Small molecules that relay and amplify signals from membrane receptors to target proteins inside the cell. These molecules, such as cAMP and calcium ions, serve as intermediates in signal transduction pathways.
The term emerged in the 1960s with Earl Sutherland's work on hormone action. 'Second' refers to their role as intermediates after the 'first messenger' (the original signal like a hormone), and 'messenger' describes their function in carrying information within the cell.
Second messengers are like cellular amplifiers that can turn a whisper into a shout! One hormone molecule binding to a receptor can generate thousands of second messenger molecules, creating a massive amplification effect that ensures the cell doesn't miss important signals.
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