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991.
992.
Mitotic cycles in dicotyledons and monocotyledons 总被引:1,自引:0,他引:1
993.
Detection of human leukaemia associated antigens in leukaemic serum and normal embryos 总被引:4,自引:0,他引:4
R Harris D Viza R Todd J Phillips R Sugar R F Jennison G Marriott M H Gleeson 《Nature》1971,233(5321):556-557
994.
995.
Crystallization of a second adenovirus protein (the fibre) 总被引:5,自引:0,他引:5
996.
997.
998.
Schödel R Ott T Genzel R Hofmann R Lehnert M Eckart A Mouawad N Alexander T Reid MJ Lenzen R Hartung M Lacombe F Rouan D Gendron E Rousset G Lagrange AM Brandner W Ageorges N Lidman C Moorwood AF Spyromilio J Hubin N Menten KM 《Nature》2002,419(6908):694-696
Many galaxies are thought to have supermassive black holes at their centres-more than a million times the mass of the Sun. Measurements of stellar velocities and the discovery of variable X-ray emission have provided strong evidence in favour of such a black hole at the centre of the Milky Way, but have hitherto been unable to rule out conclusively the presence of alternative concentrations of mass. Here we report ten years of high-resolution astrometric imaging that allows us to trace two-thirds of the orbit of the star currently closest to the compact radio source (and massive black-hole candidate) Sagittarius A*. The observations, which include both pericentre and apocentre passages, show that the star is on a bound, highly elliptical keplerian orbit around Sgr A*, with an orbital period of 15.2 years and a pericentre distance of only 17 light hours. The orbit with the best fit to the observations requires a central point mass of (3.7 +/- 1.5) x 10(6) solar masses (M(*)). The data no longer allow for a central mass composed of a dense cluster of dark stellar objects or a ball of massive, degenerate fermions. 相似文献
999.
The community of the self 总被引:4,自引:0,他引:4
Buchman TG 《Nature》2002,420(6912):246-251
Good health, which reflects the harmonious integration of molecules, cells, tissues and organs, is dynamically stable: when displaced by disease, compensation and correction are common, even without medical care. Physiology and computational biology now suggest that healthy dynamic stability arises through the combination of specific feedback mechanisms and spontaneous properties of interconnected networks. Today's physicians are already testing to 'see if the network is right'; tomorrow's physicians may well use therapies to 'make the network right'. 相似文献
1000.
Multiplicative computation in a visual neuron sensitive to looming 总被引:12,自引:0,他引:12
Multiplicative operations are important in sensory processing, but their biophysical implementation remains largely unknown. We investigated an identified neuron (the lobula giant movement detector, LGMD, of locusts) whose output firing rate in response to looming visual stimuli has been described by two models, one of which involves a multiplication. In this model, the LGMD multiplies postsynaptically two inputs (one excitatory, one inhibitory) that converge onto its dendritic tree; in the other model, inhibition is presynaptic to the LGMD. By using selective activation and inactivation of pre- and postsynaptic inhibition, we show that postsynaptic inhibition has a predominant role, suggesting that multiplication is implemented within the neuron itself. Our pharmacological experiments and measurements of firing rate versus membrane potential also reveal that sodium channels act both to advance the response of the LGMD in time and to map membrane potential to firing rate in a nearly exponential manner. These results are consistent with an implementation of multiplication based on dendritic subtraction of two converging inputs encoded logarithmically, followed by exponentiation through active membrane conductances. 相似文献