Not very supportive support cells!

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I was studying for my neuroscience midterm in a week...(which I should be doing right now and not writing this post....i also have something really important to do for this Tuesday.....) …anyways I got to a chapter of my text that talks about how everything in the nervous system works at the cellular level. I came across something that explained that when a nerve cell is damaged in the peripheral nervous system (nerve cells outside brain and spinal cord) the stump of the crushed axon will grow new sprouts and then schwann cells (the support cells that form the myelin sheath around nerve cells of the PNS) will remove the old dead cell and then form cylinders that guide the re-growth of the new axon. The axon will re-grow quickly through the cylinders and will re-establish connections with the other cells with which it was previously connected and resume its original function. (This only happens when a nerve cell is damaged in certain ways and if the nerve cell is still located close to the cells with which was once connected). Still....this means that cells nerve cells outside the brain and spinal cord have some ability to regenerate after being damaged.

Why don't our brain cells do this? According to my (undergraduate level) textbook if axons of cells in the central nervous system (brain and spinal cord) are damaged they WILL form new sprouts but a class of support cells in the CNS (that provide cells with nutrients, remove bacteria and viruses from the cells and generally help brain cells)called astrocytes will leave scar tissue after removing the dead parts of the old cell and this scar tissue will form a barrier that the budding new cell cannot penetrate. More interestingly even if the astrocytes are prevented from forming scar tissue the astrocytes STILL stop the new cell from regenerating! The astrocytes send the new sprout of an axon a chemical message that it has reached it's final destination and should stop (and form it's terminal buttons) right at the beginning of it's journey.

So when nerve cells in the PNS are damaged (under certain circumstances) they will regenerate and their support cells will help them do so… but when neurons in the brain are damaged not only will their support cells not help them regenerate these so called support cells will actually prevent them from doing so! Why?????????? Can anyone think of a possible evolutionary explanation for this?

I have absolutely no idea but I have to tell you that I have always loved reading text like what you wrote. Really good plot at the cellular level! I'm serious - do you happen to know where I can find some books like that? Not all grown-up scientific, something that a mere 60yo can read.

Let me take a guess at why - because the brain can grow so big and no bigger?
If it gets over-crowded in the cranium, maybe that individual removes itself from the gene pool?