A single-cell neuronal model was used to investigate the effect of membrane trajectory on voltage threshold (V t h ) for action potential generation. Previous results suggested that hyperpolarization of V t h could be produced by a rapid membrane depolarization, but this effect is limited to the first spike in a train. This study shows rapid current injections hyperpolarize V t h because they are more effective in activating the sodium current underlying spiking. The hyperpolarization of V t h induced by rapid membrane depolarization becomes less effective in altering V t h when other mechanisms of enhancing the fast sodium current underlying action potentials are activated.