By Lauren Lim
What are brain cells? You’ve probably heard of neurons, also called nerve cells. But neurons are only one type of cell composing the nervous system. The second main type of cell is the glial cell. Some estimates claim that there is a 10:1 ratio of glial cells, or glia, to neurons, but some more recent research estimates that glia are roughly equal in number to neurons (Jabr, 2012). When they were first discovered in the 19th century, glia were thought to only provide structure for neurons and act as the “glue” of the nervous system. By now, glia have long been known to also support neurons by supplying nutrients, removing waste products, insulating neuron signal transmissions, and performing other duties. Within the broader category of glia are specialized glial cells, such as microglia, astrocytes, oligodendrocytes, satellite cells and Schwann cells (Guy-Evans, 2021). In recent years, researchers have discovered many complex functions of glial cells that demonstrate that glial cells have a greater significance in the nervous system than previously thought.
Neurons have been the focus of neuroscientific research due to their ability to transfer and process information, making them the basis of neural functioning. Neurons communicate with each other at sites called synapses, and increasing evidence is proving that astrocytes play a major role in synaptic transmission (Araque & Navarrete, 2010). For one, they maintain the homeostasis of environments around synapses, improving synaptic efficiency. Transmission of neurotransmitters like glutamate is one way that neurons communicate with each other at synapses. However, excess concentrations of glutamate can cause hyperactivity in neuronal networks and even epileptic activity. To help prevent such pathological issues, astrocytes clear glutamate from synapses. Not only do astrocytes support neuronal communication, but studies have also found that complex information processing in neuron-to-astrocyte communication has similarities to neuron-to-neuron communication, proving that glia play an active role in neuronal networks. Variations in calcium ion concentrations inside astrocytes determine cellular excitability, allowing them to make use of electrical signals. Furthermore, astrocytes have specific responses to different neurotransmitters, and even release their own neurotransmitter-like chemicals called gliotransmitters.
Although it might sound reasonable to think that more synapses is better; in fact, the opposite is true. When children develop, their brains form synapses—many more than are needed and many more than are efficient. As a result, a process called synaptic pruning is essential for healthy brain development and function. Microglia can engulf synapses, thereby contributing to synaptic pruning (Ji et al., 2013). Aside from participating in brain development, microglia also maintain brain health (Guy-Evans, 2021). When signaled, microglia move to areas of injury or disease to clear dead cells, pathogens, and harmful waste.
Other recent studies have shown that glial cells may be involved in chronic pain. When pain signals travel from the body, or the peripheral nervous system, to the brain, or the central nervous system, glia regulate the intensity and duration of the pain (Dobbs, 2021). When this process goes awry, glia may cause neuroinflammation and prompt nerve cells into sending never-ending pain signals. Microglial, astrocyte, and satellite cell activity have all been found to contribute to chronic pain, but researchers are not yet sure where and why glial functioning goes wrong. The involvement of glial cells would explain why current painkillers aren’t effective: they only target neurons. Previously, researchers had struggled to find the biological basis of chronic pain, but now that they’ve identified glial cells as the issue, there’s more direction for finding a solution. Despite this progress, treatment for chronic pain may still be a while away. Glia perform a range of functions and are so vital for other processes in the nervous system that trying to simply incapacitate them would cause more harm than good.
As new discoveries are made, there has been increasing interest in the function of glial cells. Glia are not only vital in the development and maintenance of the brain, but have also been proven active in neuronal communication. Links between glia and certain neurodegenerative diseases plus the chronic pain issue open up new paths for potential treatments. This research, though, may progress slowly. As National Institutes of Health researcher Dr Doug Fields put it, “neuroscientists have studied neurons for over a century, but they are playing catch-up with glia” (Dobbs, 2021).
References
Araque, A., & Navarrete, M. (2010). Glial cells in neuronal network function. Philosophical Transactions of the Royal Society B: Biological Sciences, 365(1551), 2375–2381. https://doi.org/10.1098/rstb.2009.0313
Dobbs, D. (2021, November 22). How Glial Cells Are Quietly Revolutionizing Chronic Pain Study and Care. The New York Times. https://www.nytimes.com/2021/11/09/well/mind/glial-cells-chronic-pain-treatment.html
Guy-Evans, O. (2021, June 9). Glial Cells Types and Functions - Simply Psychology. https://www.simplypsychology.org/glial-cells.html
Jabr, F. (2012, June 13). Know Your Neurons: What Is the Ratio of Glia to Neurons in the Brain? Scientific American Blog Network. https://blogs.scientificamerican.com/brainwaves/know-your-neurons-what-is-the-ratio-of-glia-to-neurons-in-the-brain/
Ji, R. R., Berta, T., & Nedergaard, M. (2013). Glia and pain: Is chronic pain a gliopathy? Pain, 154(Supplement 1), S10–S28. https://doi.org/10.1016/j.pain.2013.06.022
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