Voltage and current clamp transients with membrane dielectric loss

Transient responses of a space-clamped squid axon membrane to step changes of voltage or current are often approximated by exponential functions of time, corresponding to a series resistance and a membrane capacity of 1.0 muF/cm(2). Curtis and Cole (1938, J. Gen. Physiol.21:757) found, however, that the membrane had a constant phase angle impedance z = z(1)(jomegatau)(-alpha), with a mean alpha = 0.85. (alpha = 1.0 for an ideal capacitor; alpha < 1.0 may represent dielectric loss.) This result is supported by more recently published experimental data. For comparison with experiments, we have computed functions expressing voltage and current transients with constant phase angle capacitance, a parallel leakage conductance, and a series resistance, at nine values of alpha from 0.5 to 1.0. A series in powers of t(alpha) provided a good approximation for short times; one in powers of t(-alpha), for long times; for intermediate times, a rational approximation matching both series for a finite number of terms was used. These computations may help in determining experimental series resistances and parallel leakage conductances from membrane voltage or current clamp data.