Okay, but let me go backward for a second and still go back and discuss the controversy. The controversy between two big giants Camillo Golgi, who I just mentioned developing the technique to stem cells. Santiago Ramon y Cajal who used this technique. Both received the Nobel Prize for their huge, fantastic anatomical discoveries. Detecting cells, detecting networks, detecting structures within the nervous system. And especially Ramon y Cajal was a very, very good painter, a very, very good artist, and actually wanted to be an artist, but eventually became a great scientist. But he was an artist as well, as you can see. These original drawings by Ramon y Cajal, sitting in front of his very very old, poor microscope. Looking at the microscope again and again at different pieces of the nervous system, from the spinal cord to the cerebellum, to the cortex. So here is the cerebellar neurons and this is the cortical networks, here and there, then other regions, and this is the retina inside the eyes, the eyes, with all these different layers. So Ramon Ramon y Cajal was really a giant in the sense that he liked to draw, and he liked to characterize structures. And he went to different regions in different animals, humans, rats, and others, and to the most simplest system, and try to really draw what he sees. And this is using, as I said, the method of Golgi to stain cells. Luckily, as I said, they are really very sparsely stained. So you see one and then another one and then another one. Not everything is being stained because otherwise it would be completely black. So about one percent or less of the neuron in this system are being stained. And this enabled Ramon Ramon y Cajal to follow, to draw one and then another one, and then another one. So Ramon Ramon y Cajal, when he just looked anatomically at the system, he did the following, which is a big jump conceptually. So he see a network in this case, cortical network. And he sees cells, so this is one cell and this is another cell body, and you can see the dendritic tree, and if you zoom in, as we should do later, you can see this little appendages which we will call spines, dendritic spines. But he saw it, when he drew it, and then he invented something, almost. Almost invented something. Just by looking at the anatomy of the system, he said, I think that information flows, information flows. But he didn't record any information. He just thought it. Information flows through a process, that is an axon. A long process, in this case coming from the white matter into the cortex, from the thalamus into the cortex. So we draw this little arrow here saying that information flows in this direction, along the axon. And then he did another jump. He said, from this axon to this dendrite there is a communication. He could not see any communication. But he thought that information must flow this direction. And then it means that they must be somehow connected to each other through this link. And from this dendrite, receiving the input from the axon, information flows this direction from the dendrite to the cell body of this post neuron. And then from the cell body to the axon of this neuron, and then from the axon of this neuron to the dendrite of the next neuron. So these arrows, these arrows showing information flow, direction of information flow, is a pure so to speak invention of Ramon Ramon y Cajal, because he could not record any activity. He just could look at their anatomy. But this was the beginning of his concept that one neuron and the second neuron are separated somehow. And so he can call each one of them a cell, a nerve cell, later on a neuron. Okay, so this is the neuron doctrine by Ramon y Cajal. And the other, what he called the theory of dynamic polarization, he says the receiving cell, the receiving dendrites are first polarized, somehow polarized, electrically probably, ret polarized in this region, because the input comes to the dendrite. It polarizes here and then the polarization flows from the dendrite to the soma and then to the axon and so forth. So this is called the theory of dynamic or dynamical polarization, by Ramon Y Cajal. So, to summarize Ramon y Cajal, the functional view of neurons, he says the following. Dendrites are the receptive, the input device. They receive input from other neurons. And axons are the sending, the output, device of neuron. And today, we actually basically accept, for more than 100 years, we accept this concept. We agree with Ramon Y Cajal. Golgi did not like this description. He did not feel that there is a distinction or separation between one neuron to the other. He thought that they are all connected physically, they touch each other, and it's one big connected network, so he didn't agree to the concept of neurons. And this was probably part of his romantic view that the brain is something different than any other system. How could the brain, how could the feelings, how could emotions, how could the love of opera, and everything else that we create, relies on individual elements like these, connected to each other carrying information? It must be something continues, it must be something different than any other system. It must be something different. That's what Cajal said. And both essentially fought on the podium in 1906, when each presented his Nobel lecture. Ramon Y Cajal spoke about the neuron doctrine, about the separation of cells from one another. He did not know how they talked to each other, but he realized there must be some connection. And Golgi said, my honorable colleague is mistaken. It's a very nice theory. But it's not the correct theory. So today, of course, we accept Ramon Y Cajal concept. The nervous system is built, essentially, from separate elements connected to each other via synapses, which we are going to discuss in a second. But just before that, I wanted to show you a modern view of neurons. Because today we can stain and reconstruct using computers, using modern microscope and staining techniques. This is an example, a beautiful example from Kevin Martin in the University of Zurich, where you can see very clearly an individual cell in the cortex In this case of cat. The green element is the dendritic tree. You can see that the dendritic tree is local, relatively local. And this is about one millimeter from the cell body to tip of the dendrite. And you also can see the axonal tree. The red and the white dots the, axonal tree. Axons can go sometimes out, very far away, or sometimes more local. So you can see the red thing is the axonal tree, and the white points, the white dots in on the axon are what we call the varicosities. You will later see that in each of these little varicosity, or sometimes called buttons, in each of these buttons there is the neural transmitter, the chemical transmitter, that interacts through the synapse with a postsynaptic, with the other element, with the other cell, through this little spot, the synapse. So you can see an axon full of spots. Sometimes 5,000 synapses. 5 times 5,000 spots within a given axon. So this is a typical view of a neuron. The dendritic tree which is local, the axonal ramification branching, which could be rather global, sometimes go to another part of the brain, sometimes make a lot of synapse locally.