what we see is this that a single excitatory post synaptic potential epsp is typically not strong enough to bring the neuron to that threshold voltage of 55 molts so typically a single epsp is going to be somewhere in the neighborhood of 0.5 molts and of course you realize that we need uh 15 molts uh to go from the 70 resting membrane potential to the 55 molt threshold potential on this particular slide we see the effects of various press synaptic neurons on the membrane potential and the post synaptic neuron now if we go to the left hand side here we see that a would be an excitatory presynaptic neuron B is an is an inhibitory prepic Neuron a excitatory b inhibitory and so on and so what we notice is that we don't exhibit we don't show an action potential an action potential doesn't occur until we reach threshold as you know and we reach threshold if you're following the pointer at this particular uh place on the slide and there are reasons for that so one thing that we're going to notice is that all presynaptic axons don't have equivalent input or don't have equivalent influence on the charge that's found in the post synaptic cell and we'll explain why in just a moment but the end result here is that the membrane potential of the post synaptic cell is the result of the action of all the synapses that it forms with its presynaptic neurons and so there are thousands of these most likely hundreds of which are active at any given moment and some of them are excitatory some inhibitory and what we notice is that the important thing is the vyl charge that's reached in that initial segment also referred to as the axon hilic now as you would imagine what is really important here is something referred to as summation and so summation is essentially what we just described it is the ultimate membrane potential that's re reached as a result of the input of all of these different press synaptic neurons on the single post synaptic neuron what we want to look at on this particular slide are the two types of summation that can occur in the post synaptic neuron to do that let's take a look at our the figure that's found on this particular slide and first let's look over here at the drawing on the left hand side so we have a single post synaptic neuron following the pointer on the screen here we have a recording electrode that's placed inside it and we have three to make things fairly simple in this case press synaptic neurons that are synapsing with and influencing this single post synaptic neuron two of these labeled the two labeled A and B are excitatory one of these uh C shown in red here is inhibitory now let's go to the graph and in the graph we're going to see what happen happens when we stimulate a b and c at different intervals now of course the graph shows the membrane potential at this recording electrode that's placed inside the post synaptic uh cell and if we start at the left- hand side of the graph what we see is that if we we send an electrical signal an action potential down a and that's what's being done here we send an action potential down a then it does influence the post synaptic cell and it does create some dear ization in the post synaptic cell but not a whole lot and because as you know these synaptic potentials are a type of greated potential and they're very transient they go away very quickly then that rapidly Fades away so at this point we stimulate a again and we get a nearly identical depolarization in the post synaptic cell so essentially nothing has happened um we have stimulated the postoptic cell twice we've created two graded potentials of course the type of graded potential we create as a synaptic potential and they've disappeared so no overall effect has been uh endured by the organism in which we find this neuron so in other words the organism doesn't know that it's being stimulated because no action potential made its way back to the central nervous system now let's go to two so in two uh here on our graph we do something different we stimulate a just like we did before over here uh in the beginning of our graph and we get the same same depolarization we observed here but the difference is the interval the time interval that elapses before we stimulate a again so here we only allow a very short period of time to elapse we apply a second stimulus and what we see is that we get a second graded potential but that graded potential is of Greater magnitude than the one we achieved over here because we started with a higher Baseline we hadn't gone all the way back to that resting membrane potential you may say well that doesn't make sense because of the the refractory period that we learned about but the refractory period remember that only deals with those voltage gated calcium uh rather sodium channels excuse me that are found in the axon of the neuron so these are different channels uh these These are Lian gated channels that are binding to a neurotransmitter so there is no refractory period that's in play here at all and it would make sense you know this temporal summation makes perfect sense doesn't it because we have this neurotransmitter that's binding it's opening these Pro these non-specific most likely uh sodium and pottassium channels sodium is rushing in and then that sodium is not pumped out it doesn't have a chance to be uh pumped out before we have a second stimulation so then these channels open again perhaps additional channels open as well and we have increased depolarization above what we would have had if we allowed a a greater amount of time to elapse between these two stimulatory impulses and again it's important to note that these are both stimulations that are coming down both Action potentials that are coming down Neuron a in this particular uh case so we refer to this as temporal summation because it depends on the time um we can see that this occurred this summation this greater change in the resting membrane potential occurred as a result of the time that was allowed to elapse between the two stimulation events in a so that is temporal summation let's go over to part three of our graph in part three of our graph what we see is this excitingly we stimulate the neuron here but we're stimulating it with B uh with this different PR synaptic neuron and so when we stimulate B we get some depolarization there then we go back to threshold and then then look at this we stimulate A and B simultaneously and when we do that as you would expect we have a greater concentration a greater Vol overall volume of sodium that flows into the cell so we have a greater change in the charge in the post synaptic cell so this is spatial summation because we have two presynaptic neurons in the same general area that are stimulated and obviously they have to be stimulated prettyy close to the same time but when they are stimulated then they open a greater number of channels than would be opened if just B is stimulated by itself so we have a greater degree of depolarization now so spatial summation to summarize now moving over to part four of our uh graph here what we see in part four is this um that we are experiencing both temporal and spatial summation here so we're stimulating a a b b and what we see is that we have this Progressive increase in charge up and up and up and up as a result of the opening of all of these ion channels and the rushing of sodium into the cell and this does allow us to achieve that5 Volt depolarization or that 15 volt depolarization all the way up to 55 and then the voltage gated sodium channels open and the a potential commences and so we go all the way up to positive 30 and exhibit that action potential as we've learned about previous ly so as you know backwards and forwards at this point the only important thing is if you reach that threshold voltage of 55 molts lastly if we go to part five of this particular graph what we see is this here we're stimulating C PR synaptic neuron C PR synaptic neuron c as you can see over there is an inhibitory synapse and so it of course as we talked about previously uh is either opening just potassium channels or potassium and uh chloride iion channels or perhaps just chloride ion channels and it's causing this hyperpolarization of the cell so what we see is that neutralization can occur if we stimulate a shown here C shown here simultaneously a is exciting B is inhibiting and so the ultimate result is they cancel each other out and we have stabilization of the resting membrane potenti and we call this neutralization because the excitatory and inhibitory synapse have neutralized one another the last thing we want to point out regarding synaptic integration is this that when a synaptic potential occurs this type of graded potential then you know let's go down to a here at the excitatory synapse and so what's happening in a is that we have a signal that's released we open up these sodium ion channels sodium rushes into the post synaptic cell and it spreads out now it doesn't go too far but it does spread in this direction in this direction as you can see following all these arrows as you well know at this point the really important part of the post synaptic cell is this initial segment also known as the axon hilic and so the axon hilic has this threshold voltage of 55 molts interestingly the rest of the cell does have the rest of the neur on like where I'm pointing here does have a threshold potential but it's generally much higher than 55 molt so it's very unlikely that an action potential could be achieved on other parts of the neuron and what we notice is that as these positive charges sodium ions reach this initial segment then we have this depolarization that you're familiar with and that's where summation comes in if we have enough of these stimulatory events we can eventually reach threshold now the initial segment uh has this 55 this lower which is good makes it more sensitive and more likely to undergo an action potential threshold voltage because it has a higher density of these sodium channels these voltage gated um sodium uh channels that are found within it and I did mean to say voltage gated right so the voltage gated channels are the ones that are going to open and initiate this action potential these are liated Chan channels as you know over here that are causing this synaptic potential and what we see is that the input of all presynaptic neurons is not equivalent and you can see that very easily by looking at this particular diagram so you could imagine although it's not depicted here that a presynaptic neuron that that synapses on this side of the post synaptic cell would have less influence in other words it would be less likely to initiate an action potential then would a presynaptic neuron that terminated say right here where I'm placing the cursor on the screen at this moment simply because these charges spread out within the post synaptic cell but they dissipate and so you know that from our discussion of graded potentials and how they rapidly dissipate they don't move very far they're decremental they go away very rapidly within the post- synaptic cell so this means that press synaptic cells that have their axon terminals closer to the initial segment also known as the axon hilic will have a greater influence on the post synaptic cell the last point that we want to make in this particular section is an important one and it's this that these post synaptic potentials also just known as synaptic potentials last much longer than do Action potentials Action potentials move very rapidly and of course are limited by the characteristics of of the sodium channels and the refractory periods but these last longer and so what we see is that if there's no picture uh here of this but I'm going to go ahead and point to to this initial segment as I speak so if this initial segment remains depolarized after the firing of the first action potential and so the charge is above 55 here an action potential fires and moves on down here the sodium iion channels where I have the pointer now go through the entire refractory period and then they're ready to fire again and the charge is still above 55 molts well then another action potential is going to fire and the reality is that although that may not be how we've thought about it as we we have learned about it so far in this chapter Action potentials almost nearly fire in these short bursts as opposed to a single action potential at a time in other words a a post synaptic neuron for like the one that we have pictured here will undergo depolarization to threshold and then bam bam bam bam it'll fire several action potentials those Action potentials run in close succession down the axon to their Target and then of course the cell will become repolarized and the action potentials will cease to fire but the overall message that we want to to get across here is that action potentials typically come in short bursts as a result of the fact that these synaptic Potentials in post synaptic cells we could call them post synaptic potentials last for a fairly long period of time relatively speaking and therefore that's long enough for multiple Action potentials to be initiated before the initial segment repolarizes