Electrostatic preorganization is the predominant source of catalytic power in enzymes. Indeed, Warshel and co-workers have long proposed the theory that enzymes are organized to stabilize the charge distribution of the transition state over that of the reactant state. Compared to the same reaction in water, the organized enzyme environment lowers the energy barrier, thereby accelerating the reaction rate. Since its inception in the nineties, electrostatic preorganization theory has been validated for ketosteroid isomerases, dihydrofolate reductases, methyltransferases and many others enzymes. Despite the success of electrostatic preorganization in rationalizing the catalytic performance of enzymes, a debate grew over the relative importance of non-electrostatic effects , such as conformational motions, dynamics, and enzyme deformation. Vibrational Stark spectroscopy, which enables the direct measurement of electric fields in proteins, have caused the resurgence of this debate. Pivotal experiments by Boxer and co-workers showed that ketosteroid isomerase exerts strong electric fields that correlate with its exceptional catalytic rate, emphasizing the paramount importance of electrostatic preorganization. Similar results were found in other enzymes, including dehaloperoxidase A, horse liver alcohol dehydrogenase and tyrosine kinases. Theoretical studies have provided a detailed picture of the origin of these electric fields at the molecular scale. The heterogenous enzyme environment, including protein residues and solvent molecules, generate a non-uniform electric field in active sites that facilitates catalysis. Simulations also showed that structural dynamics cause wide fluctuations of the electric field in enzymatic active sites, which further accelerate reactions rates, thereby directly linking electrostatics and conformational motion effects. Finally, theoretical studies demonstrate that electric fields could be used as a design principle to improve the catalytic performance of synthetic enzymes and explain the mutations selected by directed evolution. Although predominantly used to rationalize enzyme performance in the context of electrostatic preorganization theory, electric fields can also explain drugreceptor binding and selectivity , unidirectional electron transfer , protonation states and metal ion chelation in polymeric systems. In this paper, we propose a new application and calculate electric fields to unveil the molecular mechanisms at the origin of the selectivity and conductance of voltage-gated sodium channel (Na v s). We use these electric field calculations to investigate electrostatic preorganization in Na v s.