[Formation of impulses in an excitable medium]

The paper deals with the evolution of initial perturbation in an active distributed system, described by non-linear equations of a diffusion type. Division of all movements into the less than slow greater than and less than fast greater than ones in time and space makes it possible to give a simple analytical description of all the stages. The following cases are possible: a) Initial distribution limited in space falls into two diverging impulses, each of them consists of two sharp fronts connected by slow movements. While propagating each impulse acquires a stationary form; b) Meeting fronts are formed in the wave, the result is that the initial perturbation disappears in a finite time; c) A sharp front with zero rate of propagation is initiated; its slow evolution may lead to an autooscillation process. The solutions obtained are applicable to the description of concentration waves in oscillatory chemical reactions.