optic nerve: see vision.

nerve gas, any of several poison gases intended for military use, e.g., tabun, sarin, soman, and VX. Nerve gases were first developed by Germany during World War II but were not used at that time. These gases generally cause death by asphyxiation, often preceded by such symptoms as blurred vision, excessive salivation, and convulsions. Physiologically, the toxic effect of nerve gases arises because they inactivate the enzyme cholinesterase, which normally controls the transmission of nerve impulses; the impulses continue without control, causing breakdown of respiration and other body functions. Atropine is an effective antidote against most nerve gases. See also chemical warfare.

nerve: see nervous system.

or nerve cell

Any of the cells of the nervous system. Sensory neurons relay information from sense organs, motor neurons carry impulses to muscles and glands, and interneurons transmit impulses between sensory and motor neurons. A typical neuron consists of dendrites (fibres that receive stimuli and conduct them inward), a cell body (a nucleated body that receives input from dendrites), and an axon (a fibre that conducts the nerve impulse from the cell body outward to the axon terminals). Both axons and dendrites may be referred to as nerve fibres. Impulses are relayed by neurotransmitter chemicals released by the axon terminals across the synapses (junctions between neurons or between a neuron and an effector cell, such as a muscle cell) or, in some cases, pass directly from one neuron to the next. Large axons are insulated by a myelin sheath formed by fatty cells called Schwann cells. Bundles of fibres from neurons held together by connective tissue form nerves.

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