In this paper we construct a mathematical model for excitable membranes by introducing circuit characteristics for ion pump, ion current activation, and voltage-gating. The model is capable of reestablishing the Nernst resting potentials, all-or-nothing action potentials, absolute refraction, anode break excitation, and spike bursts. We propose to replace the Hodgkin-Huxley model by our model as the basis template for neurons and excitable membranes.1. Introduction. When Hodgkin and Huxley constructed their model for the squid giant axon ([1]) they were fully aware of their model's drawbacks because it was only a phenomenological fit to their experimental data. They commented specifically that different empirical forms should fit the same data or even better. They were right and other researchers saw the same problem too [2,3,4]. Alternative models were concocted ([2]) but never gained any traction because there was no point to replace one ad hoc model by another arbitrary one. However, replacing a phenomenological model by a mechanistic one is a different matter entirely.By mechanistic it is meant for a model to have as few hypotheses as possible that apply to physical processes or objects of a same type. Newton's inverse-distance-squared law for gravitation is the first and one perfect example of mechanistic modeling because it applies to all macroscopic bodies of mass. Goldman's derivation ([5]) of Nernst potentials across cell membranes is mechanistic because it applies to all ion species. In contrast Hodgkin and Huxley's individual treatments of the sodium and the potassium currents are not mechanistic because their hypothesis for the sodium ion does not apply to the potassium ion or vice versa. Researchers must have tried but failed because other than various variations of the HH model no mechanistic model can be found in the literature.The purpose of this paper is to fill this literature gap. The idea is to model the membrane as a circuit of devices each is defined by a current-voltage characteristics. We will model the sodium-potassium ion exchanger pump by the IV -characteristics that the time-rate of change of the current is proportional to the power of the pump. We will model the ion channel activation and the voltage-gating by one unified IV -characteristics that the voltage-rate of change of the conductance is proportional to the conductance. We will demonstrate that the resulting conductance-adaptation model is capable of reproducing all known phenomena of the HH model and much more, and hence provides a mechanistic alternative to the HH model and a model template for other types of excitable membranes in general.