Currents, recorded under different conditions for the same oocyte, to measure I_Na, I_g, and I_GP. First, Na⁺ currents were recorded in a 115-mM Na⁺ bath with analogue compensation for linear capacitance and digital P/N subtraction of residual linear leak (top row). Responses are shown for test potentials of −20 to 30 mV. The external solution was then exchanged with K⁺/TTX (R672G) or NMDG⁺/TTX (R669H or WT), and P/N leak subtraction was performed to reveal the nonlinear gating charge displacement current, I_g (middle row). Finally, all analogue and digital P/N leak subtraction was omitted to reveal the gating pore current for mutant channels and the residual leakage current for oocytes expressing WT channels (bottom row). Each column shows the responses recorded from a single oocyte (black, WT; blue, R669H; red, R672G).