Signal transduction (also known as cell signaling) is the transmission of molecular signals from a cells 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. Transmission is continued either by a series of biochemical changes within the cell or by modification of the cell membrane potential by the movement of ions in or out of the cell. Receptors that initiate biochemical changes can do so either directly via intrinsic enzymatic activities within the receptor or by activating intracellular messenger molecules.