Potassium and sodium ion current noise in the membrane of the squid giant axon.

The spectral density of current noise power from 20 mm segments of giant axons of the squid Loligo vulgaris has been measured under space-clamp and voltage-clamp conditions. From 4 to 1000 Hz the measured noise is larger by several orders of magnitude than the theoretical thermal noise. The amplifier's noise, which may yield appreciable contributions above 200 Hz, could be evaluated and subtracted from the total noise using direct measurements of the membrane impedance...