what is receptors

In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems). Cellular receptors are proteins either inside a cell or on its surface which receive a signal. In normal physiology, this is a chemical signal where a protein-ligand binds a protein receptor. Receptor proteins can be classified by their location. Cell surface receptors, also known as transmembrane receptors, include ligand-gated ion channels, G protein-coupled receptors, and enzyme-linked hormone receptors. Intracellular receptors are those found inside the cell, and include cytoplasmic receptors and nuclear receptors). A molecule that binds to a receptor is called a ligand and can be a protein, peptide (short protein), or another small molecule, such as a neurotransmitter, hormone, pharmaceutical drug, toxin, calcium ion, or parts of the outside of a virus or microbe).