okay so in this video we're talking about postsynaptic potentials now a postsynaptic potential is the word we use to describe graded potentials in postsynaptic cells you know like a postsynaptic neuron earlier we talked about how neurotransmitter when it's once it's released from your presynaptic neuron it can bind to receptors on the postsynaptic cells membrane and if it's a ligand gated ion channel this can cause graded potentials to form now these graded potentials are generated as a result of changes in permeability of ions and therefore a change in current which changes your voltage and the amount of neurotransmitter release can can alter how large these graded potentials are and how long that neurotransmitter stays in the synaptic cleft also will alter how large these graded potentials are and an example of this would be let's say if your presynaptic neuron released a tremendous amount of neurotransmitter into the synaptic cleft where you have even more neurotransmitter available to find the receptors so you get even more current in the postsynaptic cell which means even get more a larger graded potential and or we can talk about how if neurotransmitter stays in the cleft longer you get the same effect so these are both modifiable in a variety of ways now graded potentials are not always depolarizing in fact graded potentials can either be excitatory or inhibitory so these excitatory postsynaptic potentials are a type of graded potential that's due to a depolarizing current within our postsynaptic cells now the inhibitory postsynaptic potentials are actually caused by a hyperpolarizing current within our postsynaptic potentials so neurotransmitter binding opens chemically gated ion channels and in the case of excitatory postsynaptic potentials this allows for this the flow of both sodium and potassium however because of the structure of these channels sodium influx is actually much greater than potassium and so because you're bringing in more positive charge then positive charge might be leaving this influx of positively charged sodium causes excitation of the cell because it's polarizing and if you depolarize the voltage of that cell you get closer to action potential threshold and the in that regard it's actually excitatory because it might trigger the you know action potential now what happens is these excitatory postsynaptic potentials are graded potentials and so therefore they degrade over time and distance so an epsp has to be large enough that so that when it spreads throughout the neuron and by the time it gets to the axon hillock if it's above action potential threshold that can trigger opening of voltage-gated sodium channels which then causes the action potential to be generated so what this slide is showing is basically an example of an epsp where we have membrane voltage or potential on the Y and time on the x axis so you see that in response to stimulus like neurotransmitter binding to receptors you get an influx of sodium into the cell which causes the voltage become more positive but this is temporary because that sodium dissipates and that it's local effect on the voltage becomes smaller over time and distance and so in that regard graded potentials are these temporary changes in the voltage of our cells now if they're excitatory postsynaptic potentials they allow for sodium to rush into the cell and are therefore depolarizing and it's excitatory because it gets you closer to action potential threshold now in this example an action potential wasn't generated because the graded potential or epsp here wasn't quite large enough to get to threshold now opposite of that we have ipsps or inhibitory postsynaptic potentials so these inhibitory post metals are or ipsps are hyperpolarizing which means that due to the current that they generate it causes the voltage in the cell to become more negative than rest and that's what hyperpolarization is now this hyper polarization of an IPS P is due to either potassium or chloride flowing now potassium flows out of the cell due to its electrochemical gradient if it flows through a certain channel and then chloride can flow into the cell due to its electrochemical gradient if it's allowed to so what we're here that is if a neurotransmitter from let's say an inhibitory neuron binds to a certain receptor on the postsynaptic cell it might allow for potassium to flow out of the cell or for chloride to flow in and both of those types of current causes the cell to become more hyperpolarized which is essentially when the voltage becomes more negative than rest and this is inhibitory because it gets you farther away from action potential threshold so to look at what an eye PSP looks like on a voltage trace we see that we have our stimulus here and that in response to let's say neurotransmitter binding to its receptors and also we get potassium leaving the cell if all of a sudden you make potassium even more permeable it's going to flow out of your neuron and make the voltage even more negative because you're removing positively charged ions same thing with chloride except for chloride could be flowing into the cell because you're bringing in negative charge that also makes the voltage more negative but remember this is a greater potential so it degrades over time and distance and it's inhibitory because it gets you farther from action potential threshold now what's pretty interesting here is that ipsps and epsps can summit or ad together so you might have a depolarizing epsp that adds together at the same time with a hyperpolarizing ipsp and their effect could cancel out or you might have multiple epsps add together to get you to action potential threshold so most of the neurons in your brain spinal cord and around your body receive both excitatory and inhibitory inputs from thousands of other neurons so basically in order to get to action potential threshold we typically need several EPS B's to some eight that way you can generate an action potential now there's two types of summations here we have temporal and spatial now temporal summation occurs from typically about one synapse and if this one synapse transmits action potential I'm sorry generates depolarize earn or hyperpolarizing a current very frequently such that those greater potentials add together in time then they can sum eight and then we call this temporal summation because it occurs typically at one synapse now spatial summation is when you have postsynaptic neuron that's stimulated by a large number of synapses simultaneously and you might find that there's thousands of epsps and ipsps that can add together and some eight to reach some kind of net effect you know you might have five thousand and two EPs peas and four thousand and fifteen IPS peas but because of the fact that you have a lot more epsps and IPS peas being generated simultaneously that means the net effect on that postsynaptic neuron is excitatory and that could generate action potentials in that neuron so this is actually a pretty cool way that neurons can actually you know either excite or inhibit other neurons so what this isn't showing is an example of where we don't get integration or summation so you might have a circumstance where one neuron excites a postsynaptic neuron here and because the exit excitatory events or stimuli occur too far apart in time the greater potentials that are generated in the postsynaptic neuron don't add together in fact they stay separate which means you actually don't get two action potential threshold but if you added these stimuli closer together summation would occur and because these ipsps could add together it could get you above action potential threshold that's what we see here so if we actually have this one neuron generate action potentials more frequently and therefore excite the postsynaptic cell more frequently then we see here that these these stimulatory events are closer together so the first EP SP can add with the second and because that gets you two action potential threshold then we get an action potential so they get the depolarization phase in the repolarization phase of the action potential and that's going to spread throughout the axon now this is an example of temporal summation because it occurs at once amps now spatial summation would occur at many synapses and this could be like where we have an epsp from one synapse add together with an epsp of another and because they add together then they can get you to action potential threshold which then generates your action potential now what's interesting too is we can also throw in an inhibitory effect here as well so let's say that we have an inhibitory our ipsp greater potential generate because of an influx of chloride or an e flux of potassium that it hyperpolarizes the voltage and takes you farther away from action potential threshold now because this one event you know occurred way back here you know basically just made it less likely for action potentials to be generated but let's say if we have an excitatory and inhibitory actually but greater potential rather basically add together some eight at the same time they almost canceled each other out where you just see the tiniest little blip or change in voltage here because of the these greater potentials adding together and so because these add together in time you can actually can prevent this neuron from responding in fact a lot of inhibitory types of synapses are actually located closer to the axon that way they can actually can block the generation of an action potential and be a better inhibitory type of synapse now synaptic potentiation is a really interesting effect of neurons where repeated use of a synapse increases the postsynaptic cells ability to respond to that synapse so think of synaptic potentiation is like positive feedback where once you generate a response in the postsynaptic cell that response makes it far more likely that the next response will be easier to generate and so the way this works is that by repeated excitation or inhibition of a postsynaptic cell calcium concentration can increase in the presynaptic terminal which causes the release of more neurotransmitter and this leads to more epsps in your postsynaptic neuron also what happens here too is that potentiation can actually cause these voltage gates to open on the postsynaptic neuron and calcium in the postsynaptic neuron can activate these kinase enzymes and these kinases can basically alter the sensitivity of receptors on the postsynaptic cell and therefore make them more likely to respond with even less neurotransmitter so what's cool here is that this is actually the chemical correlate of learning in fact we call this long term potentiation or Alta P so long-term potentiation is how synapses can strengthen over time from repeated use and this is why it's important that if you're learning something for the first time that you have repeated repetition of that over and over and over again that way you can stimulate long-term potentiation the formation of newer stronger synapses that can help with learning and memory now we can also occur as presynaptic inhibition an inhibition is actually due to the release of excitatory neurotransmitter by one neuron but can be inhibited by another neuron through an EXO axonal synapse so it's pretty cool here is that we can see then is that you can block the excitatory effect of one neuron by causing inhibition by another and so what we see then is just basically a summary of where graded potentials can occur remember graded potential occurs in the cell body and dendrites and those can spread on over two axons now remember great potentials typically travel a pretty short distance and they get smaller over time and distance as they travel but if these graded potentials have enough depolarization by the time they get to the axon hillock they're gonna generate an action potential which travels long distance and very rapidly in fact much more rapid than a graded potential now in terms of a graded potential versus an action potential just a comparison and contrast we see that the amplitude can vary in size and that's what makes these graded however action potentials are always the same size and they're all or none and graded potentials decay over time and distance whereas action potentials don't now graded potentials are stimulated by lots of different types of stimuli it could be light pressure temperature and action potentials are only triggered by depolarization so you need enough depolarization at the axon hillock to get to action potential threshold for these to be generated now we see that great potentials don't typically respond to positive feedback types of regulation whereas action potentials can in fact that's how action potentials work is that do the opening of voltage-gated sodium channels at threshold it causes even more voltage-gated sodium channels to open and that's what triggers the depolarizing phase of the action potential we don't see this in graded potentials in fact graded potentials don't stimulate themselves which means they don't propagate and therefore degrade over time and distance but because action potentials have a positive feedback cycle they can propagate themselves and travel a long distance now repolarization of graded potentials occurs when no stimulus is present because these things get smaller over time and distance because of diffusion action potentials repolarize when the voltage-gated sodium channels close and the voltage-gated potassium channels open so you need these voltage-gated potassium channels to repolarize the action potential we see that greater potentials occur due to stimulus from other neurons at a synapse and it can either occur either temporarily or spatially now greater potentials occur in two major types we have excitatory and inhibitory postsynaptic potentials remember excitatory postsynaptic potentials generate depolarizing current which is typically sodium flowing into the cell and inhibitory postmen potentials or ipsps generate hyperpolarizing current and which is typically potassium efflux or chloride influx we find those that action potentials only have one form there's not like inhibitory or excitatory action potentials these things just travel long distance they constitute the nerve impulse they're the same way every time so long as everything is working properly now we find that greater potentials are caused by ions flowing through different channels so remember the epsps are due to sodium influx and because of that net influx of sodium you're bringing more positive charge inside the cell which makes your voltage slightly more positive and that these ipsps are generated by potassium efflux or chloride influx which is what hyperpolarizes the voltage away from rest now action potentials are caused by two types of voltage-gated channels voltage-gated sodium which is what constitutes the rising phase of the action potential and voltage-gated potassium which is what constitutes the depolarizing phase I'm sorry repolarizing or falling phase of the action