Ok, now I get it. Thanks!

I am currently working on a quite difficult s(t)imulation, would be great If you could give me some advice.

I want to replicate data obtained via stimulation with a suction electrode. Vestibular nerve fiber bundles are separately sucked inside (~500 um of nerve inside electrode) to separately stimulate them with current pulses.

The way I see it now there are three different resistance "domains".

(the fiber spreads along the x axis and starts at x=0, y and z coordinates are ignored, d is diameter of the electrode, dn is diameter of the fiber bundle,

x_xtra(x) is the current position on the x axis, the silver wire inside the electrode ends at x=-100, lstim = 500 is the end of the electrode)

1. From the end of the silver wire in the electrode to the beginning of the nerve. Here the resistance should be:

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`Ri1 = rho * ( 100 / (PI*(d/2)^2) ) * 0.01`

2. From the beginning of the nerve to the desired location at the nerve (e.g. node), here the nerve takes away a significant amount of the diameter so the effective diameter of the electrode has to be reduced by that of the nerve bundle:

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`Ri2 = rho * ( x_xtra(x) / ( PI*( (d/2)^2 - (dn/2)^2 )) ) * 0.01 `

3. From the end of the electrode to infinity:

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`Ri3 = (rho / 4 / PI)*(1/ ( x_xtra(x) - lstim ) ) * 0.01`

So the resistance at a given position should be obtained by adding up in series all other resistances, so for a position at x > Istim this would be:

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`Ri = rho * ( ( 100 / (PI*(d/2)^2) ) + ( Istim / ( PI*( (d/2)^2 - (dn/2)^2 )) ) + (1 / 4 / PI)*(1/ ( x_xtra(x) - lstim ) ) ) * 0.01`

My Questions:

A: So what really puzzles me here is that even in infinity, the extracellular voltage would be as high as at the end of the electrode tip, can this be right?

B: Is the reduction of the effective electrode diameter correct? Our would it be better to just use another rho with the value for brain tissue? By the way, where can a find such values, for example for ringer's soultion and brain tissue?

C: Do you think the capacity of the suction electrode should be taken into account? Perhaps it could done by separating the electrode into RC cylinders and calculate the effect of these at the current nerve position.

Thanks a lot!

Chris