A Model for Studying Velocity Variations in Unmyelinated Axons

A simple electrical model of the nerve axon is presented. The only non-linear element in the model is the sodium conductance, which is assumed to vary in a step-like way. This allows the propagation velocity for action potentials to be expressed analytically. The aftereffects from an action potential are generally slow. By considering them quasi-stationary, the velocity of a subsequent action potential can be calculated and compared with that of the first one. Considering the effects of afterpotential and afterconductance, it is shown that the theory adequately describes the variations in propagation velocity in the squid giant axon for action potentials separated by 3 ms to 14 ms.