Na Conductance Kinetics in the Low-Frequency Impedance of Isolated Snail Neurons

The complex impedance of isolated single neurons of Helix aspersa (snail) was measured at low frequencies (0.25-100 Hz) with a combination of a suction electrode for applying current and a microelectrode for measuring potential response. A repetitive, Fourier-synthesized signal superposed as a small perturbation (0.2 nA rms) on step current clamps was used in conjunction with Fourier transform techniques to obtain high-resolution impedance data 1 s after a step. The membrane capacitance of an 80 Ωm diameter cell, measured in the frequency range 2.5-1000 Hz after suppression of ion conduction, was 1.8 nF and had a constant phase angle of about -69°. The resting (-50 mV) extrapolated zero-frequency resistance, before suppression of conduction in the same cell, of a predominantly Na-conducting membrane was about 5 MΩ. Linear ion-conduction parameters were estimated directly by fitting an admittance model, based on the linearized Hodgkin-Huxley equations with a constant-phase-angle capacitance, to the reciprocal of the impedance data. The natural frequencies of Na activation and inactivation processes for small depolarizations (10-20 mV from rest potential) were determined to be 23-28 Hz (Tm = 7-5.7 ms) and 2.5-1.8 Hz (Th = 64-88 ms), respectively. These values are more than an order greater than the corresponding relaxation times of Na channels in the squid axon.