Receptors that couple to an intracellular second messenger cell signalling system, like voltage-gated ion channels, fall into a superfamily of receptors which all share a similar protein structure. In your textbooks these receptors are referred to variously as 7 transmembrane domain receptors or G protein- coupled receptors. These terms reflect the fact that the predicted structure of these receptors requires the amino acid chain to fold back and forth through the membrane 7 times and that they appear to couple initially to G proteins which then go on to influence a variety of second messenger systems which are collectively termed effectors. G protein coupled receptor structure and function will be covered in detail by Dr. Rutter (Element 4).