Although Hodgkin and Huxley's expression n^4 fits the kinetics of the K conductance quite well over the normal physiological range, it is not a completely satisfactory description. It fails to predict the dramatic additional delay observed by Cole and Moore (1960) in the activation of the channels when a depolarization in voltage clamp is preceded by a strong hyperpolarization.

Twenty years later, Young and Moore extended the earlier studies of the kinetics of the K conductance to a wider range of depolarizations by using tetrodotoxin to block the Na channels selectively. They found that a kinetic reaction scheme involving six states was able to fit all of their data.