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Electric Field Effects in the Brain?

r norman rsnorman_ at _comcast.net
Sat Apr 19 12:20:42 EST 2003

On 18 Apr 2003 20:26:19 -0700, y.k.y at lycos.com (yan king yin) wrote:

>Inside the brain there are ubiquitous fluctuations of
>electric fields of magnitude ~100mV, due to nerve impulses.
>Most of the standard connectionist models of the brain
>do not take this effect into account. Is this some noise
>that can be ignored?
>I think one way to know the extent of the significance
>of this E field is to induce some random ~100mV E fields
>externally from the scalp, and see if they wreck the
>mind =)
>Can anyone point me to some references or facts...
>P.S. Personally I know of 2 instances. One is the
>"ephatic coupling" of parallel nerve fibers that tends
>to synchronize nerve impulses. Second is the effect of
>E fields on growth cone dynamics. Both of these theories
>are not very mainstream it seems.

There are fields and there are fields.

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.

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