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Neuromuscular junction, motor end-plate

Video transcript
So when we're at a social gathering and we've determined that this is the most appropriate time to perform the worm, how does our brain tell our muscles to contract? Well, in this video, we'll talk about the place where neurons talk directly to muscles. That's the neuromuscular junction, the junction of where motor neurons talk to muscle cells. So that involves, first, the axon terminal. This is the end of an axon, which is the part of a neuron that casts a signal away. And it looks like this. It gets larger at the end right here. And then it tapers back off like that. Muscle cells sits adjacent to these axon terminals at the neuromuscular junction, and kind of look like a block, but not exactly. They have these in-foldings that I'm drawing right here, these in-pouchings. And why does nature cause in-pouchings to occur? What's the purpose of these guys? What function do they serve? Well, if you said that it serves to increase surface area, you're absolutely right. Because with the increased surface area, we're going to have extra space where we can have sodium channels present that will help us transmit a message into the muscle cell. And so it's not just present on the outside, but there are a bunch of sodium channels that are deep inside as well. And in addition to sodium channels, you definitely have calcium channels that are present as well. They are situated buried deep within your muscle cells, too, to make sure that the most deepest parts of your muscle cells will get an influx of calcium when it's the right time. And to foreshadow a point we'll discuss later, I'm going to draw another muscle cell right here, just kind of chilling out on its own. So now how does the axon send a message to the muscle cell? Well, if you recall, there's going to be a signal that's cast away from our motor neuron to this axon terminal. And that signal is in the form of an influx of sodium ions. So this is a depolarized membrane that propagates this signal to this axon terminal. But it's not just sodium that's influxing. You're also going to have some calcium that's running in as well. And the calcium here is actually going to play a major role. Because in our axon terminal, we have a bunch of vesicles that are sitting around in here. These are just little pockets that are waiting for something to happen. And in each of these pockets, we have a message that's waiting to be released into the space between our axon terminal and the muscle cell. This message is called a neurotransmitter, which is a very well-named scientific term. Because all this is is a molecule that the neuron uses to transmit a message. And so the neurotransmitter that we use in the neuromuscular junction is called acetyl-- like from chemistry, acetyl-- choline, acetylcholine. And oftentimes, you'll see it abbreviated as ACh. So when there's an influx of calcium into the axon terminal, what'll happen is that the calcium will actually bind to our vesicle. There are proteins that are on it that'll grab onto the calcium. And so when there's calcium attached to these proteins, the vesicle will be drawn to this axon terminal membrane and actually fuse with it to become one continuous membrane. As a result, we release the acetylcholine into this space we call the synaptic cleft. The synaptic cleft is just the space between our presynaptic membrane, the membrane of our axon terminal, as well as our postsynaptic membrane, which is just the membrane of our muscle cell. So this is our postsynaptic membrane. All right, and so we have a bunch of acetylcholine that's released into the synaptic cleft and is ready to send a message on. But let's take a minute here. What just happened here with the membrane? I mean, the vesicle literally became one with the membrane of the axon terminal. What would you call this, if you have to give it a name? Well, it looks like some compound within the cell exited the cell. So I'll say "exo." And it exited a cell, so a "cyte"-- exocytosis. Exocytosis, and that's the process of molecules, or substrates, leaving a cell by vesicles fusing with membranes. And so that's what we do when we want things to leave cells. The exact opposite process, where we have things enter cells by fusion of vesicles, is called endocytosis. And these are very important terms keep in mind. Great, so now we've got acetylcholine all over our synaptic cleft. What is it going to do here? Well, these sodium channels have receptors that sit on them that are called nicotinic acetylcholine receptors. And as the name suggests, acetylcholine can come and very snuggly sit here and send a message to this sodium channel that it's time to open and cause sodium to influx into our muscle cell. And that's going to happen across the membrane, causing a large amount of sodium to enter. And once we've depolarized the membrane enough, calcium will even start to enter. And so in this way, we'll have what's referred to as voltage-gated calcium release. All right, and so we have all this calcium that's entering the cell. Now what? Well, this is just one cell contracting. How does this make me able to do the worm, or to kick a ball, or to high five my best bro? Well, we can't just have calcium entering through the membrane. There's another reservoir within our muscle cells that releases calcium for our disposal. This reservoir-- kind of a large name, and I'll write it out right here-- is called the sarcoplasmic reticulum, R-E-T-I-C-U-L-U-M, sarcoplasmic reticulum. This guy holds a whole bunch of calcium. It's sitting in there waiting to be released, waiting to do something. So there are proteins that are attached to the membrane of the muscle cell. And they're just waiting for enough calcium to be present here so that one of these calcium cations could bind this protein complex and then effectively cause calcium to be released from the sarcoplasmic reticulum. This process is called calcium-induced calcium release. And so now we have a bunch of calcium in here, and this muscle cell will be contracting. But it's still just one muscle cell. What's the big deal? Well, this muscle cell is attached to its neighbor right here, actually. There are proteins that link muscle cells together. These proteins are called gap junctions. And they allow for cations to flow from one muscle cell into another. So these muscle cells aren't really separate at all. Actually, they're connected because of this gap junction and actually continuous. And there's a term that we use to drive home the fact that we can have our calcium cations move to this other muscle cell and to start a calcium-induced calcium release process here. And it's that our muscle cells are in a syncytium. What does that mean? Well, "cyte," just like we talked about her, just means a cell. And "syn" means that these muscle cells are in a synergy with each other. When one contracts, it causes its neighbor to contract as well, and that's how we scale up. Because when muscle cells start contracting as neighbors, you get the entire neighborhood contracting, because you can then scale up and imagine that not just muscle cells, but muscle fascicles will also be contracting, too, to produce a kick of a ball or the worm. So this signal that started from our axon terminal here that begins as just depolarization turns into an acetylcholine molecule that undergoes exocytosis to end up in the synaptic cleft, where it binds a nicotinic acetylcholine receptor to cause sodium to enter a muscle cell, and over time, calcium to enter a muscle cell, which would then cause calcium-induced calcium release, which can then go to an adjacent muscle cell to cause a synergy of muscle contraction that spans from muscle cell to muscle cell, from fascicle to fascicle. And that's what happens at the neuromuscular junction.