Action-potential propagation along the length of an axon beyond the regions of initial excitation requires current flow driven by Na+-channel activation to access remote, initially quiescent, regions of nerve. This current, and its effect on membrane potential, varies with membrane resistance and capacitance, and the electrical resistances of the adjacent extracellular and intracellular fluids. These variables quantify the spread of the consequent voltage change with time and distance through the cable equation. This in turn determines action-potential conduction velocity which, in combination with its refractory period, determines the wavelength of this advancing excitation. Conduction velocity in unmyelinated fibres increases with fibre diameter. That in myelinated fibres increases with the reduced electrical capacitance and increased resistance of their surrounding myelin sheath, resulting in a saltatory action potential conduction. Conduction is further modified by the threshold for the initial excitation, in turn dependent on the membrane Na+ channel density.