The problem of current flow in the axon of a nerve is much more complicated than that of flow in dendritic networks (Chap. 4 ). Recall from Chap. 5 that the voltage dependence of the ionic currents can lead to excitability and action potentials. When an excitable membrane is incorporated into a nonlinear cable equation, it can give rise to traveling waves of electrical excitation. Indeed, this property is one of the reasons that the Hodgkin–Huxley equations are so important. In addition to producing a realistic description of a space-clamped action potential, Hodgkin and Huxley showed that this action potential propagates along an axon with a fixed speed, which could be calculated.