so in this video we're going to focus on membrane potentials like all cells of your body neurons have a resting membrane potential also called resting voltage and what this means is basically a separation of charges across the plasma membrane so what we find is there's going to be typically more negative charge in on the inside of the cell than outside of a cell and this is due to ions you know whether they're ions like chloride or sodium potassium or even you know charges that you find on things like protein but what's weird about neurons is that unlike most other cells neurons can rapidly change their resting membrane potential and therefore rapidly change their voltage and this is done in an excitable way so neurons will change their voltages in response to a stimulus whether that's another chemical or electrical stimulus we say that neurons are highly excitable so we find is that opposite charges across the plasma membrane are attracted to each other and there's energy that's required to keep these opposite charges separated across the plasma membrane this is what we call us the potential because there's a potential for these charges to move in fact energies liberated when these charges move towards one another now when opposite charges are separated the system has potential energy and this is essentially what we call voltage so voltage is a measure of potential energy that's generated by the separation of charge and we can measure this in volts urmila fords volts so with respect to the human body in ourselves we're gonna measure this on the order of millivolts which is you know basically very very small separations of charge now this potential difference or potential is ultimately what is the resting membrane voltage or resting membrane potential now the greater the charge difference between the points the higher the voltage so when we say that there's a big difference in charge like maybe a veil a tremendous amount of negative charge inside the cell and you know some more positive charge outside if there's a large separation then you have a higher voltage just like a larger potential difference so current is the flow of electrical charge and in this case ions between two points so the current that flows through our cells isn't the same like electrical current through a wire because that's the flow of electrons but rather when you have ions that are flowing through channels across the plasma membrane that's what we call current and this can be used for work so flow is dependent on both voltage and resistance and resistance is basically the hindrance to charge flow now insulators are substances with high electrical resistance and so an example of an insulator would be you know the myelin sheath around your axons it functions as a good insulator because it has high electrical resistance and therefore doesn't allow for flow across that membrane and therefore those ions inside the axons can only flow in you know a very particular direction conductors are substances with very low electrical resistances so for instance the inside of the axon is a really good conductor but the outside layer sheath around the axon myelin is a really good insulator in that way we actually can conduct that current or flow through the axon rather than outside of the axon where the information might get dissipated or lost so Ohm's law gives us the relationship between voltage current and resistance so we see that current or flow is equal to voltage or potential difference divided by its resistance so we say that resistance and current are inversely proportional because if you increase resistance you decrease current if you decrease resistance you increase current and voltage is that potential difference or separation of charge this this law comes up a lot in physiology you know when we talk about blood flow and air flow it turns out that those those equations for flow of air and blood are basically Ohm's law now current is directly proportional to voltage so that we find that the greater the voltage the greater the current and if there's no net current flow between two points with the same potential then we say that voltage is zero so that there is actually no current so you've got to have a potential difference in order for current to exist and these are proportional however current is also inversely proportional to resistance so we mean that means they they vary in opposite direction so if you increase resistance you decrease curve now the role of membrane ion channels in the plasma membrane of your neurons is they basically help to establish this resting membrane potential now there's large selective proteins that serve as selective membrane ion channels and at rest potassium channel are mostly leaking so that potassium flows the most while these neurons are resting and ultimately there are two major types of ion channels we got leakage gate channels and gated channels leakage channels are always open which means they always allow for some current across the membrane and gated channels are conditionally open these might be chemically gated voltage-gated or even mechanically gated which means that due to a change in the condition or states like a chemical or you know a change in voltage or even mechanical stimuli can open up these channels now chemically gated channels are things like ligand gated channels which open only when a specific molecule like a neurotransmitter binds to this receptor channel voltage-gated channels open and close in response to changes in voltage or membrane potential and mechanically gated channels open and close in response to physical deformation you find a lot of sensory receptors are actually mechanically gated so things like you know pressure and vibration or mechanically gated channels so chemically gated channels only open in response to a chemical so like you know you might have a neurotransmitter by and do this receptor here and this can open up a channel that allows for sodium to flow through you know we saw an example of this back at the neuromuscular junction where the nicotinic channels or acetylcholine channels were like this so you needed to see how choline to bind this receptor allow the channel to open up and a lot of for ions to flow as a result voltage-gated channels open and close and responds to changes in voltage so at rest you might find there's more negative charge inside the cell versus outside the cell and that these channels can actually open up if this switches were if you have more positive charge inside the cell that might cause these channels to open up and laughs for ions to flow so when these gated channels are open ions diffuse pretty quickly in fact these ions will diffuse along their electrochemical gradient there's a chemical concentration gradient for these ions you might find that sodium is in higher concentration outside the cell and lower concentration inside the cell and there's also an electrical green as well where you have a separation of charges across the membrane so that also plays a role here with diffusion direction so the electrochemical gradient is basically electrical and chemical gradients combined ion flow creates current and voltage changes across the membrane as a result remember it this is actually can be expressed by rearranging Ohm's law where we can say that voltage is equal to current times resistance so as long as you have a current and there's a resistance present that you have a voltage if there is no current then voltage will be zero so we find that is that voltage represents the movement of ions and the resistance of the flow of those ions and all of these are actually proportional so we find that voltage is proportional to current voltage is also proportional to resistance in this example so a voltmeter can actually be used to measure this potential difference so scientists can actually measure the voltage or separation of charge across the plasma membranes of neurons and other cells for body resting membrane potential or RNP is basically the voltage inside of your neurons while they are not currently generating an action potential and we find that resting voltage can vary from like negative 40 to negative 90 millivolts but it's usually approximately around negative 70 millivolts or so because of a separation of charge we say that the membrane is polarized now this potential difference in charge is actually generated by a variety of factors from one that's generated by differences in the ionic composition of your ICF and ECF you find that in the interest other fluid there's different compositions of ions like you find more potassium and more protein inside your interest other fluid and in the extra southern fluid you find a lot more sodium but far less protein and we also find that due to differences in plasma membrane permeability only certain ions can actually flow at any moment in time and due to the flow of xylem that generates a current which changes the voltage so we can use this voltmeter and basically measure the difference between the inside and the outside of the cell now what does voltmeter telling us here is that relatively the inside of our axon here is more negative with respect to the extracellular fluid and this potential difference or separation of charge is due to you know basically a difference in the composition of the extracellular fluid versus the interest under fluid but remember neurons can change their voltages so that if this were like a red blood cell you know you might stay at negative 70 millivolts for forever as long as that cells living but even because neurons can change their voltage you know this value can can change as a result of ions flowing across the membrane but what changes the flow of ions are basically a change in the permeability or the opening and closing of certain membrane channels so the difference is in ionic composition are laid out here we find that the actress other fluid has a higher concentration of sodium in the intracellular fluid and this is balanced by chloride ions interest other fluid has a higher concentration of potassium than the extra southern fluid this is balanced by negatively charged protein now potassium plays the most important role in resting membrane potential because it's the leakiest wall at rest so we find in here than is that this concentration difference is actually established by the sodium potassium pump so remember the sodium potassium ATPase uses one ATP to pump three sodium's out and 2 potassium zin now if you let this run over time you find that eventually the extra cellar fluid is going to have a significant amount of sodium and the intracellular fluids gonna have a significant amount of potassium the concentrations are pretty well balanced you find about 140 million moles of sodium in the extracellular fluid and about 140 million moles of potassium inside now but there they do have agree in those you see that extra silently there's only 5 milli moles of potassium and intracellular million 15 million moles of sodium so that if you allow these ions to flow down their concentration gradients potassium would want to flow out and sodium would want to flow in however this is the sodium potassium pump which isn't allowing these ions to flow down their concentration green rather this is a primary active transporter which is uses ATP to pump these ions against their gradients that this effect effectively what keeps sodium low inside of the interest letter fluid is the fact that it's being pumped out and what keeps potassium low in the extracellular fluid is the fact that it's being pumped in so differences employment plasma member permeability can occur with opening closing of different channels you find that at rest sodium is slightly leaky through sleekest channels however potassium is 25 times more permeable than sodium at rest so as a result potassium diffuses out of the cell down its concentration gradient and because you're removing positively charged potassium from the inside of the cell it makes the inside of the cell relatively more negative now more potassium diffuses out than sodium diffuses in and as a result the inside of the cell becomes more negative with respect to the outside now this is this is what the resting membrane potential is basically generated by and this this ability of for these ions to flow is actually maintained by your sodium potassium pumps which maintain these concentration gradients even though that leakage channels exist remember the sodium potassium pump uses ATP to pump three sodium's out and two potassium is in so although potassium is diffusing out of the cell it's also being pumped back in by your sodium potassium ATPase --is so just to kind of summarize what this looks like is we have leakage channels as well as your sodium potassium ATPase now if you can isolate the effects of voltage due to the flow of potassium we see that potassium because you're removing positive charge from the cell and potassium is flowing down its concentration gradient the inside of the cell becomes relatively more negative in fact if only potassium flowed it would get to about negative 90 millivolts now if you throw sodium in here too and have a sodium leakage channel which remember is only 120 v is permeable as potassium you're bringing positive charge into the cell as well which makes the cell slightly more positive than if it were just potassium flowing so we find that as a result that our resting membrane potential is actually negative 70 millivolts and it's basically a balanced voltage in response to potassium flowing out and sodium flowing in that this is the voltage we get inside the cell now this concentration gradient is maintained by the sodium potassium ATPase over time so resting membrane potential is basically due to the concentration of ions across the membrane and the permeability of those ions so the mess the membrane potential can change we've created potentials and action potentials remember critic potentials are incoming signals that operate over short distances these things can degrade over time and distance so it gets smaller action potentials are long-distance signals that actually don't get smaller they stay the same size and will make sense this later now changes in membrane permeability are used to basically transmit information so that when we say that electrical information is transmitted along the neuron ultimately this relates to changing the membrane potential and this change is a piece of information that our neurons use to communicate so just some different terms to describe voltage changes we have depolarization and hyper-polarization depolarization is when the voltage of your cell becomes slightly less negative than resting membrane potential and hyper-polarization is when the inside of your cell like the voltage of your cell gets more negative than resting membrane potential and so if you think about these changes ultimately if you're if your neurons are depolarizing this increases the probability of causing an action potential if your neurons are hyperpolarized this actually decreases the probability of producing an action potential so we find here then is basically you know if you have a depolarizing stimulus voltage becomes more positive but then it can actually go back down to rest and if you have a hyperpolarizing stimulus voltages you can become more negative but they can go back towards rest remember rest is this you know voltage that you find when neurons are not conducting any kind of currents deliberately rather and this is the resting current that students their sodium leakage channels and your potassium leakage channels take together makes this negative 70 millivolts