Excitability with multiple thresholds, and tristability

The same analysis indicates the existence of three stable steady-state values of the product when the substrate concentration is held constant in the range where the sigmoid nullcline admits two regions of negative slope. Such a tristability is illustrated in fig. 3.13. As indicated by comparison with fig. 3.3, recycling of product into substrate induces a new steady state in conditions where the enzyme system previously admitted only two steady states at most. Examples of tristability have been reported for chemical systems (Epstein, 1984), but the phenomenon has not yet been observed in biological systems. 

4 Coexistence between a stable steady state and a stable limit cyde 

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