Glial cells in the nervous system

For many years, neurons stole the spotlight in neuroscience. As we have learned from previous posts, neurons have electrochemical properties and conduct nervous signals. While it is undeniably true that neurons are essential, there is another whole category of cells that is also quite important and actually outnumbers the neurons: These are the glial cells. Although these cells are not capable of sending electrical signals like neurons, they are important for the integrity and functioning of our nervous system, including neuronal survival and communication [1].

Glial cells can be separated in two categories: Microglia and Macroglia. 

Microglial cells provide immune protection for our nervous system. Thus, they are key for inflammatory reactions and for defending our brains from pathogens. They are also important for regulating cell death and getting rid of debris [1]. 

Macroglial cells include many different subtypes [2]. In this post, we will focus on some of the most important ones:

1. Astrocytes provide support to neurons and modulate their environment to make sure it is suitable for neuronal communication [1], which eventually will impact the neuronal circuitry. Thus, they regulate the ion levels in the cell environment, and they are also important for the uptake and turnover of some neurotransmitters* [3,4]. Moreover, they help control the blood flow and become the “communicating barrier” between the blood capillaries and our neurons [3,4].

2. Ependymal cells are the “communicating barrier” between neuronal tissue and the cerebrospinal fluid (CSF). They produce the CSF and modulate its distribution throughout the brain [5]. The CSF is important for waste removal, nutrient supply, and for providing “buoyancy” and “cushioning” to the brain and spine (that way, they are “suspended” in a type of liquid and, if there is a hit, the injury is less severe) [6].

3. Oligodendrocytes are essential for the myelin covering of some neurons in the central nervous system [1]. As we learned in previous posts, neurons conduct signals, and this process needs to be fast. In an effort to speed it up, myelin is a chemical substance rich in lipids (it is like a type of mix between fat and proteins) that provides a form of “electrical insulation” in the neuronal axon. 

4. Schwann cells are the glia in charge of covering the axon with myelin in the peripheral nervous system [1].

Research on glial cells continues advancing and these cells, which used to be relegated to a secondary role, started playing an important part in understanding the functioning of our brain and the pathophysiology of diseases. For example, multiple sclerosis is one of the diseases characterized by the loss of myelin, which is partially due to the dysfunction in the oligodendrocytes [7]. Other studies have focused on understanding the role of astrocytes in the context of depression, which would perhaps lead to new developments in antidepressants [8]. Lastly, microglial cells have been studied in the context of Alzheimer’s disease (AD). They can play a role in protecting the brain and preventing AD, but there are also studies that show that, in more advanced disease stages, activated microglial cells are involved in the loss of synapses [9].

These are just examples illustrating why glial cells deserve a second spotlight in neuroscience to further our knowledge of the nervous system. Stay tuned for future posts about glial cells and brain immunology!

*Neurotransmitters: are the chemical substances that neurons send to communicate a message in a chemical synapse.

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Written by: Nicole

Edited by: Natasha

BioDecoded is a volunteer group committed to sharing accurate scientific information. We cannot offer any specific health advice. If you have any questions about this topic or would like to learn more, please comment below, or send us your questions.

References:

1. Purves D, et al. (2001) “Neuroglial Cells”, Sunderland (MA): Sinauer Associates; 2nd edition. Available at: https://www.ncbi.nlm.nih.gov/books/NBK10869/

2. Rea, P. (2015). “Chapter 1 - Overview of the Nervous System. In Essential Clinically Applied Anatomy of the Peripheral Nervous System in the Limbs”, Academic Press (1-40). Available at: https://www.sciencedirect.com/science/article/pii/B9780128030622000012

3. Lago-Baldaia, et al. (2020). “More Than Mortar: Glia as Architects of Nervous System Development and Disease” Frontiers in cell and developmental biology, 8, 611269. Available at:  https://doi.org/10.3389/fcell.2020.611269

4. Allen, N. J., & Barres, B. A. (2009). “Neuroscience: Glia - more than just brain glue”, Nature, 457(7230), 675–677. Available at: https://doi.org/10.1038/457675a

5. Jiménez, A. et al. (2014). “Structure and function of the ependymal barrier and diseases associated with ependyma disruption”, Tissue barriers, 2, e28426. Available at:   https://doi.org/10.4161/tisb.28426

6. Telano LN & Baker S. Physiology (2023). “Cerebral Spinal Fluid”, StatPearls Publishing. Available at: https://www.ncbi.nlm.nih.gov/books/NBK519007/

7. Dulamea A. O. (2017). “Role of Oligodendrocyte Dysfunction in Demyelination, Remyelination and Neurodegeneration in Multiple Sclerosis”, Advances in experimental medicine and biology, 958, 91–127. Available at:  https://doi.org/10.1007/978-3-319-47861-6_7

8. Rajkowska, G., & Stockmeier, C. A. (2013). “Astrocyte pathology in major depressive disorder: insights from human postmortem brain tissue”, Current drug targets, 14(11), 1225–1236. Available at:  https://doi.org/10.2174/13894501113149990156

9. Hansen, D. V., Hanson, J. E., & Sheng, M. (2018). “Microglia in Alzheimer's disease”, The Journal of cell biology, 217(2), 459–472. Available at:   https://doi.org/10.1083/jcb.201709069