Okay. So let's go from here, and summarize what we've learned today. First, let's look at the network. Because remember, we have so many cells in a given tissue. Very, very dense. Let's look at this sparse representation of a piece of a cortex. There are these different cell types. I mentioned the pyramidal cells. There is a spiny stellate cell, another morphological type. And another interneurons, and so on. And these cells interact with each other via synapses. So for example, the green cell is making excitatory synapse to the white cell. The red cell is making maybe inhibitory synapse to the dendrite of the white cell. The blue cell, the blue pyramidal cell, is making excitatory synapse into the dendrite of this white cell. So this white cell, this spiney stellate cell in layer four of the cortex, receives synaptic inputs from many, many neighbors. Maybe 10,000 neighbors. And each neighbor is saying different things. Some neighbors excite the cells in this particular stellate region. Some other local neighbors, inhibitor interneurons, inhibit the dendrites at this location. So it's a very particular design of interaction between pre-synaptic, post-synaptic through different types of synapses, excitatory inhibitor. And of course, into this cell you may get axons coming from very far. In this case the yellow axon is coming from the thalamus, which is another region, it's of the cortex. So it's a long, long, long, long axon coming in, making contact also with this cell. So if I zoom into the cell, zoom in, I see something like that. This cell is absolutely decorated with different types of synapses. The blue synapses may come from the thalamus. The red synapses may come from another cell type. The yellow synapses come from another cell type. And each one of the synapse convey a set of post-synoptic potential locally. So let's say this green cell is making green synapses here. And these green synapses, when this cell is active, when this cell's axons fire sparks, the post-synaptic cell receives a set of post-synaptic potential. So pre-synaptic, you'll see dock, dock, dock, dock. And post-synaptically you will see an excitatory post-synaptic set of excitatory post-synaptic signals. So this is what you'll receive as a cell, and eventually you have to make sense out of all this and generate an output in your axle. So, you'll receive all this bombardment of different synapses from different pre-synaptic sources. Note the huge number of synapses that each type is making. So, post-synaptic cell receives, for example, from its similar type of cell. So, this is spiny stellate cell and this is spiny stellate cell. This portion of the cell receives from the population of it's neighbors. From the same type, about 1,500 synapses. For example, from the thalamus, this particular cell receives from the thalamus, these are the yellow synapses. Maybe 400 synapses from the thalamus into the cell. From other cell types you can see the large number of synapses all together maybe 5,000 6,000 synapses from the different sources. And each one conveys a particular electrical signals, sometimes inhibitory, sometimes excitatory. So let us summarize. Now, you can see much better what I started with. There is an dendritic tree which is the input part. This input part receives two types of inputs. The excitatory and the inhibitory types of synaptic inputs. The excitatory and the inhibitory. When I say excitatory, I mean that these red cells through transmission of action potentials, reaching the Soma, generate post-synaptically in the dendrite locally. They generate positive signal which I call excitatory post-synaptic potential. When I speak about inhibition, I mean this particular interneuron typically local, sends a set of spikes that reach this synapse. And when it reaches this synapse, the spike, it generates post-synaptically in this dendritic location, a negative signal, an inhibitory post-synaptic potential. So you have an excitatory post-synaptic potential due to the excitatory cells. You have an inhibitory post-synaptic potential due to the pre-synaptic inhibitory cells. Eventually all these synapses sum up at the cell body. From everything it converges, it flows, the signal flows into the cell body, and they start to generate voltage change at the Soma and at the initial segment. And they build up, one on top of the other, excitatory, inhibitory, excitatory, inhibitory, excitatory, excitatory, excitatory, excitatory. And at some point, at some voltage, they may reach the threshold for action potential generation. We'll talk about the spike. But you already see that there is a certain threshold, a positive threshold, that you may reach in order to get a spike. So all these synapses interact together, influence together, some in a positive way, some in a negative way. And eventually, the axon initial segment should decide, so to speak, did I reach the threshold, or not? If I did not reach the threshold, no spike. If I reached the threshold, boom, there is a spike, and when there is a spike or many spikes, you see them, when there is a spike, you see them flowing. You see them flowing one spike after the other spike, into this axon, effecting the next stage, the next chain, in this interaction. So, I hope that you better understand this unique element, the neuron, with its dendritic tree, axon tree, release sites, that enables the interaction between synapses. So if you know this element, now anatomically, and you all ready heard about some electrical activity, the next lesson will be focused specifically on how do electrical signals generated. What makes the membrane of this neuron an electrical device so it can generate spikes, and it generate synoptic potentials? So this is the next lesson. See you next week.
