Conditions under which Na⁺ channels can boost conduction of small graded potentials

It has recently become apparent that in the dendrites or short axons of some neurons, voltage-dependent sodium channels are used not to generate action potentials but to modulate graded potentials; graded potentials carry far more information than do action potentials. A model axon (or dendrite) is described in which sodium channels with kinetics described by equations of the Hodgkin-Huxley type boost conduction of small voltage signals. For a sodium channel density beyond a certain minimum there exists an optimal potential, depolarized with respect to the resting potential, at which there is no steady-state decrement along the axon. For an axon not longer than about 0.7 length constants, small, steady-state deviations from this optimal potential imposed at one end of the axon appear amplified in a graded and stable way at the other 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 ratio at the synapse; the axon also acts as a selective frequency filter. The same axon is capable of conducting an action potential.