An excitable medium is a nonlinear dynamical system which has the capacity to propagate a wave of some description, and which cannot support the passing of another wave until a certain amount of time has passed (known as the refractory time).

A forest is an example of an excitable medium: if a wildfire burns through the forest, no fire can return to a burnt spot until the vegetation has gone through its refractory period and regrown. Some chemical media are excitable media, for example in the Belousov-Zhabotinsky reaction. Pathological activities in the heart can be modelled as excitable media. A group of spectators at a sporting event are an excitable medium, as can be observed in a Mexican wave.

Modelling excitable media

Excitable media can be modelled using both partial differential equations and cellular automata.

With cellular automata

Cellular automata provide a simple model to aid the understanding of excitable media. Each cell of the automaton is made to represent some section of the medium (for example, a patch of trees in a forest, or stress in heart tissue). Each cell can be in one of the three following states:

• Quiescent or excitable -- the cell is unexcited, and can be excited. In the forest fire example, this corresponds to the trees being unburnt.
• Excited -- the cell is excited. The trees are on fire.
• Refractory -- the cell has recently been excited and has not yet been through the refractory period. A patch of land where the trees have burnt and the vegetation has yet to regrow.

As in all cellular automata, the state of a particular cell in the next time step depends on the state of the cells around it--its neighbours--at the current time. In an excitable medium the general function is as follows:

• If a cell is quiescent, then it stays quiescent unless one or more of its neighbours is excited. In the forest fire analogy, this means a patch of land only burns if a neighbouring patch is on fire.

• If a cell is excited, it becomes refractory at the next time period. (The trees burn, and the patch of land is left barren.)

• If a cell is refractory, then its refractory period is lessened at the next time period, until it reaches the end of the refractory period and becomes excitable once more. (The trees regrow.)

This function can be altered according to the particular medium. For example, the effect of wind can be add to the model of the forest fire.

Geometries of waves

One-dimensional waves

It is most common for a one-dimensional medium to form a closed circuit, i.e. a ring. For example, the Mexican wave can be modelled as a ring going around the stadium. If the wave moves in one direction it will eventually return to where it started. If, upon a wave's return to the origin, the original spot has gone through its refractory period, then the wave will propagate along the ring again (and will do so indefinitely). If, however, the origin is still refractory upon the wave's return, the wave will be stopped.

In the Mexican wave, for example, if, for some reason, the originators of the wave are still standing upon its return it will not continue. If the originators have sat back down then the wave can, in theory, continue.

Two-dimensional waves

Several forms of waves can be observed in a two-dimensional medium.

A spreading wave will originate at a single point in the medium and spread outwards. For example a forest fire could start from a lightning strike at the centre of a forest and spread outwards.

A spiral wave will again originate at a single point, but will spread in a spiral circuit. Spiral waves are believed to underlie phenomena such as tachycardia and fibrillation.

References

• Understanding Nonlinear Dynamics. Daniel Kaplan and Leon Glass.

External links

• An introduction to excitable media (http://www.cnd.mcgill.ca/bios/bub/excitablemain.html)
• Java applets that show excitable media in 0, 1, and 2D (http://arrhythmia.hofstra.edu/java/applets.html)