r norman <rsnorman_ at _comcast.net> wrote
> The 100 mV potentials you describe are specifically across the
> cell membrane. If you want to talk about electric fields, 100 mV
> across a 100 A membrane (10 nm) gives a field strength of some
> 10,000,000 V/m. It takes a pretty decent dielectric to hold up
> against that!
>> However, these potentials are in specific locations caused by
> sources that have the proper impedance characteristics (channel
> conductances) to produce the necessary current. Most of the brain, or
> any organ of the body for that matter, is salt water with a very high
> conductance. The electric fields in either the intracellular or the
> extracellular spaces are very small. It is very hard to induce
> potentials in these media from externally applied fields because of
> the high conductance.
>> It is, in fact, the high conductance of the extracellular medium that
> makes ephatic interaction between nerve cells so ineffective. Only if
> the adjacent cells are extremely close and only if there is some
> special confinement of extracellular space (as, for example, wrapping
> by a common glial cell) can current densities reach a high enough
> level to produce an electrical potential that significantly alters
> cell function.
>> Growth cones are influenced by electric fields. You can produce a
> strong enough field with special experimental chambers and electrodes.
> It is difficult to produce that strong a field with external
> radiation. People routinely work in areas of very high electric field
> intensity without any hint of mind altering events.
Thanks for your reply, which is very informative as always... =)
This makes a lot of sense and reassures us that the brain may not be
that mysterious. My query was specifically about electric field
strengths found in the normal brain under physiological conditions.
Things like ECT and TMS can affect the brain significantly...
Reading this post I realized once again that the brain's signal
transmission mechanism is very peculiar, quite different from
my early impression that nerve fibers are like electric wires.
I have another question which is related to making brain-computer
interfaces. Suppose I make some nano-scale tubes of diameters on
the order of 10-100nm (assuming we know how to do that). Inside
these tubes would be an ionic solution. The tubes contain no
ionic channels; basically they are closed. Will these tubes
conduct electric signals efficiently? Given that ions in solution
are different from electrons in metals, what would be the
differences between these tubes and electrodes? In terms of
conductance, and frequency-dependent impedence?
(Another assumption is that the surface of the tubes are
electrical insulators -- but this is probably not true
because carbon nanotubes, for one, are known to be conductive.)