CONDITIONS UNDER WHICH NA+ CHANNELS CAN BOOST CONDUCTION OF SMALL GRADED POTENTIALS

It has recently become apparent that in the dendrites or short axons o f some neurons, voltage-dependent sodium channels are used not to gene rate action potentials but to modulate graded potentials; graded poten tials carry far more information than do action potentials. A model ax on (or dendrite) is described in which sodium channels with kinetics d escribed by equations of the Hodgkin-Huxley type boost conduction of s mall voltage signals. For a sodium channel density beyond a certain mi nimum there exists an optimal potential, depolarized with respect to t he resting potential, at which there is no steady-state decrement alon g the axon. For an axon not longer than about 0.7 length constants, sm all, steady-slate deviations from this optimal potential imposed at on e end of the axon appear amplified in a graded and stable way at the o ther end. A small pulse of potential is propagated with amplification and more rapidly than in an axon with a passive membrane. Compared to passive propagation, there will be an improvement in signal-to-noise r atio at the synapse; the axon also acts as a selective frequency filte r. The same axon is capable of conducting an action potential.