Hi Ninja Nerds, in this video we're going to talk about the neuromuscular junction. So if you guys haven't already seen it, we made a video previously talking about the skeletal muscle structure. Then we zoomed in and we took a look specifically at the functional and structural unit of the muscle cell, skeletal muscle primarily, and that was the sarcomere.
In this video now, we're going to be talking specifically about what's called the neuromuscular junction and the neuromuscular transmission. Okay? So to start this all off, you can kind of see here we have this long motor neuron. Okay, this is a long motor neuron, which is basically, we have to figure out, okay, where is this motor neuron coming from and what is it doing?
Alright, so if you look here, what I did is I took a cross section of a spinal cord. We're going to take this here, let's say midbrain, pons, medulla, here's your spinal cord, right there. All I'm doing is, I'm cutting it like this.
And we're taking a look at it in this form. So what you see here is the cross section of the spinal cord. We're not going to go into super depth in this because we're just going to mainly focus on the muscle physiology.
But just a little bit of anatomy in this back part here, the posterior part. So this is the posterior part of the actual spinal cord. This side is the anterior side of the spinal cord. So posterior side, anterior side. Then if you look here in the anterior side, there's a clump of neurons.
Okay, so a clump of these cell bodies specifically that are right here in this horn like structure. which is on the anterior side. And it's gray when you look at it on a microscope, right? And the reason why is because there's no myelination. And myelination is actually what allows for us to be able to see the difference between gray matter and white matter.
So for example, if it's white, it means it's myelinated. It's wrapped with myelin sheaths. If it's gray, it has no myelin. So if you look at it, this is the anterior side of the spinal cord, posterior side of the spinal cord. You're gonna have some neurons in here, some cell bodies specifically.
that have no myelination, so therefore they're gray, and it looks like a horn. That is called the anterior gray horn of the spinal cord. So this specific part there is called the anterior, but don't be confused if you read in the literature and it says ventral, because these are synonyms, so they could be either anterior or ventral gray horn.
Okay, and so again, what is in the anterior or ventral gray horn? It is the cell bodies of the somatic... Motor neurons. That is a key word.
We need to talk about that real quick. What is meant by the term somatic? So somatic can give you two basic simple definitions or two points.
One, I want you guys to automatically remember that the somatic nervous system, so somatic nervous system is voluntary. What does that mean to be voluntary? So for example, if I'm deciding to move my arm right now to be able to flex and actually contract this biceps muscle. That's voluntary.
I decided I want to do it. That's one thing the somatic nervous system controls. We're consciously aware of it.
We have voluntary control of our skeletal muscles. The other part of the somatic nervous system is it mainly is supplying the skeletal muscles. Okay.
And we're talking about a skeletal muscle right here. Okay. So we're talking about, again, what is this right here before we get into all of this depth here? This right here is the anterior or ventral gray horn, which consists of the...
Cell bodies of the somatic motor neurons. What is the somatic nervous system? It's where you can it's the nervous system that controls our skeletal muscles and it's under voluntary control Okay, let's say that this muscle is my biceps muscle and I'm specifically zooming in on one of those muscle fibers or muscle cells What I want to do is I want to contract this muscle cell But in order for me to do that I have to have a stimulus if you guys remember one of the characteristics of muscles is that they're excitable They have to be able to receive stimulus in a form of neural stimulus and then respond to that by having a change in their membrane potential.
That's what we're going to talk about. So let's say that my cerebral cortex sends impulses down, right? It sends impulses down to my spinal cord.
And when it goes to the level of the spinal cord, which is going to go to my skeletal muscle, it's going to send these motor neurons out. This motor neuron, motor just meaning it's going to the muscle, is specifically referred to. They specifically refer to this motor neuron.
As the alpha motor neuron. Okay, that's extremely important, so I want you guys to remember that. The alpha motor neuron is the neuron that is actually going to be going and supplying this skeletal muscles. Alright?
So this alpha motor neuron, what's actually happening right now? Well, we're going to talk about the action potentials in a little bit more detail in the actual neurophysiology, but for right now... I want you guys to just trust me and understand that from the cerebral cortex impulses are coming down and it's stimulating These actual cell bodies.
When it stimulates the cell bodies, it can generate an action potential. So moving down this axon is going to be having this thing called an action potential. And to make it super simple, I'm showing it with up arrows to represent that this is an action potential. But really what's happening in just a super simplistic way, we're expanding out on a part of that axon. So this is still part of the axon.
What really an action potential is due to is, you see these red channels right here? These red protein channels are specifically voltage sensitive. And they open up at a specific threshold voltage.
And what happens is, sodium is responsible for flowing in to this actual axon through these protein channels. Now what happens is, action potentials are caused by a depolarizing wave. And all depolarization is, is usually the inside of the cell when it's at rest is generally partially negative. So generally the cell is negative. I'm going to represent that with just these negative charges.
What happens is, is there's usually some type of stimulus to that neuron that we talked about coming from the cerebral cortex. What happens is it brings it to threshold, and we'll talk about that a little bit here too. And once it reaches that threshold, these voltage-aiding sodium channels can open.
Sodium rushes in and causes the inside of the cell to become really, really positive. Why does it become positive? Because sodium is positive. So if you have more cations, which just means positively charged ions, right? If you have cations coming into the cell, what's that going to do to this negative charge?
It's going to get rid of that negative charge. And now what type of charge are you going to develop inside of this actual axon? You're going to develop a lot of positive charges. And this positively charged... Particles are going to move along this axon.
Okay, so they're moving along this axon. And that's like a depolarizing wave. Because again, depolarization is whenever you're making the cell increasingly more positive. Okay, less negative. But again, these positive charges are sweeping across and moving across the actual cell membrane.
Now, here's what's really cool. As the action potential gets down to this big knob-like part here. So this, you know what they call this? They can have a couple different terms that you could say.
This is an axon terminal, or they can call this the synaptic bulb, right? It doesn't matter, but really whatever one you prefer, I like to say either axon terminal or synaptic bulb. I'm probably going to refer to it as synaptic bulb throughout this video.
So here we have this big synaptic bulb, all right? As the action potentials are moving down the synaptic bulb, it's going to stimulate these black protein channels, who are very, very special. Okay, so here's the positive charges.
They're flowing across the actual cell membrane. This voltage-gated channel is specific to a special ion, and that special ion is called calcium. So calcium is usually more concentrated outside of the cell than it is inside of the cell, okay?
But these channels are normally closed because they have what's called an inactivation gate and inactivation gate. We'll talk about that more in neuro. But what happens is once the voltage moves across this membrane and stimulates a special part of this actual protein, what happens is the activation gate will open.
And this channel will open because it's reached a special voltage. Usually the voltage required to activate these channels is approximately about positive 30 millivolts. Okay, so once it hits positive 30 millivolts, this channel is then open.
And look what can flow in, calcium. Calcium can flow in because the activation gate of this channel has been opened. Because why? Because this protein channel has reached its threshold voltage of positive 30 millivolts in order for it to open and allow for the calcium ions to flood in.
Okay? So the first thing we said, what was the first thing? First thing is we had this action potential occurring from sodium ions flowing into the axon and causing a depolarizing wave.
Then what did we say was the second thing? The second thing was going to be whenever this depolarizing wave that's approximately positive 30 millivolts is moving across this actual cell membrane. We can call this the actual plasma membrane, right? This would be called the axolimma, they call it.
But this cell membrane here has special proteins, these black proteins, which are voltage-sensitive calcium channels. When the threshold of positive 30 millivolts is reached, their activation gate opens. And calcium floods in. And we'll explain why that's important in a second here.
Alright, at this synaptic bulb, you see we have a lot of these green vesicles. They're very special. They're actually... Now, here's one thing that we're not going to talk about, super important, but these vesicles are generally made where?
That is something we should know. Where are these vesicles actually made? Are they made here in the synaptic bulb?
Are they made in the axon? Or are they made here in the cell body? That is important because the vesicles are made in the actual cell body, specifically by the Golgi. Okay, and there is specific proteins in there, so it actually might have to undergo... some type of transcription translation by the DNA but the important point is is that these vesicles are actually moved down right so they're moved down by what's called an anterograde transport and an axon transport where you're moving these vesicles down towards the synaptic pole another thing that's being moved down from the actual cell body is the mitochondria so there's a lot of mitochondria in this area too and we'll talk about why that's important now these black dots are representing a special neurotransmitter.
That neurotransmitter that we're going to talk about is specifically referred to as acetylcholine. Okay, acetylcholine. Now, I'm going to refer to that a lot as ACH. Okay, I'm going to refer to it a lot as ACH.
Now, here's the important part. Acetylcholine. Most people will be like, oh, neurotransmitters, they're generally proteins.
Now, acetylcholine is not a protein. Acetylcholine is very very interesting. So it's actually made up of two constituents. And now here's the question, here's what I actually wanted to get at.
Remember I asked you these synaptic vesicles, they're actually made specifically in the cell body, but this acetylcholine is not made in the cell body. I can't stress it enough. That's that it's extremely important. You need to remember acetylcholine is a neurotransmitter, but it's not a protein neurotransmitter. So it's not made in the cell body, it's made in the synaptic bowl because it's composed of two different constituents that we're going to talk about.
One part of it is this name, choline. Choline is like a essential, like vitamin-like nutrient. Okay, so it's kind of like a vitamin-like nutrient.
So one component of acetylcholine is choline. And choline is like some type of, it's like an essential vitamin-like nutrient, okay? The other component is acetate, okay? All right, our acetyl-CoA is the other constituent that we use to combine these together.
Okay, but really we're just going to say acetate. Okay, so acetate. But we'll see where this acetate actually comes from in a second. Okay, so what are the two components of acetylcholine?
Acetate and choline. Where is choline coming from? Do we just have it in our body? Is it just there?
No, it comes from various different types of nutrient sources. We can't, okay, so what happens is you usually get through certain types of dietary sources. So let's say here's your gastrointestinal tract.
Oh, that's an ugly. Stomach, let me fix that, guys. Let's say here is your actual gastrointestinal tract, okay?
Whatever you're eating, many, many different types of foods contain the actual choline. So whenever you're eating certain types of foods and vegetables and stuff like that, it's actually absorbed across the actual GI tract and into the blood, okay? So choline is kind of an essential vitamin-like nutrient, and that's coming from the GI tract, from the diet, okay? Acetate is coming from where? Okay, remember we have here the mitochondria.
What's important about these mitochondria? What do the mitochondria have in them that's important with starting the whole Krebs cycle activity? If you guys remember, the mitochondria have a very, very, very important chemical, and that important chemical that they have is called acetyl-CoA.
But what happens is, let's say I have here my acetyl-CoA. Here's this acetyl-CoA that we have, right? We have this acetyl-CoA coming from the mitochondria because you know that there's a lot of Krebs cycle activity occurring within the mitochondria and acetyl-CoA is the basic starting point for the actual Krebs cycle to occur.
Now where is this choline that we're taking in? How is it getting in? Okay so choline is coming and moving you know through the blood and it has special transporters located in the synaptic bulb. So here's choline, let's say here's our choline, there's a special transporter that brings the choline into the synaptic bulb. So outside here we're going to have, you know, maybe it's coming from the blood and it's actually going to get moved across the actual blood-brain barrier and then moved into this synaptic bowl through this special type of choline transporter.
So this is a choline transporter. Okay, now what's really cool is this acetyl-CoA and this choline are going to get fused together. Now what's going to happen is the acetyl-CoA is going to lose the CoA. So this reaction is going to happen here. So let's take this reaction.
Let's combine the... These two molecules together. So here's our choline and here's our acetyl-CoA. What happens is these two molecules react. As they react, they produce acetylcholine, but I'm going to write that it's ACH because that's what I told you guys I was going to do.
I'm going to write it as ACH. Okay, now, but look what happens? The CoA should get released out of this reaction. Okay, so that's what happens.
Choline is coming from the diet. Let's put that right here above it. It's coming from the diet.
Certain types of foods are rich in choline. Then it's brought into the actual synaptic bulb through the choline transporter acetyl CoA is coming from the mitochondria which are down here in the synaptic bulbs because it's an intermediate for the Krebs cycle acetyl CoA and choline react with one another releasing the CoA and synthesize acetylcholine. But you know this can't happen on its own. We need enzymes for this to reoccur. So what is this special enzyme?
This enzyme is called choline acetyltransferase. Very important enzyme. Okay, so choline acetyltransferase is the enzyme responsible for stimulating this step here, for converting acetylcholine into acetylcholine.
Alright, cool. We're good there. coming to the next issue.
How the heck do I get that acetylcholine into the synaptic vesicle? Well it has special types of protein transporters. So here's this pink protein transporter that you see right there, right? That pink protein transporter is really cool.
And what it does is is we have to concentrate this acetylcholine in. So how we do that is we push this acetylcholine in, right? So I'm going to draw acetylcholine like this, right?
So let's say I just draw a couple acetylcholines that I transport in. Now here's the next thing. We're trying to bring acetylcholine in, but we have to bring up something else. Before I tell you what that is, let me actually show you something over here.
Okay, so now what we want to do is we want to pump these protons into this area. There's a special channel there to get these protons in. So, what happens is we're going to take these protons and we're going to pump them into this actual synaptic vesicle. Now, generally, the proton concentration in the synaptic vesicle is higher than the proton concentration out here.
So, if I'm pumping it from an area of what? If I'm pumping it, this concentration out here of protons is lower and the proton concentration in here is generally higher, I'm going against my concentration gradient. So, that is going to require energy. and we utilize that energy in the form of ATP so we're gonna break down the ATP into ADP and inorganic phosphate now this proton concentration is being developed in here so now we have a high proton concentration low proton concentration out here that's going to help assist bringing the actual acetylcholine in by like a secondary active transport right because this right here is the direct use of ATP so this is an example of primary And this one over here, what's going to happen to these protons now? Now the protons have established a concentration gradient, right?
Lower out here, higher in here. So they can move from high concentration to low concentration, which doesn't require energy. So that'll help to move the acetylcholine into these actual synaptic vesicles or bags of acetylcholine. Okay? So again, one more time.
Proton concentration out here is lower than the proton concentration in the synaptic vesicle. So therefore, it's going to require direct use of AT. ATP to pump it against its concentration gradient. As the protons are being concentrated in here, they establish a gradient difference, right? More protons in here than there is out here.
So they'll move from areas of high concentration to low concentration through this protein channel. And when doing that, they help bring the acetylcholine into this actual synaptic vesicle. Okay. Now we got that part. Now that we've done that, we're going to look at where the heck and how the heck is this calcium going to help us at all in this release of this neurotransmitter because that's what's really cool here and calcium is extremely important in this process.
Without calcium we would not be able to release this acetylcholine and you'll see why. So there's many proteins on this vesicle. I just showed some channel proteins but there's a lot of other proteins. You know and in the beginning of anatomy and physiology usually talk about the cell proteins. One of the big ones that I want to just briefly talk about is what's called snare proteins.
They're called snares. And snares, there's mainly two types. One is called a V-snare and that's just proteins that are on the synaptic vesicle.
Okay, V for vesicle. The other one is T-snares. T-Snares are usually on the actual cell membrane. There's two main ones. There's many, many proteins.
I'm going to talk about the more common ones or the more significant ones. First one I'm going to talk about is the V-Snares. There's two main proteins here that we're going to talk about on the synaptic vesicle.
I'm going to draw one here in this actual purple color. This purple protein right here is called synaptotagmin. It's called synapto. Tagmen.
It's a very, very important protein. Okay, so synaptotagmen is this protein that's bound to the actual synaptic vesicle. Now, there's another protein bound to the synaptic vesicle. Let's draw this one in blue.
So look here, there's another protein. Here's another protein molecule here. Very, very important protein.
And this protein is called synaptobrevin. Okay, so that's called synaptobrevin, this protein right here. Then, we have two proteins here on the actual cell membrane of the synaptic bulb.
Okay, so now let's draw these proteins. So this protein right here is going to be an important protein. And let's have one more protein. Let's do this one in the nice pink. We'll have this protein right here.
Okay, this red protein is called SNAP25. SNAP25. Alright, so we got SNAP25 and then over here this pink one is the more important one. It's called Syntaxin.
So that protein right there is called Syntaxin. Okay, so let's just review these proteins again. I know there's a lot of proteins.
I'm sorry. The V-snares, two V-snares. What are the two V-snares?
Synaptotagmin is this purple one. Synaptobrevin is this blue one. Okay, then we have two T-snares.
Snap 25 and Syntaxin. Here's what's important. What did we say? Calcium came into this actual synaptic bulb whenever its threshold was reached.
What calcium does is it acts like a cross link between these two snares. Okay so between the V snare and the T snares, calcium is acting as the link between the two. Okay so look what happens here. Calcium comes over here and when calcium comes over into this vicinity He activates through special types of mechanisms.
He activates these proteins. So when he activates these proteins they start wrapping around one another. So look what happens here.
It's a super cool thing. Synaptobrevin and Syntaxin intertwine. Okay.
SNAP25 also is involved in that intertwining. So look at this. All of these proteins start intertwining with one another. Let's draw it like this. Draw like that.
And then we can say this protein over here, this syntaxin, is coming over here, it's intertwining with this actual synaptobrevin. And imagine that these two guys are wrapping around one another and they start pulling this vesicle towards the membrane. So again, what happens here? Calcium comes in, calcium acts as the cross link that stimulates these proteins to link up with one another. So originally, during rest, they were separated, they weren't bound to each other.
But when calcium comes in, these proteins... bind onto each other and imagine it clamping and pulling this actual synaptic vesicle down towards this actual cell membrane of the synaptic bulb. Okay?
So now, once these proteins intertwine in such a way that it brings the synaptic vesicle enough that it fuses with the cell membrane. Once this cell membrane fuses with the synaptic vesicle, the inside of the synaptic vesicle is exposed. to this actual space out here called the synaptic cleft. So again, once that happens, once the SNAP25, the Syntaxin, the Synaptobrevin, and the Synaptotagmin start intertwining with one another and pull the synaptic vesicle towards the actual cell membrane, fusing the synaptic vesicle membrane with the actual synaptic bulb membrane, due to calcium's presence, that'll cause the release of this acetylcholine out into the synaptic cleft and then acetylcholine which we're going to represent as this dot here is going to diffuse across this actual synaptic cleft and cause many different types of physiological changes. So now to quickly recap in an order a quick recap in order of everything that's happening here because in the next video guys we're gonna take acetylcholine and we're gonna see how it acts on these receptors how it regulates a membrane potential and all these changes.
So let's quickly recap. Everything in an order. First thing we said, the actual cell bodies of the somatic motor neurons in the anterior ventral gray horn are stimulated. We can represent that.
They're stimulated. Okay? Once they're stimulated, action potentials are propagated down the alpha motor neuron.
What is really happening with this action potential? Voltage-gated sodium channels are open, and sodium is flowing in. causing the inside of the actual cell to become more electropositive.
This wave of positive charges is moving along the cell membrane. That is called depolarization. So again, what is this called here?
This is called depolarization. So this depolarizing current is moving along this actual, what is this whole big thing here, guys? It's called the synaptic bulb.
Once that happens, It hits around positive 30 millivolts. That's what the depolarizing wave is. What happens is that opens up the activation gate on this voltage gate of calcium channel, and calcium flows in. That was the second step. Then what else did we say?
We came over here and we talked about acetylcholine. Where is the acetylcholine coming from? Okay. Well, the third step was involving specifically the choline and the actual transport in. So this is the third step.
The third step is bringing the choline from the diet. through this choline transporter and into the actual synaptic bowl. The fourth step is taking acetyl-CoA, really in the form of acetate, right?
Because you're going to get rid of the CoA, and fuse the acetate and the choline together in the presence of this choline acetyltransferase. Then the fifth step was trying to transport this actual acetylcholine into the vesicle. But how do we do that? Since the acetylcholine has to move against its concentration gradient, We need to take these protons and actively pump them in against their concentration gradient. So that whenever they're concentrated, they move out of the synaptic vesicle passively, which provides the secondary active transport of acetylcholine into the vesicle.
Okay, so that was that part. Then what else did we say? Then we said that in order for this acetylcholine to be released, we need to have these actual V-snares and T-snares interacting.
So that was the sixth... So the sixth step was the interaction of the V-snares, which stands for the vesicle snares, with the T-snares, which is the actual proteins on the cell membrane of this whole synaptic bulb. What were those V-snares?
The V-snares were synaptotagmin and synaptobrevin. If you don't remember these two, at least remember this main one, synaptobrevin. He's the really important one clinically, and we'll talk about why. The other ones are the T-snares. And the T-Snares are SNAP25 and Syntaxin.
Again, if you don't want to remember all of these, at least remember Syntaxin. He's an extremely important protein. So the main ones I really want you guys to remember is Synaptobrevin and Syntaxin.
But here's the thing. They're normally not touching one another. When calcium is moving in from the synaptic bulb, what does he do?
He stimulates these proteins to cross-link with one another and intertwine and pull this actual synaptic vesicle membrane. towards the actual cell membrane of this axon bulb or synaptic bulb. Once they fuse, the internal environment of the synaptic vesicle is open to the external environment of the synaptic cleft and acetylcholine moves out by what process? We'll say that this is the seventh process and that is the process of exocytosis.
Okay? One more thing. Sorry, I forgot about the other thing. Now, this actual axon bulb is depolarizing, right?
Well, every cell has to rest. That's important. You see these blue channels right here? They're also really special. Now, if you guys remember, I briefly talked about it.
This guy has what's called an activation gate. Activation gate opens whenever it's at positive 30. Alright? Now, what happens is... It gets to a certain point, right, to where these calcium channels, they just keep coming in.
This channel over here is really special because we can't keep this positive 30 millivolts up there for the entire time. These channels have a special, what's called, activation gate. Their activation gate opens at positive 30 millivolts also.
And once this opens, a special ion starts flowing out. This ion is called potassium. So again, activation gates open at positive 30 millivolts on this channel, and then this potassium starts exiting out of the cell.
Now I want you guys to think about this for a second. If positive ions, or cations, are leaving the cell, what is going to happen to the inside of the cell then? It's going to become electronegative. Because if you think about it, you're losing... positive charges.
So when that happens, potassium starts leaving out according to what's called its equilibrium potential by the Nernst equation. It starts moving out and keeps moving and moving and moving out of the cell until the inside of the cell reaches a voltage of approximately negative 70 millivolts. Okay, so now again, potassium is going to move out until the inside of the cell has reached a point called re- Polarization. And once it does that it goes into refractory period. In other words, if this potassium starts leaving out, what's the voltage you're gonna reach then?
The voltage will eventually reach negative 70 millivolts. But not just over here, these potassium channels are all across the synaptic bowl. So what happens when they reach negative 70 millivolts?
The inactivation gates, right? So what happens is this this gate will close and calcium can't come in. If calcium can't come in whenever this is happening, can the acetylcholine get released?
No, because these synaptoproteins won't be able to link up together. That's what happens in the resting period. So whenever this actual axon terminal is relaxing or going into a refractory period, potassium is moving out to repolarize this whole membrane, make it negative, which shuts off these calcium channels. They can't keep coming in and acetylcholine can't continue to be released.
Okay? In the next video guys, we're going to talk about exactly how acetylcholine is affecting this actual muscle cell, alright, at this actual junction here. I'll see you guys soon.