Signal transduction (also known as cell signaling) is the transmission of molecular signals from a cell's exterior to its interior. Signals received by cells must be transmitted effectively into the cell to ensure an appropriate response. This step is initiated by cell-surface receptors. The intracellular component of signal transduction is highly receptor specific, thereby maintaining the specificity of the incoming signal inside the cell. Signal transduction pathways amplify the incoming signal by a signaling cascade using a network of enzymes that act on one another in specific ways to ultimately generate a precise and appropriate physiological response by the cell. Signal transduction involves altering the behavior of proteins in the cascade, in effect turning them on or off like a switch. Adding or removing phosphates is a fundamental mechanism for altering the shape, and therefore the behavior, of a protein. Several small molecules within the cell act as intracellular messengers (also known as second messengers). These include cAMP, cGMP, nitric oxide, lipids and Ca2+ ions. Activated receptors stimulate second messenger production, which in turn activate other enzymes and so the cascade continues.