So welcome to lesson two of the course Synapses, neurons, and brains. Last time we spoke, I gave you a general introduction to the excitement in brain research today wnd with some techniques to approach the brain in a modern way. Today we shall dwell into the ingredients of the brain. I want to discuss with you the materialistic brain, the ingredients of the brain, the neurons that the brain consists of. I want to discuss with you the axon, which is part of the neuron, a very interesting, long process, the axon. We should then discuss dendrites and dendritic spines. Later on, we'll discuss several types of neurons. I want to show you that neuron is not a neuron, but there are different types of neurons, and we'll talk about what does it mean, a type of a neuron. We later speak about synapses, the connection between neurons, the device that connects between neurons, the synapse. The electrical signals that the neuron carries, and we'll discuss two major electrical signals, the spike or action potential. The active signal, and the synaptic potentials, the postsynaptic potentials, PSPs, which is the signal that is generated between one cell to the other through the synapse. And finally, I want to summarize this session, this lesson, by showing you or discussing with you the neuron as an input/output, very interesting device. So this is the plan of the talk for today, we'll see how we progress. So but first, I want to give you a perspective, a general perspective, about the development of ideas, development of tools and concepts until we came to the day when we know that neurons are the elementary building blocks of the brain. [COUGH] So about more than 500 years ago, 400 years ago, there was a microscope. So this microscope was really a big invention because it enables for the first time to look at the microscope and see a different tissues, different type of tissues that they are consistent of cells, the notion of a cell. This came about after the invention of the microscope, and then there was this issue of cell theory that tissue, every tissue, is built from cells, individual cells together form a tissue. But is it true for the brain? Is the brain a regular tissue? A tissue that eventually becomes what it is, but it is built from little building blocks like neurons. Maybe yes, maybe no. In 1870, Camilo Golgi, which you already heard about him, and we'll talk a little bit more about him, Camilo Golgi, the anatomist, the Italian anatomist, develop a very special method using silver to stain neurons, to stain the nervous system. Luckily, this staining was very random, very, very random. So it stained only very few cells. And suddenly, as you will see soon and you already saw, you could see entities, you see ingredients inside the nervous system. But this is due to a new technique to stain nerve cells, and this is due to Camilo Golgi. This is 1870 and then the colleague and open end. The Spaniard, Ramon Y Cajal used the technique developed by Golgi. In order to really detect draw cell by cell, and he proposed that indeed like any other living system, also the nervous system is built from individual separate elements. He called it the neuron doctrine. Later on about a few years later, four years later, the term neuron was invented. So before, about 120 years ago, there was no neuron so to speak in the world, and the term neuron was invented by the German Waldeyer. And finally, this process of understanding the nervous system, characterization of the nervous system, Charles Sherrington, the British came and coined the word synapse. So all this is a very brief introduction to the development of tools, both of the microscope, the staining tools, and then concepts that there is a neuron like in any other tissue, and this neuron has a term suddenly. A neuron and then there is a new term for connection between neurons, the synapse. So one of the players interestingly in this development of ideas, was Sigmund Freud. Because Sigmund Freud as you may know started as a neurobiologist actually working in Trieste in Italy. And he was also drawing neurons, crayfish neurons. And you can see original drawings by Freud who was also like Ramon Y Cajal believing in the individual, in the neuron doctrine. Maybe later on, but that's my speculation. It took the neuron as an individual and dealt with humans as individuals. So Sigmund Freud is part of this development of concept about the neuron as an individual. But let's go to the brain. So when we look at this interesting tissue below our skull, we can characterize regions, we can speak about function of different regions, but we can zoom into the brain, very, very fine zooming into the brain. And when you zoom into the brain, what you see everywhere independent where you look at, you will see this. You will see a network of neurons, very heavy, very intense, a jungle of elements, of neurons sitting one inside the other, one on top of the other intermingled like in the jungle of trees. With a lot of wires, with a lot of interactions as we should discuss in a second, a lot of jungle of wires and jungle of neurons. And this is what we need to understand, how this jungle of neurons generates all that we know about, feelings, emotions, and so forth. Today, we can really look at the living brain, and this is a new technology, the photon imaging technology that enables us to zoom in, to look into the network, in a living system, in the living brain. And you see cells, you zoom into the depths of the brain. You see more cells, and suddenly, you see activity of the group of the cell. So we have now techniques to record the activity using visual technique, using light in order to see suddenly these green group of cells flashing. So that's what you see in the brain. You see cell after cell after cell. And there are these beautiful techniques today to look at the brain in modern way, we discuss the Brainbow technology. This is one example in the hippocampus, for example, you see the cells and the cells that are very particular structure. You can see some kind of a process. We'll discuss it, we saw dendrites. You can see axons. We'll discuss the axons. We'll see the cell body. This is in a given region, Hippocampus. We can look at other regions, and we should also see cells. This is a cortex here, just below the skull. One cell, another cell, dendritic tree, axonal tree, cell body. This is this repetition of a given building block in any area of the brain, in any system of the brain. Big brain, small brains, of cockroach, of mouse, of monkey, of cat, of human, this is the building block, a network of elements and connections and axons and wires going from one to the other, again and again. Another beautiful picture for you from the cortex of the mouse. In this case from Henri Malcolm's lab in Lucerne where they succeeded to individually stain one cell, another cell, another cell, another cell, 12 cells together with a fluorescent dye, and you can see again, this is a parameter cell. We speak about parameter cell, dendritic tree, axonal tree, cell body, and axon going all over. This is the elementary building block of any nervous system, cells and their connections.