Nonstationary Noise Analysis of Membrane Currents

Abstract

The simplest way to study membrane current fluctuations from ionic channels would be to place the channels into a steady, partly activated state and to monitor the fluctuations in the current around its steady mean value. In practice, an experiment of this kind is often not possible because of properties of the channels themselves. Acetylcholine-receptor channels, for example, show a desensitization process in which the agonist-induced membrane current declines with time. Some voltage-activated channels show a similar kind of behavior: during a steady depolarization, Na⁺ channels spontaneously inactivate. These changes in membrane current with time pose both practical and theoretical problems for noise analysis. Since the size of the fluctuations is typically only a few percent of the mean current, a serious practical problem arises whenever a small baseline drift occurs, since drift can seriously contaminate a power spectrum computed from the fluctuations (Fig. 1). In many experimental situations the baseline drift can be estimated and removed from the records; however a theoretical problem still remains: how should the fluctuations be interpreted when the mean current itself changes with time?