| natural language processing | Computer processing of a language with rules that reflect and describe current usage rather than prescribed usage. (12 Dec 1998) |
|---|---|
| natural mutation | A mutation which occurs by itself without first being affected by a mutagen, for example during the process of DNA replication. Spontaneous mutations arise at a remarkably constant rate. The rate that spontaneous mutations arise has been used as an evolutionary clock to estimate how closely related two (or more) separate species are to each other. (09 Oct 1997) |
| natural passive immunity | Immunity conferred by the mother on the foetus or newborn. (14 Nov 1997) |
| natural pigment | A naturally occurring coloured compound; absorbs light in the visible range of the electromagnetic spectrum. Compare: structural colour. Synonym: biochrome. (05 Mar 2000) |
| natural products | Naturally occurring compounds that are end products of secondary metabolism; often, they are unique compounds for particular organisms or classes of organisms. (05 Mar 2000) |
| natural selection | The hypothesis that genotype environment interactions occurring at the phenotypic level lead to differential reproductive success of individuals and hence to modification of the gene pool of a population. (18 Nov 1997) |
| immunity, natural | The capacity of a normal organism to remain unaffected by microorganisms and their toxins. It results from non-specific mechanisms such as genetic endowment, naturally occurring antiviral and bactericidal substances, phagocytosis, and constitutional factors such as body temperature, oxygen tension, and age. (12 Dec 1998) |
| killer cells, natural | Cells responsible for spontaneous cytotoxicity of a variety of tumour cells without prior immunization. These natural killer cells are found in non-immune humans and experimental animals and are thought by some to be the same as killer cells (killing by antibody-dependent cell cytotoxicity), but they can also kill in the absence of antibody. (12 Dec 1998) |
| rapid eye movement sleep | REM sleep, that state of deep sleep in which rapid eye movements, alert EEG pattern, and dreaming occur; several central and autonomic functions are distinctive during this state. (05 Mar 2000) |
| paradoxical sleep | A deep sleep, with a brain wave pattern more like that of waking states than of other states of sleep, which occurs during rapid eye movement sleep. (05 Mar 2000) |
| paroxysmal sleep | <neurology> A disorder of sleep associated with excessive daytime sleepiness, involuntary daytime sleep episodes, disturbed nocturnal sleep and cataplexy. Narcolepsy affects over 100,000 people in the United States and appears to have a genetic basis. Symptoms usually begin in the patients twenties. Treatment often includes the use of amphetamines and-or tricyclic antidepressants. (27 Sep 1997) |
| REM stage sleep | <physiology> A stage of deep sleep that is accompanied by rapid eye movement and muscle paralysis. Vivid dreams can be recalled in over 80% of patients who awake from REM stage sleep. (27 Sep 1997) |
| central sleep apnoea | <neurology> A form of sleep apnoea which from the lack of neurologic stimulation to breathe. (12 Jan 1998) |
| mixed sleep apnoea | <chest medicine> A form of sleep apnoea where there is exists a combination of obstructive and central sleep apnoea syndromes. (13 Nov 1997) |
| winter sleep | The dormant state in which some animal species pass the winter. It is characterised by narcosis and by sharp reduction in body temperature and metabolic activity and by a depression of vital signs. It is a natural physiological process in many warm-blooded animals. (12 Dec 1998) |