Amplification of small electric fields by neurons; implications for spike timing.

Small (down to 1 mV/mm) electric fields will polarize neurons by only a small amount; for this reason small electric fields have previously been considered to have no physiologically relevant effects. However, here we propose a novel mechanism by which the non-linear properties of single neurons 'amplify' very small electric fields. Specifically, an amplified change in timing of action potential firing (DeltaT) is inversely proportional to the slope of depolarizing ramp stimulation and proportional to the amount of polarization (DeltaV) caused by the electric fields: DeltaT=DeltaV/(ramp slope). Thus, when responding to slow depolarizing synaptic input, small electric fields can have significant effects on spike timing. Hippocampal CA1 pyramidal neurons were depolarized with injections of depolarizing current ramps approximating synaptic input. Simultaneously, neurons were polarized by either DC holding currents or extracellular uniform DC electrical fields and the resulting changes in spike timing quantified. Consistent with our hypothesis, the polarization induced by each method was found to affect firing time linearly with the amount of polarization, scaled (amplified) with the inverse of the injected ramp slope consistent with our hypothesis.