Conductance fluctuations from the inactivation process of sodium channels in myelinated nerve fibres*

Abstract

SUMMARY1. Na currents and fluctuations of Na currents were studied under voltage clamp in the same myelinated nerve fibres of Rana esculenta at 13 'C. The results were used to test several kinetic models for the gating process of Na channels.2. Long voltage pulses, depolarizing the membrane by 16-48 mV from a hyperpolarizing holding level of -28 mV, were applied in 4 sec intervals. The d.c. and a.c. components of the membrane current were recorded during the last 328 msec of the 473 msec pulses. For each depolarization, ninety-six trials were made with the node in Ringer solution and, again, after adding 300 nM-tetrodotoxin (TTX) in that solution.3. The TTX-sensitive d.c. component declined during the 328 msec records by 14-51 % of its time average. The a.c. component was corrected for this trend by subtracting the first from the second of each pair of subsequent records. The TTXsensitive part of its variance declined, on the average, in parallel to the current, as if the open probability rather than the conductance of the individual Na channels was reduced by a slow process.4. Single-channel conductances, y/, were calculated on the assumption that Na channels have only one non-zero conductance and were corrected for the limited band width (5 kHz) of the a.c. records. Values of y increased slightly ( < 30 % from 16 to 40 mV), and averaged 8-85 + 0-7 pS (s.E. of mean, seventeen measurements on ten fibres). This small degree of change in y suggests that deviations from the all-ornone gating are very small. 5. Power spectral densities of the fluctuations between 3 Hz and 5 kHz were calculated from the trend-free a.c. records and corrected for the TTX-insensitive noise component. Control calculations showed that the only effect of the nonstationarity in the Na current was to enhance the low-frequency points of such spectra by less than 10 %. The spectra revealed at least two Lorentzian components with cut-off frequencies in the range expected from the activation and inactivation kinetics. The low-frequency component became dominant as depolarization was increased. F. CONTI, B. NEUMCKE, TV. NONNER AND R. STAMPFLI 6. Na currents recorded during brief (< 40 msec) depolarizations were analysed in terms of various all-or-none gating models, in which inactivation either was independent of activation (Hodgkin-Huxley (HH) model) or could occur only from the partly or fully activated states (coupled models). The transient Na currents were reproduced by all models.7. With the parameters from such fits, the fluctuation spectra expected for each model were calculated. The predictions differed in the fraction, rh, of the variance contributed by the slow (inactivation) fluctuations; rh was larger in the coupled models than in the HH model.8. The experimental spectra were divided into two spectral components to yield empirical values for rh. We used as templates the spectral curves derived for the fast and for the slow fluctuations of the HH model. The empirical rh values were one (48 mV) to four (16 mV) times larger than those e...

Sodium currents and sodium‐current fluctuations in rat myelinated nerve fibres

“…h scheme. A similar discrepancy has also been described for frog nerve fibres (Conti et al 1980). Thus the independence between sodium activation and inactivation implied by the ml .…”

“…However, a significant difference exists with respect to the following lower conductances reported for sodium channels in frog nodes: y = 7-9 pS (Conti et al 1976a), 6-4 pS (Sigworth, 1980a) and 8-85 pS (Conti et al 1980). Part of the difference is due to the higher temperature (20 'C) and to the higher extracellular sodium concentration (154 mM) in our experiments on rat nerve compared with the corresponding parameters in the work on frog node (13 'C, 105 mm in Conti et al 1976aConti et al , 19802-5 'C, 100 mm in Sigworth, 1980a). To compare values obtained under different experimental conditions we take a Q10 value of 1-3 for PNa (Frankenhaeuser & Moore, 1963) and assume a linear dependence ofy on external sodium concentration.…”

“…(ii) Our experimental procedure to record mean currents and stationary current fluctuations at the end of depolarizing test pulses has been extensively described (Conti et al 1976a(Conti et al , 1980. In brief, current samples were taken at intervals of 80 Usec between 145 and 460 msec after the onset of the test pulse.…”

“…(19) and (20) ofConti et al 1976a). The fit yielded the microscopic time constants 'Tm and Th of sodium activation and inactivation, respectively, and the fraction, rh, of the variance of sodium-current fluctuations which is produced by slow h inactivation gating reactions (Conti et al 1980).…”

SingleIhChannels in Pyramidal Neuron Dendrites: Properties, Distribution, and Impact on Action Potential Output

“…NSFA of recorded, as well as simulated, I h currents was performed according to recently described routines for voltage-dependent channels (Conti et al, 1980;Sigworth, 1980;Alvarez et al, 2002) written in and analyzed with Igor Pro 5.01 (WaveMetrics, Lake Oswego, OR). In brief, assuming that the macroscopic ionic current at a given time point (t) is dependent on the single-channel properties, such that I(t) ϭ N␥P o (V,t) (V Ϫ V REV ), where I is the current, and V the membrane voltage.…”