S3E3. Brain Eaters

In this season’s first episode, I mentioned that I was in awe of the symbiotic relationship between microglia and neurons. I’m ready to explain why.

Let’s begin with the basics. We’re all familiar with neurons, the sexy cells that connect with each other, transmit electrical signals, store all of our memories and control our abilities to perceive and do things. We have about 86 billion of these cells and they form about a quadrillion (1,000 trillion) synapses with one another. And, yes, we use all of these cells, not just one-tenth of them.

There are about 100,000 miles of blood vessels providing oxygen and nutrients that enable them to work non-stop as long as you breathe.

Please take a moment to embrace the awe inherent in these numbers!

But neurons only comprise about half the cells in our brains. The rest are known as glia. Early on, these cells were just believed to provide a structure within which the neurons could function, like trellises for grape vines in a vineyard. But there are several different types of glial cells and they do a whole lot more than just provide the scaffolding and glue that hold the brain together:

Oligodendrocytes attach themselves to the long axons that stretch out from the neuron bodies and cover them with fatty tissue known as the myelin sheath. This is the brain’s white matter. It acts like insulation on a wire and improves the quality of the electrical impulses. If the myelin sheath breaks down and the signal is sufficiently compromised, that neuron may not be able to communicate with other neurons.

Astrocytes wrap around neurons and are involved in a variety of functions ranging from providing nutrients to enhancing synaptic activity to regulating blood flow to creating neurotransmitters to signaling the presence of invaders which triggers an immune response.

Pretty impressive, huh? Feel free to take another moment to let another wave of awe wash over you.

Then there are my favorites, the microglia. These cells have a variety of roles that change over one’s lifetime. 

It turns out that you are born with way too many synapses and so you experience a period of synaptic pruning where inactive connections are eliminated by microglia who eat them. 

Eat them!

Then as you mature, the microglia take on the role of janitor, cleaning up chemical garbage that accumulates between neurons, including the dreaded beta amyloid. They are also responsible for the primary immune response to local infection and injury. But that’s a two-edged sword: the inflammation caused by an immune response in your brain has been linked to dementia. They also eat damaged neurons as part of their protective role, but sometimes that process runs amok and they eat healthy neurons, as well.

Finally, the glymphatic system has a network of tubes that transports fresh fluid into your cranium, mixes it with the waste-filled fluid surrounding brain cells, and then flushes the solution out of your head and into your bloodstream where it can be excreted. All of this occurs during deep sleep.

The more I think about this elaborate, complex, elegant system, the more awestruck I am.

If any of these types of cells weren’t present, the entire system would break down pretty quickly. Or put another way, we wouldn’t exist if all of these tiny organisms hadn’t evolved together in their interdependent microscopic biome.

Try to imagine the process, played out over hundreds of millions of years, that led to this current iteration. How many combinations and permutations of chemicals stressed under different environmental conditions did Nature experiment with before stumbling upon one that would cause a cell to extend itself out from its body in search of similar cells, thereby giving birth to neurons?

And what does it take for a second…and a third…and a fourth type of cell to evolve that depends upon a neuron for its existence yet enhances the functionality of the host cell itself?

And how did they all come to coexist only inside a cranium?

And if what we see now is the culmination of a billion years of evolution, what will the next billion years bring?

Just thinking about the brain and how it works is truly mind-boggling!

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