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 ¿Am 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 M¿. 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.