A "Second Messenger" refers to a molecule that transmits signals within a cell after the initial signal, often from a hormone or neurotransmitter, binds to a receptor on the cell surface. This process is crucial for cellular communication and helps regulate various physiological functions. Common second messengers include cyclic AMP (cAMP), calcium ions (Ca²+) and inositol trisphosphate (IP3). When a first messenger, such as a hormone, attaches to its receptor, it triggers a cascade of reactions inside the cell, leading to the production of second messengers. These molecules amplify the signal and activate specific pathways, ultimately resulting in a cellular response, such as changes in gene expression or metabolic activity.