Theory of a membrane-voltage clamp with discontinuous feedback through a pulsed current clamp

Abstract

We present a voltage clamp (potentiostat) for biological membranes which works on a new principle. Its feedback loop contains a current clamp which drives a train of brief current pulses through the membrane (typically 8 μsec duration and 8 μsec intervals). In each interval between two current pulses, the difference between membrane voltage and command voltage is sampled and determines the amplitude of the next current pulse. Since the membrane capacitance acts as an analog store between current pulses, the membrane voltage oscillates near the desired value in a triangular waveform. It is independent of resistive voltage drops at electrodes or fluid layers developing in series to the membrane (or the electrode) to be clamped. The clamp does not tend to oscillate, in contrast to clamps with continuous feedback after compensation for series resistors. A precise measurement of membrane capacitance is possible.