What up, what up, this is Metacosis Perfectionellus, this is my biology playlist. In the previous video, we have talked about the neuron, but today, let's talk about nerve impulse. Today, we'll talk about the resting membrane potential and the action potential. As you know, the action potential is depolarization and then repolarization.
This video is in my biology playlist, a very brief discussion. If you want to dig deeper, check out my physiology playlist. Here's your lovely cell membrane.
Here is inside the cell. here is outside the cell. This is the intracellular fluid.
This is the extracellular fluid. Who is the dominant in the extracellular fluid? Sodium.
Who dominates the intracellular? Potassium. Why do I care about the action potential? Because the action potential is life.
How does your muscle contract? Action potential. How does your skin feel?
Action potential. How do your glands secrete? Action potential. Everything, even your brain thinking, is action potential.
As you know, the nerve impulse is unidirectional. Starts here and goes downwards in one direction only. We call this orthodromic because it starts here and goes here.
What part of the neuron is the most excitable? It's here. It's the axon heloc.
All right, here's the deal. During rest, when I am not active, we call this the polarized state by the way, the inside of the membrane is negative compared to the outside. Therefore, upon activation, we will do the exact opposite. the inside of the membrane will become more positive.
And this is opposite to this. That's why we call the activation reversal of polarity, which is the more technical term. Tongue-in-cheek, you can call it depolarization.
It's not like depolarization. The membrane is still polarized. It's just polarized in the opposite direction.
Instead of negative, we are now positive inside the neuron. So let's play this game. Imagine that potassium is leaving the cell. Okay, a positive is leaving. Okay, when a positive leaves, it will leave the inside of the membrane more negative.
Is this activation or inactivation? This is inactivation. Perfect. You can call it inactivation or you can call it hyperpolarization.
Magnificent. What if sodium is entering into the neuron? As you know, sodium is positive.
As sodium enters, the inside becomes more positive. When you become more positive on the inside, this is called activation or depolarization. Thank you.
How about chloride entering? Well, chloride is negative. If chloride enters, the inside of the membrane becomes more negative. This is inactivation. You will not need the chloride story today.
You will need it in pharmacology, so just forget the chloride. When potassium leaves, this is inactivation. When sodium enters, this is activation.
The membrane has electricity, whose potential difference can be measured in millivolts. Not volts. Volts are gonna kill you. Millivolts.
The electricity of the membrane during rest is known as the resting membrane potential. Upon stimulation, this is called the action potential, provided that you give me a threshold stimulus, a strong enough stimulus to cause activation. Here is the story morning glory.
During rest, the membrane is resting. No kidding. What's going on?
Potassium is leaving. As you know, potassium is positive. When the positive leaves, it will leave the inside of the membrane more negative.
Okay, when we measure this, this is about negative 90 millivolts. Some textbooks will say negative 90, others will say negative 70. No one cares. And we call this inactivation or rest or the polarized state.
Amazing. Okay, you started to secrete. You started to contract.
You started to get excited and think and secrete and do all kinds of crazy stuff. This is activation. How did it happen? Sodium.
has entered into the cell in huge amounts. Okay, sodium is positive. When the positive enters into the neuron, the inside will become more positive. When I become more positive on the inside, this is called activation or depolarization, because it's the exact opposite of the first stage. And when we measure this, it gives you a potential difference of positive 35 millivolts, which means the inside of the membrane is more positive compared to the outside by a magnitude of 35 millivolts.
After activation, I was excited. Now, let me go back to Earth, down to Earth. What happened? I stopped letting the sodium in, okay? I closed these sodium channels, and I will open potassium channels.
Potassium is leaving. Potassium is positive. When the positive is leaving, the inside becomes more negative, and this is called inactivation or hyperpolarization or repolarization. Unfortunately, I do not just stop at negative 90. I keep going down, down, down, down, down until I reach negative 100. Okay, how can I give myself a push until I go back up to negative 90? Well, some of that potassium will go back in.
Potassium is positive. When the positive comes in, the membrane is starting to get slightly positive. So I go from...
Negative 100 up to negative 90. Some quick questions. Quick questions. Who was the hero of the resting membrane potential? The answer is potassium.
Okay. Who is the hero of the depolarization or the activation? The answer is sodium. Who is the hero of repolarization?
Of course, you stop the sodium and you start potassium. Resting membrane potential. Who is the hero? The selective permeability for the membrane, especially for potassium.
This is the hero of resting membrane potential, as well as the sodium-potassium pump. First, we'll talk about the selective permeability, and then we'll talk about the sodium-potassium pump. Selective permeability during rest.
What does that mean? It means that during rest, your membrane is letting the potassium leave. And it's letting a very teeny tiny amount of sodium in.
But look at this. A sense of proportion will help. A huge amount of potassium is leaving, but a tiny amount of sodium is entering.
Therefore, if you have to bet the rent money, would you bet it on potassium or would you bet it on sodium? Of course, potassium. That's more important.
Jazillion potassiums are leaving. Okay, so selective permeability means that during rest, the membrane loves potassium more than sodium. This is what it means. As you know, potassium is positive.
When the positive leaves, the inside becomes more negative. That's why it's resting membrane potential. It's inactivation.
That was the first reason why we have negative resting membrane potential. What's the second reason? Sodium-potassium pump. Sodium-potassium pump pumps sodium to the outside and potassium to the inside.
Three sodiums are leaving. Two potassiums are entering. The net result is one positive going out.
If one positive is going out, Therefore, by the same token, one negative is going in. The loss of loss is gain. When I lose one positive out, it's as if I'm gaining one negative in. All right, let's activate the membrane. Action potential, baby.
All right, here you have the ascending limb like this. Boom, up, and then descending limb. Boom, down, but then I overshoot to negative 100, and then I go back to negative 90. What's the name of this? Depolarization. What's the name of this?
Repolarization. What's the name of the overshoot? Hyperpolarization, which is a continuation of repolarization. Who is the hero of depolarization?
Sodium influx. Sodium is entering into the neuron. Sodium is positive.
When the positive enters, the inside becomes more positive. Hashtag activation. who is the hero of repolarization first you have to close the sodium channels and then you open potassium channels potassium leaves the neuron when the positive leaves the inside becomes more negative i go from here the positive down here to the negative because i'm becoming more negative because the positive is leaving what is the magnitude of the action potential well if you remember the resting membrane potential was negative 90k and then after activation i reached positive 35. What is the difference from here to here? Well, all of this.
So from negative 90 to zero, this is a magnitude of 90 plus 35. 90 plus 35 is 125 millivolts. That's the magnitude of the action potential. Why do you call this firing level? Because there is life before the firing level and life after the firing level.
Before the firing level, some sodium particles were entering. But after the firing level, boom, it's fire, baby. We're on fire.
Tons of sodium ions are entering. All of my sodium channels are open to let all of that sodium in. And that's why before the firing level, we call this slow depolarization. But after the firing level, this is rapid depolarization.
And this is the threshold level of a stimulus. If you give me a robust stimulus, I'll give you an action potential. If you give me a weak stimulus, I cannot reach the firing level.
therefore there is no action potential for you. Pause and review. Now, can you put the story and the graph together?
Let's do it. Resting membrane potential is here, negative 90 millivolts. Okay, who's the hero?
Potassium is leaving, potassium efflux. Thank you so much. Then, what's that? This is the ascending limb or depolarization.
Who's the hero? Sodium influx. Sodium is entering into the neuron, and that's why I'm going from negative to positive 35. Amazing. And then repolarization, what's happening?
You close the sodium channels and then you open potassium channels. When you open potassium channels, potassium leaves. As potassium leaves, the positive is leaving. The inside is becoming more negative.
Unfortunately, I overshoot from negative 90 to negative 100. How do you reverse the overshoot? And you go back up here, inward rectifier potassium channels, let some potassium in. Potassium is positive. When the positive comes in, I go from negative 100 to negative 90. Cool.
All right, action potential is over. The party is over. How do I go back to the resting membrane potential?
Well, you can thank your sodium potassium pump. You pump three sodiums out, and you pump two potassium in. The end result is one positive to the outside, therefore one negative to the inside. You go back to the negative resting membrane potential.
What's the all or none law? It's either my way or the highway. It's either all or nothing. You treat me with respect, I give you the action potential.
You treat me with half respect, I give you nothing. I'm either all or none. You give me a million dollars, we have a deal.
Anything less than a million, you have nothing. No deal. The action potential is either generated and conducting maximally or not at all. You have to give me a threshold stimulus.
How about subthreshold? I will not give you an action potential. How about two thresholds? I'll still give you the same deal. You give me a million dollar plus, I give you a deal.
anything below million dollars, you get no deal. How about if I give you three millions, do I get three deals? No, you just get one deal. It's the all or none.
What's the refractory period? It's a period, no kidding, at which the nerve is refractory. The nerve is not listening. The nerve is just dumb, does not react.
It's refractory to stimulation. Why is this important? To protect your nerve from extremely rapid repetitive stimulation, which can kill you.
And we have two types of these, absolute refractory period and relative refractory period. The comparison between the two was discussed in my physiology playlist. If you liked this video, I have a CNS pharmacology course on my website medicosisperfectsnetics.com. Learn about anti-epileptics, anti-depressants, anti-psychotics, anti-parkinsons, opiates, anesthetics, stimulants, sedatives, and hypnotics.
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