(a) Left: the accessible inner surface15 (in grey) of the NavMs symmetric pore model, showing that there is a continuous pathway from the extracellular to the cytoplasmic surface of the membrane, which is of sufficient size to enable the passage of a fully hydrated sodium ion (except around the SF, where only a hemi-hydrated ion will fit). In this figure the NavMs (red) and NavAb (blue) structures are overlaid as ribbon diagrams. Right: plot of channel radius versus distance along the channel direction for the open NavMs (red) and closed NavAb (blue) structures, showing that in the extracellular half (including the SF and the central cavity), the diameters of the open and closed forms are nearly identical, but that in the region of the activation gate there is a substantial difference in the diameter of the cavity of the open versus closed forms. (b) Schematic model for the opening and closing of bacterial sodium channels, showing changes in the orientations of the bottom half of the S6 helices and the compensating changes required in the disordered regions of the C-terminal domain. Note only two monomers are shown for clarity.