Re: CRYO: In light of HD neurons?

Robert J. Bradbury (
Sun, 3 Oct 1999 22:23:05 -0700 (PDT)

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.