Model, based on two-photon morphometric measurements in a representative layer 5 neuron predicts diffusion rates of Na⁺ ions from the AIS to the soma and to the internode during the voltage ramp. Left, Top, Structural relationship between the axon of layer 5 pyramidal cell and the myelinating processes of oligodendrocytes. The image obtained by merging optical sections at a wavelength of 760 and 940 nm; yellow color indicates the colocalization of the SBFI (red) and EGFP (green) fluorescence, revealing the boundaries of the first myelinated internode (yellow arrows). Left, Bottom, Simulated changes in [Na⁺]_i elicited by voltage ramp to indicated voltages, plotted against distance from the soma. Voltage protocol, current traces, and [Na⁺]_i in soma (black), AIS (red), internode (cyan), and node of Ranvier (blue) are shown in right top panel. Right, Bottom, Model predicted the rate of Na⁺ diffusion from the AIS to the soma (black) and to the internode (cyan), plotted as a function of time.