Eli5: Almost every cell in our body is able to regenerate. Why are the braincells different?
In: Biology
Neurons in your brain would need to be both able to replace themselves, and then most importantly, spread their axons and dendrites to the appropriate location. Brain cells are a small cell body coupled with long tentacle like structures that stretch from place to place to form connections. This means that in a place like your spine, a single cell goes all the way from your brain to your tailbone, which means that the cell is about a meter long! Its easy enough for a new cell body to be put in the same place if the original gets damaged, but it would be very hard to make sure that cell then grew that extreme distance and connected back to the correct target.
There are some animals that have very impressive regenerative properties, but it just isn’t something humans (or most mammals) are capable of. In the ancestral environment, an accident that was bad enough to leave you paralyzed was generally bad enough to kill you pretty quickly, which means that we never evolved a way to regenerate that kind of damage.
Contrary to old beliefs, the brain [can generate new cells](https://www.scientificamerican.com/article/the-adult-brain-does-grow-new-neurons-after-all-study-says/) in the hippocampus at least.
They are not. Neurons regenerate. For a long time we thought they didn’t because they are both large and complex compared to most cells, and part of an intricate network so they are difficult to replace. But for the past several decades we’ve known that neurons in the adult brain do regenerate.
As others have pointed out, they do regenerate, just with a lot of poorly defined caveats.
The simplest way to think of it is a lot of other places are simple, but the brain is very complex. Your skin is always regenerating, but it’s more or less just a loosely organized layer, whereas the brain is a intricately connected structure. As an analogy, if I said this ball pit needs 50 balls replaced, it would be super easy to just dump in 50 new balls because it doesn’t really matter where they are in the pit. That’s skin or blood or other really regenerative really simply structured tissues. The brain would be like saying 50 wires in my computer are broken, can you replace them all? Sure you could, but it’s going to take way more time to do it so that the final product still works.
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