hi there everyone and welcome to our unit on nerve and muscle physiology in this lecture we're going to give a very brief overview of membrane potentials before we dive into the nitty-gritty details of resting membrane potentials and action potentials and here are the unit objectives and what we want to cover today so i briefly want to recap here the difference between diffusion potential and equilibrium potential so we talked about both of these concepts in our last unit and the goal here is to just do a brief summary so remember diffusion potential is that potential difference that's going to be generated across a membrane when an ion moves down its concentration gradient okay an ion can only move down its concentration gradient if the membrane is permeable to that ion so if the membrane is not permeable you will not have any diffusion and if you do not have any diffusion you're not going to have a diffusion potential okay so when you have that ion moving from the inside to the outside or from the outside to the inside you have to think of the ion as always carrying an electric charge that's by definition and therefore you have movement of electric charge that is current and if you have current you're going to have potential equilibrium potential it's a little bit different and when we say equilibrium potential we mean that when an ion is diffusing right from one end to the other down its electrochemical gradient if everything is held constant eventually you're going to reach a stage of equilibrium now if once we reach that state of equilibrium if we measure the potential difference across the membrane that potential difference at equilibrium is going to be called your equilibrium potential and we can calculate the equilibrium potential for each individual ion if we know the permeability of the membrane to that ion and if we know the concentrations across both sides of the membrane we're going to use the nermist equation in order to calculate the equilibrium potential so let's do a quick recap here so let's imagine that we have a um so here's a cell okay and let's make that cell you know permeable um right and remember that we're going to have sodium ions on the inside right lots of sodium sorry potassium my apologies a lot of potassium on the inside okay and we're also going to have a lot of sodium on the outside so that's something very important for you to remember sodium outside potassium inside now what happens is that potassium is going to want to travel from the inside to the outside now the movement the movement of these ions right is going to generate that current because again it's a positive ion and that positive ion is moving right so that electric charge is moving as a result of that the inside of the cell essentially right is going to be losing positive charge when you're losing positive charge that's going to make the inside of the cell negative relative to the outside of the cell that is what we call the diffusion potential now if everything is held constant right this diffusion is not going to happen forever right if permeability is constant if everything is constant what's going to happen is eventually we're going to reach a stage of equilibrium now once we reach that stage of equilibrium if we measure the potential difference across the membrane at this time point we're going to get a value of x millivolts that value is then going to be called the equilibrium potential so slight difference between the terms diffusion potential and equilibrium potential just make sure that you review both of these concepts and they become clear to you in your minds and what's the difference between both so here's another recap here um and so you you see we have a nerve fiber and the re in red in red your um this is going to be your chemical gradient right that's in red while what is in black is going to be your electric gradient okay and so you see here for the scenario of potassium you have a pretty strong chemical gradient pushing potassium to the outside but you also have an electric gradient pulling potassium inside and that's because the inside of the cell is negative relative to the outside so this is going to generate a diffusion potential however if we reach a state of equilibrium and we measure that potential difference across the nerve fiber we're going to find that's going to e that it's going to equal minus 94. now if we do the same with sodium imagine we only have sodium and the membrane is only permeable to sodium you see here in red this is the chemical gradient pushing sodium in you also see that we have a an electric gradient pushing sodium out now i want you to note this difference that now right the inside of the neuron or the nerve fiber is positive right because sodium is moving in the opposite direction potassium was generally moving this way so the cell is losing positive charge while now sodium will be moving this way which means the cell is gaining positive charge okay so a few points here to note think about multiple gradients it's not just one gradient moving in a single direction you have more than one gradient and they're moving in different directions and you have to um just consider both and and and think about which one is going to be the stronger driver or where is the net movement of that ion going to be now in this scenario of sodium in which the membrane is only permeable to sodium if we reach a state of equilibrium and if we measure the membrane potential across the cell membrane at equilibrium we're going to find that this potential is going to equal plus 61 millivolts for sodium we call this the equilibrium potential of sodium and we call this the equilibrium potential of potassium okay important terms to be aware of so a membrane potential is going to arise if there is a difference in electric charge difference in electric charge between both sides of the membrane this can result from either passive ion diffusion right like what like when you have an open sodium channel for example right an open sodium channel or an open potassium channel that's passive ion diffusion or or you can have electrogenic pumping like what remember the sodium potassium atpase what does the sodium potassium atpase it pumps three sodiums out of the cell and two potassiums into the cell therefore it has a net negative balance right three out to two in therefore you're always going to be losing a positive charge and that's why we say that the sodium potassium atpase contributes to the negative membrane potential now potassium is pretty important and we're going to talk about this a little more in future lectures but potassium is pretty important because basically the cell membrane is just more permeable to potassium than sodium because we have what are called the potassium leak channels we don't have leak channels for sodium but we do have leak channels for potassium making the membrane at rest recall or notice here on my words making the membrane at rest a lot more permeable to potassium than it is to sodium and that is going to have um an impact on the resting membrane potential as we will learn in the next lecture um and this is just a recap of that sodium potassium pump that we went over again remember we are moving um this is your inside this is your outside we're moving three sodiums out we're pulling two potassiums in therefore you have a net negative effect on the resting membrane potential all right and with that we end this lecture on the basics of membrane potentials thank you