字幕表 動画を再生する 英語字幕をプリント Hi. It's Mr. Andersen and welcome to biology essentials video 41. This is on the animal nervous system. And most of this podcast I'm going to be talking about theses things over here. These are called neurons. How they send messages called action potentials and how they can eventually jump off across these synapses. But I want to start with the big picture. And nothing gets more big picture in the nervous system than the brain. And so this is what your brain looks like looked from above. It has two different hemispheres. So you have a left hemisphere and a right hemisphere. And we're starting to learn more about the two different hemispheres. There are certain things like speech is clearly centered over here on the left side. Your handedness has to do with a lot of which of these is dominant. But vision is pretty interesting. And so you have eyes that we'll say are right up here. And so when you look at a sign, let's say you're looking ahead, this side of the sign actually is being picked up by this side of your eye. And that information is eventually going to the right side of your brain. And so if you look straight ahead, everything in your right side of your field of view actually goes to your left brain. Or left hemisphere. And everything on your left goes to your right hemisphere. Now luckily it is connected. In other words we have a corpus callosum that connects the two sides of our brain and so we can share information back and forth. But occasionally our brain goes haywire. And we get what's called a seizure or an electrical storm that goes all the way across your brain. And if you have epilepsy this gets be a huge problem. And so a really radical procedure that scientists will do is they'll occasionally sever this corpus callosum. And when you sever that corpus callosum the two hemispheres can't communicate. And if you look at people who have this split brain, they look really normal. But occasionally you can trick them and you start to discover how the brain is really set up. And so let's do an example of that. So what I have right here a plus sign. So if what I want you to do is stare at the plus sign and I'm going to flash an image to the right. That's going to go to your left brain, but always stare right at the plus. Don't stare off to the right. And so we had an image that showed up. And you would say that was an image of the earth. And so you can say that because you're using the left portion of your brain. Let's try it again. That was a pretty big flash. That's a flower. So you can say that it's a flower. And if you do this to one of those split brain people, they'll do the same thing. They'll say that was the earth. And then they'll say that was a flower because they're seeing that on the left side. Their speech is on the left side. And so they're good to go. But now let's put one one the left side of your field of view. So that's going to go to your right brain. So let me flash one there. And you can say that was a sun or a cartoon of the sun. You would show that to a split brain person and they would say I didn't see anything. And so that seems a little weird. And the reason why is it's going over to the right side of their brain where they can't actually make speech to explain what that was. Now what's interesting in these you can actually give them a piece of paper and say I know you didn't see anything but could you draw what you might have seen. And they'll start to draw like a picture of it. Because they did see it. And then they might get to this point and say it's a flower. I don't know what it is. Oh. It's like a sun. And so once they see it, then they could say it. So it's like playing pictionary with your self. So that's weird. But that shows you how there are different portions of our brain and those different portions of our brain do different things. And so in this podcast I'm going to talk about the nervous system. We've already talked about the brain and it has different portions, like hemispheres. But we're going to spend most of the time talking about the base unit of the nervous system which is the neuron. Neurons can send messages. Those are called action potentials. Action potentials work by polarizing a cell. In this case it's the neuron. They set up this polarization through sodium-potassium pump. If you remember that's a form of active transport. And then when they depolarize it by opening up these ion channels then we can send a message down the neuron. Now when that message gets to the end of the neuron, it jumps to another neuron. And those neurons aren't usually connected. There's a gap between the two and that's called a synapse. And so a synapse is not a chemical electrical message going through it, it's actually neurotransmitters which are chemicals. An example I'll give you is GABA. And what those do is send a message to the next neuron. Those messages can either be excitatory, so yes, you need to keep that message going or inhibitory, no you need to stop the message right here. And so the synapse, that gives us a lot of control over that message and what happens when it gets to the next neuron. And so this is a neuron. This is a basic neuron. It actually has two different parts. This part up here is going to be the dendrites. Dendrites are going to be, I always use my hand. So if a neuron is your arm, these things up here are going to be the dendrites. It's a regular cell body. But then this portion right here is called the axon. And so most of what we think of as a neuron is called the axon. I don't think I added that here. So that A X O N. Okay. So we've got the dendrites and the axons. If we label some of these other things, this right here is going to be the nucleus. It's a regular cell. So it's going to have all the parts of a cell. This is going to be the cell body or we sometimes refer to that as the soma. Then this whole thing, I would say from here to the end, is going to be called the axon, A X O N. To speed that we have it wrapped in this fat material called myelin. And those are actually made of something called a Schwann Cell that wraps its way around the axon. And then we have Nodes of Ranvier. And Nodes of Ranvier are going to be these gaps between the myelin. And really what happens, and I'll come back to that at the end is that the message actually, it doesn't go all the way through the axon, it's able to jump from spot to spot. Because we put all the ion channels right here. So it speeds it up. So if you think about a neuron like a wire, it's kind of like that. And then this myelin sheath is going to be like the insulation that wraps around the outside. And it speeds up that message as it goes. At the end we then have a terminal. What would be next? Well we'd have the next neuron that's going to be connected right there with a synapse or a gap between the two. Now if you've ever heard me talk about neurons before you've heard this. I always like to say that nerves or neurons, a nerve is just a bunch of neurons together, that a neuron is simply a salty banana. And that allows you to remember where the ions are. And so what do you know about a salty banana? So this visual over here on the left side is super important to remember. We've got a salt grinder up here. It's going to put salt on the surface of the banana. Well what do you know about salt? Salt's going to be high is sodium. And what do you know about a banana? It's going to be high in potassium. And so if we look at, let's get rid of the salty banana for a second, if we look at a neuron, I've just drawn right here a section of it. So we've just taking one portion of a neuron. So up here we're going to have the dendrites. Down here we're going to have the axon terminals. So this is just a portion of the axon. You've got sodium out here. A lot of sodium ions out here. And you've got potassium ions in the inside. You have other things in here. Like this is a protein that I've drawn in here. But, the big ones to remember are sodium and potassium. And then you have these channels. Channels are proteins that allow sodium and potassium to pass. If they're not open, you shall not pass. And so I kind of color coded those. And so if you look at it, every sodium ion is going to have a positive charge. So these are positive. And every potassium is going to have a positive charge as well. So if you count the number that we have on the outside and the number that we have on the inside, we have a lot more sodium on the outside, or excuse me, positive charges than we do on the inside. And these proteins are going to have negative charges as well. And so what does that mean? It's more negative on the inside. And it's more positive on the outside. And we can actually measure that. In a typical neuron, or a typical nerve cell, it's going to have a voltage of -70 millivolts. What does that mean? It's a battery. And so there's a battery across our neuron. And so it has energy. Or it has the ability to do work across that membrane. And so it sits at -70 millivolts. Now what happens, let me get some of this out of the way, is that something will happen to trigger the opening of this first channel. And so right now you're seeing light. And that's actually opening up sodium channels. So if you open up sodium channels, where's the sodium going to want to flow? Well there's a whole bunch of sodium out here. And so it's going to move or it's going to diffuse along its gradient. And so if we open up that sodium channel, what's going to happen? The sodium is going to flow in. Now what's that going to do? It's going to actually change the voltage. So it's going to change the voltage. Remember it used to be -70 millivolts. And now it's going to be less that that. So maybe it's going to be -50 millivolts. What does that do? Well these channels, not only allow sodium and potassium to move through, they're actually activated by changes in the voltage. And so when this goes to negative 70, what that does is it opens up the next sodium channel a little bit down the way. Now potassium channels aren't effect by this. And so what is going to happen next, well that's going to open up the sodium channel a little bit farther down. And that's going to open up the sodium channel a little bit farther down as well. And so we get flow of sodium. We get this cascade. It's almost like dominoes. This cascade of sodium which triggers the next sodium to go and the next sodium to go. And so neuron isn't passing electricity. What it's doing is it's opening up these channels. And it's allowing those chemicals to influence the next channel, which opens that up. And so you have this cascade almost like dominoes flowing up. So now where is our charge? Now we have a positive charge here and a negative charge down here. These sodium channels then are going to close up. So now sodium can't pass. But the potassium channels are going to open up. So when the potassium channels open up we have all this positive charge in here so what's going to happen to the potassium? Potassium is going to flow out. What's going to happen to our voltage again? Our voltage is going to become more negative. And we're going to kind of get back to an equal charge on either side. For just a moment. Now what happens next, well we have to reestablish that gradient. And to do that we use something called the sodium-potassium pump. And so when you see light, you're sending literally thousands of action potentials down a neuron. Between each of those action potentials, the sodium-potassium pump is going to reestablish that gradient. And so let's actually put a graph to that. And so this what it looks like at rest. Remember it's at -70 millivolts. And so here's our axon. It's a -70 millivolts. In other words it's a negative charge here and a positive charge here. What was the first thing that happened remember? That sodium opened up. And so we had sodium channels moving in here. So what happens to this phase? Well our neuron, which is polarized, is going to depolarize. And so it's going to move towards the positive. Now if it reaches this point, that's called the threshold, it's going to have an action potential. In other words some times we'll get a little bit of flow. But each of these are a failed initiation. In other words until we hit this critical point, and in all animals it's -55 millivolts, once we hit that -55 millivolts, then this is going to be an all process. And so what's going to happen is all the sodium is going to flow in. What does that do? That makes our charge go really positive. Then what happens? We're going to open up our potassium gates. That's going to flow in the other direction. And we're going to have this plunging falling phase. And then we have what's called the undershoot as it resets itself with the sodium-potassium pump. Now the reason we have this undershoot is that we don't have an action potential going in both directions. We want it to be directional. We want it to move in the direction of that axon. And so then there's another action potential. And so what does that mean? If I were to flick myself on the finger like that, that pain that I experience is a number of action potentials. That was a really bad one. A number of action potentials going to my brain. And I'm perceiving that as pain. If I were to flick it lightly, like that, it's still going to be the same size of action potentials, but they're not going to be close together. And if I were to cut my finger that would be a huge amount of action potentials really close together. And so what goes to your brain are simply action potentials. You're brain then has to decide where is that coming from. Is it coming from my nose? Is it a smell? Is it from my ears? Is it from my eyes? Is it vision? All the neurons transmit that same action potential to your brain, but your brain decides on where it's coming from and as a result of that decides on what it is. Now, before it can get to your brain it actually has to go across a number of gaps. And so a synapse is a gap between two different neurons. And so that action potential is going to go down this neuron. Then it's going to go down this neuron. In other words we're going to have opening the sodium, opening the sodium, opening the sodium, opening the sodium. It's discharging it. Then we're opening the potassium, opening the potassium, open, so the action potential is going to move in this direction down here. But eventually it gets down here to the synapse. And it doesn't just flow across the side. It doesn't just flow across this gap. What happens is that you're going to get an influx of something called calcium. So calcium ions are super important for nerves to function correctly. And then you have these things. These neurotransmitters. Neurotransmitters are going to be chemicals. And those neurotransmitters are going to float across the gap. And so once the action potential gets to the end. So this would be the pre-synaptic side. It's going to get an influx of calcium. That's going to release these neurotransmitters. And these neurotransmitters are going to float across the gap. So they're going to move from an area of high concentration to low concentration. They're going to match up right here with another ion channel on the other side. And they're going to change its shape so it can take ions in. Now we've got sodium flowing in, potassium flowing out and the action potential moves in the other direction. A very famous example of that is G A B A, GABA. We can have a number of different chemicals that form as neurotransmitters that move across. GABA is actually a negative neurotransmitter, an inhibitory neurotransmitter. And so if GABA flows across the side, it's going to actually hit receptors on the side that say don't send an action potential. Don't send an action potential. But there are likewise going to be a number of different ones that are excitatory. And they're going to send a message across that says, action potential go, action potential go. And so if your brain like a computer? Not really. A better way to do it, to think about it is like a vote. And so if we have this action, this neuron right here. Let's say this one is headed to the brain for example. It's going to get a number of different messages from a number of different neurons. So the connections are super important. Some of those messages are going to be inhibitory. Let's say inhibitory is red. And so they're going to say you need to fire. You need to fire. You need to fire. You need to, excuse me, inhibitory, so they're going to say don't fire, don't fire, don't fire. There's also going to be a number of those that are excitatory. We'll make those in blue. And so they're saying fire, fire, fire, fire. And so all those connections are saying fire. And so depending on the amount of excitatory and inhibitory that we have, it's either going to fire or it's not going to fire. Let's go back to that diagram that we have. Remember it sits at threshold. And then an action potential is going to depolarize and then it's going to repolarize and then it's going to go like this again. Or excuse, that's bad, let's do that again. So it's going to reach up. It's going to have an undershoot and it's going to come back to here. And so what's really happening. This is at -70 millivolts. Every time we get an inhibitory, that's actually going to be pushing the voltage of this in this direction. Every time we get a excitatory message it's going to be pushing it in the other direction until we hit that threshold of -55. And so again all these neurons are voting. They're voting should it fire or shouldn't it fire. And as a result of this, this one passes that message on. And so what's super important in nerves is not only sending a message, for example from my toe when I step on a tac all the way to my brain saying move your toe you're on a tac. It's the connections in your brain. It's all these inhibitory and excitatory messages that are forming memories. And so you're forming memories right now. And those memories are connections between the neurons in your brain. And the more information that you can get be it auditory, verbal, the more connections you can make, the more likely you are to remember this in the future. And so that's a lot as far as learning. A lot of information to hold on to. But that's the nervous system. It's really cool. It's really important and I hope that's helpful.