Voltage-gated sodium channels are primarily responsible for generation of the upstroke of an action potential in nerves, muscles, and heart. They share a common molecular architecture with other members of the voltage-gated ion channel family and comprise a central ion-conducting pore surrounded by four voltage-sensing domains1-3. In response to a depolarization, the voltage-sensing domains of the sodium channels undergo conformational changes that are transduced to the pore domain. These conformational changes in the voltage-sensor result in the opening of central pore and this process is referred to as “electromechanical coupling”. The molecular and structural mechanisms involved in electromechanical coupling remain poorly understood. Previous studies on voltage-gated potassium (KV) channels have suggested that the intracellular gating interfaces involving S4, S5 and S6 helices are crucial determinants of the cross-talk between the voltage-sensors and pores. We have been systematically investigating the role of these interfaces in the domain III of a Na+ channel by mutating them to tryptophan. Tryptophan substitution in this region is expected to perturb the tight packing interactions and thereby alter coupling between the pore and voltage-sensor. To assess the effects of site specific mutations on activation dynamics of the voltage-sensor and pore, we combined voltage-clamp fluorimetry and conductance measurements. The activation of the domain III voltage-sensors was measured by monitoring the voltage-dependent change in fluorescence of a dye attached to a substituted cysteine residue at the extracellular end of the S4 segment. The pore dynamics, on the other hand, was obtained by measurement of ionic currents. To further probe the role of these interfacial residues, we have examined the effect of these mutations on voltage-sensor modulation by local anesthetics. We found that almost of half of these mutations cause an effect on voltage-sensor and pore dynamics. Most of these have similar effects on voltage-sensor movement and pore opening. In other words, they concurrently stabilized (or destabilized) the activation of voltage-sensor and pore opening. But a small number of high-impact mutants (7 out of 55) had opposing effects on voltage-sensor and pore. They stabilized the activation of the voltage-sensor while destabilizing pore opening (1). To understand the role of these residues, we considered canonical models of cooperativity and find that these residues are highly likely to be involved in both the resting state and activated state coupling interactions. These experiments were complemented by examining the effect of drugs that allosterically modify the voltage-sensor movement by binding to the pore. In the wild type sodium channel, binding of lidocaine causes a large hyperpolarizing shift in the activation of domain III voltage-sensor. We find that in all seven of the previously identified high-impact mutants, this tight correlation between drug block and voltage-sensor modification is disrupted (2). Together, these experiments allow us to identify the critical residues involved in electromechanical coupling at one of the intracellular gating interface of a sodium channel. Identification of residues involved in coupling is the first step towards understanding the physical mechanisms that govern electromechanical coupling in a voltage-dependent ion channel. Upon mapping the critical residues on a sodium channel structure obtained by homology modeling, we find that many of these residues occur either in or near the flexible hinges connecting the various helices. We speculate that these regions presumably behave as elastic hinges which mediate conformational coupling between voltage-sensor and pore in a voltage-dependent ion channel.