Biology: Neuromuscular Junction: Contraction

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When a muscle contracts - think about this - this is very bizarre. There is movement in a cell. Now if you think back to your cell knowledge, you know that cells can move. We've seen phagocytosis and it has something to do with something we called the cytoskeleton and the cytoskeleton had some microtubules and microfilament involvement. But how do these things work? How do materials move?
Motor molecules must have something to do with that, huh? And therefore, ATP must have something to do with that. But the bottom line is that muscles don't move on their own. Muscles need some kind of stimulus. Muscles need to be stimulated by a nerve. Therefore, cells must communicate. Homeostasis - cell-cell interaction to maintain some form of stability. And, indeed, this represents a part of the muscle-nerve network called the neuromuscular junction. But, so what? Nerves touch muscles. Show me something important here. How is this going to work? In order to really understand this, we have to do a little bit more about the anatomy of those actin and myosin filaments, why they're not connected and how we might get them connected, and then, some other time, we'll talk about what happens, after they're connected. But we have to get them connected first. Why aren't they connected?
Let's take a look. Back to my diagram of a typical myosin filament. You remember that the myosin filament has these heads coming off of it - myosin heads. You remember that the actin filament was above it. The question to ask is why aren't they touching? It turns out that there are two proteins that interact together on the myosin filament. These proteins are called troponin and tropomyosin. What happens is, there are regions on this purple strand where the myosin and actin should be able to bond. They're there. The way I've drawn this now, they would be bound except for one thing. Except for the fact that troponin and tropomyosin are going to interact together to form a protein complex that is arranged so that it blocks any bonding between the actin and the myosin. So there is going to be no bonding between the actin and the myosin as long as those things are in place. So, somehow this neuromuscular junction must someway remove those blocks.
At least that's what your intuition will say, right? With what you know about how cells work. Well, you're absolutely right. Let's go through the steps of what's going to happen.
Number One. The first thing that's going to happen is right along the - what kind of neuron is this? Think about it. It's a neuron going to a muscle. It's a motor neuron. So, along the motor neuron, along it comes down to the terminal branch, where the muscle meets the nerve, and at the neuromuscular junction, the action potential is going to arrive. So, the action potential arrives. Now, in this case, we're showing a skeletal muscle and, therefore, a very common neurotransmitter to be secreted with skeletal muscles is acetylcholine. So the neurotransmitter will be released and since we want this thing to contract, we're obviously going to be using something like acetylcholine, which causes a contraction in a skeletal muscle, the muscles attached to your skeleton. The neurotransmitter is released. So what's going to happen next is an entire wave of depolarization. So, literally what's going to happen is along the plasma membrane of this muscle cell - remember this is a cell, one cell, and that makes this the plasma membrane. You are going to get a depolarization wave radiating out as we show with these arrows here, along the entire cell plasma membrane of the muscle cell. Here's the thing. There are these tubules, these extensions - let me show you this in a diagram. If you take a look at the muscle cell, there are places where the plasma membrane dips way down in between, into the cytoplasm of the cell. So, there are these cytoplasmic dips of the plasma membrane, and so when the neurotransmission or when the depolarization goes along the plasma membrane, it will often go down these dips. These dips are referred to microscopically to as T tubules. So they're extensions of the plasma membrane downward. You can see a T tubule in our diagram here as you see the wave of depolarization is going downward. What the depolarization wave I going to do is trigger events inside the cell. Because that's where all this has to happen. Now, remember, back to my troponin and tropomyosin block, we have to somehow remove that. And where is that? That's in here. See, here's my fiber in cross-section. That block is blocking events in between - if this is a myosin filament here, it's blocking events that are going to occur here, and here, and here, and here. So, in other words, we want to attach these actin and myosin filaments together. We want to get these things together somehow.
That being said, let's take a look at how we're going to get this done. When the impulse goes down the T tubule, it goes to something called the SR. It's kind of an ER except that it's in a muscle and it's specially adapted and it holds calcium. SR stands for the sarcoplasmic reticulum. Let me tell you about this sarcoplasmic reticulum. Anatomists, if this were a course in muscle anatomy, we would be using that term "sarco" a lot. The functional unit of the muscle is called the sarcomere. The cell membrane of this muscle cell is called the sarcolemma. The ER, the specialize ER, the sarcoplasmic reticulum. So when I say SR, realize that I'm not really inventing something. This is like the ER. I want to talk to you about what happens here. What happens here is the SR is a calcium storer and it stores calcium. What do you think that wave of depolarization does? When the depolarization hits the sarcoplasmic reticulum, the sarcoplasmic reticulum is going to give off calcium and calcium is going to flood the microfilaments, the actin and the myosin filaments. What do you think it's going to do? It's going to bond to the troponin and tropomyosin and change its configuration. And so now what can happen is something like this. Here's my resting state muscle, with its heads coming off, like so. Here's my actin filaments. When this thing was resting before, you remember that it had the troponin/tropomyosin. The troponin/tropomyosin blocked the binding situation. Well, now, we're going to allow it to happen because we are going to remove the troponin and tropomyosin and - not remove it necessarily, but change its position. In changing its position, we're going to free up the actin and myosin so that what can happen next is something like this. Now, we can form bonds. The troponin heads are going to bond to the actin, thanks to the action of calcium. So the calcium initiates it all. The calcium is what's going to allow these actin and myosin filaments to bond together. When you are said and done, what you are going to have is something that looks like this. You're going to have your myosin filaments, you actin filaments, and your filaments hooked in there like so, poised, my friends, poised to contract that muscle. We'll see that later.
Animal Systems and Homeostasis
Motor Mechanisms
The Neuromuscular Junction: The Contraction Is Triggered Page [2 of 2]