Fig. 7.

Origin of the instability. (A) Antagonist effects of an initial membrane potential fluctuation (solid line) around a stationary pattern (dotted line). Two membrane currents I₁ (blue arrows) and I₂ (red arrows) are induced. (B) The total current I₁ + I₂ tends to dissipate the fluctuation to zero (dashed and dotted lines) and propagates laterally rapidly by cable effect. (C) The initial membrane potential fluctuation generates also a concentration gradient for each ion, and electrodiffusive fluxes occur outside and inside the cell. Because the ions are assumed to diffuse at different speeds, the result is the occurrence of a lateral electric field (C), which amplifies the initial perturbation for a suitable ratio of diffusion coefficients, D₂/D₁ > 1. The characteristic dynamical coefficient of this amplification process is a diffusive one that is slower than the inhibition process. Thus, as the inhibition effect propagates rapidly, the perturbation is slowly and locally amplified.