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9. Transport Across Membranes

Ion Channels and Neurons

9. Transport Across Membranes

Ion Channels and Neurons

1

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Neuron Structure

5m

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Hi in this video we're gonna be talking about ion channels action potentials and neurons. So the very first thing that we need to talk about before we go over. Anything that has to do with neurons are signaling is just the structure and anatomy of a neuron. So this video is going to really just be a vocab list of different neuron structural features. Um So bear with me so neurons or nerve cells of course that function by receiving integrating and transmitting signals. So they have a few distinctive physical features. One is the cell body. So I'm gonna try to go over this as I describe themselves back up the cell body And it's also sometimes referred to the nucleus. That's the core of the neuron. So you're gonna see the cell body here. This is the cell body. Then you have your axons and these are the long extensions that conduct electrical signals away from the body. Um So if we're looking at what the axon is it's this long this long structure here. And um an interesting thing about the axon is the diameter. So kind of um controls the speed. So the larger the diameter the faster the speed. It's interesting. Then you have your dendrites and these are gonna be several shorter branches that radiates from the cell body to receive signals. We look where our cell body is here. We have our dendrites. So let me use a different color. Are dendrites extending here various various ways and then we have the nerve terminal. So here um gender rights. Um And then we have our nerve terminal And that's gonna be the end of the Exxon that branches to pass neurons to message to whatever cells at the end of it. So your nerve terminal is going to be here a synoptic terminal. You'll sometimes see that there's over the first four. Don't move on. So now we're focusing on some vocab referring to axon. So axons actually contain distinctive features as well. So one of these features is called Myelin and that's going to be a protective covering um that forms around the axon. And some of the cell types that are found in this are known as glial cells or Schwann cells. So just in case you see those cell types in your book you know that those make up the Myelin. So the Myelin sheath um is responsible for insulating the ion or insulating the axon so that ions passing through the axon can't leak out of the membrane and wreak havoc in the body. And then finally there's this other term called the note of or the nodes of ranveer. And these are actually just patches of ion channels that interrupt the myelin sheath and are really responsible for uh neuronal signaling. So the signal is passed between these nodes of ranveer down a neuron. So we're looking at what this looks like. So remember we're focusing on the axon which is gonna be this long terminal here and you can see that there's this myelin sheath. Just this blue covering here that protects the um axon from leaking out ions but it still does need to be able to import and export ions. So you have these nose of ranveer here that are interspersed sort of groups of ions. So then finally let's talk about the junction between the neurons which also contains distinctive physical features. So the terms here that you need to know our synapse which is the junction through which the cell the signal is transmitted. So that is going to be this region here. The synapse then you have the pre synaptic cell and the post synaptic cell which are the cells that either have the signal and release it or the cell that receives it. So you can see here the pre synaptic cell has the signal and it releases it into the synapse. And that synapse then or synaptic cleft then can the post synaptic cell can then accept it. And so finally you have the synaptic cleft which is the space between the pre and post synaptic cell which is what I refer to as the synapse here over here you can also do synaptic cleft technically the synapse can be this entire thing but you will see them as both ways. So don't get confused. Um So the synapse is gonna be referring to the whole thing. The pre synaptic and post synaptic cell. And then you have your synaptic cleft. So I know that's a lot of vocab sort of bear with me maybe review the video a few times until you get all that vocab down. So now let's turn the page.

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Action Potentials

14m

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Okay, so now we're gonna talk about action potentials. And essentially what action potentials are is they're just these waves of sort of this electrical impulse which carries a message from the neuron. So I like to think of this as kind of the electrical wires that you may see around cities or they may be buried underground. But we all know that electricity gets to house because of electrical wires. And what those wires do is they take they send electricity from the power company to our house. And neurons are exactly like that. Except for that. It's a I mean it is an electrical impulse, but that electrical impulse isn't just pure electricity, it has a message with it, it's supposed to do do something and it comes from our brain to wherever it's supposed to go. So if we're touching something hot, then our brain is triggered. This impulse saying, hey, that's hot. Move your hand and that travels through our neurons from the brain to our hand and causes the muscles and the bones and everything in our hand to move. And so action potentials control that the action potential is that actual sort of electrical pulse that is being sent through the neurons so that our brain can communicate with other parts of our body. And you can imagine that this is super fast, right fast. Because when we touch a hot stove, there's not a delay from the moment we touch it too when we move our hand, I mean it's almost instantaneous, right? We feel it's hot, we move it. And it's because these electrical impulses that are passed through these neurons, These action potentials are so fast, they travel up to 100 m a second, which is almost I can't even imagine how fast that is. That's how fast they move. And importantly. Also, not only are they moving fast, but they are also not weakening the message, which means when we touch a hot stove and our brain realizes that's hot, then it's not that it as it's the electrical impulse as it slowly travels from our brain to our hand, it gets weaker and weaker until we just think it's warm. Now it's hot and we know it's hot and that message does not ever weaken in the time that it takes or in the distance that it takes to travel from our brain to our hand. So it's fast and it is strong. These messages that are transported through neurons. And those messages are what we call these action potentials. Because there there are kind of electrical messages that get sent. So what causes an action potential? So an action potential is this like kind of this electrical impulse that gets travels from our brain to our hand or to wherever else. But something has to actually cause that, right, we have to generate this electrical impulse. So what is what generates that? And the thing that generates it is called a voltage gated cat iron channel. Now we've gone over sort of terms of different um transport channels before. But let's just remind ourselves what this is voltage gated. So what does that mean? That means that this is this protein is going to be controlled by positive and negative charges. Like voltage gated. So it's gonna open with certain charges and it's gonna close with other charges. It's a cat ion channel which makes sense that its voltage gated. Right because cat ions are going to be um positive ions, right, positively charged ions. And so this voltage gated channel is gonna be controlled through these positive ions and they're gonna open so that positive ions can move through them and they are what control this action potential. So they are what controls this electrical impulse. Which is good because if I mean what is an electrical impulse? It's just these um these influxes of electrons which are negatively charged. And so um so electricity is essentially just these these charged differences. And therefore the proteins, these voltage gated cast iron channels are opening to allow positively charged ions to get in. So how does this how does this actually work to control an action potential? And we're gonna go over individual steps in future videos but I just want to introduce it here. So what happens is that normally a neuron has a certain charge? And that charge is negative, 60 milli volts. And that means that the intracellular space. So inside the neuron is more negatively charged. Okay, now this voltage gated cat ion channel let's positively charge things in. Well at the inside of the neuron is negatively charged. That means that most of the time this voltage gated cast iron channel is closed, it's not letting these positive charges in because as soon as it opens those positive charges are going to go straight in to be with those negative charges. And so most of the time these voltage gated ion channels are closed. But what happens is that there's something that there's some kind of trigger. Maybe you're touching a hot stove, maybe you stub your toe, I don't know, something triggers this cat ion channel to open. And what happens is when that opens all those positive ions come in and that rapidly changes this charge. This charge the neurons usually used to with negative 62 plus 40. So if you go to negative 62 plus 40 and just a split second that is a rapid change. And it triggers a lot of things to happen. So if I'm just to draw um we'll just say that this is a this is an axon, remember that's that long part of the neuron that where the signals travel from the dendrites to um to the nerve terminal. And so what happens is if the signal, this sort of trigger comes in here. So we'll just say that this is a heat trigger, then we have these pad ion channels that then open and that causes a rush of positive ions into this little section of the neuron, right? But what happens is eventually this trigger goes and it starts bringing the positive ions to the next one and that will trigger this one to open. And so these positive ions begin coming in and coming in and they travel along until they reach the next one where this one will open and then this will go in. So now we've started this huge process where all of these are opening. All of these cat ions are coming in. This is a huge electrical signal that is traveling this way through the axon. So when eventually though we need to shut off this impulse, right? Because when we move our hand off the stove then our brain doesn't need to keep telling us our hands burning, Right? Because that would stop. We don't know, we no longer need that message. Well, the only way we can stop that message is if we counteract the This positive charge. And so how we counteract this positive charge is through a series of potassium channels. Now remember potassium is also positively charged. And so how do we use potassium to get the membrane back to its resting membrane potential, which is negative 60. It's what we started with. So how we do that is that some of these channels open at different times and allow for the positively charged potassium ions to flow out of the neuron. And that really can help get the neuron back to its normal thing. So there's a few, I want to talk about I want to talk about the delayed potassium channel and so it is returning the neuron to its original state. So if we have these um these are the voltage gated channels remember we started and they're open now and they're all in flux saying a bunch of cat ions into the cell. But we also have potassium ions here. And so these delayed ones open after this one open. So if this one opens first this one will be delayed and open second and that allows these potassium ions to flow out of the south. And when that does we're lowering the amount of positive charge here the second one. So this is really the one that's responsible for getting the positive charge out of the neuron so it's delayed, it happens after this one opens so that the signal can still be passed but that we're not just continually accumulating these positive charges. So we have the voltage gated channel opening first and then we have a delayed potassium channel to allow those positive charges to get out. We also have another one called a rapidly activating. So obviously that's going to activate pretty quickly. And um this is a really important one because it controls the relationship of firing to intensity. So what do I mean by that? Well when we when these voltage gated calcium channels open and those positive charges rush in if we allow that to just continually build, that's going to be a huge signal so that if we're touching a stove, if we just allow those positive charge charges to accumulate then what will happen is that when we realize our hand is touching the stove and our fingers are hot, it will feel like our whole body is on fire and we don't want that. We don't want to feel like our whole body is on fire when only our hand is touching the stove. So these rapidly activating potassium channels are making sure that the positive charge inside of the axon never gets so much that it's just way too much way too much intensity. Right? So it's allowing those potassium is to come out of the axon so that we make sure that we have a positive charge in the axon but that it's not so large as it's just overbearing our entire nervous system. And then the third really important potassium channel is the calcium activated potassium channel. And we'll talk a lot more about how calcium is involved in this process when we go over the detailed steps but just know for now that it is and this activated channel. What it does is it it puts a delay between one action potential and the next. So what does that mean? Is that when we have this we have I don't want to draw it like that. Hold on, let me erase this. Okay, so when we have a neuron this is an axon and we open this one and we get lots of positive charges and that travels to the next one which causes it to open right positive charges. And then we have our delay channels potassium channels that allow the potassium to come out and eventually this will go back to its negative 60. But the calcium activated potassium channels make sure that there's a delay between this action potential and the one that's going to come after it. So you need that delay. And so there's more potassium that comes out to ensure that the axon gets back to its resting membrane potential which is negative 60 so that it will delight from one action potential to the next. So I know it's really confusing. We are going to go over individual steps in the future. But what I want you to get from this is what an action potential is which is just that electrical impulse that's kind of being passed along through the neurons from your brain to wherever it's signaling like your hand touching a stove The action potential. Those electrical impulses are controlled through the positive ions getting into this into the axon. And that's controlled through these voltage gated cattle ion channels. And then what happens is that there are potassium channels that regulate it. They make sure that it can return to its original negative 60 state in the case of delayed it makes sure that the intensity never gets too low large for us to handle and it makes sure that we're not just constantly firing and just sending more and more electrical signals through these just more and more action potentials. It says wait, we'll finish this one before we start the next one. So that's really what I want to get from you. I really want you to understand from this topic. I know there's a lot of vocab words and like I said, the individual steps of this we're gonna talk about later but I just want you to kind of get the overall process of what action potentials are, how the positive charges really controlling this and how these potassium channels are really about regulating the different facets of it, whether it's returning to its original state, controlling the intensity or in controlling how fast all these action potentials can happen one right after another. And so if we are to look at what this looks like, you know, I've drawn it out a lot but I like these pictures better because they're much better. What happens is we have some kind of trigger potentially this is the trigger which comes in and now we have the axon which has all these voltage gated cat iron channels right here and they're opening and that's causing lots of positive charges to flow through the neuron and those positive charges that are flowing through the neuron those are the action potentials and after they begin to flow through the neuron, What we get is we get these potassium channels that are regulating this process, either helping it return to its normal resting state, is making sure that the positive charges never get too intense so that we have an appropriate response and then making sure that this returns to normal before it starts its next one. And you can see that these action potentials, they travel all the way down until they trigger the next you're on. So that's how it happens. It starts in our brain. We're triggering these neurons and it travels from neuron to neuron. These positive charges all the way until it gets to your hand. And that triggers that says, hey, there's a big positive charge here, there's some action potential. It triggers your muscles and your bones to move your hand off that stove. And so that is what an action potential is and that's how it travels from the brain to your hand to tell you to move. So if that's a little complicated, I will be going over the individual steps later if you're still a little confused. But hopefully you get just the basics of how these positive charges are being sent from neuron to neuron and that's called an action potential. And that is really that electrical impulse that tells that allows your brain to tell your hand to move and along many of other movements and things that our brain tells our body to do so with that let's move on.

3

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Neuronal Signaling Steps

7m

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So in this video we're gonna be talking about the seven steps to um neuronal signaling. So what happens when a neuron needs to signal happens in seven steps, we're gonna walk through each step, it's kind of detail just carry stay with me. Um Cell biology involves a lot of steps and this is just another one. So um like I said the signal propagation between neurons occurs in seven steps. So the first step is the neuron itself receives some type of signal which triggers the opening of these cast iron channels. And the caddy on channel that's opened. In the first step are sodium channels. And so when the sodium channels open, what happened is sodium rushes into the cell and when it does it creates this sort of influx of positive charge. And when there's an influx of positive charge we call that um deep polarization. And so when this positive charge just rushes in so quickly, what happens is that the sodium channels then become inactivated and it happens within a millisecond, I mean so fast that these channels open, sodium Russian then they're closed. Um To prevent this axon from continually just you know signaling needs to be regulated a short pulse. So the first step is this sodium receive some type of signal sodium channels open comes to polarized and now there's this positive charge sitting in the neuron. So this deep polarization triggers more voltage gated ion channels to open and there can be a variety of different ion channels calcium potassium whatever. Um And this causes the deep polarization to continually travel down the Exxon. So the axon has all these different ion channels at different regions along the axon. So as soon as the first one is open, it triggers the opening of all of them and then just tumbles down the axon. So the third step that happens is voltage gated potassium channels open. But remember this is the third step. So it's important here that these are delayed in opening. And so because now they're delayed, there's this really high positive charge inside the cell. And so when these channels open, the potassium is actually transported out of the cell instead of in the cell. And this helps restore the neuron to its resting state. So it's delayed so that most of the positive charge has already left its already traveled down so it's somewhere else. But there's still this positive charge left behind where it just was. So there's potassium channels open, the potassium flows out and returns that section of the neuron to its original state. And um eventually this traveling wave of deep polarization reaches the end of the axon and the nerve terminal. So what you're gonna see here in this example, these are gonna be kind of the first four steps. So the first thing you'll see is up here, we're looking at sodium channels um which are start out as closed and then open and sodium rushes into the cell. But then because there it only stays open for a really short amount of time. So it then closes and resets for the next action potential. And so down here you're looking at this graph of membrane potential. So this is going to be charged. Remember that more positive is going to be inside the sell side and more negative is going to be more negative inside the cell. So this is kind of over time. So again you have step one where the sodium channels open, sodium enters. So then it becomes more positive. The calcium channels are delayed in opening so it begins to leave. But the sodium channels here are still open so it eventually reaches some type of peak but then it starts traveling down the potassium is leaving the cell. So the membrane potential is becoming more negative now and everything eventually resets itself for another action potential. So those are the first four steps. But like I said, there were seven. So now where we are is we've had this action potential. It's traveled down the Exxon now, it's at the nerve terminal. So what happens now? So this electrical signal at the nerve terminal is then converted into a chemical signal. Because the synaptic cleft which is the region between the two cells, the pre synaptic and post synaptic cell can't pass electrical signal so they can't pass an electrical charge. So it has to turn into a chemical signal. So the chemical signal that it uses is called a neurotransmitter. And how this works is neurotransmitters are already sitting in vesicles near the nerve terminal plasma membrane. So they're already just sitting there waiting for a signal to release them. So when the deep polarization or this positive charge arrives, the triggers again more voltage gated calcium channels and influx of calcium causes the vesicles diffused with the plasma membrane and release the neurotransmitters into the synaptic cleft. So the neurotransmitters then are in the synaptic cleft and they bind to receptors on the post synaptic cell. Then this triggers the cell to fire action potentials in the cycle is repeated. But what happens to the neurotransmitter because now you have the signal, this chemical signals out there and it doesn't need to continually be out there, something needs to get rid of it. So it's um it needs to be removed and it's done through one of two methods. First is the post synaptic cell can destroy it. So it gets sort of uptake or taken in and destroyed or it's released and sort of pumped back out and the pre synaptic takes it up and reuses it. So that's how the neurotransmitter is released. So if we're watching this, what we get is we have our action potential arrives and you have your neurotransmitters already sitting in these vesicles ready to fuse. So when the action potential gets here that triggers the the neurotransmitter or the vesicles fusion which gets released into the synaptic cleft. And when it does this will move down here now so now the neurotransmitters are in the synaptic cleft so then they can bind to various receptors on the post synaptic cell and trigger more action potential signaling further away. And remember the neurotransmitters are destroyed either by the post synaptic cell or they're recycled again, pumped out and back into the pre synaptic cell to be reused. So those are the seven steps to um uh neuronal signaling. Feel free to review the steps again. Hopefully I made it very clear. Um So with that let's now move on.

4

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Neurotransmitters

4m

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So in this video we're going to talk more about neurotransmitters and specifically neurotransmitter signaling. So neurons must be able to interpret Brett complex combinations of neurotransmitters. So there's a bunch of different neurotransmitters and they fall into two classes um excitatory or inhibitory. So excitatory neurotransmitter trigger an action potential in a post synaptic cell and so that action potential then goes on to trigger more action potentials. So an example of an excitatory neurotransmitter is going to be acetylcholine and this is actually found in neuro muscular junctions and acetylcholine will actually induce action potentials which result in muscle contraction. So um the other classes the inhibitory neurotransmitter. And usually what happens is inhibitory neurotransmitters trigger chloride channels to open and that results in this influx of negative charge which makes deep polarization harder. And so it reduces the impact of the action potential by kind of helping to either decrease the intensity or reduce the chance of an action potential occurring. So an example of an inhibitory neurotransmitter you might see is called Gaba. And so some toxins and drugs work by targeting neurotransmitters. So here we have our excitatory and inhibitory so here you have an excitatory neuron that's passing some type of exciting neuro transmitter and this results in an action potential which you can see because there's this peak of membrane potential. So this is membrane potential but an inhibitory one is going to have other types of neurotransmitters. So they actually make it a different color which is going to result in a more negative charge which makes deep polarization harder. And so um for this single cell is receiving both excitatory and inhibitory neurotransmitters. So what does it do? Well there are these kind of huge neuronal networks that receive large combinations of neurotransmitters. These are combinations of both excitatory and inhibitory signals. And each neuron contains it's own set of receptors and ion channels which can respond differently to all these different neurotransmitters. So neuron have to be able to combine and interpret all these signals. And you can imagine that it's hard for us to be able to determine how the neuron is actually responding to all of these different combinations of signals. But it does and it's what allows us to do the things that we do. Now when we talk about this there is one term that I want to mention and that synaptic plasticity and what that means is that the magnitude of a neuron's response depends on how much it has been used in the past. So a neuron that has been used or like has received the same neurotransmitter, a ton in the past is going to respond differently than a neuron that just receives it for the first time. I think the best analogy to this is going to be um I think smell. So if you walk into a room and there's this overwhelming smell of trash you, that magnitude is you smell that smell and it's intense whereas if you've been in that room for a couple of hours you lose that sensitivity and the same things happen with neurons. But it's called synaptic plasticity. And so this is just an example of neurons or a network um with different types of um neurotransmitters. So you have some excitatory some um inhibitory and this post synaptic cell is going to take all this information and respond appropriately for whatever condition is being stimulated for. So that's a neurotransmitter signaling. Let's now move on.

5

Problem

Problem

Match the following neuron structure with its definition
i. Cell Body
ii. Myelin Sheath
iii. Nerve Terminal
iv. Nodes of Ranvier
v. Synapse

a. Junction through which the signal is transmitted
b. End of the axon that branches to pass the neurons message to many cells
c. Core of the neuron
d. Insulates the axon so ions do not leak out of the membrane
e. Patches of ion channels that interrupt the sheath for neuron signaling

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6

Problem

Problem

What neuron structure is responsible for ensuring that ions do not leak out of the axon membrane?

A

Cell body

B

Dendrites

C

Myelin sheath

D

Nodes of Ranvier

7

Problem

Problem

The opening of which type of channel causes depolarization of the neuron?

A

Voltage gated K⁺ channels

B

Voltage gated Na²⁺ channels

C

Voltage gated Ca²⁺ channels

D

K⁺ leak channels

8

Problem

Problem

An influx of calcium at the synapse causes what to happen?

A

Neurotransmitter release

B

Neurotransmitter binding

C

Depolarization

D

Opening of the voltage gated K+ channels

9

Problem

Problem

Which of the following neurotransmitter types blocks Cl- channels and makes depolarization harder?

A

Excitatory

B

Inhibitory

10

Problem

Problem

Potassium channels are mainly responsible for what?

A

Mediating action potentials

B

Returning the neuron to a resting membrane potential

C

Releasing neurotransmitters

D

Depolarization