Receptor:1. In cellular biology, receptors are structures found on or within a cell that specifically recognize and bind to certain substances. There are numerous types of receptors, each dedicated to a particular substance. For instance, the receptor for substance P, which is a molecule involved in transmitting pain signals, is a specialized site on the cell's surface where substance P attaches. Without this receptor, substance P cannot connect to the cell or communicate its pain message. Different forms of nuclear hormone receptors play roles in processes such as fatty acid production. Some of these hormone receptors are associated with diseases, including certain cancers. A receptor known as PXR is believed to activate the body's defense against unfamiliar chemicals and may be linked to interactions between drugs.2. In sensory biology, a receptor is an endpoint of a sensory nerve responsible for receiving and reacting to external stimuli.