signal transduction

Signal transduction is how a cell hears the outside world and acts on it. An extracellular signal — a hormone, neurotransmitter, or drug — arrives at the cell surface, and the cell converts that arrival into a specific internal change: a contraction, a secretion, a shift in metabolism, or a change in which genes are read. "Transduction" means converting one form of signal into another, just as a microphone turns sound into electricity.

The chain usually runs: ligand binds receptor, receptor changes shape and recruits a transducer (such as a G protein or an enzyme), the transducer activates an effector, the effector produces a second messenger, and the messenger drives kinases and other proteins to alter cell function. Different receptor classes route this differently — GPCRs through G proteins and second messengers, kinase-linked receptors through phosphorylation cascades, and nuclear receptors by altering gene transcription directly.

Signal transduction is the conceptual home of pharmacodynamics, because most drugs work by entering this process at some point. Understanding the pathway tells you not only what a drug does but why it has the side effects, the time course, and the tendency to desensitize that it does. It also explains drug specificity: the same first messenger can produce different outcomes depending on which receptors and pathways a given cell expresses.