves relax. Let's throw another stimulus in there. We start to relax and then we're like, oh dang it, I really was looking forward to that couch. Throw in another stimulus. Do you see what's happening? We're throwing in a stimulus before the muscle fiber can actually put everything back to where it was. And in doing that, we are actually increasing the amount of tension that we can generate. So what we're doing here is throwing in, here was one stimulus. What we're doing is we're throwing in a stimulus here. That's where we'll get like this. We started to kind of relax and no, we're going to go back up again because we had another stimulus. If we waited and threw the stimulus in at this point, then our twitch is going to be just what you would expect. Because we already got to relax. This is called summation. Summation is the fact that you can increase the amount of tension that you have by speeding up the frequency of stimulus. So you stimulate more often. Does this work with neurons? Doesn't matter how often you stimulate them. They're going to fire. You can only go so fast with a neuron. You can't actually get a neuron to fire in the middle of an action potential. So you can't, you don't have like a bigger, a bigger, a bigger action potential that's created. An action potential is all or nothing. Skeletal muscles are different. Now watch what happens eventually over time. We keep stimulating at this pace. Well eventually we're going to reach this concept where you know what? You really can't create more tension than the maximum tension. And if you keep stimulating at the same rate, you create this scene, this phenomenon called tetanus. Now in this situation, my firings are happening, my stimuli are happening at a pace that causes, we're still getting some bumpiness. And that's called unfused tetanus. But if all of a sudden your body's like, unfused, but uh-huh, we are going to start doing like, we're not messing around, we're sending messages that you guys better get your do-do together and contract faster, faster. Guess what happens? You get a phenomenon called fused tetanus and it's a smooth series of contractions that maintain maximal tension. How fantastic is that? All right. Okay. Let's talk next about how motor units come into this mix.