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There is now general agreement that dilution of the internal potassium solution with a non-electrolyte shifts the threshold and the inactivation curve in the direction of a more positive internal potential (1-5). At first it was not clear whether the shift was due to dilution of potassium or to a change of ionic strength. Baker et al. (3) considered that the second alternative was more likely, and very clear evidence in support of this view was obtained last year by Chandler and Meves (5). They found that the change from 300 mM KCI to 50 mM KC1 and 250 mM choline C1 (or NaCl) caused no detectable shift in the inactivation'curve, whereas dilution with sucrose from 300 to 50 mM KCI gave a shift of 20 my. A similar conclusion applies to the relation between sodium conductance and membrane potential, and presumably to the threshold. Fig. 1 illustrates a possible explanation. It is assumed that there are phosphate or carboxyl groups on both sides of the membrane and that those on the outside are neutralized by Ca and Mg (the second assumption is not necessary, but it is simpler to consider only one set of charges). Since ions can pass through the membrane, the fixed charges do not make any direct contribution to the resting potential, but they do alter the distribution of potential. With a charge density of 1.4 X 1013 electronic charges/cm 2 and 300 mM KCI as the internal solution, the potential dip associated with the charges would be-17 my; the space constant of the ion atmosphere in the internal solution is 5.5 A. These figures were calculated from the Debye-Hickel theory by the method of Verwey and Overbeek (6). On diluting the internal solution from 300 to 6 m, the space constant of the ion atmosphere should increase from 5.5 to 39 A and the potential dip should change from-17 to-80 my. Hence the p.d. across the membrane, as opposed to the total p.d. between internal and external solutions, would be the same in C (6 mM KC1, no resting potential) as it is in B (300 mM KC1, resting potential of-63 mv). Since changes in sodium permeability might depend on the p.d. across the 27 The Journal of General Physiology

Effects of Changes in Ionic Strength on Inactivation and Threshold in Perfused Nerve Fibers of Loligo