Let's get into our central nervous system. Our central nervous system, or CNS, refers to our brain and spinal cord here.And the spinal nerves. These are spinal nerves. (notice: they are not involved in the CNS although they are in the picture) First of all, let's look at our brain. There're four parts, four big parts of our brain. One is telencephalon here, the biggest part of our brain including cortex. Diencephalon here, the middle part, the inside part, of our brain. Cerebellum, this part here and brain stem. Brain stem connects our brain with the spinal cord. Brain stem. Before we get into the four major parts of our brain, let's look at the blood-brain barrier. What is this blood brain-barrier? Blood-brain barrier is a highly selective barrier that localizes just underneath our brain besides the spinal cord. It separates the circulation of blood from the brain to the CNS. The blood-brain barrier is formed by epithelial cells. Back to early 20th century, researchers that studied the brain function and then they injected animals with this certain dye called trypan blue, here. The most of the animal body can be stained with this trypan blue except the brain and spinal cord. From this evidence, the scientists think maybe there's a barrier between our CNS, central nervous system, and the circulating blood system. The blood-brain barrier will prevent the infectious passages including bacterial yeast from getting into the CNS system. The blood-brain barrier allows the passage of some molecules, including water, gases, and lipids. But lots of things, including drugs, big molecule proteins cannot pass through this blood-brain barrier. What is this blood-brain barrier that is circulating blood here and then the top part here is our CNS system, is our brain, is our brain cells. And then the light green part is so-called blood-brain barrier. Astrocytes are necessary to create this blood-brain barrier. The blood-brain barrier occurs along all epithelial cells and form this very tight junction around capillaries that do not exist in normal circulation. These epithelial cells they are very close to each other and than they form this very tight junction with each other and then lots of the molecule's passages cannot pass through this barrier, so this barrier can form this blood-brain barrier.