And so that stimulus induces the release of that particular signal molecule. And this signal molecule is known as the primary messenger molecule. So step number one is the release of the appropriate primary messenger molecule as a result of some type of external stimulus. Step number two is now that the epinephrine is released, it travels through the bloodstream. Eventually it diffuses into the extracellular matrix around the cells and then it binds until specific receptor transmembrane proteins found on the membranes of those cells. So the primary messenger locates and attaches onto a receptor which are usually transmembrane proteins.