Okay. So we saw in that last video how individual EPSPs and IPSPs are actually kind of tiny. And by themselves, they're probably not going to have a super significant impact on our membrane potential. But lucky for them, they are rarely by themselves because, remember, our neuron has connections, synapses with 100, if not thousands of other neurons. And so all of those neurons sending us postsynaptic potentials are going to have a cumulative effect on our membrane potential, and that is the process of summation. So summation is adding multiple postsynaptic potentials at the initial segment. So all of those synapses are sending us EPSPs, IPSPs. It's going to be changing our membrane potential and that current will travel down our cell body and when it gets to the initial segment, all that change literally is going to get added up and if it adds up to a depolarization that gets us to negative 55 millivolts threshold, we will have an action potential. And if it doesn't quite make it there, nothing's going to happen, right?

Now, there are 2 types of summation. The first type is temporal summation and temporal summation is the summation of graded potentials at 1 synapse, so one neuron talking to our neuron, overlapping in time. So if you look at this graph here, this is depicting temporal summation, and this is basically just one neuron and he is very excited. He is just yelling at us fire, fire, fire. He's sending us EPSPs. We can tell because it's depolarizing our neuron, right? And what's happening here is that he's sending those signals so fast that they're getting added together. So he's sending us an EPSP, it's depolarizing our membrane, and then before we can start losing that current, boom, he sent another one. Before we can lose current, boom, he sent another one. So they're basically having this additive effect on each other because they're happening so close in time. So temporal summation is basically just one neuron talking to our neuron very fast is what's happening there.

Now, we can also have spatial summation and this is the summation of multiple graded potentials that are happening in close proximity. So this is basically when we have a bunch of synapses that are really close to each other on one chunk of our membrane. And because they're all so close, when they're having this change on our membrane potential when they're affecting it in some way and that current is dispersing, it's going to kind of start bleeding together and getting basically summed up. And so this is our graph depicting spatial summation. And spatial summation is a bit unique because what can be happening is all of those neurons could be sending EPSPs. They could all be sending IPSPs, or like we have depicted here, we could be getting a combination of EPSPs and IPSPs. So, you can see here just for simplicity's sake, we have 10 neurons talking to us and some of them are sending us EPSPs and they're saying fire, fire and they're depolarizing our membrane and then some of them are sending us IPSPs and they're saying don't fire, don't fire and they're trying to hyperpolarize that membrane. And you can see these are coming in at different strengths, different intensities. Some of them are pretty big. Some of them are kind of little but basically what's going to happen is they're going to get summed together because they're all happening so close in proximity and when we get down to that initial segment, again, if we hit threshold, if we have depolarized up to negative 55 millivolts, we will have our action potential, which is the topic of our next video, so I'll see you there.