 EPSP and IPSP that is the excitatory post-synaptic potential and inhibitory post-synaptic potential are the graded potentials which develop on the post-synaptic membrane. We know that synapse is basically a connection between the two neurons that consists of basically a pre-synaptic membrane that is the exon terminal of one neuron and a post-synaptic membrane which is the dendrite of another neuron. So the signal travels from the pre-synaptic neuron to the post-synaptic neuron and because of this there is a change in potential on the post-synaptic membrane and this change in potential can be either depolarization. Depolarization is basically change in potential towards the positive side or towards less negative right or there can be hyperpolarization as well okay. So hyperpolarization is change in potential towards more negative side. So if an RMP is minus 70 millivolt, change in potential towards minus 60 millivolt is depolarization and change in potential towards minus 80 millivolt is hyperpolarization. So when the change in potential is towards the positive side or towards less negative that is basically EPSP, excitatory post-synaptic potential because this is the synapse and the potential is developing on the post-synaptic membrane so excitatory post-synaptic potential and the hyperpolarization that is known as IPSP that is inhibitory post-synaptic potential. So how these potentials are developing and how EPSP and IPSP either may lead to generation of action potential or not lead to generation of action potential we will see. So first of all just the basic concept of synaptic transmission how it happens well whenever there is a action potential which reaches to the exon terminal there is opening of the voltage gated calcium channels. So in green here are shown voltage gated calcium channels which open causing entry of calcium from the extracellular fluid into the exon terminal and this calcium causes release of the neurotransmitter which is present in the vesicles. So because of this calcium these vesicles move towards the membrane and fuse with the presynaptic membrane causing the release of the neurotransmitter which is present in the vesicles and the neurotransmitter in turn binds with the receptors which are present on the post-synaptic membrane and upon binding with the receptors there are certain changes which happen on the post-synaptic membrane that lead to change in the movement of the ions and whenever there is change in the movement of the ions there is change in the potential. So let us see that how this is occurring. So first is how excitatory post-synaptic potential is occurring. So these neurotransmitters which are released they bind to the receptors and generally the neurotransmitter which leads to excitatory post-synaptic potential very common in central nervous system is glutamide. There are obviously other neurotransmitters also but this lecture is not about in detail about the neurotransmitters okay. So we are just giving a classic example of a neurotransmitter which leads to excitatory post-synaptic potential. Now when this glutamate binds to the receptors which are present on the post-synaptic membrane there are mechanisms which can lead to movement of ions and of these the mechanism is opening of ligand gated sodium channels and when there is opening of ligand gated sodium channels sodium enters from outside into the post-synaptic membrane. So a positive ion is entering and this leads to positive change in potential okay. Now this is very common we all know about it. Second mechanism can be closure of potassium channels. So these potassium channels are open before okay. In resting state they are open and the binding of the neurotransmitter causes closure of these potassium channels. So in the open state what would have happened the potassium might be continuously leaking from the cell right. So when these channels close what will happen? Potassium will not be able to leak out of the cell and there will be potassium increase in the post-synaptic neuron right. So positive ion is being retained so that is the decreased efflux of potassium ions and that can also lead to de-polarization. So these are two mechanisms for de-polarization or EPSP to occur that is entry of sodium ion and decreased efflux of potassium ions. So here in this graphical manner what we are seeing is that on x-axis is shown time in milliseconds. So here this is time in milliseconds and on y-axis is membrane potential in millivolts right. And what we are seeing that this blue line is showing the resting membrane potential so minus 70 millivolts is the resting membrane potential and this green line is showing the threshold potential that means for action potential to occur there should be a change in potential towards the positive side such that it reaches the threshold isn't it. But here you see what is happening because of the signal which is coming from the pre-synaptic neuron there is EPSP. Excitatory post-synaptic potential that is basically changing the potential towards the positive side but this EPSP is not leading to action potential remember okay so this is a graded potential occurring which is not able to generate the action potential. So please remember that this EPSP and IPSP are actually graded potential we will see the properties very soon so these are graded potentials fine. So that was about EPSP. Let's move on to IPSP. So how will IPSP occur? IPSP we said is basically change in potential towards a more negative side right hyperbolarization. So that occurs when there is either opening of a chloride channel so opening of ligand gated chloride channel when the neurotransmitter binds to the receptor there is opening of this chloride channel and because of this there is entry of chloride ions into the dendrite and this negative ion being inside leads to hyperbolarization and classical neurotransmitters in central nervous system are GABA and glycine which cause inhibitory post-synaptic potential. But there is another mechanism also of inhibitory post-synaptic potential and that is opening of ligand gated potassium channels. So opening of ligand gated chloride channels and opening of ligand gated potassium channels why because you see potassium ion is more inside right so when these potassium channels open what will happen there will be more efflux of potassium ions from inside to the outside thus causing less positivity inside that is there will be negativity less positivity is nothing but negativity right. So opening of ligand gated chloride channels opening of ligand gated potassium channels causes IPSP on the other hand IPSP was due to opening of ligand gated sodium channels and closure of ligand gated potassium channels understanding so these two mechanisms each for IPSP and IPSP we should remember. So let us see the same way in diagrammatic manner what is happening here again the x-axis is showing time in milliseconds and y-axis is showing membrane potential in millivolt and what we are seeing here this is the resting membrane potential minus 70 millivolt and there is decrease in the potential due to this signal which is coming from the presynaptic neuron to the post-synaptic neuron. So here also again it is written activation of the synapse but here basically any potential change which is occurring that is a synaptic transmission which is occurring and causing that potential change but you see here that when IPSP is occurring it is taking the potential away from the threshold potential IPSP was taking the potential towards the threshold potential but IPSP is taking the potential away from the threshold potential so if this neuron has to generate an action potential now the change in potential which will be required is from much below maybe minus 80 millivolt till here right so it is becoming less responsive okay so maybe there might be another stimulus which is causing maybe excitatory change but you know that now that excitatory change required is much more because IPSP is also there so that is the significance of this IPSP and IPSP that is making the neuron either more responsive or less responsive to another stimulus right. Now this IPSP and IPSP remember their graded potential that means they have the same property as of any graded potential and what are the properties one very important property is that that if the signal increases if the signal increases that means the potential change will increase meaning that means if the amount of the neurotransmitters which are released from the presynaptic neuron increase then the potential change in the postsynaptic neuron will be more in case of IPSP and much more hyper polarization in case of IPSP so increase in this stimulus is leading to increase in the potential change that is why it is known as graded potential right and it is not an all or none law following potential that is action potential fine. Second very important property is that it travels with decrement what does this mean that means if at the site of the synapse maybe the potential change is minus 10 millivolt that is IPSP right so from minus 70 millivolt the potential has actually changed to minus 80 millivolt. Now this potential change travels along the exon right as it travels along the exon what happens that slowly slowly there is loss in the potential so here the potential change maybe minus 80 millivolt here again it may be minus 79 millivolt then here it may be minus 78 millivolt so you see as it is traveling along the length of the tendrite the loss in potential is occurring so that is travels with decrement action potential does it travel like that no action potential travels without decrement and third very important property is that it can be summed up it can be added up okay that we'll see in detail what is that see now here is one neuron and this is the axon and this is the cell body and there are so many dendrites right now for simplicity sake I have just shown one dendrite with one synapse okay now you see that maybe this synapse there is generation of EPSP other synapse there may be generation of IPSP and you see the quantity also EPSP here changes 8 millivolt IPSP here changes minus 10 millivolt then there may be another synapse where EPSP is 15 millivolt okay and maybe other synapse which is not active and so in that case there will be no change in potential now because these potential change travel with decrement you see by the time they reach the cell body what is happening IPSP from minus 10 millivolt it has become minus 7 millivolt EPSP from 8 it has become 5 millivolt and similarly EPSP here from 15 millivolt it has become 10 millivolt and you can very logically say that if the dendrite is long then what will happen the loss in the potential which will be much longer by the time it reaches the cell body now they have reached here at a common point right and maybe they will travel like that so by the time they come together at a particular point on the membrane what will happen that all these potential will get added up why see these are nothing but ionic changes so whatever is the summed up ionic change that will be the actual potential so that is known as summation of the postsynaptic potential all the graded potentials can be added up while action potential it cannot be added up it is an all or none phenomena and that is because of the inactivation of the sodium channels and all we are not going into details of that but just remember here that graded potential can be added up it doesn't follow all or none law so just little somebody about that different EPSP and IPSP travel with decrement towards cell body the longer the dendrite the decrement will be more finally they will be added up and depending on the net result of summation of EPSP and IPSP action potential may or may not be generated fine now let us see the types of synaptic summation of these EPSP and IPSP so first is summation in space also known as a spatial summation so here what we are seeing that there are three the synapses okay so the line diagram I have drawn for simplicity and they are impinging another another neuron now suppose that all these three synapses are activated simultaneously so all these three synapses will cause change in the potential on the postsynaptic membrane one may be EPSP one may be IPSP or all three may be EPSP or all three may be IPSP that is different but suppose they come together so what will be the ultimate change in potential on the postsynaptic neuron they will be added up and that will determine so this is known as summation in a space where the potential change caused by three different neurons in this example that is being added up so these three different neurons are differently located in space as you may say right so this adding up of change in potential caused by different synapses that is known as spatial summation and let us see it graphically how it will be so here is an example where you see that these are the presynaptic neurons which are making contact with the postsynaptic neuron and EX is showing basically excitatory synapse and in INS inhibitory synapse now suppose EX1 gets excited only only EX1 right so here this green line is showing the threshold and say suppose this is the resting membrane potential suppose only EX1 is excited or the synapse is activated so what will happen we will get a positive change in potential but it is not reaching to the threshold now given a condition where both EX1 and EX2 are excited so the summed up change in potential will occur right so they are acting together and what will happen we will get maybe a graded potential which reaches to the threshold and there will be generation of action potential okay so this is the point where two excitatory synapses are excited together so what is this this is summation let's take another example say suppose excited three synapse one and inhibitory synapse one are excited and maybe quantitatively for simplicity let us say that this causes plus 10 millivolt change in potential and this one causes minus 10 change in potential so if both of them are excited simultaneously how much will be the potential change nothing there will be no potential change right adding up is not causing any potential change so that is spatial summation next is temporal summation here temporal when we are talking it is in relation with time so here i have shown only one synapse okay so now if this synapse is activated repeatedly so suppose there is a train of action potentials here right so lot of action potentials are coming one after the other and they are activating this particular synapse so maybe one action potential from the presynaptic neuron is causing a EPSP on the post synaptic neuron of three millivolt suppose so it will fade with time remember fade with time also so one is that it travels with decrement and second is that at a particular position it will not stay forever right so after sometime it will become two millivolt after sometime it will become one millivolt and it will decay with time right now as it is decaying before it's complete decay to zero millivolt suppose another action potential comes right so this action potential will again generate a EPSP of three millivolt and EPSP due to the previous signal was suppose two millivolt because one millivolt has faded so how much will be the EPSP now it will be three plus two is equal to five millivolt so the excited triposynaptic potential from the two signals have actually added up right so that is known as summation with time that is temporal summation similar will be the case in IPSP also so maybe initially there was a change of minus 10 millivolt okay and then there will be decay so it is going to become minus nine millivolt minus eight millivolt and maybe now another signal is added up here that is minus 10 millivolt so how much will be the EPSP it will be much more negative that is minus 19 millivolt understanding so that is summation of the EPSPs and IPSPs fine so if we see it graphically in this same example so here excitatory stimulus is there only one stimulus comes so it will be like this right and then it will start decaying right now as it is decaying again maybe the next stimulus comes in the same synapse so what will happen it will add up and it will increase to this level and again if the same stimulus comes right so it was decaying and again it will add up and you see the third time it may reach the threshold so if the frequency of the stimuli is more in case of excitatory synapse ultimately it will lead to generation of the action potential so what is the point of all this thing point is basically that generally stimulation in a single synapse doesn't lead to action potential it is the combination of the synapse which are being activated that ultimately will determine whether the post synaptic neuron will fire or not and depending on how much is the added potential so here you see EPSP is much more than that of the threshold right you see here is the threshold so this is the action potential frequency another example you see that if the threshold is here and the summated potential is much more higher than that of the threshold here it is only this much okay then the frequency of action potentials is much much more so here this diagram is showing only the action potential so in this the action potential frequency is less in the other one action potential frequency is much more now before we end this concept on EPSP and IPSP I want to talk about the presynaptic inhibition what is presynaptic inhibition here you see this diagram is showing one excitatory synapse right and this excitatory synapse is being inhibited right so on the exon there is an inhibitory signal this synapse is not here right so if the synapse was here that is known as direct inhibition now this is what is known as presynaptic inhibition because it is the presynaptic neuron which is being inhibited okay now suppose one action potential causes the release of maybe 100 neurotransmitters right just simplistic values we are taking and 100 neurotransmitters lead to say suppose this much potential change okay now suppose this excitatory synapse the presynaptic inhibition is present that means this one is activated but by means of another neuron this is inhibited also understanding so maybe instead of 100 neurotransmitters now only 50 neurotransmitters will be released because of this inhibition the amount of voltage gated calcium channels which will be opening are very less so the fusion of the vesicles will also decrease so that is why the neurotransmitters are going to decrease okay and here in this diagram if we see it has shown that okay with inhibition total inhibition of the neurotransmitters is occurring that means no release is occurring but generally it doesn't happen like that it may be partial inhibition also right so if EPSP without this presynaptic inhibition was say plus 10 millivolt with the presynaptic inhibition the EPSP maybe only say suppose plus 8 millivolt or plus 7 millivolt understanding so that is the concept of presynaptic inhibition this is not direct inhibition but it's still the amount of EPSP which is being generated at a particular synapse is being less because of the presynaptic inhibition so that was all about the concept of EPSP and IPSP how they are generated what are they how they are summed up and when they will lead to the generation of action potential thanks for watching the video if you liked it do press the like button share the video with others and don't forget to subscribe to the channel if physiology open thank you