Membrane Ionic Currents, Current Noise, and Admittance in Isolated Cockroach Axons
The application of fluctuation analysis to conduction processes in excitable membranes has developed rapidly since the early work of Derksen and Verveen (1966) on the node of Ranvier. First recordings of membrane current noise were made on the giant axon of the lobster (Poussart, 1969, 1971) and showed that the spontaneous noise consisted mainly of a 1/f component with an intensity related to the driving force for potassium ions. A second noise component with the apparent form of a relaxation process, 1/[1 + (f/fc)2], was later observed in both squid axons (Fishman, 1973; Conti et al., 1975; Fishman et al., 1975a) and frog nodes of Ranvier (Siebenga et al., 1973). Since then, numerous experiments have been done on these last two preparations and noise spectra arising from the transitions between “open” and “closed” states of sodium and potassium channels have been characterized. The relationship between the frequency characteristics of this noise and the observed or computed kinetics of the sodium and potassium conductances is, however, still controversial. In large-area noise measurements of squid axon, several technical problems such as low input impedance, potassium accumulation in the periaxonal space, and electrode polarization provide some impediments to a good quantitative analysis of ionic channel noise. In nodes of Ranvier, the situation is better but extrinsic noise is quite high and potassium also accumulates externally during long-lasting depolarizations.
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