PHYSOSTIGMINE IS A REVERSIBLE ANTICHOLINESTERASE WHICH EFFECTIVELY INCREASE THE CONCENTRATION OF ACETYLCHOLINE AT THE SITE OF CHOLINERGIC TRANSMISSION. THE ACTION OF ACETYLCHOLINE IS NORMALLY VERY TRANSIENT BECAUSE OF ITS HYDROLYSIS BY THE ENZYME, ACETYLCHOLINESTERASE. PHYSOSTIGMINE CONTAINS A TERTAIRY AMINE AND EASILY PENETRATES THE BLOOD-BRAIN BARRIER, WHILE AN ANTICHOLINESTERASE, SUCH AS NEOSTIGMINE WHICH HAS A QUATERNARY AMMONIUM ION IS NOT CAPABLE OF CROSSING THE BARRIER.