Asymmetry currents in the mammalian myelinated nerve

1. Asymmetrical displacement currents were recorded in the rabbit node of Ranvier by averaging the currents associated with depolarizing and hyperpolarizing pulses in the temperature range 15-25 degrees C with the ends of the fibre cut in 160 mM-CsCl. 2. The identification of the asymmetrical currents as intramembranous current is supported by the findings that (a) the "on' and "off' current transients were equal for short and small depolarizing pulses and (b) the total charge displaced reached saturation at (32-111 x 10(-15) C/node) when sufficiently large depolarizations were applied. 3. After a large depolarization to around 50 mV, the "off' response consisted of a fast phase followed by a slow phase. Lengthening the depolarization reduced the size of the fast response but enhanced the slow response. 4. The steady-state rearrangement of the charges can be described by a Boltzmann distribution of charges with an effective valence of 1.86 and a midpoint potential of -33 mV. The time course of rearrangement of these charges following a change in membrane potential could be fitted reasonably well with a single exponential though a double exponential might have been better at large depolarizations. 5. The steady-state activation curve for Na conductance was measured with various procedures to eliminate the effects of series resistance, which include decreasing peak Na current by TTX, and electronic compensation of the series resistance. The measured steepness of the Na-activation curve corresponds to moving a minimum charge of about 5e to open each Na channel at 22.7 degrees C. 6. The time constant (Tm) for activation of Na current and time constant (Ton) for declining phase of the asymmetry current were of the same order of magnitude but not identical over the potential range -50 to +25 mV. 7. The time course of charge displacement determined from the asymmetry current occurred earlier than did Na activation. Raising the charge displacement curve to second or third power did not yield a curve that matched the time course of Na activation. 8. If all the observed asymmetrical currents are related to the Na-gating system, an upper limit for the number of Na channels per rabbit node is 82,000 and a lower limit for the single channel conductance is 9.8 pS at 18.8 degrees C.