The Other Brain
Synapse #3: It's time to appreciate the other 90% of your brain
It’s interesting to me that even though a football team has 52 players, the quarterback gets by far the most credit for team success and blame for team failure. Sure, the quarterback is a very important position, but without the offensive linemen protecting him and wide receivers working to get open, how could the quarterback succeed?
If the brain is a football team, then its neurons are the quarterback—integral to its function yet far out-numbered by other vital team members: the glial cells.
You may have never heard of glial cells, yet they make up nearly 90% of the cells in the brain. For over a century, scientists thought that they didn’t play any role in cognition and simply acted as glue (‘glia’ is Greek for ‘glue’) between neurons. One type of glial cell, the astrocyte, gets its name from its star-like shape. Many astrocytes can can be seen below:
Photo credit: Nancy Kedersha
Only in the last 20 years have we begun to appreciate the incredibly important role of glial cells in nearly every aspect of brain function. So why have they gone unappreciated for so long?
Why glial cells have been neglected
The answer to why glial cells have gone underappreciated may be as simple they were not discovered first. Neurons have long axonal projections that make them easier to study without advanced imaging technology. In 1791, Luigi Galvani first discovered the electrical properties of the nervous system and how electrical stimulation causes muscles to contract using frog spinal cords.
Later, in the 19th century, Ramon y Cajal famously studied and mapped out a scheme of the anatomy of the neuron and all of its connections:
This work led to what is now known as the “Neuron Doctrine”—that thoughts are stored in individual cells called neurons. As you can imagine, this research must have been painstakingly difficult. There’s an interesting phenomenon in social psychology called the “Justification of Effort” which states that humans tend to ascribe high value to something that received a lot of effort—regardless of its objective importance.
I think this is one reason why glial cells were neglected in neuroscience for so long. That is, we started down a difficult path studying neurons and used this as justification for their importance instead of the other way around.
Now, of course, neurons are important, but don’t you think that the 85 billion glial cells in the human brain are doing something important?
You are your glial cells
In the last two decades, we’ve discovered many critical functions of glial cells in the brain, but I think one of the more interesting ones is their role in learning and memory. Last week, I wrote about how stimulation of certain neurons can cause artificial memories. In that case, the connections between neurons were strengthened to form an association which is believed to be the basis of how learning works in the brain.
What strengthens the connections?
It turns out that glial cells are one of the ways that connections are reinforced in the brain. One type of glial cell, the oligodendrocyte, produces a fatty substance called myelin. Myelin is to axons like rubber insulation is to electrical wire except that the presence of myelin dramatically increases the conductance velocity of neurons such that myelinated axons are like 5-lane highways and non-myelinated axons are like country roads.
Glial cells don’t just produce myelin indiscriminately but actually strategically myelinate brain connections during learning. For example, it’s been shown that when someone is learning a complex motor task, glial cells are activated to myelinate the pathways involved in that task.
Much of the myelination process is believed to occur during childhood development such that much of who you are as a person is largely determined by which connections in your brain are heavily myelinated. For me, whichever connection that is between the taste of pepperoni pizza and happiness must be myelinated more than anything else.
↩️ Follow Up: Diversity in Science
Just this month, Nature Neuroscience wrote an editorial on widening its scope on diversity. I wrote a response to it which you can check out here.
🔗 Links, links, links
Scientists have built brain-like organoids to study diseases like autism, but some people are concerned that they may become too much like the real thing.
More on why glial cells are important in learning and memory
How neuroscience can inform criminal justice
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