hey everyone welcome to professor long selectors in anatomy and physiology again where this coronavirus shut down so these I'm doing these lectures for my students in my classes if anyone out there in YouTube land finds these helpful outstanding I'm glad it helps you hope you can learn something from it anyway I am doing these quick impromptu videos this video is intended for my students taking my 2401 or human anatomy and physiology one course and we're working on the nervous system this video is going to be the fourth video in the series of nervous system lectures the last video I completed all this I just finished that video but I want to keep them to around 20 to 25 minutes every now and then I got to go a little longer anyway so what we were talking about in the last lecture was the establishment of the resting membrane potential why is it that neurons are sitting at minus 70 millivolts which is the unequal distribution of sodium and potassium ions there are other ions inside the cell that can affect things like action potentials and other things but they are minor players when you do all the equations the mathematics behind it anyway and the experiments bear that out but what we saw is anytime we open a sodium ion channel a gated channel that we can alter resting membrane potential because of the unequal distribution sodium ions come rushing in the cell become more positively charged any time I open potassium channels because of the concentration gradient potassium exits the cell this cell will hyperpolarize okay becomes more negatively charged inside and the sodium potassium pump is always fighting this I always imagine being in a boat in a lake or something and let's say the boats leaking a bucket of water every you know 10 seconds well then you're gonna have to scoop that bucket let it fill up and get that water out the leaking is the leaky channels you are the pump and requires energy to pump the ions out now if you can do that and you can keep up with that you know let's say every second you have to scoop a cup of water out in order to keep up what if someone opened a bunch of hatches in the boat at the bottom of the boat and let water rustle fast then you can keep up with then that boat would start to take on water okay so that would be sort of the sodium ion channels opening the gated channels and we're letting sodium rush in faster than the sodium potassium pump can keep up with but we see changes in resting membrane potential students have to understand the concentrations of which ions where do we have more sodium where do we have more potassium where's the higher concentration which direction the ions move you need to understand that leaky or passive channels let the ions flow through to see the sodium potassium pump is trying to fight them holding it steady resting membrane potential for our neurons that we're studying right now it's minus 70 millivolts and what you also have to understand is if I open sodium channels because of the concentration gradient the cell will always move more positive and depolarize sodium would come rushing in faster than the pump can keep up with and we'll see a slight positive change in voltage by open potassium channels every single time based on the potassium gradient potassium will exit the cell the neuron will become more negative in charge because I'm taking away positive charge from an already negative situation so now we establish that and we establish the different types of ion channels chemically a voltage and mechanically gated channels what we need to understand is how do action potentials occur so in my note set I have a little table here and we're going to talk about some things here and we're going to fill this in in order to fill it in and we'll do this eventually I need to kind of review some of this or actually talk about the neuron itself and we're gonna measure the action potential diagram and a multipolar neuron so all this was left over from the last video if you didn't if you don't understand all that you certainly need to go back and watch video number three okay the nervous system lecture number three which is ion channels and ion movement okay so get all this off here I got a newer bigger board although it doesn't stick to the wall as well I'm gonna have to figure something out but anyway um so let's look at our multipolar neuron okay i'm gonna draw this I'm going to draw a multipolar neuron across the top I'm going to draw the action potential down here and how and why everything's happened okay so here's our multipolar neuron these would be the dendrites if you don't know the anatomy of the multipolar neuron go back and watch the first or actually the second nervous system lecture but I have a bunch of dendrites here I'm actually going to race this dendrite I'm just gonna make a big empty piece of the cell membrane here because I need this okay it's gonna look goofy but just bear with me neurons can look brother goofy the axon hillock we have the axon coming all the way down here and then at the very end the neuron will split into a bunch of Tilo tendría then ends up in synaptic knobs as you guys should know from studying the nervous dub the neuromuscular Junction the synaptic knobs are filled with thousands of these tiny vesicles these are way too big I'm not drawing anything to scale but those little vesicles are going to be filled with chemicals called neurotransmitters and so there's a whole bunch of neurotransmitter in here thousands of molecules each when one of these neurotransmitters one of these vesicles fuses with the cell membrane and what exocytosed the neurotransmitter over here would be the dendrites of my next multipolar neuron making their massive dinner dendritic tree and then this neuron will go and form to load indriya and synaptic knobs and to communicate to the next neuron and so on and so forth and rather ugly neuron down there but you can understand how neurons communicate across small gaps called synapses or synaptic cleft and the postsynaptic membrane this is our excuse me sorry my allergies are killing me and I'm not going to start this video over look as the actual potential arrives down here we talked about voltage-gated channels in the last video we have some voltage-gated calcium channels the concentration of calcium ions CH e plus let me rewrite that a little more neatly for you the concentration of calcium is much higher outside the cell than it is inside and most of the neuron doesn't have a lot of calcium ion channels on it but we do have some voltage-gated calcium ion channels all over the synaptic knobs I'm only drawing one channel but there are thousands in a little patch of memory these are voltage-gated calcium ion channels and like I talked about in the last video when a voltage arrives they will open and because of the concentration gradient calcium will rush into the cell cause the synaptic vesicles to fuse with the synaptic membrane of this presynaptic neuron the neuron before the synaptic cleft the presynaptic membrane or the presynaptic neuron when depolarized will release neurotransmitter into the synaptic cleft that neurotransmitter will flood the synaptic cleft if released and because of the nature of the voltage all the synaptic knobs would release neurotransmitter and then all of these receptors on the postsynaptic membrane if these are the correct receptors for that neurotransmitter then they will bind and it will alter the resting membrane potential of this neuron those receptors can be attached to chemically gated sodium channels chemically gated potassium channels in some cases chemically gated calcium channels depending on what we're looking at at the human body usually sodium and potassium channels we're going to keep it simple for now alright so we know that neurons can communicate to a skeletal muscle cell at a neuromuscular Junction across a synaptic cleft if you don't understand that then you need to go back and watch the neuromuscular Junction videos so anyway that will be what muscle lecture three or four so now we understand on this and I told you before if you watch those videos once you understand one synapse you understand them all we looked at a neuromuscular Junction that releases the neurotransmitter acetylcholine the receptors are acetylcholine receptors and there's an enzyme that lives in the synaptic cleft called acetylcholine esterase that breaks down the neurotransmitter all synapses work the same 'we essentially the name of the neurotransmitter can change the name of the receptor changes the name of the enzyme and the synaptic cleft changes and then whether that neurotransmitter is excitatory causing the cell to depolarize or inhibitory causing the cell to hyperpolarize depends on the nature of the channel and which are the receptor and which ion channel is either is or is attached to nonetheless today's lecture the action potential and if you want to follow along I'm going to be going along the page of 66 and 67 in my notes set so today's lecture we're going to talk about how do action potentials occur in the neuron and how are they propagated or propelled all the way down to the end of the neuron because it turns out once we start an action potential in a neuron we can't stop it it's because of the nature of the channels and their distribution from each other it's going to cause the action potential to spread all the way down the axon every single time so bear with me while we do this so I'm not going to do leaky channels we understand the leaky channels and the sodium potassium pump are doing their job keeping the neuron at rest and if I were to draw an action potential diagram down here the time would be done in milliseconds I'm not going to measure them we'll talk about the length of neuron action potentials over here I'm going to measure the millivolts thousandth of a bolt these ions carry tiny charges it takes thousands of ions to move just a fraction of a millions but when we do the measurement like I talked about in the last video the neuron is sitting at minus 70 millivolts we call that resting membrane potential I reavie ate that as our MP resting membrane potential and then there's a reason I'm going to put minus 90 millivolts down here and I'm going to try to do this sort of this 20 millivolts so that would be honest 50th well I'm not going to do this quite to scale let me move my minus 70 a little bit further down don't alter your if you've already done this just know that it's not going to be the scale let's say this is about zero millivolts and this is like plus 30 millivolts again I know that's probably not to scale but just bear with me and right about here minus 60 millivolts minus 60 millivolts we're gonna call the threshold or the threshold potential because once we cross that threshold there's no turning back and it was explained to me by one of my very fantastic sometimes eccentric but really brilliant professors at taught my anatomy and physiology at UTSA dr. Nichols Grimes I hope he's doing wonderfully well it's been many years since I've spoken to him but he was a huge influence on me and a brilliant lecturer so anyway the reason they call this the threshold is because I guess an ancient law a couple that said their vows was not considered married until they crossed the threshold of the men's abode so in ancient England if you got married but you didn't move in with the man you weren't technically married it was kind of consummated not by a physical act or a sexual act like people think of these days but it was consummated by the woman moving in with the man supposedly once you cross that threshold the threshold being the doorstep the door jamb at the front of the home once you cross that step there's no turning back so we call that the threshold potential because once we cross this the action potential is going to occur I'll explain to you why now when I look at this neuron and again let me just erase this a goofy-looking theme right here but I have a multipolar neuron and I need some space here to draw on it okay let's say everywhere that I make a blue line I'm gonna have a chemically gated sodium channel so any blue marker or any blue mark on here is going to equal a chemically gated sodium ion channel okay now I lost the lid to my marker forgive me for a second but I'm not losing this ten minutes sorry I told you these were all impromptu and they are they're not very good so everywhere that I have a purple line let's say is a chemically gated potassium channel if I make a green line that's going to be a voltage-gated sodium channel it will open and close do the Kim to the voltage is changing and let's say anywhere that I do this funky-looking red color that is a potassium I'm sorry a voltage-gated potassium channel I apologize for the knocking of this thing I'm really gonna make a need to stick something back there's piece of sponge or something but for the time being forgive me so now I have all these different ion channels that I'm gonna sort of demarcate with certain colors so everywhere that I have a blue line is a chemically gated sodium channel so let's say I have a couple of chemically gated sodium channels over here throughout the dendrites and the soma of my neuron okay I have all these chemically gated sodium channels I also have some chemically gated potassium channels spread out all over the neurons sometimes in the very close proximity proximity of my sodium channels and I mean there's thousands of these things all over the dendrites and the axon I can tell you because we didn't are on morphology and I was an undergrad I worked in a lab where we actually saw where these things were there's a thing on a dendrite called the thorny excrescence and where those little thorny looking things stick out turns out there's always a synapse there and there's tons of these channels anyway so I have all these chemically gated sodium potassium channels blue is sodium purple is potassium okay now when I get to the axon hillock I can have some chemically gated channels I'm still gonna have some of these chemically gated sodium channels but that's about where they start to end I don't have a lot of chemically gated channels down here and it makes sense I'm not going to because another neuron is not going to synapse down there now this is where I start switching to the voltage-gated channels I start having abundances of voltage-gated sodium channels all throughout the entire axon and there are a certain distance apart until I get all the way down to the voltage-gated calcium channels down there so I would have these voltage-gated sodium channels literally all the way down my neuron all the way up to the calcium channels okay and then I'm gonna have voltage-gated potassium channels also all down on my neuron here and these things are all interspersed with each other like this so essentially we have a lot of chemically gated channels all over the soma and the dendrites and going up to the axon hillock I have voltage-gated channels from the axon hillock all the way down I hope these colors are appearing for you I really can't see the color very well on my monitor my screen but nonetheless so we're set up with all of this and we also understand from the last video that every time I open a chemically gated sodium channel if I do nothing if I don't stimulate this neuron at all the cell will be sitting here at minus 70 millivolts and the voltage will not change very much at all might waver a slight bit but nothing traceable now let's say I have another neuron here and this neuron has the ability to dump a chemical that excites sodium ion channels so when it dumps this neurotransmitter into the synaptic cleft this blue neurotransmitter can start to bind to these chemically gated sodium channels as you know when I start to open or activate a chemically gated sodium channel so I'm just gonna put kim sodium here for a chemically gated sodium ion channel okay it's not a very good sodium but nonetheless I know that the cell will start to depolarize and the more of those channels I have open and the longer they are open the more positive charge that's going to allow to flow into my neuron and that positive charge is going to cause a slight change in voltage here if I closed if I remove the neurotransmitter I stop releasing it it's broken down and those channels closed then the sodium potassium pump would restore everything back to rest and the neuron would be sitting here at rest again now since I did not reach my threshold here this is a small change in voltage but it's not enough to stimulate the voltage-gated channels to open because it's a small local event and not enough sodium ions are going to diffuse down that far to cause enough voltage change to open the first voltage-gated channel and one of the reasons is this as we go through these channels there's a table in my notes set it's on page 66 the leak channels their stimulus to open and close there is none they're always open for chemically gated sodium channels their stimulus to open is usually what we call an excitatory chemical or an excitatory neurotransmitter the comment is they can depolarize to threshold depolarize to threshold I'll fill in this table or put some notes on a worksheet for you guys that are in my class chemically gated potassium channels that what stimulates them to open and close is the presence of an inhibitory chemical an inhibitory neurotransmitter or something that will hyperpolarized a cell and so that will cause ions to flow out of the cell causing the cell to hyperpolarize forgive me for a second I apologize but I'm not losing this video because I don't want to reshoot this we're 20 minutes in and I had to let my dog in the house anyway so potassium gated channels are always going to hyperpolarize the cell or inhibit it they will cause it to move away from resting membrane potential voltage-gated sodium channels they actually open the stimulus to open is at minus 60 millivolts or threshold voltage gated channels if I stimulate a piece of membrane where one of those channels is to hit minus 60 millivolts that channel is going to open the nature of these channels is that they stay open until so much IO and rushes through them that they close at plus 30 millivolts so voltage-gated sodium channels open at threshold minus 60 and close at plus 30 millivolts technically they start closing at zero millivolts but by the time they all closed so much sodium is rushed in we actually kind of have a little overshoot to about plus 30 millivolts by the time they closed voltage-gated potassium channels begin opening so we're in this range and as potassium starts to rush out of the cell we've closed all the voltage-gated sodium channels now potassium is going to rush out of the cell well if the potassium rushes out that would actually bring the cell back down if I'm adding sodium in and potassium out it could bring the cell back towards resting membrane potential or repolarize it and the voltage-gated potassium channels begin closing at minus 70 millivolts they close so slow that excess potassium leaks out and they'll actually hyperpolarize all the way down to minus 90 millivolts for voltage-gated calcium channels you simply need to know that they are present at the synaptic knob and cost neurotransmitter release for now okay so I'll go over that table in detail on the worksheet so you guys know what to write in there and we're on the bottom of page 66 we're going to talk about graded or sub-threshold potentials a graded potential is any change in resting membrane potential that does not reach thresholds they're also called sub threshold potentials because they're below threshold we can have excitatory gated start excitatory sub threshold potentials like anytime I open a chemically gated sodium channel I'm exciting this cell I'm getting it closer to firing off so it's starting to get excited kind of like if you're pulling up closer and closer and closer to Fiesta Texas with your kids they start getting excited and the closer you get to the gate the more excited they are once you open the gate and they're in they're gone when fire an action potential so we call those excitatory postsynaptic potentials at this postsynaptic membrane my presynaptic neuron my postsynaptic neuron at the postsynaptic membrane if I open chemically gated sodium channels I'm getting an excitatory postsynaptic potential I've opened chemically gated sodium channels so I get what's called an EP SP epsp let me rewrite that a little bit more neatly stands for excitatory postsynaptic potential excitatory postsynaptic potential epsps epsps are always going to cost the cell to become more positive inside because we open the sodium channel sodium rushes into the cell the cell becomes a more positively charged or said to be excited it's getting closer to threshold now if I close that channel sodium potassium pump restores us back to resting membrane potential now what if I opened a chemically gated potassium channel let's say this neuron releases an transmitter that binds to chemically gated potassium channels and opens them because of the unequal ion distribution potassium will exit the cell i'm already negative if I lose positive charge then the neuron starts to become more negative the more channels I open and the longer I open them or have them held open the more potassium exits itself the more negative it will become and I will move away from the threshold if I close the channels sodium potassium pump will restore us right back to rest okay now the further away I go from the threshold normally it takes a 10 millivolt change to reach threshold but if I started right down here if I opened a whole bunch of potassium channels and then I dumped a whole bunch of an excitatory neurotransmitter that open sodium channels if it only took ten millivolts to go from rest to threshold to fire an action potential but I start down here let's say I'm at minus 85 and I have a 10 millivolt change is that gonna get me to threshold no so essentially every time I open potassium channels I'm just gonna put kim potassium here chemically gated potassium channels opening allow potassium to exit the cell hyperpolarizing it making it harder to reach threshold or inhibiting us reaching threshold under normal conditions so they call those ipsps inhibitory postsynaptic potential they are a type of graded potential I have depolarizing graded potentials and hyperpolarizing graded potentials in depolarize and graded potentials they are excitatory epsps open sodium channels sodium enters the cell the cell becomes more positively charged or said to become excited moving closer to threshold ipsps are hyperpolarizing sub-threshold potentials happen when i open chemically gated potassium channels potassium exits the cell the cell becomes more negatively charged or said to be inhibited hence the term IPS okay knowing that I know I can mess with the neuron and what happens is there are many many synapses all over these dendrites and even at the axon hillock and these neurons are releasing different neurotransmitters opening different channels and some of them might start to excite it but then a lot of potassium can bring it back down and they're kind of adding up all of this information now one of the tricks is because of the voltage-gated channels if enough sodium ions if enough positive charge flows in here and as it flows in if enough of this positive charge even though somes being kicked out some will diffuse from areas of higher concentration to areas of lower concentration if enough of this positive charge reaches the first voltage-gated sodium channel then it will open so let's say I have a whole bunch of neurons here dumping excitatory neurotransmitters chemicals that open chemically gated sodium channels and I hold those channels open long enough that the cell reaches threshold once the cell reaches threshold then I begin opening the first voltage-gated sodium channel and because of their nature they'll open at minus 60 millivolts and they will not close until we get to around zero to plus 30 millivolts they start to close and enough sodium is entered that the neuron will actually have so much positive charge it moves all the way up to plus 30 millivolts now granted that's after we've already stopped all of this other information so eventually we excite it and then the excitement goes away they opened and in this little patch of membrane I'm going to add tons of positive charge this is a hundred millivolts from minus 72 plus 30 and that's billions of sodium ions in here all that green is positive charged because it's all diffusing in the same direction it's going to hit the next voltage-gated channel we'll get to this in a moment but if this little patch of membrane right here I've got all the sodium now I've closed the sodium channels but remember at plus 30 millivolts that's when we get vegan opening voltage-gated potassium channels when I start opening voltage-gated potassium channels they open and they will open around plus 30 millivolts and so much potassium will leak out of the cell that the cell will start to move back down towards resting membrane potential it's literally like erasing all of these sodium ions and I move back down in a more negative direction they start closing at minus 70 millivolts we give back to resting membrane potential I don't know if that's the right height but it's close okay now the sodium channels it turns out physically they've worked out the physical structure have a channel on top and a channel the door on the top and a gate on the bottom the gate the bottom gate is open when we open them sodium ions rush in and then they close rather quickly and then they reset they open and level with the certain voltage voltage-gated sodium channels open sodium ions come rushing through and then they close and then they reset potassium channels seem to have only one gate so they open and when they close they close so slowly excess potassium is leaking out of the cell before they close because the excess potassium is leaking out it actually causes the the voltage inside the cell to go beyond rest and it dips all the way down to minus 90 millivolts by the time we hit minus 90 millivolts all those voltage-gated sodium I'm sorry potassium channels have all closed and then our sodium potassium pump will restore everything back to rest this is an action potential and it turns out when you study neurons and you fire action potentials once I fire an action potential in this neuron it will always go from from resting membrane potential to threshold once I hit the result that has to go to plus 30 millivolts because of the nature of the chemically I'm sorry of the voltage-gated sodium channels when they're all closed and I open the voltage-gated potassium channels and it has to swing all the way back down to minus 90 and then it can be restored back to rest that event takes about two milliseconds to one thousandth of a second for this patch of membranes so an action potential is sort of this little local event here graded potentials are local events that don't reach threshold and they're called graded potentials because for example if I open a sodium channel for just a fraction of a second I might get a small bump if I open a sodium channel for longer I might get closer to threshold so there's different gradations or sizes of these and my potassium ones my IPS piece can be very small or very large events okay so one of the differences between epsps and our graded potentials and action potentials is graded potentials can vary in size but they're always below threshold action potentials must always reach threshold and once they do they have the exact same shape the same numbers based on the nature of the opening and closing of the voltage-gated channels graded potentials are always small localized events action potentials will spread all the way down the neuron which is the last part of the selection I know it's starting to get a little long but follow along here this action potential is occurring at one patch of memory but remember in order to go from minus 60 millivolts to plus 30 once I hit threshold and opened the first voltage-gated sodium channel so much sodium is gonna rush in and because the wave of sodium that caused it to open is coming in from behind it then when the sodium rushes in the concentration gradient causes the sodium to diffuse in one direction and all these sodium ions entering here can only diffuse in this direction now it's gonna take the next voltage-gated sodium channel only ten millivolts but I have a hundred millivolts worth of ions here so naturally the next voltage-gated sodium channel is going to be tripped in open and the next patch of membrane is going to have an action potential and that's going to trip the next patch of membrane and the next patch of membrane so once I start an action potential at the axon hillock I can't stop it it's going to continue from here and every patch of membrane will be forced to depolarize until I get to the calcium channels and then it will release a neurotransmitter what happens at the next neuron depends on the nature of the neurotransmitter and the ion channels is associated with those receptors if their sodium channels this neuron can have an epsp if their potassium channels they don't have an IPS P no if the epsps are strong enough to cause the next cell to reach threshold this cell would fire an action potential and we would see the exact actual potential there's an all-or-none principle to this they all are known principles in neuron states that any stimulus any stimulation strong enough to cause the neuron to reach at and reach threshold will cause an identical action potential and that action potential will spread all the way down the neuron we can't stop it okay so that's the all-or-none principle all stimuli strong enough to cause the neuron to reach threshold will cause the neuron to fire an action potential and that action potential will spread all the way down the axon until it reaches the synaptic knobs and it's either all once I cross threshold I can't stop here and go back down because of the nature of the channels what causes them to open what causes them to close I can't close them early I can't open potassium channels early if they're voltage-gated channels got it okay now there's a couple of more principles I need to throw in here and then I'm gonna explain one last detail how this works this is gonna be about a 40-minute video and I'm sorry I'm trying to keep them around 20 but this is a difficult concept all right now mm-hmm when I look at this there are some numbers that I'm going to put on here and these numbers go along with my notes set but I'll tell you what's in the numbers the first number is this little area right here in blue that is called depolarization the threshold what we know about depolarization of thresholds usually involves excitatory post-synaptic potentials or opening chemically gated sodium channels to reach threshold okay so once we reach threshold though all of this green part of the graph is the number two on my series of the dance once I reach threshold then I begin opening voltage-gated channels and this section is called rapid depolarization I have depolarization the threshold chemically gated sodium channels I have rapid depolarization voltage-gated sodium channels allowing sodium to rush into the cell till it hits plus 30 millivolts then I get to this part of the graph which is repolarization repolarization is when we open the voltage-gated potassium channels as potassium exits the cell the cell starts to be to lose positive charge and return towards resting membrane potential this part of the graph is called hyper polarization the hyper-polarization along the action potential occurs because as the potassium channels the voltage-gated potassium channels begin to close they close so slowly that excess potassium leaks out and by the time we close them all enough has reached out to hit minus 90 millivolts and then we repolarize back to resting membrane potential but these are the four parts of the graph that you really need to know depolarization a threshold chemically gated sodium channels once we hit threshold its voltage-gated sodium channels that allow rapid depolarization we close those channels and open voltage-gated potassium channels for repolarization the potassium channels begin closing at minus 70 millivolts but do so slowly that excess potassium leaks out causing hyperpolarization that's one actual potential of one patch of membrane now remember each patch of membrane is going to depolarize and go through its action potential then the next patch of membrane and then the next patch of membrane and next patch of membrane so the action potential once started will spread like a wave all the way down the neuron until it reaches the not acknowledged and then we released neurotransmitter starting this whole process over in the next neuroma so let me draw one last thing for you and then I'll wrap this video up imagine this okay you guys know that if you look at the edge of a swimming pool the lip of the pool would be here and the water in the pool would be like this if you stood at the edge of a swimming pool with your toes sticking over there's going to be a rule if any water just barely nips your toe or even if the water comes flooding over it but anything that causes your foot to get wet in any way shape or form just a little touch of a wave or some big wave that's gonna be our depolarization of threshold this is our resting membrane potential this is our threshold so to speak you with me now imagine it like this okay I have a very long swimming pool and then I have a gap and then I have another swimming pool here okay the first person standing here has the ability to turn a dial and that's going to open a container that has thousands of marbles in it let's say 500 pounds worth of marbles almost like those little jellybeans distributors at the store if I turn it just a little bit I'll only get a few marbles out then I'll hit the water and cause just a little bit of a ripple okay if I crank it wide open a whole bunch fall in and I get a much bigger splash so this is my chemically gated sodium channel so to speak and then a certain distance away along the pool I have another person standing here with their toes over the edge this person is holding a rope and when they pull that rope all the chawl the marbles fall all 500 pounds they don't have the ability to control it that would be like a voltage-gated sodium channel chemically gated sodium channels can control how much they release voltage-gated sodium channels have to release them all so I'm gonna get a big splash and then every so often once I have my first voltage-gated sodium channel I'm going to have more voltage-gated sodium channels with the exact same scenario every time one gets stimulated if the water just barely touches their toe which would be threshold or if it comes rushing over as soon as they hit threshold the rule is if you're blindfolded and I have earmuffs on you and you're standing at a pool and the rule says as soon as the water touches your toes in any way shape or form you pull your cord you're a voltage-gated sodium channel and if these are so equally distributed throughout the length of the axon and then when I reach here if the water hits this guy he stimulates somehow the next sodium chemically gated sodium channel and this repeats itself so this is my first neuron this is my second neuron so as I shock this guy or stimulate him somehow or poke him with a stick if I poke him a little bit he flashes open a few ions we get a very weak sub-threshold potential or epsp if I poke him a little bit harder he flashes open a little bit longer I get a little bit larger epsp if I stab him with a spear so that he cranks it open and lets them all fall in that's depolarizing the threshold once the water the wave starts to wash over here and hit this next guy the first voltage-gated sodium channel there's no stopping it he pulls the cord BAM all 500 pounds I get a big wave a big wave a big wave and eventually I released neurotransmitter onto the next neuron you can see how once I opened the first voltage-gated sodium channel the action potential spreads like a wave all the way down it rises and falls and can flow down the neuron because of the nature of the location and the distribution of these ion channels the voltage-gated and potassium sodium and potassium voltage-gated channels causing depolarization and repolarization depolarize repolarize by the time the action potential here down here is back at rest and we're ready for the next action potential now some instructors will have you look at the graph and they'll have a couple of extra areas of this graph okay they'll do this I know this graph is already busy but you'll see this you'll see the sodium channels opening and beginning to close and then you'll see the potassium channels opening and closing like this and this is where the sodium is rushing into the cell causing this to happen then when the sodium stops rushing in potassium rushes in and you see all of this happening I'm not going to do that on my graphs you know there's enough to know about this without making it a little bit more complicated but you will learn that if you take some higher-level neuro or physic lasses and then the last thing I want to talk about really quickly is this there's a thing called oh my drawing a bling refractory refractory period and we're going to talk about this briefly I know we're getting on at 45 minutes here but this is a tough topic there's a theme called the absolute refractory period meaning absolutely positively without a doubt you cannot have another action potential so the absolute refractory period refractory means to refrain or not do something so there's a period of time where absolutely without a doubt I cannot fire another action potential and the idea is this once I'm past past threshold let's say I'm right here on this particular patch of membrane and another dose of excitatory neurotransmitter is released can I go back and make this even bigger and no I can't because the chemically gated sodium channels as they open and close they have to start to reset and it takes a little bit of for that resetting to occur so I can't add any more sodium to the cell whatsoever so there's a period of time until I get to about here that is the absolute refractory period usually we draw down here there's a period of time when I'm above minus 60 until I get back to minus 60 that we call the absolute refractory period I'm sorry getting a little messy here I absolutely positively without a doubt cannot fire another action potential okay no matter what I do to this neuron I can't make it go back up because all my sodium channels have been some one deactivated my chemically gated sodium channels the voltage ones are being reactivated as well now there's another period of time that goes from let me see if I can find a different color monitor from one I hit minus 60 until I get back to resting membrane potential called the relative refractory period I will abbreviate it party but it's defined as the relative refractory period this is a period of time where I can fire another action potential but the ability to do so is related or relative to how strong the stimulus is and where I am along the graph for example let's say I'm down here at minus 85 millivolts and I have another 10 millivolt stimulation so I can go back to here is that going to cause a second action potential to occur before I've completely reset the answer is no but let's say I'm right here let's say I'm at - I don't know 68 millivolts and I have a 10 millivolts stimulus I have a bunch of neurotransmitter release I actually can go back up to threshold and fire another action potential so the absolute refractory period is a period of time during the actual potential when I absolutely cannot fire a second action potential because the neuron is not capable of doing so we said enough the relative refractory period is a period of time during this action potential where I can't fly our second action potential but it's relative to the strength the stimulus if I'm too far down and I stimulate it's not going to fire another action potential that will be overwhelmed by all of this event but if I'm down here a certain stimulus might cause me to fire a second action potential before I've completely reset it so absolute cannot fire another action potential during this period of time refractory I'm sorry relative refractory period I can't fire a second action potential without finishing everything but it's relative to the strength of the stimulus that's needed to reach the rush hold again anyway I hope that all of this makes some sense to you I hope that this was helpful I hope that you learned something I hope you have as much fun as I did because I always have a blast talking about this stuff sorry for the length of the video but nonetheless we got to get this stuff done the next video will talk about some other things related to this and we'll get on to the nervous system and how it really works as a whole so thanks for watching again I hope you have as much fun as I did see you on the flip side in the video number 5