Calcium and magnesium dependence of spontaneous and evoked afferent neural activity in the lateral-line organ of Xenopus laevis.

1. Rates of spontaneous and mechanically evoked neural impulses from the Xenopus laevis lateral line were compared in the presence of calcium and/or magnesium or in the absence of either divalent cation, bath-applied to the synaptic side of the mechanoreceptors. 2. Spontaneous activity and total activity during mechanical stimulation decreased with increasing calcium or magnesium concentration. 3. Activity during stimulation minus spontaneous activity increased with increasing calcium concentration and decreased with increasing magnesium concentration. 4. At constant divalent cation concentration, increasing calcium/magnesium increased total stimulated activity minus spontaneous activity, while spontaneous activity remained essentially constant. 5. Membrane charge screening and mass action of divalent cations at voltage-dependent cation channels are hypothesized to account for the effects observed.