Asymmetrical displacement currents in the membrane of frog myelinated nerve: early time course and effects of membrane potential

Abstract

1. Asymmetrical displacement currents were studied in myelinated nerve fibres from Rana esculenta with a voltage clamp technique. 2. For brief pulses symmetrical with respect to a holding potential of--97mV, the asymmetry current flowing during pulses (on-response) exhibited a rising phase to a peak followed by an approximately exponential decline. After the pulses the rising phase in the off-response could not be resolved; the time constant varied about 2-fold with either size or duration of the pulse. 3. For longer pulses a second slower component could be detected both in on- and off-responses. 4. The rapidly declining on- and off-responses associated with brief pulses carried about the same charges Qon and Qoff. Increasing the duration of the pulse reduced Qoff. For all pulses tested Qoff approached about one fifth of Qmax. The reduction of Qoff was roughly characterised by time constants ranging between 1.5 and 0.5 ms for potentials between--25 and + 23 mV. Analysis of individual membrane currents confirmed that the capacity current after depolarizing pulses decreased with pulse length. 5. The effects of membrane potential on asymmetry current were studied by varying the level from which pulses were applied during 46.9ms prepulses in the range from--97 to--29mV. The fast and slow components of asymmetry current were affected differently. For potentials more positive than--90mV the fast on-response was reduced and reversed its sign at a potential 25mV more negative than the potential estimated from the steady-state charge distribution measured from--97mV.