On the interpretation of voltage noise in small cells

Noise analysis is a powerful technique for studying membrane conductance mechanisms, but it can be applied straightforwardly only to current noise recorded under voltage clamp. Many small cells, however, such as some sensory receptors and interneurons, are not suited for voltage clamping. In such cases it is of great interest to be able to interpret recordings of voltage noise. Voltage noise must be corrected for the effects of: (1) changes in membrane potential, (2) the cells' input impedance, and (3) noise contributed by conductances other than the one of interest. This paper describes in detail the factors--including in particular voltage-dependent K+-conductances--that affect voltage-noise recordings, and describes how these factors may be measured and corrected for in practice. A novel method allows membrane impedance to be measured at the same time as voltage noise. An example is given, showing how this approach may be applied to voltage noise from an insect photoreceptor.