[Neuroscience] Re: Effects of weak electric fields on the activity of neurons and neuronal networks

r norman via neur-sci%40net.bio.net (by r_s_norman from _comcast.net)
Tue Nov 6 09:11:09 EST 2007

On Tue, 06 Nov 2007 09:28:15 -0500, Andy Resnick
<andy.resnick from op.case.edu> wrote:

>ayaz wrote:
>> if you want the full text, you'll have to pay for it...
>> http://rpd.oxfordjournals.org/cgi/content/abstract/106/4/321?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&andorexactfulltext=and&searchid=1077029641510_173&stored_search=&FIRSTINDEX=0&sortspec=relevance&volume=106&firstpage=321&resourcetype=1&journalcode=rpd
>> Radiation Protection Dosimetry 106:321-323 (2003)
>> © 2003 Oxford University Press
>> Effects of weak electric fields on the activity of neurons and
>> neuronal networks
>> J.G.R. Jefferys, J. Deans, M. Bikson and J. Fox
>> Electric fields applied to brain tissue will affect cellular
>> properties. They will hyperpolarise the ends of cells closest to the
>> positive part of the field, and depolarise ends closest to the
>> negative. In the case of neurons this affects excitability. How these
>> changes in transmembrane potential are distributed depends on the
>> length constant of the neuron, and on its geometry; if the neuron is
>> electrically compact, the change in transmembrane potential becomes an
>> almost linear function of distance in the direction of the field.
>> Neurons from the mammalian hippocampus, maintained in tissue slices in
>> vitro, are significantly affected by fields of around 1-5 Vm-1.
>Remarkable.  They present no actual data, summarize previous results 
>with no way to decipher what was actually done, and make numerous 
>unsubstantiated claims.
>For example:
>"More importantly, we were able to measure that transmembrane
>potential at the cell body changed by an average
>of 0.12 mV for each V m
1 of applied field(16)A"
>AFAIK, the action potential is around 100 mV- so they claim a 0.1% 
>effect is meaningful and statistically significant.
>But it *is* open source, so some would say this is a model effort.

If you look at the papers that cite the particular one you find some
truly experimental work that suggests a true effect.

For example:
   T. Radman, Y. Su, J. H. An, L. C. Parra, and M. Bikson
   Spike Timing Amplifies the Effect of Electric Fields on 
       Neurons: Implications for Endogenous Field Effects
   J. Neurosci., March 14, 2007; 27(11): 3030 - 3036. 
"We found that a 1 mV/mm uniform field induced on average a
transmembrane potential change of 0.1 mV. Compared with the scale of
depolarization necessary to bring a neuron from rest to threshold (15
mV), these fields were previously considered insignificant with
respect to action potential initiation. Previous action potential
threshold studies identified changes attributable to electric fields
of <5 mV/mm (Jefferys, 1981). Rather than spike generation, here we
demonstrated changes in timing, consistent with the proposed
amplification mechanism. The present results provide a potential
mechanism for the effects on network spike timing demonstrated
previously in vitro with exogenous uniform fields as low as 0.1 mV/mm
(Deans et al., 2003; Francis et al., 2003; Fujisawa et al., 2004) and
in vivo with calculated fields of 1.2 mV/mm (Marshall et al., 2006). "

Just how significant these effects  are in a free-living animal moving
around in an external field so that any small effect is variable and
transient is another question.  Fields of 1 mV/mm (1 V/m)  are pretty
strong because tissue is a relatively good conductor, resistivity = 60
ohm-cm.  That means that to get a field of 1 mV/mm = 10 mV/cm you need
a current flow of  0.17 mA/sq-cm.  I haven't calculated what kind of
external electric field would be necessary to produce that type of
current flow in a volume as large as a human head, but it must be
rather large.  I can say from direct experimental measurements that
you do NOT see currents and fields like that in ordinary
neurophysiological experiments in rooms filled with electronic
equipment even if the experimental setup is not enclosed in a Faraday
cage.  The purpose of the cage is to eliminate potentials induced by
external fields across relatively high resistance electrodes, not
potentials induced within a saline bath.

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