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.

Studies demonstrating, characterising and thereby clarifying our understanding of nerve function began from the experimental availability of electophysiological methods for recording and stimulation of bio-electric signals. The classical recording methods were developed to measure intracellular potentials directly from cephalopod giant axons, skeletal muscle fibres and other excitable cell types. These consistently demonstrated strongly negative resting potentials and monophasic action potentials in response to stimulation, whose detailed waveforms varied with different excitable tissue types through a wide range of species. Measurement of extracellular potential differences between different recording sites in the nervous system permitted study both of electrical events occurring at a point, and their propagation along lengths of nerve. This demonstrated and characterised the observed compound action potentials. It separated their components by conduction velocity attributing this to their different fibre diameters and degrees of myelination. It also demonstrated their threshold excitation, all-or-none and refractoriness properties.