ANTICHOLINESTERASE

any of several drugs that prevent destruction of the neurotransmitter acetylcholine by the enzyme acetylcholinesterase within the nervous system. Acetylcholine acts to transmit nerve impulses within the parasympathetic nervous systemi.e., that part of the autonomic nervous system that contains chiefly cholinergic fibres and that tends to induce secretion, to contract smooth muscles, and to dilate blood vessels. In preventing the destruction of acetylcholine, anticholinesterase permits high levels of this neurotransmitter to build up at the sites of its action, thus stimulating the parasympathetic nervous system and in turn slowing the heart action, lowering blood pressure, increasing secretion, and inducing contraction of the smooth muscles. Physostigmine and neostigmine are among the principal anticholinesterases. These drugs have only a few clinical uses, mainly in augmenting gastric and intestinal contractions (in treatment of obstructions of the digestive tract), in generally augmenting muscular contractions (in the treatment of myasthenia gravis), and in constricting the eye pupils (in the treatment of glaucoma). Other anticholinesterases in larger doses, however, are widely used as toxins that achieve their effects by causing a continual stimulation of the parasympathetic nervous system. Parathion and malathion are thus highly effective agricultural insecticides, while tabun and soman are nerve gases used in chemical warfare to induce nausea, vomiting, convulsions, and death in humans.