Now let's look at what happens if you stimulate a single muscle fiber with just enough stimulation to cause a single contraction cycle. This is called a twitch. So let's assume now that we have an action potential from a motor neuron that releases enough acetylcholine at the neuromuscular junction to generate an action potential within the sarcolemma.
And that sarcolemma is going to propagate the action potential down the t-tubules. and be able to release enough calcium from the sarcoplasmic reticulum that it will then cause the contraction. So let's look at a theoretical thing that we call a twitch.
And this is a real thing. We can make a single muscle fiber twitch. But the twitch in and of itself is not going to accomplish a lot of work. It's almost the same thing that you would feel if you had like a tick in your eye or a fasciculation or a quick brief stimulation of a part of a muscle or a single muscle fiber.
So when we talk about twitches, we're not talking about doing any real work with the muscle, but it's something that we can study in the lab. We can induce them in the lab and study them and look at the dynamics of how muscle stimulation occurs. So when we get a threshold stimulus, then we can cause a very brief contraction, and we have a force of contraction that's going to be proportional to stimulus. But in this case, remember, this is a single contraction of a single muscle fiber. But what we can do is look at different muscle fibers of different muscle types throughout the body.
So, for example, our eye muscles, which move very, very quickly and will generate a very brief amount of contraction, a fairly large amount of force for the type of muscle or the size of the muscle that we're dealing with, versus, say, a muscle in your calf, like the soleus, which is going to be holding you up all day and be one of your postural muscles. We would expect the type of stimulation, though they might be the same, the contraction profiles would be quite different. In any case, we have something called a latent period. And the latent period is the time it takes between stimulation to the point at which the contraction begins. And the latent period is the time that it takes for the acetylcholine to diffuse across the synaptic cleft, open the sodium channels, have the sodium diffuse into the sarcolemma, Depolarize the sarcolemma, initiate the action potential.
That action potential will spread across the sarcolemma down into the t-tubules and then it will cause the release of the calcium of the sarcoplasmic reticulum. At that point the calcium will then bind to troponin, which will start that whole contraction cycle by moving the tropomyosin or causing it to rotate around thus unblocking the active sites on actin so that the myohate myosin heads can start their cross-bridging. So all of that is contained within this latent period. And then once the cross-bridging starts and the contraction starts, that's when we have our period of contraction. And that will peak until such time as the calcium starts to diffuse back into the sarcoplasmic reticulum.
Because remember, if we've only done a single stimulus to cause a single contraction, then we've had just enough stimulation. to release just enough calcium to cause a single contraction. At the peak of that contraction, now that calcium is going to go back into the sarcoplasmic reticulum because we're no longer stimulating the muscle. And so once that happens, then that's going to unbind the troponin, and then the tropomyosin will roll back over the active sites on actin, and then the myosin heads can no longer cross-bridge, and so then we have our relaxation period.
And in different muscles, these periods of contractions will differ. depending on what type of muscles they are. We have things called fast twitch muscles, which are specialized for very quick ballistic movements, and we have slow twitch muscles, which are important for things like postural control and endurance and these kinds of things.
And though we're not going to get into that in this course, it's important to notice that there are differences and the length of contraction is going to vary depending on the type of muscle. fiber that we're dealing with. Now if we do a single stimulation and we get a single contraction, well then we could wait and do another stimulation and get another contraction or another twitch of this muscle fiber.
We keep doing that. Now this isn't going to produce much force. It's not going to be able to do much work.
But imagine that if we keep stimulating the muscle fiber more quickly so that we stimulate the muscle fibers such that Before it has a chance completely to relax, we stimulate it again, and then we stimulate it again. Then we have this type of profile right here, and this is called summation. And if you stimulate quickly enough, you can actually make the muscle fiber stay contracted because the cross bridging will continue.
And basically, each sarcomere will contract with smallest length. And then the muscle will stay in its shortest, the muscle fiber, I should say, will stay in its shortest configuration. And there'll still be a lot of tension on it because those cross bridges are still happening.
And so what's going to happen is what we call tetany or tetany. And this is where we have complete contraction of that muscle fiber. Now we can translate this to an entire motor unit and possibly even an entire muscle. So, so far we have looked at just a single muscle fiber.
But remember, muscle cells are made up of hundreds of muscle fibers. And one of the things we learn about muscle fibers is that Each muscle fiber is innervated by one and only one motor neuron. However, one motor neuron can innervate several muscle fibers, and they do. They will innervate many, many muscle fibers. And all the muscle fibers innervated by a single motor neuron is called a motor unit.
And as we will see in the next video, that the greater the force of contraction that you need to generate, the more motor units you will bring online because then you can bring on more muscle fibers so that the more units you have contracting then the more force you can generate in the muscle.