whenever you're trying to learn a complex concept as complex as an action potential you should always try to think of a memory technique and for this i want you to think of a salty banana how's it going to help us stick around [Music] all right guys welcome to psych explained in this video we're gonna break down an action potential now if you look at different textbooks you might see different names for it you might see the phrase a neural impulse or nerve impulse or spark but essentially it means the same thing it is a brief electrical charge that travels down a neuron or more specifically the axon and this is going to allow neurons to communicate with each other and get the message across whatever that message may be in this video we're going to go step by step by step by step how this process works but let's begin by understanding what are we actually looking at behind me let's actually start with this visual right here this is a neuron now i do have a separate video on the parts of a neuron and their functions i recommend to watch that first and then maybe come back to this video we have different parts we have our soma and cell body that sits in the middle we have our dendrites that receive the message from the sending neuron we have the axon which takes the action potential the charge away from the soma and our axon terminal which contains the neurotransmitters this visual right here is essentially a close-up of different parts of the neuron this is considered the beginning of the neuron so this is going to represent our soma our cell body and the receptor sites on the dendrites okay so what we're looking at right here is a close-up of this area and this long tube is going to represent our axon okay because this is where the electrical charge or action potential is going to travel down all in the same direction you'll also notice there's a membrane this outside this protects the neuron right it keeps things in it keep things out the only way to get things in and out are through these little channels right there we'll talk about those in a moment now another thing to pay attention to is that a neuron is surrounded by ions what's an ion it's a charged particle or molecule all right some of them are negative and some of them are positive when referring to a neuron there's two that are really important to us sodium and potassium you'll notice there's a high concentration of positively charged sodium ions that sit outside the neuron right we have sodium that sits outside on the inside of the neuron here's our axon you have a higher concentration of potassium positively charged potassium ions all right do you remember our memory technique earlier we have our salty banana let's think about that together here's our salty banana right here's our banana how does this help us we have potassium on the inside potassium on the inside what are in the outside sodium right imagine we're pouring salt all over the banana salty banana there is our memory technique so when a neuron is at rest it's not doing anything we have positive uh potassium on the inside positively charged sodium ions on the outside okay so there's where we're at all right now here's what i want us to know when a neuron is at rest right the neuron is not doing anything what we have is a negative 70 millivolts in other words if you take the voltage inside the neuron we're at negative 70. it's very negative on the inside okay that's where we're starting the next question is how do we then excite the neuron right how do we actually have the neuron do something well let's just imagine that we are reaching for a glass of water right there's a lot of things that can stimulate a neuron it could be thinking about something it could be smelling something it could be putting cold water on my on my arm but in this case let's imagine that you're reaching for a glass of water what's going to spark that we have these neurotransmitters we're going to represent these as acetylcholine what is acetylcholine this is a neurotransmitter that controls our muscles to move right to move our muscles pick something up to bend down this is acetylcholine and what's going to actually happen is our first step we'll write this in together is there has to be some sort of stimulus that excites the neuron the stimulus is going to be acetylcholine reaching for something these acetylcholine are going to bind to the receptors on the dendrites all right and what's going to happen is these are going to open the channel and take with it sodium are positively charged sodium ions all right and these are very positive okay so there we go once we bind the receptor this is going to allow sodium to flow into the neuron as well and then we have another acetylcholine is going to bind to another receptor site what's going to happen it's going to allow this channel to open up this gate to open up and more sodium is going to rush in okay once again it is very positive and this is going to happen over and over and over again all on the receptor sites they're going to bind to those little pockets we have our sodium that's going to rush in becoming more positive positive and positive so right now this area is extremely positive what does this mean this graph here is showing the voltage from negative to positive right as we go up it gets more positive right we're getting close to zero and then as we go down we're getting more negative the neuron is starting to become more positive so what's going to happen the inside of the neuron the charge is going to start going up up and up and up okay it's getting more positive until it reaches negative 55 okay this is considered the threshold what is it considered the threshold this is the magic number if a neuron gets a positive enough to reach negative 55 something amazing happens the action potential is likely to fire we call this the all-or-none principle because if we do reach that threshold a neuron's going to fire every time now not every neuron does reset threshold there could be kind of false alarms where it's like it's almost there and then nope it's almost there and then nope right there wasn't enough stimulation but if we use the threshold that's the magic number so what's that going to do all these sodium positively positive sodium ions are then going to trigger these voltage-gated sodium channels that go all along the axon what's going to happen this is going to cause these gated channels to open up these are going to open up the lid right thing about like a channel that opens up like boop right little gate that opens up and this is going to draw more very quickly sodium into the neuron okay once again very positive okay and this is going to go to negative 55. what's going to happen this is going to trigger then the next gate to open up the next voltage channel open up and then this sodium is going to enter okay you get the idea here right all this sodium is entering it's going to become more positive and then finally it's going to open up the last one all right this is going to open up it's going to open this and our sodium is going to rush in as well okay we got positive positive positive all right so what's happened here we have so much positive sodium ions within the neuron that this is going to take the charge not just a negative 55 but it's going to change it all the way up to positive 30.
it is so positive now on the inside it's reached positive 30. okay we'll go right there okay so what does this mean right now what we have is what we call depolarization that is the next phase in our action potential d [Music] polarization okay or depolarized what this means is when a neurons start to become more positive the neuron is depolarized this is going to start our action potential so what does this mean it means our sodium ions are going to start rushing in to our neuron and it's going to create more positive so as they rush in everything starts to become more positive on the inside what do you think happens on the outside it's going to become more negative on the outside the voltages have switched this is the beginning of our this is our action potential right this is our depolarization is going to cause or evoke or trigger the action potential so whenever we think of electrical electricity traveling throughout the body essentially what we're just saying is all these sodium gates opening up and letting sodium into the cell all right so that's where we're at right now now what happens once it reaches positive 30 well the neuron has to go back to being negative right has to go back to the resting state so what happens is these sodium gates are going to close right it's almost like we've had we have enough sodium inside the cell we don't anymore we're going to close the gate what's going to open up the potassium voltage-gated ions are going to open up these channels are going to open up okay and this is going to cause these positively charged sodium ions are going to rush outside of the cell okay all outside potassium rushes out why is that important because this is going to make the neuron more negative and this is going to rush back down all the way to negative 90. okay i'll explain why it's a negative 90 there the process of closing the sodium positive solar positive sodium ions and opening up the potassium we call this phase repolarization re-polarization okay repolarization and this is when our potassium rushes out of our cell okay because we want to go back to the original right we want to go back to the original resting state so let's rewrite this in together this charge going up this graph going up is sodium rushing into the cell and the voltage going back down to negative is our potassium rushing out okay this whole thing together right this whole process together is our action potential okay the rising and the following now you'll notice the voltage goes even more negative than it rests why is that it's because one we have so much potassium leaving the cell and also it takes a long time for these to close that actually undershoots and goes even more negative and it goes to negative 90.
okay why is that important this phase right here is what we call hyper polarized what is it called hyper polarized okay you can also call this the refractory period refractory period okay what happens here we have the neuron recharging right it's like trying to flush a toilet twice in a row it can't right you have to let the toilet fill up the water has to fill up and then it could fire again well it's the same thing with a neuron it can't fire twice in a row it has to recharge the charges have to return back to normal they have to switch and then we can fire again and eventually we're going to go back to our resting potential and we'll be ready to charge again so there we go we have our resting state or resting potential we have a threshold at negative 55.
