We may start with the obvious classification of conditions where there seems to be far more nervous activity than is normal, and those for which there is far less than normal. Now of course you will not find the second half of these lists in the medical books as problems . But I am puting them in to emphasise a point that is obvious to many classes of psychotherapists, but not necesssarily to pharmacotherapists, and that is that very often there is almost NO physiological difference between states that are perceived as very good and those that are perceived as very bad. You would be VERY hard put to distinguish between the body chemistry of two people on a roller coaster one of whom is extremely excited and the other equally frightened. And as far as I know experimentalists do not consider this aspect.
Barbiturates are similar.
GABA is the most common transmitter in the brain and acts to inhibit activity. It opens up channels in the cell membrane which allow negatively charged calcium ions to pass in to the cell. The effect of this is to hyperpolarise the cell - to make it more negative than before. Thus the overall effect of GABA is to calm down any cells to which it binds. Now benzodiazepines and barbiturates have the ability also to bind to those sites on nerve cells which are designed to be used by GABA molecules, and by doing so enhance the calming or hyperpolarising qualities of the GABA molecules which will be binding nearby.