The sodium-to-potassium ratio was calculated as the ratio between the sodium current when holding the membrane potential at −20 mV in voltage clamp mode and the potassium current when holding the membrane at 20 mV in voltage clamp mode. The sodium current is obtained by subtracting the minimum current, representing the inward sodium current from the current after stabilizing from the transient sodium current. The sodium at −20 mV is then divided by the slow potassium (sustained) current at 20 mV. One reason for choosing these currents is that, at −20 mV, the gating currents are small and the measured sodium currents are close to the actual sodium currents. The second reason is that, at −20 mV, many sodium channels have opened. At 20 mV, on the other hand, many potassium channels have opened, but the depolarization is not too large, and most cells would have no problem sustaining this potential.
