On Mon, Jul 11, 2005 at 10:38:35PM -0700, George Levy wrote:
> Stathis Papaioannou wrote:
> >The ionic gradients across cell membranes determine the transmembrane 
> >potential and how close the neuron is to the voltage threshold which 
> >will trigger an action potential by opening transmembrane ion 

Stop your heart now. See EEG collapse to zero in 20-30 sec. Your transient 
gradients (spikes) are gone with the oxygen, until circulation is restored.
All you see is a brief blackout.

I wonder for whom I'm writing all these mails.

> >channels. Other factors influencing this include the exact geometry of 
> >the neuron and composition of the cell membrane (which determines 
> >capacitance and the shape and speed of propagation of the action 
> >potential), the number, type and location of voltage-activated ion 
> >channels, the number, type and location of various neurotransmitter 
> >receptors, the local concentration of enzymes that break down 
> >neurotransmitters, and many other things besides. The ionic gradients 

You might be surprised:


> >across cell membranes (all cell membranes, not just neurons) are 
> >actively maintained within tight limits by energy-requiring 
> >transmembrane proteins, such as Na/K ATPase, and if this suddenly 
> >stops working, the cell will quickly die. The moment to moment 

Define quickly. You can culture human neurons for several days after death.  
At deep hypothermia and flushout, the empirical canine viability window is 
several hours
(the ceiling could be at 12 h or more, nobody knows yet).


> >variations in ion fluxes and membrane potential may be allowed to 
> >collapse and the neuron will remain structurally intact, so to this 
> >extent the exact cellular chemistry may not be necessary for long term 

You can safely remove the conditional here. 

> >memories. However, all the other things I have mentioned are important 
> >in determining the "wiring diagram and strength of connections", and 
> >could easily be maintained over decades. Look up "action potential" in 

Yes, and all of them seem to be present in vitrified brain tissue -- a
snapshot with geological shelf half-life.

> >Wikipedia, and think about how you would design an equivalent circuit 
> >for even one neuron. It may be a ridiculously complex way to design a 

You don't design a circuit for a specific neuron. That would be moot, as a
circuit is fixed, and a neuron is not (an understatement: see cell migration in

What you need is a computational engine capable of emulating arbitrary cells
in the CNS. The hardware layer of such a system can be very simple, the
complexity being contained in several emulation layers.

> >computer that would be able to run and maintain a human body, but 
> >whereas I would happily trade my heart or my kidneys for more 
> >efficiently engineered models, I would like any brain replacement to 
> >be an exact functional analogue of my present one.

Nobody is trying to sell anything else.

> Stathis,
> you don't have to get down to that level of complexity. As long as the 
> high level function remains the same, you can still say "yes doctor" to 
> a substitution experiment. Example: artificial eye lenses made of 
> plastic and not of tissue, prostheses made of titanium steel and not of 
> bone.

I seem to not be coming through, so this will be the last post on my part 
in this thread, for a long while.

Eugen* Leitl <a href="http://leitl.org";>leitl</a>
ICBM: 48.07100, 11.36820            http://www.leitl.org
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