Receptor occupancy produces a response at the receptor (R = E r) However, the response is not necessarily identical to the occupancy even for a pure agonist because many receptor systems have silent receptors. Silent receptors represent a proportion of the total receptor pool that must be occupied before there is any response at all. This is a conceptual model and the silent receptors are not a discrete subgroup. Interactions between receptors can also alter the shape of the plots from the theoretical.
Hysteresis is a feature of changes in a dynamic system in which the response to a stimulus in one direction does not follow the same path when in the opposing direction. The binding of a drug to a receptor confers a degree of stability and energy must be supplied to break this bond. A rise in drug concentration results in diffusion of drug towards receptors which facilitates binding. During decreases in drug concentration, there is a lag phase before which drug-receptor bonds will break and thus the system demonstrates 'hysteresis'.
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