Figure 2.

Classes of gating models for Shaker potassium channels. For each model, each of four subunits undergoes one, two, or three transitions between subunit states, designated by S₀, S₁, etc. In some of the models, the channel undergoes one, two, or three additional concerted transitions. The models are named by the number of subunit transitions and additional concerted transitions. For models with no concerted transitions, the channel is taken to be open after the fourth subunit has undergone the last subunit transition; this is indicated by the dashed line to the open state.