What is second messenger?

A second messenger is a molecule that transmits signals received by cell surface receptors to target molecules inside the cell, facilitating cellular responses to external stimuli. Common second messengers include cyclic AMP (cAMP), calcium ions (Ca²⁺), and inositol trisphosphate (IP3). They play crucial roles in various physiological processes, such as hormone action, neurotransmission, and cell proliferation. By amplifying the signal, second messengers help ensure a rapid and effective cellular response.