On the Initiation of Spiral Waves in Excitable Media.

Excitable media are systems which are at rest in the absence of external input but which respond to a sufficiently strong stimulus by sending a wave of "excitation" across the medium. Examples include cardiac, cortical, and other tissue in the central nervous system. In each of these examples evidence of rotating spiral waves has been observed and associated with abnormalities. How such waves are initiated is not well understood. In this numerical study of a standard mathematical model of excitable media, we obtain spirals and other oscillatory patterns by a method, simple in design, which had previously been ruled out. This method applies in a parameter range of "low excitability." We analyze the early stages of this process and show that long term stable oscillatory behavior, including spiral waves, can start with very simple initial conditions, such as two small spots of excitation and all other areas at rest, and no subsequent input or disturbance. Thus, there are no refractory cells in the initial condition. For this model, even random spots of stimulation result in periodic patterns relatively often, leading us to suggest that this could happen in living tissue. [ABSTRACT FROM AUTHOR]