If the intersection is at C the system is excitable, and a single large amplitude excursion can occur if a fluctuation happens. This happens to be called the cooperative Michaelis-Menten expression, where n is the index of cooperatively . The origin of oscillations of calcium concentration in biological cells can be modeled by the mechanism shown in figure 3. An external signal S hits a receptor R, which releases a messenger, IP3. Oscillations arise just as they did in the simpler systems.
Cells are sensitive to the frequency of calcium oscillations.
Complex biological and neurological process are controlled by variations in frequency of calcium oscillations. For given values of the parameters we may get rapid, small oscillations or slower, larger ones. A system displaying one type of oscillation can be kicked to the other type of oscillation by some perturbation. This is called birythmicity. This model demonstrates a possible origin of calcium oscillations of different frequencies, such as are involved in signaling.