Analysis of adaptation in Helix pomatia neurons at different extracellular potassium concentrations

Adaptation plays an important role in the firing pattern of nervous cells. This phenomenon has been studied stimulating Helix pomatia neurons by long lasting (9-18 s) currents at different strengths. The results were interpreted by means of a behavioural model allowing the determination of some parameters (proportionality constant between excitatory current and instantaneous firing frequency, constants of the inhibitory current decay, for the incremental inhibitory current per impulse and of the frequency response decay) characterizing the neuron. Changes in these parameters have been investigated perfusing with solutions at higher [K]. A [Ko] increase leads to a membrane depolarization and, in absence of other effects, the current required to reach the firing threshold is lower than in normal conditions. In this case the initial and steady-state frequency values are higher and the neuron discharges more rapidly during the stimulation period. Opposite results are probably due to a decrease in the rate of exchange with the external solution at high current densities and to the high [Ko] decreasing the chemical gradient.