hello folks we're back again I'm out in the backyard again and I was thinking about how in our previous lectures we've been trying to set up how the cell utilizes voltage differences across the membrane and concentration differences across the membrane to get work done for the cell well cell also will use this voltage change across the membrane and concentration difference across the membrane to Signal itself to let it know that maybe there's other work that needs to be done as well and the cell can actually utilize this this change in voltage across the membrane to Signal other cells well this change in voltage that I'm talking about has a specific name associated with it the name let's bring it up here it's called action potential action potential let me get my laser pointer right here action potential action refers to that there is a change in both the concentration of particular ions and voltage across the membrane of the cell this change just very quick an acute change very quick change cell wants to maintain its resting membrane potential that's what we've been talking about for the last three many lectures that resting membrane potential this action potential is a change or or a move away from that resting potential now overall I want you to remember that resting membrane potential is kind of the cell's set point it's the homeostasis that's where the cell wants to stay at that resting membrane potential so this action potential is something special now talk about this action potential and how it actually happens we're going to have to talk about the changes in the membrane permeability to particular ions because that's what's going to cause the voltage change in this case sodium and potassium when we get a change in permeability across the membrane for any of these ions that means there's a change in the conductance for that ion its ability to get across the membrane so that's why the term conductance is here now to talk about the action potential we're going to focus on a on a single type of cell in in our body a nerve cell a nerve cell because that's where you hear the term action potential usually associated with it with the nerve cell but I want to make sure everyone realizes that any cell in the body because every cell has a resting membrane potential they also have the ability to set off an action potential it just so happens that the action potential of nerve cells seems to be a real special thing that folks like to focus on well let's get started in this in this image here or this slide we have several images we're going to focus on the upper right hand Image First in that image up to the right I want you to notice on the bottom axis time in milliseconds on the left axis membrane potential the voltage across the membrane if you remember in our previous lectures we talked about the the cell having a resting membrane potential a resting voltage across the membrane of minus 70 or negative inside the cell negative inside the cell minus 70 well for most cells that's what you see for some cells it could be a little bit more negative and others a little more positive depending on the cell overall sitting at about minus 70 that's its homeostasis that's its set point well something is quite something quite interesting about this in a neuron if we change the voltage on the inside of the cell to become a little more positive and in this case it's trying to show you we push it up to a minus 55. the properties of the membrane change well what changes actual opening of particular channels for ions like sodium or potassium in this case if we make the inside a little more positive doors for sodium open up channels for sodium open up now remember sodium high concentration outside the cell versus inside the cell so if the doors open up sodium is going to want to rush into the cell look what happens here as the door is open up for sodium sodium rushes into the cell and all those positive ions moving into the cell make the inside more positive more positive so look at look at here the voltage on the inside of the cell is just skyrocketing now well let me tell you remember the inside of the cell was negative at the beginning right minus 70 minus 55 and we open the doors for sodium not only does it want to move across the membrane because of concentration it also wants to move across because well positive sodium is positive and the inside's negative so sodium has two forces on it a charge difference across the membrane and a concentration difference across the membrane about that that's why we see this Spike occur this change in voltage from being really negative on the inside to being very positive on the inside well as the cell becomes more positive on the inside that's a signal to those doors that opened up to close so as sodium rushes into the cell and it becomes more positive on the inside the door is for sodium close and as they close you can see here we've reached a peak sodium can't come in anymore we're not getting any more positive at the same time as those doors closing other doors for potassium are opening up well the inside of the cell Now is really positive hmm potassium what's the charge on potassium it's positive because it is positive and the inside of the cell is positive it wants to leave it wants to get out of that cell as fast as possible right because remember light charges repel so being positive inside the cell potassium leaves the cell leaves the cell well remember the concentration for potassium on the inside of the cell is higher than it is outside the cell so what do you think it's going to want to do it wants to leave so when the doors for potassium open up up in this area here potassium starts flying out of the cell and as potassium is leaving look at the charge on the inside of the cell it drops way negative again very quickly very very quickly at the same time sodium is getting kicked out of the cell by the sodium potassium pump that drives the voltage on the inside of the cell to go even more negative than it normally does well as it gets to that third phase there the sodium potassium pump well you should say the doors for potassium start to close the sodium potassium pump that's sitting in the membrane starts kicking out sodium bringing in potassium back into the cell and brings the membrane back to its resting potential its resting membrane potential this whole process that we just talked about is called the action potential this action potential is a signal to the cell hey we need to do something all right now this whole setup that we just talked about phase one two and three well in in in class we don't talk about the S phase one phase two and phase three they actually have more specific names phase one where we have sodium rushing into the cell and the cell changing voltage from negative to positive that's called the depolarization phase or the depolarization stage all right we're polarized of being very negative on the inside and now we've flipped it to the other end well once we get up to the top here at this positive end here and we start to drive down back towards what our resting membrane potential that phase two it has a name as well it's called the re polarization phase depolarization repolarization this area that we see where the charge on the inside of the cell goes way negative that's called hyper polarization hyperpolarization and that hyperpolarization will eventually be brought back to the Rusty membrane potential by the sodium potassium pump all right and the graph down below it's trying to show you conductances Which ion has the greater ability to move across the membrane at what time so in red is our action potential that we just showed you above up here that's in red and in blue it's showing you the permeability of sodium the permeability of sodium and you can see during that depolarization phase the sodium has a huge amount of permeability you can get across the membrane very easily and then it stops because the doors close and then it permits its permeability drops off when its permeability is dropping off because the doors are closing remember potassium stores are opening now it has the ability to move across the membrane better and then its doors start to close as we get into the hyperpolarization phase and now we're back to our normal levels back to our normal levels all of this can be looked at using the equations that we started using in our last couple of lectures now am I asking you to do that here no but in lab we'll play with this a little bit all right so that you understand this action potential a little bit better I want to jump over and try to give you a little more visuals associated with the action potential so here we have an image showing you a bilipid membrane so this is the cell membrane inside of the cell negative outside of the cell positive in the membrane we have channels we have channels for sodium and potassium but at this point ah the doors are closed the doors are closed formal and over here we have the sodium potassium pump now I'll be labeling these here in a few moments for you all right now again inside of the cell negative in comparison to outside how much around -70 is what we're saying over here everybody got that now this is at rest this is the homeostasis for the cell this is where it wants to stay so the action potential must be something that happens to Zone something's pushing it to move all right we're going to show you how that works here first and foremost again show you our channels here let's show you some of our other components here like sodium if you look over the left here is a little Legend the red dots refer to sodium now where does sodium have a higher concentration outside of the cell higher concentration outside versus inside so which way should they want to move if they had the ability to get across the membrane they should want to move into the cell remember equal out concentrations but the membrane is not permeable to sodium they need to have doors open for them or Channel special channels for them to get across well they're not open right now so they're not getting in they're not getting it how about potassium where is it higher inside or outside inside the cell now because of that higher concentration inside versus outside which way should they want to go they should want to leave they should want to leave now there are some doors open for potassium and so they can leave if they really want to however remember the charge on the inside of the cell is negative because it's negative on the inside and potassium has a positive charge it actually hangs out it likes to stay remember differences in charge positive versus negative that attracts each other so the positive potassium is attracted to the inside of the cell now what's creating that negative inside of the cell versus outside for for the most part it has to do with negative ions mainly negative proteins but chloride well it's negative and it's an ion but it's the concentration is pretty equal across the membrane but what what what isn't equal is of proteins negative proteins and negative proteins tend to be more highly concentrated on the inside of the cell helps Drive the inside of the cell to be more negative so we have this difference across the membrane minus 70 millivolts across the membrane that's our graph over on the right showing you our resting or homeostasis for this cell all right now this homeostasis is maintained by the sodium potassium pump because the membrane is imperfect and so there's so much pressure on sodium to move across well it's going to find a crack someplace and sneak in well sodium potassium pump soon as it sees it come in it's going to kick it out potassium well because sodium is sneaking in it doesn't terribly like sodium so it may decide to leave because it has some doors open well guess what the sodium potassium pump it'll it'll entice or grab potassium and drag it back in it's trying to keep this different so again the cell can get work done now at rest at rest let's recap higher concentration of sodium outside the cell higher concentration of potassium inside the cell negatively charged ions on both sides but mainly negatively charged proteins on the inside the membrane is permeable to potassium it does have some channels open for it not a lot but it does so potassium can leave but it's not leaving why isn't it leaving well because the inside of the cell is negative and it's attracted to that all right so remember that and of course the sodium potassium pump is helping to maintain this difference all right let's take this to the next step remember from our first uh graph that we were looking at if we change the voltage on the inside of the cell to become a little more positive a little more positive reaching a particular threshold it's going to open doors for sodium here on the left you can see the doors have opened for sodium because there's a concentration difference for sodium and because the inside is negative compared to the outside sodium once in badly and it will come in in bucket loads into the cell everybody got that now it is the only ion that's permeable to the membrane at this time only ion so it's moving potassium it can't move it can't go anywhere the other negatively charged ions they can't go anywhere either so sodium is coming in causing the inside of the cell to become more positive look at it drive the charge on the inside of the cell to become more positive so let's put some more positive marks there there we go so the inside becomes more positive remember what happens when it gets up to that point that that positive area doors for sodium starts start to close doors for sodium start to close remember that hmm alrighty now let's talk about our depolarization because that's what we're talking about remember that depolarization depolarization voltage inside the cell reaches a threshold okay going more positive and that is going to open doors for sodium open doors for sodium sodium moves into the cell changing the concentration inside of the cell and the charge all right the inside of the cell becomes more positive and somewhere in the area of about a positive 30 I guess you could say inside of the cell this is a signal to the cell this will set off all kinds of of signaling Pathways inside of the cell to kind of start doing different things maybe set off some genetic markers or many many things inside of the cell but the cell doesn't want to stay this way it needs to get back to its resting state so now we have to talk about repolarization so with repolarization remember doors for sodium have started to close the doors for potassium are opening up and why would potassium want to move remember now the inside has become more positive the Insiders become more positive and that means sodium or excuse me potassium doesn't like that because it's a positive ion it wants to leave now it wants to leave so potassium will start to leave because doors are open for it that potassium leaving and sodium being pumped out because of the sodium potassium pump drives the voltage on the inside of the cell to become more negative all right so potassium leaves sodium is getting kicked out the charge on the inside becomes more negative more negative all right and in fact it's going to get so negative it's going to go beyond where it normally would be so repolarization main points the doors the sodium close potassium moves out of the cell inside of the cell becomes more negative more and there because potassium is leaving sodium is going to be starting to get kicked out it goes from a positive 30 bat way back down now to a minus 90 millivolts a minus 90 millivolts the cell is trying to get back to its homeostatic set point it's homeostatic set point all right now in doing so it gets a little more negative than it normally would all right and remember we called up hyper polarization hyperpolarization now getting back to that hyperpolarized state what's maintaining it the sodium potassium pump the sodium potassium pump all right now that sodium potassium pump is pretty important to us maintaining this whole thing so sodium outside potassium inside we have our negative ions all sitting around good we're getting back to where we wanted to be back at our resting level doors sodium are closed sodium potassium pump is taking care of all the differences that may be occurring across the membrane the inside is back to its minus 70 millivolts ah we're back to where we started from everybody got that back to where we started from and it's doing this so that well maybe we can have another action potential come up in a few moments if the cell needs to have that signal spent a lot of time on this folks go over this again and ask me about it in lab because we're going to play with this in lab okay folks talk to you soon