Re: What is the advantage of neurons not being able to regenerate?

I sought the help of Dr. Andrea Missias at U. Penn, who is more familiar with neuroregeneration, for the answer to this question. Her response was:

It seems like there are 4 answer-bits:

1. The inability to regenerate is not inherent to neurons -- many can, in fact regrow. However, in higher vertebrates, these are limited to the periphery. For example, if you cut your finger, you can regrow the nerve endings to the damaged area.

2. However, neurons in the central nervous system do not appear able to regrow. The best-known example is the spinal cord -- if neurons here are cut, they appear unable to regenerate. This inhibition appears to be due to some aspect of the surrounding cells, rather than to a difference in the neurons themselves. I am not aware that this inhibition is of any benefit to the animal -- probably most animals that have a spinal cord injury die quickly anyway -- it is probably a side-effect of some other process or purpose for those inhibitory molecules.

3. It is entirely possible that new connections are constantly being grown and withdrawn within the brain -- these subtle changes would be very hard to measure. Certainly there is plenty of evidence for both short-term and long-term adaptations by the brain to a variety of circumstances and learning experiences, some of which would seem to require novel connectivity.

4. Why new cells (as opposed to connections) are not continually generated in the nervous system is a trickier question. In the olfactory system, there is, in fact, ongoing production of new neurons, which may be related to the high degree of stimulation (fatigue?) which those neurons continually receive. However, it seems that much of the nervous system prefers to generate its cells over a limited time period, after which the refinements occur at the level of connections. It's sort of like keeping the same components, and just adding or altering the wiring. The trick with adding new components (cells) after the wiring diagram is set, is how you integrate them -- how you fit them into circuits which were shaped by experience over time (rather than by a set wiring diagram) -- there's no way to create "instant experience" for the newcomers. Thus, I think the strategy is to generate an excess initially, which can either be eliminated later, or kept around as redundant pathways which can be rewired later to compensate for any future injury -- as when the brain learns to "work around" a stroke lesion. But this is where the brain differs from other tissues -- the structure itself carries information (in the way that things are interconnected), so it can't be simply regenerated in the way that a more mechanistic organ (like, say, the liver) can be. New tissue would never really be the same as the damaged tissue it was replacing, because it wouldn't be carrying the same information. So that's not an advantageous process (and has other risks... a brain tumor might be more serious than a stomach tumor, for example).