Nonuniform response in the squid axon membrana under voltage-clamp.

TASAKT, I. AND C. S. SPYROPOULOS. Nonunijorm response in the squid axon membrane under ‘voltage-clamp.’ Am. J. Physiol. 193(2): 309-317. 1958.-By cooling a portion of the squid giant axon at a time, it was shown that the repetitive membrane currents observed under the so-called voltageclamp conditions derived from the main portion of the ‘clamped’ axon membrane and not from the lateral unclamped portion. An improvement of the guard system by the use of the method of ‘double voltage-clamp’ also supported this conclusion. Using a microelectrode introduced through the axon surface, it was found that, when the clamping level was between 20 and 35 mv, some part of the membrane became fully active while other parts remained at rest. In this range of membrane depolarization, the active area was found to increase with increasing depolarization. It was stressed that spatial nonuniformity of the squid axon membrane has to be taken into consideration in the interpretation of the results of so-called voltage-clamp experiments. I THA~BEENREPORTED in aprecedingpaper (I) that a repetition of discrete inward membrane currents can be observed in the squid axon under the so-called voltage-clamp conditions. The present investigation is an extension of our study on the discrete phenomena under ‘voltage-clamp’ by the use of several new techniques. Evidence will be presented indicating that the excitable membrane of the squid axon is spatially nonuniform. As a consequence of this nonuniformity, a mixed state of the membrane (a state in which only some parts of the membrane are responding) can be produced when an attempt is made to fix the potential difference across the membrane at a constant level.