After uploading ephap.zip, I had some additional thoughts about this topic, so I have now revised notes.txt accordingly (fixing some typos at the same time) and put a new ephap.zip on the web. Here, for those of you who might be curious about whether it's worth getting the new file, are my additional comments:
Finally, a big caveat: in this particular implementation, the two interacting segments are parts of sections that have nseg>1. "What's wrong with that," you ask. Here's what: change nseg and you change not only the size of these segments but also the coupling between them. In other words, nseg is no longer just a simulation parameter--it has now become part of the model specification, that is, part of the specification of the anatomical and biophysical properties of the biological original that are represented in the model. Change nseg and you not only change accuracy of the solution in space, but you also change the model itself. Very bad idea.
It is much better for the coupled segments to belong to sections that have nseg = 1. For this particular model, this means
1. decide how long the coupled segments will be
2. break each axon into three pieces, like so
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(this sketch not to scale)
proximal part distal part
where each piece is a separate section.
Make the section that represents the region of ephaptic interaction long enough to allow the degree of coupling you want, but short enough to be electrotonically compact so that its nseg can be set to 1. The proximal and distal parts can have whatever nseg values are necessary for spatial accuracy.
"Well, exactly what length, diameter, Ra and membrane properties should I assign to the region of ephaptic interaction?"
That's where your scientific expertise and judgment come in.