So lets go from the axon to the dendrites. First of all, dendrites are really beautiful. You can see examples of three types of dendrites, in terms of the morphology. This is one dendritic tree, the red dendritic tree is a dendritic tree of a Purkinje cell. From the cerebellum, the small brain, the cerebellum, you can see how beautiful. So this is the cell body, this is the axon which will then branch. And this is the dendritic tree, which is in this case flat, almost flat, two dimensional. And you already see, in this beautiful image of this beautiful reconstruction, you can see also an axon. The green process is an axon from another cell. Intervening, going through the axon, the dendrite, making contacts at specific locations, which you cannot see. So that's one beautiful dendritic tree from the Purkinje cells of the cerebellum. That's another beautiful dendritic tree, very unique dendritic tree. This is called the starburst amacrine cell in the retina, in our retina. One layer consists of this particularly beautiful dendritic tree, the amacrine cells. In another area of the brain, in this case from the hippocampus, in the particular region of the hippocampus, the CA1, you can see the pyramidal cell of the hippocampus. It has pyramidal structure cell body and we see that, and we say this must be a pyramidal cell. And because of the unique structure, it must be pyramidal cell from the hippocampus. So as you see, I'm using the terms Purnkinje cell, amacrine, pyramidal set in the hippocampus. In order to give them names, we use, many times, the structure of the dendritic tree. So when you look at it, the face of the neurons in so to speak, in terms of nomenclature, in term of naming, is very much dependent on the structure of the dendritic tree. So these are the names, but we'll talk about different types of neurons in a second. But just to give you the beauty, the beauty of dendrite and again each dendritic tree will receive many, many synaptic inputs from many, many axons going through, across, or in between the dendritic tree. And already Ramon y Cajal, as we saw before, reconstructed dendritic trees, you can see a classical reconstruction of Ramon y Cajal, using Golgi staining method. You can see the dendritic tree of this pyramidal cell, in this case of the cortex, so again, there is the pyramidal structure of the cell body. There is this dendritic tree, we call it the apical part. And then from the cell body there are many other branches. The puzzle part of the dendritic tree of the pyramidal cell, and then you can see the axon going and branching. So this is what Ramon y Cajal only saw with his old fashioned, very, very poor microscope. And he drew this pyramidal cell, but he also could see something else that is very important. And if you zoom into here, and you already can see, just by looking at it that the distance, the dendritic tree is not smooth. But if you zoom in, you see the dendritic tree consists of all these little appendages, all these small branches. Many, many of them coming out from the main dendritic tree here. So one dendritic spine, another dendritic spine, and a third dendritic spine, so many little dendritic spines emerging out from the dendrites. So in this case, we will call this dendritic tree a spiny dendritic tree. Because if you zoom in, you will see many, many little appendages, and later on I will show you that this little appendage, the dendritic spine, is the region where synapses are made onto. So it's a very important little process, this dendritic spine. Not all neurons are spiny. Some of them are non-spiny, so they are smooth. And indeed, in general, you can characterize anatomically spiny cells, pyramidal cells are spiny, and other cell types that are not spiny, that are smooth. So he already show that there are this garden, the garden of spines. Sometimes 10,000 spines per pyramidal cell. And so, Ramon y Cajal drew spines. You can see spines drawn from different cell types. All of them are spiny cell types. In one cell, in another cell, in a third cell, and so forth. And Ramon y Cajal actually, on his deathbed, just before he died, in a little notebook, Ramon y Cajal drew spines. He was dying and he drew spines. He knew that spines are extremely important. He felt that they must be very important, but he didn't know why. Today we know why. To give you typical numbers for a dendritic tree, but they're very, very typical. Of course there are different sizes and different types of dendritic trees. But just to give you, let's say, for a pyramidal cell in the cortex, let's say of a mouse or a rat, just ballpark numbers. So the total dendritic area of both the epical and the basal tree are, let's say, around 20,000 square microns. The total membrane area of the dendritic tree. The number of dendritic spines per cell, as I said before, there are so many of them, could reach maybe 8,000 maybe 5,000, maybe 10,000 depending on the cell type. Each spine, each dendritic spine, and I emphasize dendritic spines because they are so important. Each dendritic spine has an area of about one square micrometer. And totally, if you want to count how many synapses, how many contacts impinges, connect to this dendritic tree, I would say on the order of 10,000, could be 8,000. Sometimes it could be less, in some cells like the Purkinje cell we have 200,000 synapses per one dendritic tree. So, but this is a ballpark number, just to give you the intensity of connections. So each one dendritic tree, imagine that you are a dendritic tree, and you get synapses, you would listen to 10,000 synapses on average. And it's a big part of your brain. So if you ask me, in terms of area in your cortex, maybe 50, maybe 60% of the area are dendrites. I can say that you, in terms of cortex, you is mostly dendrites, in terms of membrane area, not in terms of length cuz axons are longer, but axons are thinner, on average. Let's look at the very recent work, beautiful work, on dendrites in spines of humans. So this is work by Javier DeFelipe, from Madrid. He reconstructed, or stained and reconstructed as well, pyramidal sense from human cortex. And you can, again, see this is a pyramidal cell. You can see the apical tree with its branches. You can see the many basal trees with their branches. And when you zoom in, you can see again, a piece of dendrite from here. A piece of dendrite completely decorated. Completely covered with, in this case, maybe 30,000 spines, dendritic spines. So you can see this is a repeated phenomena in the rat, in the cat, in the mouse, in the monkey, in humans. Cortical pyramidal cells are covered, the dendritic tree of cortical pyramidal cells, are covered with these unique structures. Absolutely unique structures, the dendritic spines.
