We present a so-called adaptive Ising model (AIM) to provide a unifyingexplanation for sensitivity and perfect adaptation in bacterial chemotacticsignalling, based on coupling among receptor dimers. In an AIM, an externalfield, representing ligand binding, is randomly applied to a fraction of spins,representing the states of the receptor dimers, and there is a delayed negativefeedback from the spin value on the local field. This model is solved in anadiabatic approach. If the feedback is slow and weak enough, as indeed inchemotactic signalling, the system evolves through quasi-equilibrium states andthe ``magnetization'', representing the signal, always attenuates towards zeroand is always sensitive to a subsequent stimulus.
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