Nerve agents, (also known as nerve gases, though thse chemicals are liquid at room temperature) are a class of phosphorus-containing organic chemicals that inhibit the acetylcholinesterase enzyme in animals. Muscle contraction is stimulated by the release of acetylcholine molecules at the motor nerve endings; within a fraction of a second, the acetylcholine is normally destroyed by acetylcholinesterase to end the muscle contraction until the next nerve impulse. When nerve agents compounds block the action of the enzyme, acetylcholine is not removed, with the result that muscle contractions do not stop, with extremely unpleasant or fatal effects.
Poisoning by a nerve agent leads to contraction of pupils, profuse salivation, convulsions, involuntary urination and defecation, and eventual death by asphyxiation as control is lost over respiratory muscles. Atropine and related"anticholinergic" drugs act as antidotes to the poisoning by their action in blocking acetylcholine receptors, but they are poisonous in their own right. One of the many hypotheses advanced for the causes of Gulf War Syndrome is the widespread use of nerve agent protective drugs during that conflict.
This class of compounds was first discovered in Germany during World War II; three of the most widely known agents, sarin, soman, and tabun were developed at that time for used as chemical warfare agents. At that time, the Germans believed that the Allies also knew of these compounds, based on the belief that because these compounds were not discussed in the Allies' scientific journals, information about them was being suppressed. In actuality, the first that the Allies knew about these agents was when shells filled with the compounds were captured towards the end of the war.
Nerve agents have not been used on large scales in wars, though there have been persistent reports of entire Kurdish villages in Iraq being killed by the use of nerve agents during the 1980s. Although in the event, nerve agents were not actually used by Iraq in the Gulf War, one of the many hypotheses advanced for the cause of Gulf War Syndrome was the widespread use of anticholinergic drugs as a protective measure against any such attack.
A number of insecticides, such as dichlorvos, malathion and parathion are nerve agents. The metabolism of insects is sufficiently different from mammals that these compounds are relatively innocuous to humans and their food animals; but there is considerable concern about the safety of these chemicals where farm workers are exposed to concentrated solutions of these chemicals when used as insecticidal sheep dips etc.
 We All Fall Down: The Prospects of Biological and Chemical Warfare, Robin Clarke, Allen Lane the Penguin Press , 1968 , quotes Brigadier General Rothschilds's book Tomorrow's Weapons to the effect that the Allies knew nothing about these agents until they captured German munitions near the end of the war.