Huperzine A is extracted from Huperzia serrata.[2] It is a reversible acetylcholinesterase inhibitor[6][7][8][9] and NMDA receptor antagonist[10] that crosses the blood-brain barrier.[11] Acetylcholinesterase is an enzyme that catalyzes the breakdown of the neurotransmitter acetylcholine and of some other choline esters that function as neurotransmitters. The structure of the complex of huperzine A with acetylcholinesterase has been determined by X-ray crystallography (PDB code: 1VOT; see the 3D structure).[12]