Asymmetry currents in the mammalian myelinated nerve.
- 1 December 1980
- journal article
- research article
- Published by Wiley in The Journal of Physiology
- Vol. 309 (1), 499-519
- https://doi.org/10.1113/jphysiol.1980.sp013523
Abstract
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.degree. C with the ends of the fiber cut in 160 mM-CsCl. The asymmetrical currents were supported as intramembranous current; the on and off current transients were equal for short and small depolarizing pulses and the total charge displaced reached saturation at (32-111 .times. 10-15 C/node) when sufficiently large depolarizations were applied. 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 and enhanced the slow response. The steady-state rearrangement of the charges was 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 was fitted well with a single exponential, although a double exponential was possible at large depolarizations. 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 [tetrodotoxin] and electronic compensation of the series resistance. The measured steepness of the Na-activation curve corresponded to moving a minimum charge of about 5e-to open each Na channel at 22.7.degree. C. The time constant (.tau.m) for activation of Na current and time constant (.tau.on) for declining phase of the asymmetry current was of the same order of magnitude but not identical over the potential range -50 +25 mV. The time course of charge displacement determined from the asymmetry current occurred earlier than did Na activation. Raising the charge displacement curve to 2nd-3rd power did not yield a curve matching the time course of Na activation. If all the asymmetrical currents are related to the Na-gating system, an upper limit for the number of Na channels/rabbit node is 82,000 and a lower limit for the single channel conductance is 9.8 pS at 18.8.degree. C.This publication has 23 references indexed in Scilit:
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