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1. The effect of different polyvalent metal ions in the external solution upon the threshold membrane potential for spike initiation in crayfish axons has been studied by means of intracellular micro-electrodes. The metal ions tested included six divalent cations (Ca²⁺, Mg²⁺, Sr²⁺, Ba²⁺, Co²⁺, and Ni²⁺) and three trivalent cations (La³⁺, Y³⁺ and Eu³⁺).

2. Identical extracellular concentrations of different cations with the same valence had essentially the same effect on threshold membrane potential. However, a very low concentration of trivalent cations (about 225 µM) was found to be equivalent to a much higher divalent cation concentration (13·5 mM) as measured by their effects on threshold potential.

3. Upon a tenfold increase in concentration, the threshold potential for spike initiation was shifted in a positive direction by 30·6 mV with divalent cations and by 20·8 mV with trivalent cations.

4. It is shown that a hypothesis involving screening of negative charges at the axonal membrane surface, based on Gouy—Chapman theory, predicts these various experimental results rather closely.

5. It is concluded that a high negative charge density, sufficient to render a screening mechanism possible, exists at the surface of crayfish axons in the region of the sodium `gates'.

6. The density of surface charges is calculated to be approximately 1e^-/43 Ų. This calculation is discussed in connexion with the possible molecular identity of the sodium `gates' in crayfish axons and other excitable systems.

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