It attaches onto the receptor of the protein membrane found on the extracellular side of that membrane. And in most cases, this is what takes place. In most cases, our first messenger doesn't actually go into our cell and create that response. Now, in the pathway that we described so far, this pathway involves only our first messenger. And the first messenger binds and causes that action potential to take place. Now, in most other signal transduction processes we have the use of another molecule known as the secondary messenger.