all right in this video we're going to look at how graded potentials can give rise to action potentials so not all stimulation not all depolarizing events are capable of producing action potentials and this again boils down to the fact that action potentials are initiated by the opening of voltage-gated channels and so we've established in the previous video that graded potentials decrease in strength the further away you go from the source of the stimulus so graded potentials decrease in strength and i like to use the analogy of firing a gun so in order to fire a gun or to like fire an action potential in this case you need to place your finger on the trigger now just because your finger is on the trigger does not mean that the gun naturally fires you have to pull on the trigger with sufficient strength to initiate the firing mechanism once you reach this critical point which we will call threshold that initiates the firing mechanism and it doesn't matter with how much additional strength you pull on the trigger you can't pull on the trigger and fire a bullet twice as fast so when we talk about graded potentials and action potentials we're going to use this term threshold any time you hear the word threshold it correlates to the membrane potential at which a voltage voltage-gated channel will undergo a conformational change so when we look at these greater potentials we're focusing on depolarizing greater potentials because only depolarizing greater potentials will get us closer to threshold hyperpolarization tend to be inhibitory they move us away from threshold so we're going to look at two critical areas of the neuron at the dendrites and where the axon meets the cell body and we've called that the axon hill lock or the trigger zone okay and the reason why we focus on this is recall that in the dendrites this is where you have your chemically gated channels responsible for those greater potentials and in the trigger zone this is the first time you encounter voltage-gated channels and you're going to have voltage-gated channels down the length of the axon but the first time we encounter a voltage-gated channel is at specifically the trigger zone so the trigger zone really will act as sort of our integrating center to determine whether or not a particular stimulus is strong enough to generate an action potential or not so let's break down the two types of depolarizing graded potentials so let's look at the first scenario where the graded potential is not strong enough so we're going to call these sub threshold graded potentials so in a sub threshold graded potential keeping in mind minus 70 was our resting membrane potential if i checked the membrane potential locally you would see here that okay the stimulus is fairly strong the membrane potential is at minus 40. okay but as we progress through the cell body on this recording here notice how there's a loss in strength well yes there's a loss in strength because remember some of the leakage of the influx of sodium that has occurred so the intensity of that stimulus decreases the further away from the stimulus site at the dendrites by the time that signal reaches the trigger zone it is even lower let's say it's minus 56 millivolts well minus 56 millivolts is below what is threshold so for the voltage gated and let me actually use red because i tend to use red for sodium so for the voltage-gated sodium channel the threshold is going to be minus 55 millivolts so it's only when the membrane potential goes from minus 70 to -55 at that minus 55 that's when the voltage-gated sodium channel undergoes a conformational change and allows for the additional influx of sodium but for this particular stimulus for this particular graded potential it is not minus 55 it's minus 56 okay now it could be minus 60 it could be minus 69 it doesn't really matter it is more negative than threshold so the voltage-gated sodium channel here remains closed so it's whether or not this voltage-gated sodium channel opens that dictates whether or not a graded potential gets converted into an action potential if we look at scenario number two let's say the graded potential is strong enough so we're going to call those supra threshold graded potentials so if i look at a super threshold graded potential it is going to have a similar profile to the sub threshold its highest at the stimulus site it decreases in intensity as we progress down the cell body to the trigger zone however at the trigger zone there is a notable difference even though you lost signal strength the overall stimulus was stronger so by the time that signal reaches the trigger zone it is still more positive or less negative than -55 so in this case the voltage-gated sodium channel will open and once you open that voltage-gated sodium channel at that point at the trigger zone that is where you convert the graded potential into an action potential so this is a one-way conversion graded potentials could become action potentials but only when the graded potential is supra threshold as integrated or as determined at the trigger zone via the voltage-gated sodium channel so let's place a special focus then on that voltage-gated sodium channel the voltage-gated sodium channel actually has three unique conformations three unique shapes now the author tries to show these unique shapes by talking about activation gates and inactivation gates and if that way of describing the protein works for you great i like to focus on it in a slightly different way so in confirmation number one the voltage-gated sodium channel is closed and so the author shows this by the activation gate being closed and the inactivation gate being open but bottom line is sodium cannot go inside therefore there is no sodium permeability through this particular channel the leaky channels are king and we maintain a resting membrane potential during confirmation number one but confirmation number one is also key to determining threshold confirmation number one can assess the strength of the graded potential so that is a unique attribute of the first confirmation so if that graded potential comes along and it causes the membrane potential which was at minus 70 to reach minus 55 so in other words it's a supra threshold greater potential that's going to cause a change in shape to our voltage-gated sodium channel so the second confirmation that we have for our voltage-gated sodium channel is an open confirmation so the author shows this open confirmation by having this activation gate open and the inactivation gate staying open so there's an influx of sodium down its electrochemical gradient and we see that there's a rapid depolarization because of that influx of sodium so since there was a critical membrane potential that caused a conformational change in this voltage-gated protein to change shape so it opens then by extension there should be a membrane potential that should cause this voltage-gated channel to change shape again into a closed conformation and that's what happens over here when you reach positive 30 millivolts so at positive 30 millivolts for the membrane potential the voltage-gated sodium channel will go into a conformation that is closed and the author shows this by having the activation gate open but the inactivation gate is now closed so it's a unique confirmation from the first one so from the first confirmation it was closed and it could assess the strength of the greater potential the third confirmation cannot assess the strength of a new greater potential so it lacks this ability okay so at positive 30 when the voltage-gated sodium channel closes our permeability to sodium drops now of course what's not shown in this particular figure that positive 30 millivolt is threshold for the voltage-gated potassium channel so at positive 30 the voltage-gated potassium channel opens and there's an efflux of potassium which allows for the repolarization so we see that in this particular figure here we go back to that minus 70. at -70 [Music] millivolts the voltage-gated potassium channel will close so the voltage-gated potassium channel has two conformations either open or closed okay positive 30 is the threshold for this specific voltage-gated channel minus 70 causes the confirmation to change and we close and we are back at the resting membrane potential so the last aspect of the voltage-gated potassium channel that we need to remember is it has to be reusable right it has to be able to assess the next signal and for this we need time the voltage-gated sodium channel over time will move back to the number one confirmation so it's like a sort of a built-in timer this time period where the voltage-gated sodium channel goes from the third conformation back to the first conformation this is known as the refractory period and this is what allows the action potential to be propagated this is what allows the action potential to go in one direction and we'll focus a little bit more on this refractory period in a later video