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.