Wiki entry for essential oils gives some basic information for the LD50 of these compounds. These numbers seem to be fairly high, but I note with some alarm that they are abortefacients in very small quantity.
The psychoactive property of some essential oils comes from the presence of eugenol, isoeugenol, safrole, isosafole, piperonal, indole, and others.
You can read an article by Dr. Andrew T. Weil about the the use of essential oil of nutmeg (for example) as a psychotropic agent here.
Not all essential oils are neurotoxic. Some are hepatotoxic, some are carcinogens, etc. You can find a list of many oils and their safety info here and here.