From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Sun Oct 03 1999 - 23:23:05 MDT
On Sun, 3 Oct 1999, Eliezer S. Yudkowsky wrote:
> What does cryonic freezing damage do to those little chemical patches on
> the surfaces of individual neurons?
Generally speaking as you cool down, you are going to get the lipid membrane
becoming less fluid, so receptor proteins are going to get locked in place.
Now the problem will occur if ice crystals form that punch through the
membrane or become large enough to physically move the synapses around.
(Thats why we have all this emphasis on cryoprotectants, ice-blockers, etc.)
> Synaptic connections are one thing - are we still talking about perfect
> reconstruction?
Ralph Merkle goes into some discussion about this in his reanimation
paper. He essentially assumes that nanobots will be able to go in
do localized reliquification after establishing anchors. Then they
map the surfaces and gradually pull things back into the proper locations.
If you have ever seen a freeze fracture electron micrograph you can
easily imagine how mating the two ~mirror-image surfaces back together
should be straight forward. If the chemical cocktails work really
well, then you should be able to do the reanimation without nanotech.
(Though you may still have to solve the biological "cause" of death.)
In this situation develops, we will be looking at hibernation or outright
suspension as being very legitimate things to do as medical procedures.
Suspension seems cheaper than hibernation because in hibernation you
probably have to remain in the equivalent of an Intensive Care Unit
with everything being carefully monitored (so its going to be expensive).
If it turned out that the physical location of the receptors within
a synapse (rather than just their overall "density") was significant
things would be more difficult. But I doubt very much that the cells
have mechanisms for locking receptors at specified locations. Most
cells are designed so that receptors have a random distribution
and can move around in the membrane. Some cells have ways of directing
receptors to one side or the other (say in intestinal epithelial cells).
Presumably neurons use similar mechanisms to send specific "quantities"
of receptors down specific dendrites. Since synaptic transmission is
diffusion mediated and diffusion is a random process I can't see any
point to controlling anything more than the receptor density
(which relates either to rate or strength of transmission) on a
synaptic surface.
The brain has a lot of redundancy and can probably tolerate a fair
amount of "mismatching" before you would experience significant
memory or self-awareness loss. You might start out fuzzy, but
over time neuronal maintenance & repair should normalize things.
Much more serious would be large regions of damage in some specific
critical brain region. I'm under the impression however that the
brain is one of the tissues more tolerant of freezing. The kidney
is the difficult one due to the microstructure it requires for
filtering & pumping small molecules.
One of the people more cryo-literate may want to talk in more
detail about the relative difficulties of various organs.
Robert
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