okay in this video I'm going to talk about action potentials now remember action potential is the principal way that neurons send signals long distance and really rapidly so we talked about neuronal communication it involved action potentials these occur only briefly within muscles and axons of neurons and we find is actually the action potential itself is a brief and rapid change in membrane voltage that could be as great as a hundred millivolts difference so action potentials differ than graded potentials because they don't decay over time and distance whether these action potentials are regenerated as they move so that's why they maintain the same size and shape as they travel now in neurons you're refer to these as nerve impulses and the action potential itself is due to a change in current which changes the voltage and the channels that open that can change your current are the specific voltage-gated channels now at a resting state all of our voltage-gated channels are closed so the voltage-gated sodium channels are closed voltage-gated potassium channels are also closed so the only ones that are open are leakage channels remember we have sodium leak and a potassium leak and potassium leakage is far more leaky than sodium so we talked about how because potassium flows out of the cell due to its electrochemical gradient how that makes the inside of the cell slightly more negative and this is actually what maintains the resting membrane potential now each sodium channel voltage-gated channel rather has activation gates in an inactivation gate the activation gates are closed at rest but open with depolarization which allows for sodium to flow and then the inactivation kick gates are open at rest but then blocked the channel once it's open to prevent additional sodium from entering so think of the activation gates is basically allowing sodium to flow and then the inactivation gate stopped at sodium so will we find in here some of the key players are in the action potential are the voltage-gated sodium channels and the voltage-gated potassium channels for the voltage-gated sodium channel it's closed at rest right we're saying that you find a lot of sodium outside the cell the inside of the cell has less sodium so if sodium wants to flow down its diffusion gradient into the cell however because this channel is blocked by the activation Gate sodium can't flow but if there's enough depolarization these activation gates can swing open and allow for sodium to rush into the cell so that as sodium rushes in it causes further depolarization but after a certain amount of time the inactivation gates blocked that channel and they prevent further sodium from rushing in so we find is that the activation gates open due to a change in voltage at a sufficient amount of depolarization and the inactivation gates closed due to time so they actually put clothes pretty soon after they open but these actually get reset you know later on now the voltage-gated potassium channels are you know closed at a resting state and they open at a very depolarized voltage once they open allows for sodium to flow out I'm sorry potassium rather so potassium flows out of the cell into the extracellular fluid down its electrochemical gradient which makes the inside of the cell more negative because you're removing positive charge now eventually these voltage-gated potassium channels also closed at a certain voltage so each potassium channel has one voltage sensitive gate it's closed at rest and opens slowly with depolarization so if we zoom in on this these voltage gated sodium channels remember we have we have an activation gate and an inactivation gate Remi the activation gates are closed at rest while the inactivation gate is open however this is the activation gate still blocked this channel if there's enough depolarization sodium flows in and depolarizes the instead of a cell but after a certain amount of time the inactivation gate closes this channel and prevents further sodium from rushing it now at a certain voltage these activation gates reset in the inactivation gate also resets now in terms of the events of the action potential we find that at rest only the leakage channels are open here so that because of potassium leakage the voltage is fairly negative because potassium is leaving the cell through that leakage channel but if there's enough depolarizing stimulus these voltage-gated sodium channels swim open and so we call this the depolarization phase of the action potential so when these voltage gated sodium channels open depolarization allows for sodium the rush in the cell sodium activation gates open and sodium influx causes even more depolarization which causes the voltage to go from like negative 55 all the way up to positive 30 millivolts because as sodium rushes in the cell it's bringing in more positive charge so that's what we're seeing here is that the rising phase of the action potential is due to positively charged sodium rushing in a Cell sodium Delta flow from this voltage-gated channel now because these activation gates have opened and because they're open now that the channel is open allows sodium to run down its electrochemical gradient now eventually this inactivation gates going to close and it closes due to time dependent manner and so once these inactivation gates closed near the peak of the action potential it prevents further depolarization so it you know determines the peak here so you can't just depolarize forever and get positive forever so right around that peak of the action potential this is what the repolarization phase occurs this is when the voltage-gated sodium channels start in activating to the did those inactivation gates and right around the same time the voltage-gated potassium channels open so sodium channel inactivation gates closed membrane permeability to sodium declines so that eight action potential stops rising in terms of voltage because you're preventing sodium from rushing into the cell here now right around the same time voltage-gated potassium channels start to open due to their voltage that they open at and so what we find is that they allow for potassium to now flow out of the cell and because you're having potassium exit down its electrochemical gradient it causes repolarization which is when the membrane returns back to a resting voltage so think of repolarization is basically when voltage starts to get more negative back towards resting membrane voltage and so we find is that the falling phase or the repolarization phase of their action potential here represents an e flux of potassium out of the cell and because you're removing positive charge it makes our voltage more negative back down towards a resting state however what happens is that because so voltage-gated potassium channels open here some potassium channels remain open allowing excessive potassium efflux and that because of excessive potassium loss we get basically a membrane voltage that becomes even more negative than its resting state and what we call this is hyper-polarization so hyperpolarization is when the membrane dips below resting voltage but this is what resets the voltage-gated sodium channels back into their resting state so that way they actually can be able to act be activated again so hyper polarization is when when voltage of your neuron here is even more negative than rest this is because so much potassium is left it's actually made the inside of the cell even more negative than a resting voltage so in terms of the events of the action potential we have rest which is due to leakage channels we have the rising phase or depolarization phase which is due to the opening of voltage-gated sodium channels once those activation gates open sodium can actually rush in causing depolarization to occur and then near the peak of the action potential the inactivation gate of your voltage-gated sodium channel closes right around the same time that the activation gates of your voltage-gated potassium channels open which allows for potassium now to exit the cell but because you're removing positively charged potassium out of the cell it makes our voltage negative back down towards rest but because so much potassium flowed out of the cell there's a phase here called hyper polarization which is essentially when the inside of the cell becomes slightly more negative than the rest and this is important because if first of all it helps to reset the voltage-gated sodium channels because the inactivation gate actually opens again which then allows for our voltage-gated sodium channels to be stimulated once more now eventually these voltage-gated potassium channels will close near the end of hyperpolarization and at that point resting membrane voltage will slowly go back down towards you know it's negative 70 millivolts or so away from hyper-polarization and around this time the this gated sodium channels are ready to be activated again and you can generate another action potential so what we find then is that repolarization resets electrical conditions but not the ionic conditions so in order to re-establish these concentrations sodium potassium pumps are working in over time in fact they're always pumping sodium and potassium even during the action potential and their function here is to not really contribute to the action potential directly but rather the sodium potassium pumps maintain the concentration gradient that allows for the action potential to occur you know if we didn't have a gradient of more sodium outside the cell and less inside the cell then you wouldn't get the rising phase of the action potential because sodium wouldn't flow stay with potassium if you didn't have potassium in excess concentration within the cell you know it wouldn't want to flow out so we need these concentration gradients and order for action potentials to occur but what's also important to note is that each action potential doesn't allow for so many ions to flow that it changes the you know concentration gradients significantly in fact it would take hundreds if not thousands of action potentials before you ever affected concentration significantly so that each individual action potential doesn't change the concentration gradient it just changes the voltage but over time we need these sodium potassium pumps to maintain those gradients so remember how they work is they use the chemical energy in ATP to basically pump sodium out of the cell which is why it's in higher concentration outside the cell than it is in and simultaneously pumping potassium in which is what makes potassium and higher concentration within the cell