When people refer to “brain cells” they are often talking about neurons. While these might be the more well known cells in the brain, there are actually other types of cells too! These non-neuronal brain cells are called glia, and they include astrocytes, microglia, and oligodendrocytes.

NEURONS

Neurons are the cells that carry and store information in the brain. They help you experience the world around you, move your body, and even help store your thoughts and feelings!

There are a few different shapes of neurons, but they all have the same basic parts. Neurons have cell bodies, called the soma, that house the nucleus of the cell. Dendrites are root-like projections that receive information. Most neurons have one long projection called an axon that acts like a highway for information to move down the neuron. Myelin sheaths can wrap around these axons and help signals travel along the cell more quickly.

Four differently shaped neurons labelled unipolar, bipolar, pseudounipolar, and multipolar — Neuronal shape is different depending on the location and function within the brain

There are a few basic shapes of neurons which depend on their location and function within the brain. If you look up a picture of a neuron, you will probably see a Multipolar shaped neuron, since these are very common within the brain. These neurons have a cell body that is surrounded by dendrites, and one long axon with many branches at the end. Unipolar neurons are shaped similarly to multipolar neurons, but do not have dendrites surrounding the cell body. These neurons are found in invertebrates, such as insects, and not in human brains. Bipolar neurons have a cell body with one long projection, with either a dendrite or a sensory receptor on one side and an axon on the other side. These cells are found in the retina, and transfer visual information. Finally, a pseudounipolar neuron has an axon that branches out in two directions. These cells collect sensory information from the body and so instead of dendrites, they have special sensory receptors. There are different types of sensory receptors that can detect pain when you are hurt, or touch through pressure or vibrations.

When neurons are connected together, the axon of one neuron will release signaling molecules called neurotransmitters onto the dendrites of the next neuron. This is how neurons send information to each other. The connection between the projections of two neurons is called a synapse. When a neuron receives information from another neuron it can become activated. An electrical signal goes down the length of the neuron and then stimulates the release of neurotransmitters onto the next neuron.

A diagram of a blue neuron with markers to each part of the neuron including the dendrites, soma (cell body), nucleus, axon, myelin sheaths, and axon terminals. There is a red arrow which shows that messages travel from the dendrites to the cell body — Neurons have a very unique cell structure which is important for transmitting messages down the length of the cell and to the next neuron. This helps these messages travel large distances.

The interaction between neurons allows information to be communicated throughout the brain and between the brain and body. When you perform any movement like raising your hand, it is because your brain is sending signals through chains of neurons, telling your muscles to move. Another example of neuron communication is vision! You can see because special receptors in your eye react to light and send information through neurons about the light to a specific part of your brain where different neurons process that information. Neurons are very impressive and important cells!

ASTROCYTES

A yellow cell with many large projections. There is a blue dot in the middle of the cell which is the nucleus. Along the projections there are blue dogs which mark the nuclei of other cells — This is a picture of an astrocyte taken with a microscope. Astrocytes have many long projections that make contact with other cells. The blue dots in the picture mark the nuclei of the astrocyte as well as other cells.

Although neurons may be the most well known cells, they do not make up most of the brain. In fact, cells called astrocytes are the most common cell type of the brain! There are about 100 BILLION neurons, and although we don’t know the exact number, even more astrocytes! The word “astrocyte” comes from Greek origins and means “star cell”. They were named this because they are shaped like stars. Astrocytes are the main support cells of the brain, and they have a lot of jobs. They help to control the blood brain barrier which prevents the entrance of things from the body that would be harmful to the brain. Astrocytes also provide structure to and closely interact with neurons. They provide energy and nutrition to neurons and also help manage synapses and neuron communication. They can even help neurons repair themselves after they are injured!

MICROGLIA

A red cell with long projections and a red nucleus — Representation of a resting microglia

Microglia are the smallest of the brain cells which is why they are called micro-glia. Their main job is eating things that should not be in the brain. This process is called phagocytosis and it is involved in many different processes. During brain development, microglia get rid of parts of neurons that are not being used anymore. By getting rid of those extra parts, microglia help make the neuron connections that we need stronger and more meaningful. Similarly, if there is waste or dead cells floating around in the brain, microglia eat it so that the brain is clean and can function properly.

Microglia are also the immune cells of the brain. If there is a germ (like a bacteria or a virus) that has made its way into the brain, the microglia become activated. They change shape and become more circular and look like a blob, instead of their usual shape which has many processes sticking out from the cell body. The microglia help to remove the invading germ. This is very important for clearing infections. However, if microglia are activated when they are not supposed to be, or are activated for too long, this can hurt neurons or other brain cells.

There are three black cells labeled Cell 1, Cell 2, and Cell 3. From left to right the cells have fewer processes and look more blobby. An arrow goes from left to right and is labeled activation — Microglia change morphology depending on what is going on in the brain. At a resting state they have many processes but become more circular as they are more activated.

OLIGODENDROCYTES

A blue oligodendrocyte cell has projections that are wrapping around the axons of two orange neurons — Oligodendrocytes have a cell body and then projections that wrap around axons to create myelin sheaths.

Oligodendrocytes also have a very specific role in the brain. Oligodendrocytes have several arms that wrap around axons to create myelin sheaths. This is called myelination. One oligodendrocyte can create myelin sheaths for more than one neuron, and one neuron can be myelinated by multiple oligodendrocytes. Myelination is important for transmitting information down an axon quickly. As the electrical pulse travels down the axon, the myelin allows the charge to essentially “jump” to the small spaces between the myelin sheaths. This makes neurons with myelinated axons much faster at sending information than neurons without myelination. When the information has to travel long distances, or an immediate action is required, speed is important. For example, if someone throws a ball towards you, your eyes have to transmit the information about the ball’s location to your brain, and then your brain has to coordinate your movements so that you can catch the ball. All while the ball is still moving through the air! This is why neurons with long axons are usually myelinated.

As you can see now, there are more types of brain cells than just neurons! Although neurons are the primary transmitters of information, they would not be able to do their jobs without the different glial cells.

Other Resources

Here are some links to other resources that may help you better understand brain cells

References:

An Easy Guide to Neuron Diagrams and Types. (2022, February 28). Healthline. https://www.healthline.com/health/neurons
Bachiller, S., Jiménez-Ferrer, I., Paulus, A., Yang, Y., Swanberg, M., Deierborg, T., & Boza-Serrano, A. (2018). Microglia in Neurological Diseases: A Road Map to Brain-Disease Dependent-Inflammatory Response. Frontiers in Cellular Neuroscience, 12. https://www.frontiersin.org/articles/10.3389/fncel.2018.00488
Siracusa, R., Fusco, R., & Cuzzocrea, S. (2019). Astrocytes: Role and Functions in Brain Pathologies. Frontiers in Pharmacology, 10. https://www.frontiersin.org/articles/10.3389/fphar.2019.01114
Williamson, J. M., & Lyons, D. A. (2018). Myelin Dynamics Throughout Life: An Ever-Changing Landscape? Frontiers in Cellular Neuroscience, 12. https://www.frontiersin.org/articles/10.3389/fncel.2018.00424

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Author: Elise Lemanski

Elise is a graduate student pursuing a PhD in Behavioral Neuroscience at the University of Delaware. Her current research focuses on the effects of Zika Virus infection during early development on maturation of the brain. Outside of the lab and classes she enjoys spending time outside as well as crafting and can often be found hanging out with her two cats, Dusk and Muffin.

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