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A naturally occurring quaternary ammonium cation ester, with the formula CH3(O)COC2H4N(CH)3+, that plays a prominent role in nervous system function. The great importance of acetylcholine derives from its role as a neurotransmitter for cholinergic neurons, which innervate many tissues, including smooth muscle and skeletal muscle, the heart, ganglia, and glands. The effect of stimulating a cholinergic nerve, for example, the contraction of skeletal muscle or the slowing of the heartbeat, results from the release of acetylcholine from the nerve endings.
Acetylcholine is synthesized at axon endings from acetyl coenzyme A and choline by the enzyme choline acetyltransferase, and is stored at each ending in hundreds of thousands of membrane-enclosed synaptic vesicles. When a nerve impulse reaches an axon ending, voltage-gated calcium channels in the axonal membrane open and calcium, which is extremely low inside the cell, enters the nerve ending. The increase in calcium-ion concentration causes hundreds of synaptic vesicles to fuse with the cell membrane and expel acetylcholine into the synaptic cleft (exocytosis). The acetylcholine released at a neuromuscular junction binds reversibly to acetylcholine receptors in the muscle endplate membrane, a postsynaptic membrane that is separated from the nerve ending by a very short distance. The receptor is a cation channel which opens when two acetylcholine molecules are bound, allowing a sodium current to enter the muscle cell and depolarize the membrane. The resulting impulse indirectly causes the muscle to contract.
Acetylcholine must be rapidly removed from a synapse in order to restore it to its resting state. This is accomplished in part by diffusion but mainly by the enzyme acetylcholinesterase, which hydrolyzes acetylcholine.
Acetylcholinesterase is a very fast enzyme: one enzyme molecule can hydrolyze 10,000 molecules of acetylcholine in 1 s. Any substance that efficiently inhibits acetylcholinesterase will be extremely toxic.
an acetic acid ester of choline: CH3COOCH2CH2N(CH3)3OH. Colorless crystals, readily soluble in water, alcohol, and chloroform; insoluble in ether. Its molecular weight is 163.2.
Acetylcholine is a biologically active substance widely distributed in nature. It is found in the tissues of organisms in small quantities (fractions of a microgram) in the form of an inactive compound with proteins and lipides; in certain pathological states, the acetylcholine content of the blood is increased. Acetylcholine in its active state is formed in the organism from acetic acid and choline under the action of the enzyme cholinacetylase; it is readily decomposed by enzymes of the cholinesterase group. Acetylcholine belongs to the group of mediators—transmitters of nerve stimuli in the peripheral and central nervous systems. It is secreted by the endings of the autonomic and motor nerve fibers and causes a specific reaction on the part of the innervating organ to stimulate a given nerve. Tiny sacs (vesicles) containing acetylcholine have been discovered in the presynaptic nerve endings. When a nerve is stimulated, acetylcholine enters the synaptic gap from these sacs; this effects transmission of the nerve impulse. When acetylcholine penetrates organs and tissues, it may cause effects characteristic of the excitation of the parasympathetic elements of the autonomic nervous system (lowering of blood pressure, slowing of heartbeat, increased peristalsis of stomach and intestines, pupilar contraction, and so on). The action of certain cholinesterase inhibitors (carbamates, organophosphorous insecticides, and certain poisonous substances) leads to the accumulation of excessive quantities of acetylcholine in the organism, which at first causes acceleration of nerve-impulse transmission (excitation) and later leads to termination of transmission—that is, the blocking of impulses (paralysis). Determination of acetylcholine is made principally by means of biological indicators—contraction of the spinal muscle in leeches and of the straight muscle of the abdomen in frogs, decrease in blood pressure in cats, and so on.
REFERENCEFiziologicheskaia rol’ atsetilkholina i izyskanie novykh lekarstvennykh veshchestv Leningrad, 1957.
Al’pern, D. E. Kholinergicheskie protsessy ν patologii. Moscow, 1963.
G. N. KASSIL’