Okay so we saw the picture of a neuron, but let's think about the neuron as an input output device for a second. It will be too early, but then we'll talk about the different elements and we summarize everything and it will become understood. So I want to just conceptually show you for a second how we think about the neuron today as an input/output device and then we'll go step by step to study it. So here is a schematic description of a neuron. The blue neuron is the post synoptic neuron, the receiving neuron. The red neurons are the pre-synaptic neurons that make synaptic contact to the blue neuron. So the red neuron has the synapse here with the blue neuron. This red neuron has the synapse here with the blue neuron. So many, many many, many thousands of cells. Pre-synaptic to the blue cell connect to the dendritic tree of the post-synaptic cell. So imagine that you are a tree, you are the blue tree. Many, many, many wires approaching you. Many, many axons approaching you, making synaptic contact, we'll talk about synapse. And when they make synaptic contact, they locally, in the dendrite, make a little change in voltage, a little voltage change, a little potential. This is called the synaptic potential, locally. So these are all the red neurons. There is also another group of neurons, let's call it the green neurons or the magenta neurons. Many of them, I just show one of them, this also may conduct to the dendritic tree of the blue neuron, but this cell is a different type of cell, because instead of, and it has a synapse here, but instead of making a positive voltage, a synaptic potential that is positive, that is excitatory like the red cells that are excitatory cells, this cell tries to inhibit the postsynaptic cell. It generates a negative current, so to speak, in order to dampen the activity of the post-synaptic cell. So you see that there are these signals here. These are the signals that we should discuss a lot, called action potentials. They carry information from the red cell into the synapse. In the synapse there is another voltage another potential, it is called synaptic potential from this cell here, from this cell into dendritic tree here and so on. And all of them together some the plus the excitatory and the minus the negatory all sum over the dendritic tree. And eventually the postsynaptic cells so to speak decides whether he generates an output, yes or no. The output is again a set of action potential, of spikes, that we should discuss later on. So to summarize what I just said, I said that there are two types of inputs, general types of inputs to add into the tree. Excitatory input coming from the so to speak red cells. Of course they are not red in reality, they are red in this figure. They tend to excite the post synaptic cell by generating locally a positive potential. So these are the excitatory cell. There is the inhibitory cell, which tries to inhibit the post-synaptic cell by generating locally a negative voltage. And the output of this cell, if there will be an output, it will look like a set Of these bits of these spikes. [NOISE] This will be the output of the cell. The spikes output of this cell. Of course this axon may connect certainly connect to another post synaptic cell. So these are the pre synaptic cell to the blue cell and the blue cell is pre-synaptic to the next cell. Okay, so post-synaptic potential that are excitatory. Postsynaptic potential that are inhibitory. All this region is the input region of the cell and this already we spoke, I told you that Cajal already thought that the dendritic tree is the input site of the cell. He didn't know that there are synapses. He did not know that there are two types of synapses, excitatory and inhibitory. But he realized that the dendritic tree is the receiving, receptive part of the cell. And that the axonal tree is the output part of the cell. So this is a very general, modern view summarizing concepts from Cajal, but now with more details because I'm telling you to summarize. That the cell is an input/output device, the input is received over the dendritic tree. The input consists of two types of synapses, of two types of inputs. The excitatory input, the excitatory synapses, they tend to excite to try to make the cell fire. And the inhibitory inputs try to dampen to inhibit, to control, the output. So this is the input which consists two types of input, excitatory, red and inhibatory, magenta. And the output in the axle looks like set of spikes if there will be spikes. You can think about this today, in a more modern way so to speak. In a direct way you can record today inside the brain with an electrode impaling the cell body near the axon and you can see that this cell may generate spikes that will look like this. So this set of spikes at the soma will flow along the axon, make connection through synapses to the post-synaptic cell. So if you want to summarize it in a different way, thiis is now a reconstructed cell. This is a pyramidal cell from the neocortex. You will see a lot of these cell because it's a very principle cell in the cortex. This is layer five pyramidal cell. The cell body, the green dendritic tree, the red axon. So you see the axon comes out from the cell body and then branches and branches. And there are all these little spots. All these. Okay? And if you recalled, with an electrode in the axon, so if I impale, if I implant electrodes, small electrode into the axon, and I will try to listen to the action potential to see what is the electrical activity in the axon when the cell is active, I will hear something like that [SOUND]. So then I will see the action potentials in the axon, all over the axon they will propagate, we talked about propagation. So the axon is the output device and this device is making this [SOUND]. This is the elementary bow chord. The elementary spike. The elementary bit of information. Zero or one. Each spike may exist or not. That's the very fundamental basic building blocks of the brain.