Signal transduction is the transmission of molecular signals from the exterior of a cell to its interior. This begins when a ligand binds to a cell-surface receptor, causing the receptor’s intracellular domain to change, and this change initiates a series of signalling events. Components of a signalling pathway are classified according to the stimulus. The stimulating ligand, which can be a hormone or neurotransmitter, is the first messenger, while the receptors are the signal transducers. Once activated these are linked to second messengers, agents that act within the cell to trigger a response. Second messengers include cyclic AMP, cyclic GMP, nitric oxide, lipids and calcium. Second messengers in turn activate secondary effectors which are often other enzymes, and the cascade continues.
Signal transducing receptors are of four main types, enzyme-linked receptors, that have associated or intrinsic enzymatic activity, 7-TM receptors, which are coupled to G proteins, nuclear receptors that directly alter gene transcription upon activation, and ligand-gated ion channels, which change conformation to allow ions to pass though the membrane. In healthy organisms, the processes of cellular growth and differentiation are highly controlled, but become uncoupled in disease states, making unregulated signalling a key factor in disorders such as cancer, inflammation, arteriosclerosis, arthritis and neurodegenerative diseases.