Excitability: Types I, II, and III

In 1948, Hodgkin distinguished between three classes of excitability on the basis of spiking patterns observed in crustacean axons. This classification scheme is still applied today because it captures fundamental differences in excitability that stem from how action potentials, or spikes, are generated. The nonlinear dynamical mechanism responsible for spike initiation in each cell class has been worked out, and the biophysical implementation of those dynamical mechanisms is reasonably well understood. Moreover, the differences in spike initiation that define each class confer differences in metrics like the phase-response curve (PRC) and spike-triggered average (STA), meaning that this simple classification scheme is predictive of important differences in neural coding.