Is this from Molecular Biology of the Cell. 4th edition?
Read this:In responding to many types of stimuli, cells and organisms are able to detect the same percentage of change in a signal over a very wide range of stimulus intensities. This requires that the target cells undergo a reversible process of adaptation, or desensitization, whereby a prolonged exposure to a stimulus decreases the cells' response to that level of exposure. In chemical signaling, adaptation enables cells to respond to changes in the concentration of a signaling ligand (rather than to the absolute concentration of the ligand) over a very wide range of ligand concentrations. The general principle is one of a negative feedback that operates with a delay. A strong response modifies the machinery for making that response, such that the machinery resets itself to an off position. Owing to the delay, however, a sudden change in the stimulus is able to make itself felt strongly for a short period before the negative feedback has time to kick in.
Desensitization to a signal molecule can occur in various ways. Ligand binding to cell-surface receptors, for example, may induce their endocytosis and temporary sequestration in endosomes. Such ligand-induced receptor endocytosis can lead to the destruction of the receptors in lysosomes, a process referred to as receptor down-regulation. In other cases, desensitization results from a rapid inactivation of the receptors—for example, as a result of a receptor phosphorylation that follows its activation, with a delay. Desensitization can also be caused by a change in a protein involved in transducing the signal or by the production of an inhibitor that blocks the transduction process (Figure below).
http://www.ncbi.nlm.nih.gov/books/NBK26813/bin/ch15f25.jpgThe inactivation mechanisms shown here for both the receptor and the intracellular signaling protein often involve phosphorylation of the protein that is inactivated, although other types of modification are also known to occur. In bacterial chemotaxis, which we discuss later, desensitization depends on methylation of the receptor protein.Having discussed some of the general principles of cell signaling, we now turn to the G-protein-linked receptors. These are by far the largest class of cell-surface receptors, and they mediate the responses to the great majority of extracellular signals. This superfamily of receptor proteins not only mediates intercellular communication; it is also central to vision, smell, and taste perception.
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