
Do adult human brains renew their neurons? - jkuria
https://www.economist.com/news/science-and-technology/21740397-one-paper-says-yes-another-says-no-do-adult-human-brains-renew-their-neurons
======
truculation
Say an old neuron dies. Could a 'replacement' neuron ensure continuity of the
original 1000s of precise connections?

Assuming that it can't, how is it to compete with the other neurons? (There's
a school of thought that neurons compete with other neurons for resources by
making useful connections.)

~~~
maxander
Well, no, of course not- when a neuron dies, the information encoded in it's
connections is lost. [1]

But replicating the original connections isn't necessary, and certainly not
necessary for the neuron to "compete." For example, say an old corporation
dies, e.g. Microsoft [2]. Could a new corporation replicate the 1000s of
precise products, services, and contracts of the original? Not at all. But
replacement corporation could find _new_ things to offer, potentially taking
advantage of the economic shortfalls and new demands precipitated by the
demise of Microsoft. Eventually the economy returns to full efficiency, though
in a slightly different form than before. And just so with neurons- the
information passed through the neuron that died will now be in demand by
neurons downstream, and this demand can be fulfilled by new connections by the
new neuron (or new connections from old neurons, or simply not.) But the
information may come from different places, or have slightly different
semantic value, or other differences. [3] Brains, like economies, change, and
are good at changing gracefully.

[1] At least, as far as we know- with glial cell participation and functional
viral capsids and whatever else is going on, anything is possible, I guess.

[2] Example chosen without malice. Honest!

[3] All assuming the competition model of neural organization. Which is a
plausible model last I checked, but see [1].

~~~
fspeech
Except corporations don't really die that way. Even if the parent organization
dies (goes bankrupt say), the children organizations normally survive with
different ownership. So as long as a product or service has a market it won't
really go away. A replacement would have to be in place first to kill the
obsolete product or service.

------
untilHellbanned
More relevant: Does it matter if human brains renew their neurons?

We know that there is lots of redundant networks and plasticity, so if one
cell goes down does it matter whether the gap gets filled by and old vs. new
cell?

With 100 billion neurons we seem fine with what we have. The bigger question
seems to be how to protect these 100 billion cells from degenerating.

~~~
nine_k
Frankly, we have a rather vague idea whether the networks are redundant, and
how well spare capacity can be reused to amend a problem in another part of
the brain.

Having a known ability to rebuild neural pathways would be important.

~~~
chiefalchemist
Mind you, this book is 10+ years old but that's not exactly the impression I
got from it. That is, the brain __is plastic__ and __does__ have a high
capacity for "re-configuring" itself. The story of the woman who was born with
only one hemisphere of here brain comes to mind. There are other examples as
well in the book.

"The Brain That Changes Itself: Stories of Personal Triumph from the Frontiers
of Brain Science"

[https://www.amazon.com/Brain-That-Changes-Itself-
Frontiers/d...](https://www.amazon.com/Brain-That-Changes-Itself-
Frontiers/dp/0143113100)

------
chiefalchemist
Question: Are there any other cells in the body that are not, over time,
replaced?

Might it be possible that some brains have the ability to generate new
neurons, while others do not? Couldn't it be possible that evolution created
such situations?

~~~
textor
> Are there any other cells in the body that are not, over time, replaced?

Sure. For instance, female ova. The stock is limited.

> Might it be possible that some brains have the ability to generate new
> neurons, while others do not?

The former is obviously the case for rodents at least. Could it differ within
the species? I can't see why not, in principle.

> Couldn't it be possible that evolution created such situations?

I imagine it's more about gradual loss of function. For example, in larger
brains it's progressively harder (and less probable) to transport the
neuroblast to the area where its growth would be beneficial for the brain at
large. Thus, eventually adult neurogenesis ceased to increase fitness in large
mammals, became effectively an atavism, and mutation load disabled some
relevant genes, but it still could hang on a few pathways that either work or
do net depending on specific alleles present in the population.

