The cerebral cortex is the surface of the brain. We talked about this a little bit in the introduction, the gross anatomy of the brain. We pointed out all of the different, these sulci, which are the great big divots in between and the gyri which are kind of the bones. This is your neocortex. Your neocortex means new cortex, and it's unique to mammals. So, reptiles don't have it, birds don't have it, fish don't have it. It's a mammal thing. So, be proud that you're a mammal and you have neocortex. The surface area of the cortex, when we talked about gross anatomy, I mentioned that your brain is too big for your head. That's why it's in all these folds, these sulci and gyri. So, if you could take your cortex and flatten it out, it would be about 2.5 square feet. So, about the size of a newspaper. So, if you think about that, you think about taking a newspaper and turning it into a balloon, that's how big your head would have to be if we didn't have sulci and gyri. The cortex is about 80 percent the mass of your brain, and it's where you think. It is really where you as a person exist in your nervous system. So, everything that's unique to you is because of your cortex and is a property of your cortex. It's influenced greatly by all of the sensory information that's coming in from the outside, everything you see, and hear, and smell, and feel, and internal stuff, the internal workings of your own body, the things that your central nervous system consents. You can self-generate. You think your own thoughts. So, there are multiple levels of stimulation that are being integrated within the central nervous system and most of it is going to be integrated in the cerebral cortex. So, as we said, neocortex is new cortex and it's six layers. We can see some of those layers here. In this cartoon, we can see there's layers one through six. There's an outer molecular layer. There are layers three and five, which are these great big pyramidal cell layers. Then there's layers two, and four, and six, which are receptive layers. We'll talk a little bit more about those when we talk about cytoarchitecture. Archicortex or paleocortex, think about archaeology or the Paleolithic period. Those are both terms that mean old. So, there is old cortex in your brain leftover from evolutionarily speaking. One area of this is found in the hippocampus. Old cortex only has three layers. So, again, layers one through six are superficial too deep. So, layer one is out here. Layer six is here. They are basically layers that send information and layers that receive information. The layers that send information are these layers that have the great big cell bodies because if you think about it, if you're sending information all the way from your cortex, think about the motor system. All the way down to the bottom of your spinal cord, you've got really long axon to maintain. If you have a really long axon to maintain, you've got have a really big cell body to maintain that axon. If you are a layer that receives, so you're getting all kinds of information, you're going to have a whole bunch of different neurons in there. So, areas like this would receive, and area four is big into receiving. You have big dendritic arbors. So, lots and lots areas for synapses to take place. I'm going to erase and turn my drawing tool back on. What I also want you to note is it depends on how you look at the cortex. So, this image is a Golgi stain, it's an artist's rendition of a Golgi stain. Golgi stains were discovered accidentally by dumping a chunk of the brain into some heavy metals. We don't know exactly how it works. Still to this day, we don't know how it works. But the bottom line is that the heavy metals will fill up the entire cell body, and its axons and dendrites. It doesn't fill up every cell because if it did, it would be just one big black chunk of tissue. But it does fill up the odd neuron here and there. Nobody knows why some neurons are filled and some aren't. But it works really well to see the entire architecture of a single cell. The middle panel is a Nissl stain. Nissl substance is big stacks of rough endoplasmic reticulum, which are responsible for making proteins. So, neurons make a lot of proteins. They make neurotransmitters, they make all kinds of proteins to put into their channels and their receptors. So, this stain sit in mostly the cell body. So, you don't see a lot of processes, but you see the shape of the cell body. It stains almost all of them. So, you have a different look if you use a Nissl stain versus a Golgi stain. In this last panel, we stain for myelin. Myelin is that lipid rich coding that surrounds all of the axons to keep cross talk from happening. So, if you have a signal going down one wire, you want to go in only down that wire and not down another one. So, you have to keep them insulated. It's just like the insulation on the wires in your house. So, two axon side-by-side have to be insulated from one another. That is what myelin does. If you stain for the myelin, what you notice are these columns of cells separated by these bands of fibers that are making these columns within the cortex. It turns out that these columns are the functional unit of the cortex. So, neurons within a column are working together, and then they talk to the neurons in the next column or in the next column this way. But these guys are all working together. In the next section, we'll talk about more of the cytoarchitectonics and what that means with regard to function.
