Second messengers are small molecules that relay signals received by cell surface receptors to target molecules inside the cell. They play a crucial role in cellular communication, amplifying the effects of hormones and neurotransmitters. Common examples of second messengers include cyclic AMP (cAMP), calcium ions (Ca²+) and inositol trisphosphate (IP3). When a signaling molecule binds to a receptor on the cell membrane, it triggers a cascade of reactions that produce second messengers. These molecules then activate various proteins and enzymes, leading to specific cellular responses such as changes in gene expression, metabolism, or cell growth.