action potential for beginners

welcome to anatomy and physiology at Glen Oaks Community College I'm Dr Ren Hartung for this video I'm going to talk about Action potentials um these are charge changes that cell membranes undergo especially in neurons and in muscle cells and they are part of a signaling mechanism either for neurons in terms of getting a communication somewhere else in the body or for muscle cells um it's along the route of the signals to cause the muscle cell to contract so that's kind of the important of them to represent that I've got a cell over here and I've got the charge across the membrane this would be resting potential and by the way I've covered membrane Potentials in a short video um that you might want to check out so this is membrane potential and it's ever most basic um a little bit positive on the outside a little bit negative on the inside and this would represent resting potential and again that's the cell over here we have a graph and on the graph the Y AIS represents voltage and this little line here is resting potential that's that first voltage when the cell is not being stimulated and it's just at rest it maintains this voltage um in order to put some numbers on here I'm going to put um around -70 molts most neurons maintain that type of resting potential and even though I'm putting that number on here I don't want you to think that all cells maintain this particular resting potential for some cells it's more negative than this and for other cells it might be a little more a little less negative than this um so in in other words don't think this applies to all cells it depends on the particular cell the other part of this graph the xaxis this is time in very tiny increments like microseconds because these membrane potential changes they occur very quickly um I'll put two other numbers on here up here we'll say 30 molts and then this dashed line here is around - 55 molts um and I'll explain that dash line shortly um the other thing I wanted to make sure I mentioned here this is negative this this the reason this is negative during resting potential we're measuring the voltage of the cell and you could imagine me having a voltage meter up here maybe and that voltage meter is measuring the charge in the cell compared to the outside of the cell it's me measuring the charge difference across the cell but measuring from the inside and that's why this starts out as negative and most of the time when we do measure and they they do literally measure the membrane potential across cell membranes um and again we measure from the inside just in case you're wondering why this is negative okay now I think we're ready to get started we've already talked about resting potential um here's what basically happens and I'll do it on the cell first during an action potential um some kind of stimulus comes and causes this to happen the outside becomes negative the inside becomes positive and then right after that very very quickly after that we go back to resting so that's it that is an action potential the charge flips and then flips right back again on my graph it looks something like this some kind of stimulus comes and that stimulus is basically from somewhere positive charge enters the the cell enough positive charge to cause this negative charge to approach this dashed line that is what's called threshold the importance of that once we reach threshold there are proteins in this membrane they're going to that are going to take care of the rest of it that are going to flip the charge and flip it back again and I'll get to those in a little bit once we reach threshold then those proteins in the membrane cause the voltage to go way up to around plus 30 molts and then immediately the Char charge comes back down and actually becomes more negative than it was at rest and then we get back to resting potential again so if we represent an action potential on a graph such as this one it looks something like this so the real question and if you to really understand it how does this happen what's causing this to happen um I won't get into what stimulates it that's kind of a separate subject but it also involves channels in the membrane um and stimulus either another neuron or something else causing that positive charge to come into the cell that's the stimulus again and you should know that that's the stimulus but uh at this point oh sorry I told myself instead of labeling things I'm just going to talk about them um by the way if you're looking for spelling to words and stuff like that when I do a YouTube video usually within a few days maybe a couple of weeks I go in and I make sure that the subtitles are correct so um click on subtitles if you want to look for the spellings of words and things like that or of course refer to your textbook or somewhere else um let's talk about what happens when we reach threshold and for that I'm going to change my picture slightly and I'm going to include the proteins of action potential here's my first protein I got to make sure it's [Music] closed and then here's my second protein and then the other thing that we need to make sure we understand before we move on from here um you need to understand the basics of membrane potential so you might go back and look at that video um but I'll explain it real quick um the sodium potassium pump was operating and when the sodium pottassium pump is working it pumps sodium out of the cell so there's a higher concentration of sodium out here there's lots of oops I drew potassium there's lots of sodium ions out here there are also pottassium ions out here but there's a lot fewer of them um and then inside of the cell the sodium pottassium pump pumps out sodium and it pumps potassium into the cell so inside of the cell we've got a higher concentration of potassium and we do have some sodium in here for the but there's a lot more potassium than there is sodium these two channels that I drew these are both what we call voltage gated channels when the voltage reaches this threshold that signals these channels to open that is the way in which they are voltage gated channels whether the channel is open or not depends on the voltage and threshold this minus 55 molts roughly um that is the threshold voltage the voltage above which these channels will be opened so once we reach threshold what causes the membrane to go positive the membrane goes positive because this first channel the voltage gated sodium Channel opens in response to that reaching of threshold that causes sodium to come into the cell sodium is positively charged so the charge on the membrane on the inside the charge goes up the positive charge goes up so once we reach threshold those voltage gated sodium channels open and we get depolarization this increase of charge going a lot more positive so I'll go ahead and label this one this is called um depolarization and it's caused by f voltage gated sodium channels na A+ channels The Next Step how come it once that voltage reaches this peak of around plus 30 Mill volts or 30 molts um why doesn't go in why does it not go any higher than this once once we've opened that channel once we're Above This switch of threshold that causes that channel to open why doesn't it just stay way up here somewhere and of course that would be bad because this muscle or nerve cell would just not be able to Signal anymore it'd be stuck um that's this other piece of the channel That I drew here it's called an inhibition gate and by the way it doesn't really look like this in terms of its molecular structure um but I draw it this way just for Simplicity and here's what's going to happen once we reach this peak of around plus 30 this inhibition gate comes into play and the channel is inhibited no more sodium gets to come in so that's why we only get up to plus3 we don't go any more positive than that and now we can come back down again because of that inhibition gate um another way to think of this if you want to take the gate out of it is those sodium channels they're simulated to open but then they're inhibited really quickly afterwards so that they don't make the membrane go too positive so this peak Peak voltage is due to inhibition of these voltage gated sodium channels inhibition of those channels um when I'm studying I always like to draw pictures and then label them up it's how my brain works I guess um sometimes it gets messy as you can obviously see all right Next Step how do we repolarize the membrane and that's what this next part of this graphic is this is repolarizing ization repolarization that's where the next Channel comes in this is a voltage gated pottassium Channel and this one was also signaled to open once we reach threshold but apparently it takes it just a little bit more time to open um than the sodium Channel than the voltage gated sodium Channel takes to open so this one opens at any rate and remember that pottassium is at a higher concentration on the inside of the cell so when this gate opens the potassium diffuses out of the cell potassium diffuses quickly out of the cell that's going to cause the voltage inside of the cell to become more negative because we're losing positive charges so the membrane potential becomes more negative so repolarization is [Music] due to the opening of voltage voltage gated potassium channels potassium diffus es out of the cell and the inside of the cell becomes less positive the inside of the cell becomes more negative remember that these channels were a little bit slower than the sodium channels once we get below threshold both of these channels are signaled to close again um so the sodium the voltage gated sodium Channel closes and the voltage gated Tsum Channel closes but we end up being a little bit hyperpolarized here more negative than we were at rest because the potassium Channel apparently takes a little bit of time to close that's what causes this hyperpolarization um so if I take it down here below threshold we basically go back to the state That these channels were in before we started in my sodium Channel goes back and the inhibition gate is back to where it was before ready to they're ready to do this whole thing over again um so I guess to finish up my labeling here this hyperpolarization um which leads to what's called a um a refractory period um not an absolute refractory period that's that's what happens under the actual action potential itself this area would be absolute refractory period whereas this other one over here where we've slightly hyperpolarized that's the relative refractory period and by saying refractory period we're saying it's less likely for this cell to be stimulated to undergo another action potential right then this one's the absolute refractory period because the action potential absolutely must go positive and then absolutely must go negative again before we can have another action potential so while an action potential is happening it's not possible to stimulate another action potential to happen it's just not possible so that's the absolute refractor period the relative refractory period is called the relative refractory period because what's what's preventing it from undergoing an action potential at this point is just that it's more negative more difficult to stimulate than it was back here at resting potential so here you might get it to do an action potential again but you'd have to have a lot more of a stimulus to cause it to happen so that's what's going on with the relative refractory period and again that hyp polarization that um relative refractory period is due to the fact that these um potassium channels apparently take a little bit longer to close than the voltage gated sodium channels after that hyperpolarization we return to resting potential and we return to resting potential by the activity of the sodium potassium pump and the potassium leak Channel that I talked about in the other video about membrane potentials the basics of membrane potentials I hope this makes sense um let's do a quick quiz okay so we're back to zero here ready for questions um here's a living cell that's the cell that's going to undergo Action potentials um while we're at rest without any stimulus or anything um what's the charge on the inside of the cell relative to the outside of the cell the answer to that question is the inside is more negative compared to the outside of the cell more negative on the inside um the number I gave you over here was around minus 70 molts and again not all cells have that resting potential but it's not a bad starting point either so the cell is more negative on the inside more positive on the outside this is that voltage right resting potential negative on the inside of the cell on my graph here what does this part of the graph represent this approach to the dashed line the answer to that is this part of the graph graph represents the stimulus something has introduced some positive charge in here it's introduced enough positive charge in here so that we reach that dashed line the next question is what do we call that dashed line and what does it basically represent the answer to that is this dashed line here we call threshold and the reason we call it that is once the charge goes to this line or above that stimulates these voltage gated channels to open that's the threshold for causing those channels to open next part of the graphic this part of the line where we went positive what do we call that the answer to that is we call that depolarization next question staying with this part this depolarization part of the action potential what caus is that why does the membrane go positive like that the answer to that is it is the opening of these voltage gated sodium channels and once these voltage gated sodium channels are opened they let sodium come in they let positive charge come in and cause the inside to become positive and if enough positive charges come in of course then we end up negative on the outside positive on the inside and then just continuing down our action potential graphic um if that's what's happening we get this channel open and there's sodium just flowing in there why does this stop at plus 30 why doesn't it go up to plus 100 or whatever and the answer to that the reason the membrane voltage Peaks at this point point is because these voltage gated sodium channels become inhibited and just because I like drawings probably I like to draw the inhibition gate as this little kind of ball and stick deal but the big picture is that voltage gated sodium Channel becomes inhibited and that's why there's a peak here the next step in the action potential graphic that we've got here what do we call this part where the voltage is coming back down again and the answer to that is this is repolarization repolarization and why do we call it or sorry what what is happening to cause that repolarization the answer to that is our next voltage gated Channel over here the voltage gated potassium Channel this channel opens and that allows potassium to leave taking away positive charge from inside of the cell and returning the voltage on the inside to being negative that's repolarization next question what do we call this period here where the cell membrane is actually undergoing the action potential or it's undergoing these charge changes the answer to that is this is the absolute refractory period what does that mean the absolute refractory period what we mean by that is we're saying that there's no way no matter what kind of stimulus you apply at this point you're not going to get this cell to undergo another action potential right now you absolutely can't it's the absolute refractory period if an action potential is happening then it's got to happen it's got to go through its steps before we can stimulate another action potential and then while we're since we mentioned that one let's mention this one this period here what do we call that and the answer to that excuse me the answer to that is this is the relative refractory period we might still be able to cause an action potential during this period but because the membrane is hyperpolarized at this point it would be a lot more difficult to cause an action potential to happen during this relative refractory period next question what causes that relative refractory period why does the membrane become hyperpolarized at that point more negative and the answer to that question is it takes a little more time for these voltage gated potassium channels to close and that allows more positive charge to leave for a little while and we become hyperpolarized more negative than we are at rest or during the resting potential but it does close eventually of course so I should show that once once we're back down below threshold this channel closes again and so does the voltage gated sodium Channel and we reset and get ready for another action potential and then the last thing to understand about action potential we were hyperpolarized back here in the relative refractory period what is it that causes us to go back back to resting potential and the the answer to that the reason we go back to resting potential is because of the action of sodium potassium pumps and the potassium leak Channel which I covered in basic membrane potentials or membrane potential for beginners but I'll run through it real quick see if I can do it fast here's the membrane and imagine that we're hyperpolarized here's the sodium potassium pump it's using ATP to pump out three sodiums and to pump in two potassiums and then over here is the potassium leak Channel a transmembrane channel protein that is selective for pottassium and it lets potassium diffuse out those two together return us to resting potential and of course they were they were what set up resting potential in the first place in the first place this is complicated if you don't understand it yet don't feel bad as long as you kind of got what what I was talking about as I went through um this if the video was helpful to you you might come back and look at the questions sections a few time as you're really learning and getting an getting a good understanding of action potential um and as always please feel free to leave comments or questions down below thank you once again for watching

Channel: Ren Hartung

Views: 131,530

Keywords: human anatomy & physiology, nervous system, muscular system, science, membrane potentials, voltage gated channels, sodium ion, potassium ion, voltage, voltage changes, neuron, physiology, refractory period, relative refractory period, absolute refractory period, resting potential, action potential, Na+, K+, voltage gated sodium channels, voltage gated potassium channels

Id: 8g1RBj03Syo

Length: 25min 43sec (1543 seconds)

Published: Wed Mar 02 2016