Did you know that neurons aren’t the only kind of brain cell? In fact, these celebrated biological information processors only comprise 10% of the cell population of the brain. The other 90% of brain cells, called glial cells or glia, don’t have as many “intelligent” characteristics as their complex neuronal cousins, but the nervous system wouldn’t be able to function without them. Up until recently, glia, whose name derives from the Greek word for glue, were written off by the scientific community as a simple, gelatinous substance whose sole purpose was to act as packing peanuts to cushion our precious neurons. With more advanced modern research methods, however, we are starting to recognize the subtle yet essential contributions that glia make to our cerebral performance.
The Society of the Nervous System
Just as there are multiple kinds of neurons, there are multiple kinds of glia. These different subcategories of glia are found throughout the nervous system, but are mostly concentrated in the central nervous system, also known as the brain and spinal cord. Despite being categorized as the same type of cell, each subcategory of glia serves a very distinct role from its other glial relatives. In order to understand the diverse roles of glia in comparison with the roles of neurons, it’s helpful to imagine the nervous system as a society with the neurons as a selective and elite class of businesspeople, lawyers, entrepreneurs, professors, and other esteemed careers that deal with information processing and transactions, and then to imagine glia as the rest of society, with careers that mostly have to do with serving the upper class of neurons. In this society, some glia act as janitors, others as chefs, and some even as secretaries to neuronal executives, as well as many other occupations of the like.
Many of the roles that glia take on revolve around the maintenance and enhancement of neurons and their intracellular communication. To truly appreciate the importance of glia, it’s necessary to understand the basic anatomy and cellular processes of the neuron. Pay particular attention the myelin sheath and the synapse, because both of these neuronal components absolutely depend on the presence of glial cells.
Glia, specifically oligodendrocytes in the brain and Schwann cells in the peripheral nervous system, form and maintain the insulating myelin sheaths around neuronal axons. You can think of oligodendrocytes and Schwann cells as the electricians of the nervous system: the myelin sheath increases the conductivity of an action potential (the way that neurons communicate with each other) throughout the length of the axon so the signal can be transmitted efficiently to the next neuron and beyond, just as the coating of an electrical wire enriches the current flow throughout the wire. The oligodendrocytes and Schwann cells ensure that the correct axons are myelinated, and that the signals are efficiently reaching their intended destinations. Without myelin sheaths, many of our brain’s critical messages would fizzle out before reaching their intended destinations. Multiple sclerosis, a painful autoimmune disease that that severely impairs one’s motor skills and other important bodily functions, is the result of the systematic degeneration of one’s myelin sheaths. In other words, if your glia malfunction, your whole body malfunctions.
Aside from their humble yet essential role in myelin sheath formation, glia serve the neurons in many other ways. For example, glial cells called astrocytes (the most common type of glia in the central nervous system) regulate blood flow in active neurons, in addition to providing them with energy in the form of the indispensable neurotransmitter, glutamate. Astrocytes that take on these roles can be viewed as nutritionists and chefs: they make sure that neurons are getting proper nutrients, and cook them up some glutamate when they are running low.
Another subset of glia, microglia, constitute a specialized cranial immune system. Microglia attack and consume harmful neurotoxins, thus protecting the fragile nervous system when these toxins breach the blood-brain barrier. Though the job of microglia is comparable to that of a janitor, i.e. cleaning up unwanted waste, one’s neurons would get very sick in a toxin-polluted brain, just as a person would get sick in an unclean, mold-infested building. Microglia are especially important because neurons don’t know how to clean up after themselves. Microglia can also be thought of as the firefighters of the brain, because when an infection strikes, they excrete chemicals that reduce the inflammation and quell the infection. Without microglia, we would be much more prone to dangerous diseases such as meningitis.
Though glia are very important in practical neuronal maintenance, some glia are capable of assisting neurons in information transmission. An advanced subset of astrocytes are known to hang out around neurons’ synapses and relay information-carrying neurotransmitters from one neuron to another. In this sense, astrocytes’ role in synaptic transmission is comparable to a secretarial job. The synapse is the infinitesimal space between an axon terminal of one neuron and an adjacent receiving dendrite of another neuron, and is the location where instructive neurotransmitters are activated and influence the receiving neuron. By latching onto multiple axon terminals of multiple distinct neurons, astrocytes are able to “listen” to the neurotransmitter activity as if they were listening in on a conference call, and modulate the levels of neurotransmitters to optimize synaptic activity through their heavily ramified dendrites, thus keeping the conversation going. This enables them to “respond to synapse activity by making localized and global changes in intracellular calcium ion concentration” . This is a crucial task because calcium is the most important ion involved in intercellular signaling in the brain, and without it, neurons wouldn’t be able to communicate with one another. Astrocytes, like secretaries, ensure that the message gets passed along.
Overall, glia are paramount members of the “society” of the nervous system. Their various contributions to cranial functioning are constantly being discovered, but we still don’t know the full extent of their capabilities and neural influences. It very well may be the case that glia play essential roles in the onset and/or cure for various psychological disorders, but neuroscientists will have to devote more time to researching them, which may be difficult when neurons seem so important and exciting. Despite the misinformed origins of their name, glia are truly the “glue” of the nervous system: their functions hold the entire system together, and without them, everything would surely fall apart.