As long as we are using spherical electrodes calculation of transfer resistance is easy and the xtra mechanism can be used unmodified. However, if we are using cylindrical pippets as electrodes, calculating resistance is a bit more tricky. One method to overcome this problem is to find as expression for the potential V(r,z) (in cylindrical coordinates) for the pippet (cylindrcal) electrode in terms of the applied potential and use the xtra mechanism to assign the values of e_extracellular based on distances of various segments from the electrode. Here the input to the program is voltage rather than current
For an electrode with simple geometric shape of diameter D, if the distance X from the
electrode is large compared to D (i.e. X/D > 10 or so), the field can be approximated quite
closely by that which is produced by the same total current applied to a spherical electrode.
If more precision is necessary, the proper approach is current based, and starts from
the fact that a point source of current I produces a field whose potential is
V = I rho / 4 PI r
where r is the distance from the point and rho is the resistivity of the conductive medium.
If the electrode cannot be approximated by a point source, then we must instead do a
surface integral
V = (rho / 4 PI) INTEGRAL J dS / r
where r is the distance from the location of V in the conductive medium to a point on the
surface of the electrode, J is the current density at that point on the electrode, and dS is
the infinitesimal for area on the electrode.
An equivalent approach is to convolve the potential of a point source with the geometry
of the electrode and the current density function.