Low frequency voltage clamp: recording of voltage transients at constant average command voltage.

We implemented a simple feedback system that modifies the conventional current clamp mode of a patch clamp amplifier so that transient potentials, such as action potentials and synaptic potentials, can be measured as in the usual current clamp, while the average membrane potential is kept constant at a value chosen by the user. The circuit thus works like the current clamp for high frequency signals and like a voltage clamp for low frequency signals. We delineate its transfer properties and give application examples.