The brain is a chemical powerhouse, with each of its several billion cells churning out messages in the form of molecules, stored and ready to go. Precisely released in exquisitely tiny amounts, these molecular messages control the workings of the entire body. And it is the chemical nature of brain activity that reacts and adjusts to a person's use of an addicting drug.

The basic building block of the body's nervous system is the neuron, a cell with long fibers that make contact with other neurons. In spite of the fact that most of the signaling activity of a neuron is electrical, neurons use chemical messengers to communicate with each other. This is because neurons do not actually touch each other. Instead, they use different molecules to send messages that end up causing other neurons to either stay quiet, or activate an electric current. In turn...