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Physostigma is a genus of flowering plants in the legume group. It includes the source of the Calabar bean, historically important in the study of physostigmine. A purified compound and the chemically mixed seed are different experimental materials.

Physostigmine is a carbamate alkaloid that inhibits acetylcholinesterase. Normally this enzyme hydrolyses acetylcholine; inhibition prolongs the availability of acetylcholine at cholinergic signalling sites. Physostigmine is therefore an indirect enhancer of cholinergic signalling, rather than simply a substitute ligand for every acetylcholine receptor. ^[ref]

A mechanistic distinction matters: increasing transmitter persistence can affect both muscarinic and nicotinic signalling wherever transmitter release and susceptible receptors occur. The outcome therefore depends on location, receptor type and baseline activity. This example separates an enzyme-directed drug action from the eventual physiological response, without assuming one universal effect in every tissue.