A, spontaneous action potentials used as voltage clamp command waveform in the same cell. B, current recorded under control conditions. C, current recorded during superfusion of 50 nm TTX. D, TTX-sensitive current (obtained by subtracting C from B). E, action potential command waveform and the TTX-sensitive current at a fast time base. F, current–voltage relationship of TTX-sensitive iNa obtained from E (the TTX-sensitive current is plotted against the membrane potential).