The role of low sodium ion concentrations in the generation of action potentials by neurons of the apple snail (Helix pomatia)

Abstract

We conducted a comparative investigation of the restorative action of different sodium ion concentrations on generation of action potentials by apple snail neurons of the central nervous system kept for a prolonged period in a solution in which such ions were lacking. Of the 180 neurons investigated, 60% of the cells had lost all excitability, while 40% retained the ability to generate action potentials of normal amplitude. In neurons that ceased under these conditions to generate action potentials both independently and as the result of direct stimulation, amplitude of the action potentials and of the "overshoot" was restored after adding only 2.5–10 mM of sodium to the solution. Analogous concentrations of lithium ions did not exert a similar restorative action. They repressed the capacity of a neuron to regain excitability in the presence of small amounts of sodium ions. Increasing the external concentration of sodium after restoration of the action potentials led to a proportional decline of their amplitude. Keeping neurons in a sodium-containing solution for periods of 25 min and more caused restoration of the neuron's ability to increase linearly the amplitude of action potentials upon raising the external concentration of sodium ions.