High resolution recording of asymmetry currents from the squid giant axon: technical aspects of voltage clamp design.

The design, realisation and performance of a voltage clamp system dedicated to recording asymmetry currents from the squid giant axon is presented. The design has been optimised with respect to dynamic response, signal-to-noise ratio and linearity. Analytical expressions are given for both the dynamic performance and noise characteristics which simplify the design and setting up of the clamp and provide excellent agreement between design theory and practice. A 0.5 cm2 area of membrane can be voltage clamped in response to a command input voltage step within 10 microseconds with smooth settling and up to 100% of the effective series resistance being compensated. The noise contributions of the recording chamber, voltage sensing electrodes and clamp electronics have been reduced such that recording of gating currents with a more than 10-fold reduction in the number of averages required compared with previous clamp designs is possible. The overall system nonlinearity results in typically less than 1% contribution to the measured asymmetry charge.