X-Message-Number: 5186
From:  (Thomas Donaldson)
Subject: Re: CryoNet #5174 - #5181
Date: Wed, 15 Nov 1995 12:51:15 -0800 (PST)

Hi again:

Mr Lynch makes several statements about how our brains work, some of which 
need amplification and others are leaning a bit in the wrong direction.

First, LTP seems to be a stage in the formation of long term memory. In that
sense, it is important, but the REAL long term memory persists for other
reasons entirely. What apparently happens is that after a time (one thing
LTP does is to hold a memory while that time passes) relevant synapses can
increase in number, form dendritic spines where they formerly did not, and
new dendrites can be grown, with synapses, to make more contacts between the
two communicating neurons. These changes take place on a scale much larger
than that of molecules.

Just what the effect of releasing NO here may be remains unknown. A lot more
experiments need to be done to find out (I am referring to his reference to
the SCIENCE paper in which this was found to happen, at least for one part of
our brain). Since we are dealing with LTP rather than the changes that lead
to long term memory, just what the effect will be on long term memory needs
to be examined specifically. (Yes, one reason there is so much attention to
LTP is that these other changes are much harder to follow).

Although I feel sure that one way or another we will learn someday how to
make neural nets which form new connections rather than simply strengthen
old ones, that isn't presently the case. If we have such a neural net (which
is our brain) it becomes impossible to work out memory capacity simply by
counting the number of synapses. Given the fact that our brains are enclosed
by bone, and that limits the size to which they would grow, there must be
an upper limit ... but even so, any calculation would have to include the
fact that brain connectivity can change slowly over time, and even increase.

Furthermore, the problem very much does NOT stop just with the information
we need to reconstruct a HEALTHY brain. One reason we will want to be able
to at least identify the locations of critical molecules is that for cryonics
we will ultimately (hopefully not for everyone, but always for some) have
to put together a DAMAGED brain. The distribution of various brain chemicals
gives us important clues as to what is connected to what. We are just 
beginning to get some handle on what chemicals these may be. AT least 2
classes, together with the different neurotransmitters and their receptors 
(most neurotransmitters have more than one chemical receptor) may provide
such clues. (The classes are neurexins and connexins). It's very important
to have and use such clues --- otherwise the problem of reconstruction 
becomes virtually impossible. (If you tried to put together a puzzle with
pieces which were identical in every way that you could see, you'd have
a severe combinatorial problem. Each piece might even have a label, and
somewhere inaccessible to you the correct arrangement is stored, but that
would not help you at all).

I will finally say that while analyzing a brain on a molecular level will
prove helpful, analyzing one on an atomic level will NOT. It is important to
know how those atoms connect to one another. Simply knowing their position
is not enough. At a maximum, if we only knew about the atoms, we might be
able to work out how they were connected ---- at the cost of a great deal
of totally unnecessary computer processing. At a minimum, we might not even
be able to do that.

			Long long life,

				Thomas Donaldson


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