Membrane Capacitance Measurement
Actually there's not too much we can say here, because our objective of making measurements of membrane capacitance simple, accurate, fast and sensitive (and of course in real time too), determined the design of much of the main electronics, so there are relatively few aspects that we can describe separately. Instead, most information will be given in connection with the detailed description of the system operation. However, we can explain the general approach in this section.
Basically, we have provided facilities for the accurate measurement of membrane capacitance (and also of the access resistance, which needs to be determined at the same time), by extending the operation of the compensation controls that patch clamps traditionally provide for removal of the membrane capacitance charging currents from the recorded current signals. In our system, 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, which measures the currents produced by modulation of the command voltage by the built-in frequency generator, generates error voltages that maintain the control settings at their correct values. These voltages are direct linear measurements of changes in membrane capacitance and access conductance, so no software or other signal-processing is required in order to follow changes in either parameter.
The lock-in amplifier technique is very powerful, and we'll discuss it in detail in a later section, but for now we just need to note that it involves gain-switching in synchrony with the command potential frequency, and for best operation it is important that the phase of the switching is set correctly relative to that of the command frequency. The correct phase depends on the membrane capacitance and access resistance, so it may change during an experiment. We have therefore provided an additional facility, using a low-frequency modulating signal, that automatically maintains the correct phase at all times.
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. In some respects it is
difficult to say how unique our system is, because the software writers might
be able to argue that their algorithms are doing the same thing. However, this
is difficult for us to assess, because it is not always clear what the
algorithms actually are! Nevertheless, we believe we are the first to design an
integrated solution to the problem in hardware, and the way we have gone about
it most certainly is novel. We don't want to make extravagant claims about the
performance of our system, but its results on model systems do give us some
cause for optimism. Emulation's can be great, but on the other hand you can't
beat the real thing....
