Cell Membrane Capacitance Measurement
The Optopatch breaks new ground in the very demanding area of accurate membrane capacitance measurement, where the highest resolution is required while minimising artefacts due to changes in the access resistance. It is generally accepted that the lock-in amplifier technique gives best results, so one was incorporated into the amplifier design.
The Optopatch provides the facility for the accurate measurement of membrane capacitance and series resistance by extending the operation of the compensation controls which traditionally remove the membrane capacitance charging currents from the recorded current signals. A sinusoidal command voltage is applied to the electrode, with the cell capacitance and access resistance compensation controls adjusted so that no signal appears on the current output. If either parameter changes, a signal will reappear, but its phase will differ by 90 degrees in the two cases, and the lock-in amplifier can resolve the two components if its switching phase is correctly set.
The traditional method works best only for small changes, which in practice may tend to favour the various software-based alternatives, so we have introduced an important improvement. In the Optopatch, these controls are adjusted to (approximately) their correct settings, and the system can then be switched into an automatic mode. In this mode a built-in lock-in amplifier measures the currents produced by modulation of the command voltage by the built-in frequency generator. Error voltages are then generated that maintain the control settings at their correct values. These voltages are direct linear measurements of changes in membrane capacitance and series resistance, so no software or other signal-processing is required in order to follow changes in either parameter. Thus we can provide a linear calibrated capacitance output.
As a further refinement, the switching phase of the lock-in amplifier can also be automatically adjusted by using a low-frequency conductance modulation signal that has no detectable effect on the capacitance output. No external hardware or software are required for membrane capacitance measurement with the Optopatch, as all the necessary signal generation and signal processing can be carried out internally. These modifications give lock-in amplifiers the user-friendliness of software methods for membrane capacitance measurement. The sensitivity of these amplifiers can be extraordinarily high, and although in principle anything can be emulated in software, the amount of data and processing power required to emulate their performance would be very considerable.
Membrane capacitance measurement in practice
The use of the capacitance measurement circuitry is the subject of a joint publication between Cairn and Corne Cros in Pflügers Arch - Eur J Physiol (2002) 443: 653-663, where we made detailed performance tests of the membrane capacitance measurement system, both on model cells and real ones- mouse inner hair cells in this case. These tests very clearly demonstrated the massively improved discrimination between series resistance and membrane capacitance changes that our exclusive "track-in" mode achieves in comparison with conventional lock-in amplifier operation. The full article can be found online at http://link.springer.de/, or you can contact us for a reprint.