Nervous system would not exist without the human brain. Most neurons send signals via their axons, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called amacrine cells have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves called action potentials, which produce cell-to-cell signals at points where axon terminals make synaptic contact with other cells. Synapses may be electrical or chemical. Electrical synapses make direct electrical connections between neurons, but chemical synapses are much more common, and much more diverse in function. At a chemical
Excitable medium brain 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 (so-called from its initial appearance in the 1986 World Cup in Mexico). Excitable media can be modelled using both partial differential equations and cellular automata. Cellular automata provide a simple model to aid in the understanding of excitable media. Perhaps the simplest such model is in. See Greenberg-Hastings cellular automaton for this model. Each cell of the automaton is made to represent some section of the medium being modelled (for example, a patch