> >>Brent Meeker writes:
> >
> >>I find it hard to believe that something as stable as memories that last
> >>for
> >>decades is encoded in a way dependent on ionic gradients across cell
> >>membranes
> >>and the type, number, distribution and conformation of receptor and ion
> >>channel
> >>proteins. What evidence is there for this? It seems much more likely
>that
> >>long term memory would be stored as configuration of neuronal
>connections.
> >
> >If anything,
> >parameters such as ionic gradients and protein conformation are more
>closely
> >regulated over time than gross anatomy.
>
>Gradients where? Surely you aren't saying that there is a pattern of
>gradients
>in the brain that is relative to (x,y,z) coordinates and is independent of
>the
>neuronal structures.
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 Papaioannou
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Received on Tue Jul 12 2005 - 01:11:26 PDT