Formation of fast spirals on heterogeneities of an excitable medium.

We study the process of formation of spiral waves in a heterogeneous excitable medium under external stimulation, using numerical and analytical methods. We show that in an excitable medium with several heterogeneities with respect to refractory period, fast rotating spiral waves can be generated. These fast spirals are formed as a result of a phenomenon of period decrease, which is the generation by a heterogeneity of waves with a period shorter than the period of the external stimulation.