Receptors are vital to cell communication in various different forms of signalling - paracrine, synaptic, endocrine etc. They are proteins that bind specific signal molecules to trigger a response from the target cell. Receptors can be transmembrane proteins found on the extracellular surface of target cells, or they can be found inside the cell (intracellular). There are three major cell surface receptors, namely, the ion-channel ligand receptors, G-protein linked receptor and Enzyme-linked receptor. In the case of intracellular receptors the signalling molecule must enter the cell to activate the receptor. Examples are Nuclear Hormone Receptors and Nitric Oxide Receptors.
There can also be more than one receptor for a specific ligand, which causes different effects and cellular responses depending on the tissue type and receptor isoform. For example, there are 5 subtypes of cholinergic muscarinic receptors (M1, M2, M3, M4 and M5). When stimulated by the ligand Acetylcholine, each receptor type has a different function and downstream effect.