Cholinergic antagonists, or parasympatholytics, block ACh activity at both muscarinic and nicotinic receptors. Agents that selectively block muscarinic receptors are called antimuscarinics. The antimuscarinics include naturally occurring alkaloids; their semisynthetic derivatives—which differ in their distribution and duration of action from the parent compound; and synthetic compounds—which are selective to specific receptor subtypes. Atropine—the prototype antimuscarinic—is an ester of tropic acid and tropine. Its structure resembles acetylcholine and is used as a reference for designing many antimuscarinic agents. In general, most antimuscarinics contain an ester and a basic amine group separated by a two-to-four carbon linker. Compounds having aromatic, cyclic, or –OH substitutions in the acyl portion of the ester enhance the antimuscarinic effect. Further, quaternary ammonium derivatives are comparatively more potent than parent compounds. In nature, atropine exists as an l(_) isomer which is 100-fold more potent than a d(+) isomer. As it racemizes rapidly, the racemic mixture of atropine is used therapeutically.