 We're all probably familiar with the fact that neurons are the signal transmitting cells of the nervous system, but there are a few extra details that you need to know about neurons and the other cells of the nervous system when you're preparing for step one. We'll start by going over the anatomy of a neuron. A given neuron will receive information from other neurons via its dendrites. The dendrites stem from the cell body of the neuron, which contains most of its organelles. The signal received by the dendrites then travels along the axon. I want to emphasize something about neurons that often gets tested on step one. That's the nisselstain. This is a stain that pathologists can use on nervous tissue. The nisselstain is specific to the cell bodies of axons because it binds to the nuclei and the rough endoplasmic reticulum of neurons, which are present only in the cell body. You should at least be familiar with the fact that the nisselstain is specific for the cell bodies of neurons. Now there are a few other cells of the central nervous system that are essential to its function. These are astrocytes, microglia, oligodendrocytes, Schwann cells, and appendable cells. Collectively, all these cells are known as glial cells. Glial cells is the general term for non-neuronal cells in the central nervous system. Now let's go through these cell types one by one and discuss their function. Starting with astrocytes. Here's an example of an astrocyte in our image here. It has these foot processes that look almost like the dendrites of an axon. Astrocytes are very versatile cells within the CNS. The foot processes of astrocytes are what make up the blood-brain barrier. An additional function of astrocytes is in the repair of the central nervous system. When neurons are damaged, for example, in a stroke, astrocytes step in and form the equivalent of a scar in the brain. This is a phenomenon known as reactive gliosis, which forms what is known as a glial scar. Astrocytes are also responsible for metabolizing excess neurotransmitters to maintain a proper chemical balance in the brain. In addition to helping maintain neurotransmitter balance, astrocytes maintain potassium balance. They act as a potassium buffer by taking up excess potassium, ensuring that neuronal signal transduction happens efficiently. One last thing I want to mention about astrocytes is their relationship to glioblastoma multiforme, or GBM. Glioblastoma multiforme is an aggressive malignant CNS tumor that is notoriously difficult to treat. GBMs are tumors of astrocytes. Glioblastoma multiforme has infamously led to the death of several celebrities and political figures, including the late John McCain. Next up, we'll talk about microglia. There are the phagocytic cells of the central nervous system. You can think of them as being similar to macrophages. They serve as antigen-presenting cells within the brain and the spinal cord, and they're responsible for cleaning up cell debris after damage or infection. You can see an example of what microglia look like in the image above. Just like astrocytes, they have this branching appearance. Moving on, we have oligodendrocytes. The purpose of oligodendrocytes is to myelinate neurons. It is important to note that oligodendrocytes myelinate neurons only in the central nervous system. The peripheral nervous system has its own type of myelinating cell, called the Schwann cell, which we'll discuss shortly. Myelin is a protein that surrounds axons and serves to speed up the transduction of action potentials between neurons. Myelin is essential for normal function of neurons. In fact, in demyelinating diseases like multiple sclerosis, the loss of myelin can cause neurologic symptoms like weakness and vision loss. An important thing you need to know about oligodendrocytes for step one is that a single oligodendrocyte can myelinate the axons of multiple neurons, up to 30 actually. This is another way that oligodendrocytes differ from Schwann cells, since a single Schwann cell can only myelinate a single neuron's axon. You can see what oligodendrocytes look like in this image here. They are also said to classically have a fried egg appearance on histologic slices. Now let's take a look at Schwann cells. Once again, they are the myelinating cells of the peripheral nervous system. They lay down myelin on axons of neurons in the peripheral nervous system in order to increase the speed of action potentials. A single Schwann cell is responsible for the myelination of a single neuron. This image is a good depiction of the function of myelin. It coats the axon like the coating of a wire. But it leaves these open spaces called nodes of Ranvier. The action potentials in myelinated axons skip from node to node, increasing the speed of signal transduction along the axon. Finally, we have ependymal cells. Ependymal cells line the ventricles of the brain and their function is to produce and circulate cerebrospinal fluid. When you look at them under a microscope, you may be able to see that they have small cilia just like in the GI tract that helps circulate CSF through the ventricles. You'll also hear about ependymal cells in the context of ependymomas, which are a type of CNS tumor that commonly occurs in pediatric populations. This has been a review of the various types of glial cells that you need to know for USMLE Step 1. These concepts are frequently tested and you need to have a good understanding of them in order to do well in neuropathology as well, because several of these cells we discussed can undergo mutations to form brain tumors. Thanks again for tuning in. We hope you found this review helpful.