fractionation /frak′shənā″shən/ [L, frangere, to break] , 1. (in neurology) a mechanism within the neural arch of the vertebrae whereby only a portion of the efferent nerves innervating a muscle reacts to a stimulus, even when the reflex requirement is maximal, so that a reserve of neurons remains to respond to additional stimuli. Through this phenomenon muscle tension is maintained. 2. (in chemistry) the separation of a substance into its basic constituents by using such procedures as fractional distillation or crystallization. 3. (in bacteriology) the process of isolating a pure culture by successive culturing of a small portion of a colony of bacteria. 4. (in histology) the process of isolating the different components of living cells by centrifugation. 5. (in radiology) the process of administering a dose of radiation in smaller units over time to minimize tissue damage rather than in a single large dose. Also called dose fractionation.
