Spike frequency of the nodal membrane generated by high-frequency alternating current

Changes in membrane potential of single frog motor nerve fibres due to alternating current (ac) between 4 kHz and 20 kHz were recorded in the air gap equipment under constant current conditions at 20 degrees C. The experimental findings were compared with the results of computations on the basis of potential clamp data. Ac shifted mean membrane potential (averaged for every ac period) in the direction of depolarization. The mean depolarization Vm depended on current strength I; it disappeared when the sodium permeability was blocked, in the experiments by tetrodotoxin. In a current range between about 1 and 3 fold threshold strength the ac initiated repetitive activity with response frequencies v between averaged 120 Hz and 820 Hz or in the computations even higher; v depended logarithimically on current strength, but was independent of ac frequency. Elimination of current amplitude I from the nonlinear realtions v(I) and Vm(I) led to a linear function between v and Vm. Both v and Vm depended markedly on prepolarization of the node. The results were attributed to the preferred activation of the sodium permeability under maintained high frequency ac stimulation. Differences between computations and constant current experiments occurred for very long stimulus duration when rhythmical discharges died out in the experiment.