Neurotransmitters are chemical molecules released by the neurons to bridge the gap between and to be able to communicate. This is a process in which one side of the communication is the source of the neurotransmitters and the other is the receptor which is often used as a lock-key example. Since the neurotransmitter is the component which bridges the gap and in a sense unlocks the door, it is the key. The receptor is the part affected by the neurotransmitter which means that is the lock part of this example.