CONDUCTANCE FLUCTUATIONS AND IONIC PORES IN MEMBRANES

Abstract

This review describes recent progress in the analysis of statistical fluctuations in membrane conductance. The discussion is limited to cases where noise analysis was performed for the sake of obtaining information about the molecular properties of the membrane constituents and, therefore, does not cover work concerned with systems analysis that uses noise driving. The presentation is divided into two parts. The first part begins with a simplified treatment of the mathematical theory, based on the restrictive assumption that ionic channels open and close independently and can exist in only two states (open and closed). This minimal mathematical theory provides the basis for making certain kinds of inferences about the properties of single channels in artificial biJayers and post junctional synaptic membranes. Specifically, in these relatively simple instances, analysis of conductance fluctuations can give estimates of two important channel characteristics, the mean length of time a channel remains open and the conduc­ tance of its open state. The second part treats the mathematical basis for fluctuation analysis from a more general point of view without the restrictions imposed in the simplified development: channels are permitted to have more than two states. In this part, the analysis is extended to include inferences beyond those involving simply amplitude and duration as discussed in the first part, and we stress the relation between the kinetics of channel gating as revealed by fluctuation analysis and the relaxation kinetics measured in standard voltage clamp experiments. The extended theory provides the background for surveying applications of fluctuation analysis to more complicated situations in the pharmacologically modified postsynaptic mem­ brane and in gating of sodium and potassium channels of axons.