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atyy
#2
Dec21-11, 05:27 PM
Sci Advisor
P: 8,656
In standard circuit theory, the speed of light is infinite. This is why the potential (V) on both sides (x, y) of a capacitor (C) change at the same time (t) even though they are spatially separated, and we simply write current I=Cd(V(x)-V(y))/dt.

There is a more careful way of stating this as an approximation of the full Maxwell's equations under the assumption of low frequency and long wavelength.

Another way to see this in passive neurite theory is that there is no true propagation speed. http://www.jhu.edu/motn/coursenotes/cable.pdf: "This value can be though of as the speed of spread of electrotonic disturbances. Of course, it is not a true propagation speed, in the sense of the action potential propagation speed, because there is no fixed waveshape that is propagating, i.e. this is not a true wave."

An action potential in Hodgkin-Huxley theory has a true speed, because the cable is not passive, but has voltage dependent sodium and potassium channels.