Page 15 ”The NPE gives the explicit expression of ionic current in terms of concentration and electric potential gradients.” As we discuss in section 2.4.5, NPE is valid only in the ’mean-field’ approximation, i.e., when we assume that the diffusion and electrical interactions have the same speed. That is, we can safely use if only in equilibrium state. Furthermore, although it was derived for equilibrium state, it implies that for the short time of forming an AP, the concentration also changes, given that the potential changes. According to their Table 2.1, for a short period, up to two orders of magnitude change in the concentration can take place. See also section 2.6.5.