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Second Messenger

Meaning

A second messenger is an intracellular signaling molecule that is released inside the cell in response to an extracellular signal, known as the first messenger, which is typically a hormone or neurotransmitter. Since many peptide hormones are water-soluble and cannot cross the cell membrane, they bind to receptors on the cell surface. This binding triggers the production or release of the second messenger inside the cell, which then relays the signal to various intracellular targets, leading to a specific physiological response. Common examples include cyclic AMP (cAMP), calcium ions (Ca2+), and inositol triphosphate (IP3).