1. The resting potentials of giant neurones in the abdominal ganglia of the snail, Helix aspersa, have been measured, and their dependence on the extracellular concentration of potassium ions has been investigated.

2. The behaviour of the resting potentials differs considerably from that of a potassium electrode, as given by the Nernst equation. A modification of the equation derived from the ‘constant-field’ theory is described, which transforms the equation for the resting potential into a linear relationship; it is found that the experimental results can be fitted quite well by this equation, for potassium concentrations up to 12 mM. This is taken as evidence that the relative permeabilities of the cell membrane to potassium and sodium ions are independent of the external potassium concentration over the range of concentrations used.

3. By fitting ‘best straight lines’ to the experimental data, estimates can be obtained from the ‘constant-field’ equation of the intracellular potassium concentration, and the ratio of permeabilities of the cell membrane to sodium and potassium ions, PNa/PK The average results from experiments on fifty-seven neurones are:

intracellular potassium concentration, [K+] = (92.9±4.3) mM.

ratio of permeabilities, PNa/PK = 0.180±0.015,

the error in each case being one standard deviation, calculated from the spread of the results.

4. The average intracellular potassium concentrations of snail neurones show a distinct seasonal variation, being highest in the spring and lowest in the autumn. These changes are accompanied by changes in the selectivity of the cell membranes, as characterized by the ratio of permeabilities to sodium and potassium ions, so that the resting potentials of the cells are comparatively little affected. The changes could be connected with changes in the pattern of activity of the animals.

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