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 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 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 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 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 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 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.

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.

George

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