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

John H. via neur-sci%40net.bio.net (by j_hasenkam from yahoo.com.au)
Wed Nov 7 19:48:53 EST 2007

There is something strange going on with these sort of studies. Had a
good look at them a few years ago and clearly there is an effect.
Additionally, pulsed magnetic fields have been tried as a therapeutic
measure for a variety of neurological conditions and these experiments
do suggest promise. A friend of mine who is a physiotherapist has
suggested to me that physios have been using magnetic fields for
years, recently I came across a trial which suggested promise in this
regard. One of the stranger effects noted from magnetic fields is the
impact on mast cells, a matter I was particularly interested in
because there are neuroimmune implications.

Sadly this area of research tends to be neglected, possibly because it
strikes too many people are spooky. Why ignore such things? ECT is
arguably the quickest way to reduce depression with psychotic features
and we still don't have a clue as to what is going on there. Pity it
can also damage the hippocampus ... .


On Nov 7, 12:11 am, r norman <r_s_norman from _comcast.net> wrote:
> On Tue, 06 Nov 2007 09:28:15 -0500, Andy Resnick
> <andy.resn... 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?maxtosho...
> >> 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.- Hide quoted text -
> - Show quoted text -

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