We're going to discuss action potentials and how they are propagated down the axon of a neuron. Now remember, action potentials can only begin if we have summation of graded potentials within the somas and dendrites. The summation has to reach threshold, which is negative 55, and it has to occur within the axon hillock, also known as the trigger zone.
As long as these stipulations are met, negative 55, which is threshold, and it's occurring in the axon hillock, we will produce an action potential. So just a little better picture, we can see here is the somas and the dendrites. This is where we're going to have summation of GPs. We want the summation to occur up to negative 55. That's the minimum within the hillock before we can actually start a new action potential down the neuron. To go over a few of the parts, remember the axon is this inner portion that's yellow.
Surrounding this particular axon, we have myelin sheath. This myelin sheath is actually made out of Schwann cells, so this is telling us it's in the peripheral nervous system. And then we see that we have gaps or areas without myelin sheaths. And this is the node of Ranvir.
You see that we have many of them. At the end of the axon, we have these little branches called the axon terminals, and then we have the swellings called the synaptic anvals. bulbs.
Now for action potentials to occur, they're going to depend on a different type of gate. They're going to depend on voltage gates. Voltage gates will open to a change in charge. So as we can see in this picture, we have resting, which is negative 70. Oops, sorry about that.
And then We can see that the gate is closed so nothing can get through because it's going to hit this block. Here we have a change in membrane potential which causes the opening so now things can move through. So we went from resting to negative 50 which is actually past threshold. So as long as we're passing threshold there's a chance that these voltage gates will open.
Potassium voltage gates are going to allow potassium to go through them. You have to remember that potassium is located on the inside of the cell, so potassium is going to exit and move towards the outside of the cell. We also have sodium voltage gates. Sodium voltage gates are also located on the axon, but they're going to be a little bit more complicated because if you see here, we have two gates on the sodium voltage gate.
Those two gates have to both be open for sodium to move through. So once both of these gates are open, sodium will be able to slide. Since sodium voltage gates have two different gates, they actually control most of the action within action potentials. Now there is also a calcium voltage gate. So over the entire axon, we're going to have sodium.
But on just the synaptic end bulbs, we're going to have calcium voltage. Calcium voltage gates are going to allow calcium through. And if you remember calcium.
So now that we know where all the voltage gates are, let's talk a little bit about. action potentials as a whole. We have to remember that graded potentials were decremental and that they died out and they only occurred on the somas and dendrites. Graded potentials are also small movements from resting. Now action potentials are different because action potentials are what we consider all or nothing.
Once they start, they will travel the entire length of the neuron. So as long as they start, it's going to happen all the way. If it doesn't start, then nothing's going to happen. So we call this the all or nothing phenomenon. Also, action potentials basically travel one direction.
So they're going to travel from the hillock all the way down to the synaptic end bulbs. Action potentials are going to be controlled by positive feedback, which we'll discuss a little bit in more detail. And action potentials are only going to occur on axons. So action potentials, all or nothing. They're controlled by positive feedback.
They occur on axons. And they're going to be controlled by voltage gain. Graded potentials are going to be decremental, meaning they die out.
They occur in the osomas and dendrites. They're primarily going to be controlled by ligand gates and also participate. So now that we've kind of discussed... These basic facts about action potentials, let's move to the number line and talk about terms that we've discussed previously and add new terms in it.
Action potentials are a series of events that will bring a neuron, its axon, through depolarization, repolarization, and usually hyperpolarization. So these series of events are going to repeat each other over every single segment of the axon. depolarization, repolarization, hyperpolarization.
The next segment, same thing, depolarization, repolarization, hyperpolarization. So let's kind of talk about what this means. So we know that resting is negative 70. This is when the inside is more negative than the outside, and there's no signal being sent. Our goal is to hit threshold so we can send a new AP.
For us to start a new AP, which is all or nothing, remember we have to reach... threshold at the hillock. So as long as we hit threshold at the hillock, then we will have a new AP that will travel the entire axon. Now, anytime we go more positive, we call this depolarization.
So going to threshold and past it towards positive 30, we're getting more positive. Now for this to even start, we have to have graded potentials summate within the somas and dendrites and reach that hillock. As long as they reach the hillock right at negative 55, an AP will begin. At this point at the hillock, we're going to have sodium voltage gates open.
When sodium voltage gates open, sodium is going to rush in. And when the sodium rushes in, it continues depolarizing the membrane until positive 30. At positive 30, the sodium voltage gates will close and the potassium voltage gates will open. Since sodium voltage gates are closed, no more sodium rushes in, and potassium voltage gates are open, and potassium is on the inside, so potassium is going to move out. This step is called repolarization. An easy way to remember repolarization is re-resting.
Repolarization's goal is to bring that segment of the neuron's axon back to negative 70. To do this, those potassium voltage gates open. and the potassium will flow out. Now you'll notice I still have spots over here from negative 70 to negative 80 which is past resting. This is because usually what happens is we actually go into hyperpolarization which means we're getting more negative. But why would that happen?
It usually happens because those potassium voltage gates fail to close in a timely manner. So too much potassium rushes out causing us to drop below resting. So a series of events for an action potential is going to be depolarization, repolarization, hyperpolarization, derehyper, derehyper. And these events occur over every single segment of the axon.
So again, I'm going to go back to this picture up here. Let me clear the writing on it. Each segment of the axon.
that we see that's not covered by the myelin sheath has to go through D, Re, and hyperpolarization. This repeats itself until we reach the 10 bulbs. Now D, Re always occur, hyperpolarization does not have to occur, but for our sakes and for our understandings let's just say it is going to occur every time. So this is what an actual action potential will look like if you were measuring it with an electrode in millivolts.
So we're going to go over these steps. Remember, what we're seeing here is d rehyper, d rehyper. So everything that we see here is just going to repeat itself over and over again down the axon. So this would be one segment of an axon.
Here's the second, here's the third, here's the fourth. So this axon is having these events occur all the way down it. So the first step, which we've already discussed, it's only going to happen in the very beginning, is we're going to be at resting. So the somas and dendrites are at resting, and the axon and axon hillock are at resting.
So what this tells us is that all the voltage gates are closed in the axon. Our goal, though, is to have depolarization to threshold. So we need summation of GPs. This is going to occur on the somas and dendrites.
And as long as they summate to negative 55 at the hillock, we will have a all or nothing action potential. Now, once we hit threshold at the hillock. We are going to go directly into the depolarization phase. That's the up phase going towards positive 30. For this phase to occur, sodium voltage gates will open so sodium can rush in to the cell.
Remember, sodium is naturally on the outside. With all that sodium rushing in, we go from negative 55 to positive 30, which finishes the depolarization phase. Now, One thing we haven't talked about is that when sodium voltage gates start to open, so do potassium. But I like to think of potassium as little old people. They take a little bit longer to do a lot of things.
So even though the potassium voltage gates start to open at negative 55, they are not fully opened until positive 30. At positive 30, our sodium voltage gates will close and our potassium voltage gates will open. So no more sodium can come in. But since the potassium gates are open, potassium will rush out.
When potassium rushes out, this is going to bring us to our repolarization phase, going from positive 30 back to negative 70. So repolarization, re-resting. Problem is, we often are going to bypass resting because those potassium voltage gates fail to close in a timely manner. So they're slow to open, so they're also going to be slow to close.
So when they do close, They're going to close at this point. Once they close, this particular section one, we're going to go back to resting. And here, section two is going to be in depolarization.
Take us back to resting. sodium and resting. So if we look at this overall and we kind of look at what happens at the hillock, remember we have our resting potential.
We're going to add graded potentials together to meet threshold at the hillock and as long as we hit threshold at the hillock we will have an AP occur. So what we're about to go over are the steps that we just discussed but looking at gates occurring. So at resting at the hillock, we are at negative 70. The hillock we must hit negative to occur.
These gates right here are sodium. These gates are potassium. Notice that sodium has no base.
All of these are potassium. All of these purples are sodium. So at resting, we're more negative on the inside than we are on the outside. Okay, our goal at the HELOC is to depolarize the threshold through graded summation, graded potential summation. So here we have GPs getting us closer to threshold.
Okay, as they're getting us closer to threshold, we'll still notice that these gains are... closed because what do we need to be to be considered thresholds with the gates open? If you said negative 55, you're right.
We're almost there, but still a little bit off. Well, notice that we went way past negative 55 and now we're in the positives towards positive 30. When that happens, notice that your sodium voltage gates are open, so sodium compression. I also want you to notice that sodium is not just staying in one spot.
Sodium diffuses to the next area. That's really important. So we're going to go all the way to positive 30, rapid depolarization.
And once we hit positive 30, what do you think is going to happen? You're right. The sodium gates no longer allow sodium in, but the potassium gates are open.
So sodium gates are closed, potassium gates are open. Now, why did the potassium gates open? Well, potassium gates fully open at positive 30, and all those sodiums moving and diffusing towards the potassium gates change the charge.
When that charge reaches positive 30, those gates fly open, and potassium is going to begin to exit. So all of a sudden, we're losing all these positives on the inside. So what are we going into?
We're going into repolarization. Now, repolarization is back to resting. Our resting is negative 70. So we paused resting. So what does that mean we're actually in?
If you said hyperpolarization, you're wrong. So now we're in hyperpolarization. So you can see that once this gate closes, we'll be able to get back to resting negative 70 easier because we're going to be using that pump to forward. There is a fantastic interaction on WileyPlus called Membrane Potentials. I highly suggest you watching it.