<P> When a normally functioning motor nerve is stimulated, it releases the neurotransmitter acetylcholine, which transmits the impulse to a muscle or organ . Once the impulse is sent, the enzyme acetylcholinesterase immediately breaks down the acetylcholine in order to allow the muscle or organ to relax . </P> <P> Nerve agents disrupt the nervous system by inhibiting the function of the enzyme acetylcholinesterase by forming a covalent bond with its active site, where acetylcholine would normally be broken down (undergo hydrolysis). Acetylcholine thus builds up and continues to act so that any nerve impulses are continually transmitted and muscle contractions do not stop . This same action also occurs at the gland and organ levels, resulting in uncontrolled drooling, tearing of the eyes (lacrimation) and excess production of mucus from the nose (rhinorrhea). </P> <P> The reaction products of the most important nerve agents, including soman, sarin, tabun and VX, with acetylcholinesterase were solved by the U.S. Army using X-ray crystallography in the 1990s . The reaction products have been confirmed subsequently using different sources of acetylcholinesterase and the closely related target enzyme, butyrylcholinesterase . The X-ray structures clarify important aspects of the reaction mechanism (e.g., stereochemical inversion) at atomic resolution and provide a key tool for antidote development . </P> <P> Atropine and related anticholinergic drugs act as antidotes to nerve agent poisoning because they block acetylcholine receptors, but they are poisonous in their own right . Some synthetic anticholinergics, such as biperiden, may counteract the central symptoms of nerve agent poisoning better than atropine, since they pass the blood--brain barrier better than atropine . While these drugs will save the life of a person affected by nerve agents, that person may be incapacitated briefly or for an extended period, depending on the extent of exposure . The endpoint of atropine administration is the clearing of bronchial secretions . Atropine for field use by military personnel is often loaded in an autoinjector (e.g. ATNAA), for ease of use in stressful conditions . </P>

When was a nerve agent used in ww2