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The signal that travels along the length of a nerve fibre and is the means by which information is transmitted through the nervous system. It is marked by the flow of ions across the membrane of the axon caused by changes in the permeability of the membrane, producing a reduction in potential difference that can be detected as the action potential. The strength of the impulse produced in any nerve fibre is constant (see all-or-none response).
http://nerve.bsd.uchicago.edu/ Details of nerve impulses and other aspects of neurophysiology
From: impulse in A Dictionary of Biology »
Subjects: Science and Mathematics — Sports and Exercise Medicine.