okay so here we are so in the last video I kind of introduced you to the idea of the action potential and that this action potential this spike is an electrochemical impulse which is the basic unit of information used by the brain and it's the it unit of information that is actually generated by neurons you know it's the the basic the fundamental function of a neuron is to generate these action potentials so in this video I want to spend quite a bit of time talking about how the action potential is constructed why is there an
action potential because it's a absolutely fundamental importance okay so let's go back to this diagram so here we see the the action potentials you see before again just to reiterate why not can't do any harm the the membrane potential when the neuron is kind of doing nothing sits here the rest potential then when an action potential is initiated and in this video and probably an X we'll talk about what initiates it it shoots off very very rapidly past zero up to about plus 40 and then comes all the way back down overshoots a bit and
then goes back to the resting potential until it fires again and it might do that maybe you know in in some of the fast fastest firing neurons that can happen up to maybe you know 250 times per second okay so let's talk about how the action potential is constructed so in order to do that we so this is where okay back to the camera so this is where these kind of prerequisites come in and particularly the idea of ions and the idea of concentration gradients so we're going to look at in particular the membrane potential
first of all and what that is you know I said that the action potential occurs when the membrane potential goes from this from minus 70 millivolts shoots up to plus 40 and then comes back down again you know what what does that mean so in this video we'll cover exactly what I mean by that okay so let's go to the board and look at how all this works okay clean board so first of all we will first of all membrane dead so this is your membrane this is in fact no the plasma membrane of the
neuron and we will say this is inside the neuron and this is the outside of the neuron now in neurons particularly well in all cells there are ions but what's special about neurons is that there is this this imbalance between ions inside and outside of the cell so we'll talk about two ions in this video but we will later will introduce another eye as well so why not yellow so we'll first we'll talk about sodium ions so sodium of course is an a which carry a positive charge and potassium ions which I will do in
oh I don't know I don't know orange will do it potassium iron potassium cause which is okay and you're gonna find potassium ions and sodium ions both inside and outside of the cell however and this is the key point in a neuron what you tend to find what you always find is that there is a very high concentration of sodium ions outside the neuron and a low concentration of sodium ions carrying positive charge on the inside for potassium ions however this situation is reversed and we have a high concentration of potassium ions on the inside
and a low concentration lower concentration of potassium ions on the outside so already your mind should be thinking all this is a concentration gradient it is not just the concentration gradient is also an electrical gradient right because you've got more positive charge more positively charged sodium ions on the outside now in fact if you want to kind of get the total charge carried by sodium and potassium you simply have to add up the charges and what you find is that there is a higher number of positive charge in total on the outside of the membrane
so we would say because right this is high positive charge and a lower number of sodium potassium overall so even though there's a higher concentration of potassium on the inside overall the total positive charge on the inside is lower so if we take the number of positive charges on the inside so on the inside and we subtract okay not very much space the total positive charge on the outside that is going to equal a negative number simply because there's more total positive charge on the outside and this is where that resting membrane potential came from
remember the action potential started at the camera here so the action potential remember started from a negative level so the the way that this imbalance of charges is measured is using voltage that basically simply measures the the the the uneven distribution of charge the what's called the charge separation from the in between the inside and the outside neuron so what we say is that overall we say that a neuron is negatively charged on the inside and that's simply because of the uneven distribution of positive charges with more positive charge on the outside overall lower positive
charge on the inside okay good so so let's go back to the board and let's kind of formalize this a little bit better so we'll clear this so so now we can draw a nice graph and remember this should ring a bell ring up the hell so we had zero here and if you recall the neuron when it wasn't doing anything had a membrane potential kind of down here which was minus 70 millivolts again millivolts is just the units of measurement used to to measure that separation charge separation so so now you know why that
sits there this is called the resting potential and all neurons and this is again a slight simplification but we'll talk more about this in a later unit when a neuron is doing nothing the neuron is resting its membrane potential will tend to sit around minus 70 because of this charge separation that's not always the case and again that will become important later so what's happening then with the action potential so let's draw an action potential so you're now familiar with what an action potential looks like I think that your attention should be should be red
shouldn't it of course it should so the neuron membrane potential and we've got time here the units of time aren't important sure the neuron sets over time no once it's know once its resting potential doing nothing and then for some reason suddenly the membrane potential jumps up to around 40 millivolts and then just as quickly comes back down overshoots a bit so what have we learned here so the action potential is a reversal of the membrane potential the membrane potential goes from being negative because of this uneven distribution of positive charges to being positive very
very briefly and then comes back down again so this is the way the neuron generates information is by rapidly flipping its membrane potential from negative positive and then back to negative again so the question then is how is that action potential generated or in other words what makes the membrane potential suddenly flip to positive and then back it's a negative again so to explain that we need to think about channels channels are what ions use to get through the membrane now if you remember from your biology class about the structure of the plasma membrane it's
it's fatty which means it all hydrophobic it doesn't like water like a rabid dog and the Cujo gave me nightmares that did anyway Stephen King anyway you just put me off anyway yes the heat map of state talking about oh my god dog saliva scent burner doesn't it scent Bernard such a sweet dog it's like a bit by a bat that's saying stay away from bats clearly we've learned that haven't we yes so so what causes yes I in general that was it so so normally because the the membrane is fatty it doesn't like water
that was where I got to Cujo and he doesn't like ions so ions can't get through the membrane this is actually how because of course if you think about it the sodium ions because they're imbalance high on the outside low concentration on the inside they want to move down that concentration gradient don't they but they can't because the membrane is in the way that's what keeping the the charge separation there so in order to reverse the membrane retention or to initiate one of these spikes we need to introduce the idea of channels and in particular
a special type of channel and a channel is a protein a special type of channel voltage-gated potassium channel and voltage-gated sodium channels so let's look at these draw these see this make sense okay so again we will have our membrane so here is our membrane thus and of course it is a fatty membrane sort of main draw so normally sodium ions they're unable to get through the membrane and the same applies to potassium ions as well but a channel is a type of protein that as you might expect I think let's draw a voltage-gated sodium
channel first so a sodium channel is simply you kind of sliced through it it would kind of look like this ish now this is a channel that is in a closed conformation so so channels are actually quite complicated but we will keep it simple and say let's get rid of potassium for the time being we can it's just confusing us there we go so this channel is a gated channel so gated means it has a gate in other words the gate can be open the gate can be closed the gate is closed sodium cannot get
through no however the channel can also be in an open form so the gate is open this and the sodium ions can then pass through now this is the key point what determines whether a voltage gated sodium channel is open or closed well the clue is in the name it's gated by voltage so let's go back to our drawing of the membrane potential so you'll get used to this graph after a while we're using it for a while so we have our usual points go here's our zero and of course you're now familiar this is
our resting potential we'll just abbreviate that to our peep now sodium channels they open at a specific membrane potential so when the neuron is at its resting potential so it's sat at kind of minus 70 these voltage-gated so these voltage-gated sodium channel voltage voltage-gated sodium channel is closed which means sodium ions cannot travel through the channel cannot travel down that concentration gradient however this is where it gets interesting and I'll do this in yellow so important there is a particular membrane potential a particular voltage at which these voltage-gated sodium channels that is around minus 55
millivolts so if the membrane potential for some reason which we will get to if the membrane potential for some reason says reaches this minus 55 then the voltage-gated sodium channels will open now let's think about that so what would happen if suddenly all of these voltage-gated sodium channels opened well you need to think about the concentration gradient of these of the sodium ions so we know that this is high concentration of sodium ions on the outside of the cell and a low concentration on the inside so if suddenly all of these voltage-gated sodium channels opened
what would happen to the sodium ions of course they would suddenly start flooding in down their concentration gradient through the channels and into the cell and that is indeed exactly what they do however can we then explain well then think about what's gonna happen to the membrane potential now remember when we first introduced the idea of the membrane potential the idea was that it's the imbalance of positive and negative charge which generates the membrane potential this charge separation and there is more positive charge on the outside compared to the inside which gives the the overall
the neuron the inside of the neuron has an overall negative charge compared to the outside so what happens when positive charge starts ie sodium ions starts flooding into the neuron through these freshly opened voltage-gated sodium channels well the membrane potential is going to go up it's going to go more positive right so we can see this draw this on the diagram voltage-gated sodium channels they open here the membrane potential suddenly shoots outwards whoa what is that that of course is the first leg or the first arm arm or leg of the action potential now of
course this will keep going up and up and up in theory until there is a the charges the concentration of sodium in such a little bit more complicating that but you can imagine it would keep going up until the concentration of sodium is balanced right until it reaches an equilibrium but actually something interesting happens around about this point here when the membrane potential reaches about 40 millivolts what you think happens there well the sodium stat channels actually start to close so then that they're not open permanently they actually start to close again so the open
briefly will allow sodium in then they closed but then something else happens another type of voltage-gated ion channel opens this is the voltage-gated potassium channel so here this point voltage-gated potassium channel open now what's going to happen there well let's think about it we know that the sodium channels the voltage-gated sodium channels have closed so so sodium has stopped flowing into the cell and now these voltage-gated potassium channels have opened now let's think about the concentration gradients involved here so again there's a concentration gradient between the inside and outside with potassium ions just like those
with sodium ions however it is in the opposite direction so there is a higher concentration of potassium on the inside so potassium ions flow out so this time in the opposite direction to sodium so potassium ions flow out takes positive charge out of the cell what happens to the membrane potential it comes back down again goes all the way back down it will actually overshoot a bit and then there are systems again the potassium channels will then close again and then there are these special pumps that will kind of restore everything back to the resting
potential so that is the action potential then is the mechanics the basic mechanics of the the action potential when the membrane potential reaches around 55 minus 55 millivolts voltage-gated sodium channels suddenly open sodium rushes in membrane potential shoots up goes up to about 40 millivolts then the sodium channels shut potassium channels then open potassium rushes out of the cell taking positive charge with it and the membrane potential shoots back down overshoots a bit and then it settles back to its resting potential that is an action potential you
