NEUROTRANSMISSION
It is best to start by explaining the way things usually work. Below is a diagram of a synapse (two nerve cells meeting) and we will say that this synapse is in the nucleus accumbens or median forebrain bundle which is where we thinks that all pathways involved in addiction commonly lead. This is a dopamineric system which means the neurotransmitter involved is dopamine. The following description will be generic and can apply to any neurotransmitter affected by any drug.
Signals are sent down a particular nerve cell by a flow of electron on the outside membrane of the cell. There is a positive charge maintained on the cell by a set of simple chemicals on the inside and outside of the cell creating a weak electrical potential, much like a weak battery that you may make out baking soda and vinegar in 8th grade science class.
That electrical energy does not cross the synapse (the space between cells above) and transmission of a signal or impulse across that space from one nerve cell to another is by chemicals (neurotransmitters). In a large electrical discharge such as a seizure electrical energy may cross over this space but not in other circumstances.
When that flow of electrons gets to the end of the cell it activates a transport protein. Neurotransmitters such a dopamine are made in the cell and packaged like small water filled balloons. They then migrate toward the end of small peninsula's on the cell called foot processes. The packages of dopamine are illustrated in red above.
On the other cell are structures called receptors. The transport protein carries the packages of dopamine through the cell membrane and they break open in the synapse releasing dopamine into the space. That dopamine can:
 | cross the synapse and bind to a receptor site (the small blue squares which look like they have an old TV antenna on them). That binding causes a change in the shape of the receptor which causes the membrane to depolarize and the flow of electrons to begin down the second neuron above |
| be metabolized by enzymes in the synapse to its basic components of carbon, hydrogen, oxygen, and nitrogen atoms |
| bind back to the transport protein and be taken back into the first neuron to be repackaged and reused. |
