After the transmitters have been released due to exocytosis, they bind to proteins called receptor molecules (or receptors). This binding is similar to how a key fits into a lock. If a transmitter, known as a ligand during this process, is not the correct shape, it will not be able to bind with a receptor. If a transmitter is the correct shape, it will be able to bind with the receptor at ligand-binding sites. Each transmitter is able to bind to a group of different receptors.