second messenger
A second messenger is the inside-the-house messenger who picks up news from the doorbell and runs it to every room. The hormone is the “first messenger” that arrives at the cell surface but cannot enter; the second messenger is a small molecule made or released inside the cell that carries the message onward to its targets.
When a water-soluble hormone activates a surface receptor, the cell responds by generating a burst of second messenger — such as cyclic AMP, IP3, diacylglycerol, or calcium ions. These small molecules spread quickly through the cytoplasm, switching on enzymes (often kinases) that change the cell's activity. One activated receptor can spawn many second-messenger molecules, which is the first major step of signal amplification.
Second messengers also make signaling fast and reversible: they can be produced in seconds and destroyed just as quickly, so the cell's response can rise and fall in step with the hormone. This system explains how a tiny amount of hormone outside can produce a large, rapid, and tunable effect inside.
Glucagon binding to liver cells raises cyclic AMP as a second messenger, which switches on the enzymes that break down glycogen and release glucose into the blood.
An external hormone signal converted into an internal chemical message.
Calcium counts as a second messenger too, but unlike the others it is not synthesized; the cell raises its concentration by opening channels from internal stores or the outside.