Why do neurons not regenerate?
If you cut your skin, it can regenerate, but after a stroke or brain injury, you can't replace neurons. Why is that?
Well, the answer is that the brain has an architecture which is what's called post- mitotic. There are only a few restricted areas in the brain and central nervous system where there are new nerve cells being born. For the most part, you rely on the compliment of nerve cells you are born with and which continue to divide for a very short window after you were born and then stopped. So basically, what you're born with is what you have to make last a lifetime. And there's a reason for that because if brain cells were dividing all over the place, remember that brain cells have long connections that they make from one cell to the other. And those connections are crucial to you being able to do the right thing, say the right thing have memories and for your brain to be able to work properly. If those cells were dividing all over the place and making aberrant connections, then it will be very, very difficult to preserve that architecture. So there's kind of method in the madness. The problem is that as that is a fixed structure, it's very hard to repair it by getting the cells to re-divide because basically, if you have an injury, evolutionary speaking, that's bad enough to destroy a part of your brain or your nervous system. The chances are you'd be dead anyway. So, we haven't really evolved the ability to repair the brain and spinal cord. In some animals though, that can happen and things like gold fish, lampreys, and also even salamanders can restore whole limbs, bits of their nervous system. If you take the eye out of a frog, turn it around and put it back in again, it will rewire itself back into the brain, only because the eyes now are upside down. The animal see upside down and it does the wrong thing. If you hold a fly in front of it, instead of jumping forward at the fly, it jumps backwards and takes a bite out of the deck and that won a Nobel Prize for Roger Sperry a few years ago and proves that some animals can regenerate their nervous system, but certainly, not us unfortunately.