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yanou wrote:I have another question regarding the xtra mechanism.

The code you excerpted has nothing to do with the xtra mechanism. It merely uses a couple of tricks to show a few things of interest in a Shape plot--specifically a set of calibration bars parallel with the x, y, and z axes, and a dot to indicate the location of a monopolar electrode. These visualization aids are completely irrelevant to the model cell and simulations. If you have questions about this code, first please seek answers in the Programmers' Reference.