In biochemistry and pharmacology, a receptor is a protein molecule usually found embedded within the plasma membrane surface of a cell that receives chemical signals from outside the cell. When such chemical signals bind to a receptor, they cause some form of cellular/tissue response, e.g. a change in the electrical activity of the cell. In this sense, a receptor is a protein molecule that recognises and responds to endogenous chemical signals, e.g. the acetylcholine receptor recognizes and responds to its endogenous ligand, acetylcholine. However sometimes in pharmacology, the term is also used to include other proteins that are drug targets, such as enzymes, transporters and ion channels. Receptor proteins are embedded in the cell’s plasma membranes; facing extracellular(cell surface receptors), cytoplasmic (cytoplasmic receptors), or in the nucleus (nuclear receptors). A molecule that binds to a receptor is called a ligand, and can be a peptide (short protein) or another small molecule such as a neurotransmitter, hormone, pharmaceutical drug, toxin, or parts of the outside of a virus or microbe. The endogenously designated molecule for a particular receptor is referred to as its endogenous ligand. E.g. the endogenous ligand for the nicotinic acetylcholine receptor is acetylcholine but the receptor can also be activated by nicotine and blocked by curare. Each receptor is linked to a specific cellular biochemical pathway. While numerous receptors are found in most cells, each receptor will only bind with ligands of a particular structure, much like how locks will only accept specifically shaped keys. When a ligand binds to its corresponding receptor, it activates or inhibits the receptor’s associated biochemical pathway.