hello everyone welcome back to a.s and a-level biology with dr demi i am dr jimmy and in this video i am going to zoom in on resting and action potential when i taught this to a group of students last year i noticed that they really struggled to understand what i was saying they just felt oh my goodness what's going on it seems like a lot of information and indeed it is a lot of information so i made this slide deck which is just a powerpoint animation but one the students could sort of scroll through at their own pace to make sure that they understood whatever was going on so i figured it would be best for me to also show you that in a video as slowly as possible so that you understand rest in an action potential in very easy to follow steps so i hope that you find this helpful so even if you watched the previous video and you were just like oh my goodness what just happened i hope that this one sort of helps you get it a lot better so let us get into it all right so if you look here i'm going to use a red pen which i think would be the ideal pen if you look here i've said this is the outside of the accent and over here is the inside all right so the outside is on top and the inside is at the bottom this is the axon membrane and this is the membrane that helps with the generation of an action potential so this membrane is the one that plays a role in whether or not an action potential will be gen in how an action potential is generated because the things like the sodium potassium pumps and so on are on this membrane so there we go that is our sodium potassium pump right and for every three sodium ions that are pumped out we have two potassium ions that are pumped in and what this means is that the axon the inside of the axon is losing more positive charges than it is getting in return all right so i hope everyone gets that then we have what we call the sodium channels and we also have the potassium channels as you might see there are more potassium channels than they are sodium channels these channels are open all the time they are different from the voltage-gated channels these are just channels that are in the cell membrane and they're open all the time which means that sodium and potassium can choose to diffuse in and out depending on the concentration gradient now as we pump in uh potassium ions and sodium ions and we pump out sodium ions rather and we pump in potassium ions because these channels are open all the time surely we know that some of the sodium ions will sort of diffuse back in but there are more potassium channels in the membrane than there are sodium channels that means more potassium will diffuse out compared to sodium all right some potassium ions will remain inside but the overall thing is that the inside of the membrane will become more negative than the outside simply because we are losing more positive charges from the inside than we are getting back and this is what is called the resting potential at resting potential the membrane of the axon is about minus six to minus 70 millivolts and that is what you will see on the graphs that you are given when you are asked to explain this now let's continue with the story we've just discussed resting potential on one slide so what then happens when we generate an action potential so here we go again we have the sodium and potassium channels that are open all the time right and we have a lot of sodium and potassium ions on the outside of the axon because remember from the previous slide we had the action of the sodium potassium pump where we had sodium being pumped out and potassium being pumped in now because we had a lot of these potassium channels over here lots of potassium also moved out but not enough to make um the inside terribly negative it's just about minus 60 to minus 70 millivolts because also we have some sodium ions that diffuse in but not as many as the potassium that diffuses out in addition to these channels um which are open all the time we also have the gated channels so they are voltage-gated channels so we have the sodium-gated channels and the potassium-gated channels yellow is for sodium so these are the sodium-gated channels over here i'm just going to write n a over there for sodium and these are the potassium gated channels over here all right when a stimulus reaches the axon some of these sodium-gated channels will open and they will cause sodium ions to flow from the outside where they are abundant already into the axon all right so the sodium moves in through these gated channels that way at first you have only few sodium-gated channels that are open and so basically they start to make the axon on the inside less negative so they increase the potential difference from minus 70 millivolts to minus 50 millivolts minus 50 millivolts is considered to be the threshold potential once we reach the threshold potential more sodium gated channels will open and more sodium will flow into the membrane and that would increase the inside of the axon to plus 30 millivolts this is what we call depolarization and the reason depolarization on the graph is that increasing trend or the ascending part of the graph is because it is increasing the potential difference inside the axon after one millisecond so i'm just going to erase this over here so that you don't get confused so just rather write it on the side here just to let you know this is the potassium these are the potassium gated channels after one millisecond the sodium gated channels will close and what once they close they cause the potassium gated channels to open potassium ions that were held within the cell by large negative ions which i explained in the main video would then start to flow out okay so potassium ions start to move out and that again starts to make the inside of the membrane more negative so that is what leads to the decreasing trend on the graph that i showed you in the main video and i'm going to show it to you again when i finish this explainer so basically the potassium ions start to move out so they decrease the positive charge that is on the inside remember i just want to take you a step back that when we said that the sodium ions diffuse in and that increases the membrane potential we also need to take note that that membrane potential is also a result of some of the potassium ions that stayed inside the axon so when they start to move out they are now taken away from that positive charge and that then results in the membrane potential again decreasing to minus 70 millivolts which is the resting potential the decrease back to resting potential is called repolarization the combination of depolarization and repolarization is what we call the action potential but now let's go back again so now we're back at resting potential now we have potassium ions on the outside because they've moved through the gaelic channels and we have sodium ions on the inside because they moved in through the gated channels so we have basically created again the platform for the sodium potassium pump to work if sodium ions are on the inside then the sodium potassium pump is able to pump out three sodium ions and bring in two potassium ions from the outside and that again starts off things for the resting potential and once a stimulus hits that then helps us take off with action potential so what i usually tell my students is that after they've watched that um that animation well they don't usually watch it i give it to them as a slide back so that they can control the pace of the animation but for your sake let's say after they've watched it i tell them to try to fit the sequence of events onto this graph so that it makes sense because usually you would find that this graph is what is most likely to come forth in your exams so let's look at this at resting potential we said that we have the sodium potassium pump right so i'm going to try to animate here it's not going to be pretty but we'll try to make it work so here is our axon membrane and then we have what we call a sodium potassium pump and what this pump does is that it takes out three sodium ions over there so three sodiums will go out and it brings into potassium okay basically because sodium is plus one and potassium is plus one in terms of charge we are losing more positive charges than we are getting back all right so we we have less every time we pump out three sodiums we take into potassium so we've lost one positive charge on the inside of the axon in addition to the sodium potassium pump we also have what we call the sodium channels and the potassium channels the sodium channels are open all the time as well as the potassium channels so i'm just going to draw them here as big blobs um so let's say that's potassium and that is sodium now when we bring in potassium ions over here because these channels are open all the time and there's more potassium than this sodium um some of the potassium ions will diffuse out all right and that further increases the negativity on the inside of the axon so the axon on the inside becomes more negative obviously some sodium ions will diffuse in but not enough to balance things out all right because also we have less sodium channels compared to the potassium channels so i hope that helps you understand why the inside becomes more negative this is what we call the resting potential over here now when when the stimulus hits the axon when the stimulus reaches the accent uh that we have what we call the sodium gated channels i'm going to use a green thing to draw this one so we have the sodium gated channels so they're like voltage-gated channels which means they would respond only to electrical impulse so this year i'm going to call this sodium just write in it as n and call this one potassium when an axon is um stimulated by a stimulus the sodium-gated channels will open now most of the sodium that we pumped out here that has been trying to trickle in back you know through the channels then see that oh my goodness there's a better space for us to get in through so they start to flow in through the sodium gated channel by flowing in through the sodium gated channel they start to offset this negative charge so that making the inside more positive something else to note is that with these potassium ions some of them flow out but some of them are still on the inside okay so they're not all out some of them are still here on the inside now the negative ions here and the negativity here is offset by the influx of sodium ions whenever we have a stimulus heat in the axon that then starts to increase the charge on the inside of the axon and that is why we go from -70 here to minus 50 so this over here that change minus 50 millivolts is referred to as the threshold potential because once that threshold potential is reached more sodium ion channels more sodium gated channels will open and as soon as more sodium get it channels open it means more sodium will flow into the axon as soon as that happens after one millisecond because look at this here it's time in milliseconds so you can see as more sodium ions flow in that is what we call depolarization obviously because it increases the potential difference to about plus 30 millivolts over there all right after one millisecond the sodium gated channels will close and once they close the potassium gated channels will open the potassium gated channels will allow potassium ions that have been inside the cell to start to flow out all right so now on the inside we have a lot of sodium that has flown in okay we have lots of sodium that has come into the inside and now we have our potassium ions flowing out as the potassium ions start to flow out they start to reduce that positive charge that was gained and they reduce it until we get back to resting potential so at resting potential basically we will have potassium ions on the outside sodium ions on the inside and that then takes us back to the action of the sodium potassium pump where sodium is then pumped out potassium is pumped in to achieve resting potential hyperpolarization here is usually something students tend to miss out on and it's basically the potassium ion channels don't close quickly enough when potassium is being pumped out of the of the cell of the axon and so it causes more positive ions to flow out than necessary but that is often restored by the sodium potassium pump when it starts to bring in potassium and pump out sodium i hope this has been helpful i know that this can be very very difficult for students most of the time but i hope haven't heard it twice that it makes sense you can watch the video over and over again just to make sure that you get it but i promise you once you understand it it just really sticks and it is easy for you to explain this graph without being afraid of it that is it from me for this video i'm going to move on to the next video which is about synapses which is always fun once you get to understand it thank you so much for watching until the next video have a good time goodbye