Neuron is the word we use to describe nerve cells.  In the brain, they are responsible for relaying messages that ultimately turn into thoughts and feelings.  Check out this picture.  It shows that Ecstasy kills these neurons ability to make Serotonin.  Serotonin is a chemical in the brain related to depression and anxiety.  So, it is no wonder that some individuals who abuse Ecstasy can have life long depression and anxiety even after they quit using the drug.