Biophysical basis for monophasic action potential

We welcome Dr. Franz's comments relative to our recent publication dealing with the monophasic action potential (MAP) [1]. We agree and fully appreciate the fact that the Franz contact electrode technique has been successfully used for over 20 years by Dr. Franz and other investigators. This success stems from the fact that the two electrodes of the contact catheter are relatively close to each other so that their combined field of view is fairly localized. However, we do not agree that this success can be used as an argument against our conclusion that it is not the tip electrode that records the MAP. Our study investigated the physical principles behind the MAP recording. We started with a simple definition of voltage as the difference of potential between two points or two electrodes. To determine which of these two potentials carries time-dependent information, we placed the two electrodes at sites with different cellular activity and observed which activity is reflected in the MAP signal. The observations demonstrated in a very straightforward manner that the tip or inactivating electrode is not the recording electrode. This conclusion provides support for theories introduced as early as 1934 [2].