How do second messengers work?

Second messengers are molecules which relay signals received at receptors on the cell surface to target molecules in the cytosol and/or nucleus. These messengers relay the initial signal that occurs when a ligand binds to a specific cellular receptor. Ligand binding changes the protein conformation of the receptor in a manner that it stimulates neighboring effector proteins, which catalyze the generation, or in the case of ions, release or influx of the second messenger. The second messenger then diffuses to protein targets in other areas within the cell, altering the activities as a response to the new signal received by the receptor.  Second messengers are intended to activate intracellular signaling pathways that amplify the signal. These messengers may either activate or inhibit transcription factors, inducing a cellular response.