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nerve impulse[′nərv ‚im‚pəls]
a wave of excitation that spreads along a nerve fiber; it transmits information from the peripheral receptor (sensory) endings to the nerve centers inside the central nervous system, and from there to the performing apparatus—the skeletal musculature, the smooth muscles of the internal organs and vessels, and the glands of external and internal secretion.
The main bioelectric manifestation of a nerve impulse is the action potential (AP), or peaked fluctuation of electrical potential, caused by changes in the ionic permeability of the membrane. Increase in permeability during AP leads to intensification of cation flow (Na+ and Ca2+) to and from the interior of the nerve fiber (K+). As a result, the decomposition of compounds rich in energy—adenosine triphosphate and phos-phocreatine—and the decomposition and synthesis of proteins and lipids are intensified; glycolysis and tissue respiration are activated; certain biologically active compounds (acetylcholine, norepinephrine) are freed from the bound state; and the heat production of the nerve fiber is increased.
The velocity at which a nerve impulse travels varies from 0.5 m/sec (in the thinnest fibers of the autonomic nervous system) to 100–120 m/sec (in the thickest motor and sensory nerve fibers). The propagation of a nerve impulse is ensured by local currents that arise between the excited, negatively charged parts and the resting parts of the fiber.
Under natural conditions, series of nerve impulses run continuously along the nerve fibers, both in the peripheral parts of the nervous system and in its central parts. The frequency of these rhythmic discharges depends on the strength of the stimulus that produces them. During moderate motor activity the discharge frequency is 50–100 impulses per second in the motor nerve fibers; in the majority of sensory fibers it attains 200 a second. Some nerve cells (for example, the interneurons of the spinal cord) discharge at a frequency reaching 1,000–1,500 a second.
B. I. KHODOROV