[Neuroscience] Re: Basic question about electrical currents in EEG

Kalman Rubinson via neur-sci%40net.bio.net (by kr4 from earthlink.net)
Mon Nov 30 11:43:27 EST 2009


On Mon, 30 Nov 2009 00:38:42 -0800 (PST), cwenhoo
<ilangobi from yahoo.com> wrote:

>From Luck's book, p.32:
>
>"When a dipole is present in a conductive medium such as the brain,
>current is conducted throughout that medium until it reaches the
>surface...Electricity does not just run directly between two poles of
>a dipole in a conductive medium, but instead spreads out through the
>conductor."
>
>"Another important point is that electricity travels at nearly the
>speed of light. For all practical purposes, the voltages recorded at
>the scalp reflect what is happening in the brain at the same moment in
>time."
>
>My question is, what generates this current flow? I mean I can
>understand if the conductive medium is a piece of metal, then a dipole
>results in direct movement of electrons.

The dipoles are the neurons which, when activated, have
different voltages across different parts of their surface.

>But what about the brain? What is even more confusing to me is that
>action potentials are not even near the speed of light, so how can
>this current resulting from the dipoles in the brain travel that fast?

Action potentials are slowed by capacitance and by the time
taken for their underlying mechanisms.  Intra-axonal passive
current flow occurs at a rate equivalent to a substantial
proportion of the speed of light.

Kal


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