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  • 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.

Hi. It's Mr. Andersen and welcome to biology essentials video 41. This is on

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神経系 (The Nervous System)

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    wshuang999 に公開 2021 年 01 月 14 日