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951.
The detection of image motion is fundamental to vision. In many species, unique classes of retinal ganglion cells selectively respond to visual stimuli that move in specific directions. It is not known which retinal cell first performs the neural computations that give rise to directional selectivity in the ganglion cell. A prominent candidate has been an interneuron called the 'starburst amacrine cell'. Using two-photon optical recordings of intracellular calcium concentration, here we find that individual dendritic branches of starburst cells act as independent computation modules. Dendritic calcium signals, but not somatic membrane voltage, are directionally selective for stimuli that move centrifugally from the cell soma. This demonstrates that direction selectivity is computed locally in dendritic branches at a stage before ganglion cells. 相似文献
952.
953.
Manganese oxides in association with paleo- weathering may provide significant insights into the multiple factors affecting the formation and evolution of weathering profiles, such as temperature, precipitation, and biodiversity. Laser probe step-heating analysis of supergene hollandite and cryptomelane samples collected from central Queensland, Australia, yield well-defined plateaus andconsistent isochron ages, confirming the feasibility dating very-fined supergene manganese oxides by 40Ar/39Ar technique. Two distinct structural sites hostingAr isotopes can be identified in light of their degassing behaviors obtained byincremental heating analyses. The first site, releasing its gas fraction at thelaser power 0.2-0.4 W, yields primarily 40Aratm, 38Aratm, and 36Aratm (atmospheric Ar isotopes). The second sites yield predominantly 40Ar* (radiogenic 40Ar),39ArK, and 38ArK (nucleogenic components), at ~0.5-1.0 W. There is no significant Ar gas released at the laser power higher than 1.0 W, indicating the breakdown of the tunnel sites hosting the radiogenic and nucleogenic components. The excellent match between the degassing behaviors of 40Ar*, 39ArK, and 38ArK suggests that these isotopes occupy the same crystallographic sites and that 39ArK lossfrom the tunnel site by recoil during neutron irradiation and/or bake-out procedure preceding isotopic analysis does not occur. Present investigation supports that neither the overwhelming atmospheric 40Ar nor the very-fined nature of the supergene manganese oxides poses problems in extracting meaningful weathering geochronological information by analyzing supergene manganese oxides minerals. 相似文献
954.
光突发交换网络的数据信道分组抢占调度策略研究 总被引:8,自引:3,他引:8
光突发交换是面向下一代因特网的光交换模式,它结合了光电路交换和光分组交换的优点,同时克服了他们的一些缺点.介绍了光突发交换网络中一种支持区分服务的数据信道调度策略,通过仿真分析了数据信道分组策略对网络性能的影响,为进一步研究提供了有价值的参考. 相似文献
955.
半导体热电材料Bi1-xSbx薄膜的电化学制备 总被引:2,自引:0,他引:2
B I1-XSBX半导体合金是性能优异的热电和磁电功能材料,为制备固态电制冷器件、温差发电器件和磁电器件的重要材料。电化学沉积薄膜技术工艺设备简单成本低,在半导体薄膜制备方面有很好的应用前景。系统研究了高浓度盐酸(2.4 MOL/L)的B I和SB盐酸溶液,成份从纯铋逐步变化到纯锑的B I1-XSBX合金半导体薄膜电化学沉积特性。测试了沉积过程I-V循环曲线和和沉积电荷效率等电化学参数。结果表明在所有成份范围内都可以得到典型的B I1-XSBX固溶体结构的高质量薄膜。薄膜生长为典型的溶液扩散控制过程,具有高的沉积电荷效率。薄膜沉积和溶解之间的电位差随溶液中SB(Ⅲ)离子浓度增加而增大,生长的薄膜越来越稳定。在30%SB浓度附近,电化学过程、薄膜结构和性能发生明显的突变。应用X射线衍射和电子显微镜研究薄膜结构,发现薄膜具有明显的(012)择优取向,薄膜晶粒尺寸也随SB浓度的增加而变化。 相似文献
956.
通过一系列实验分析了温度和pH值对羊毛纤维的膨胀和羊毛织物尺寸变化的影响.在pH2.1的溶液中,羊毛织物的尺寸随温度的升高而下降,变化的幅度取决于织物组织和织物的定型率.在羊毛等电点pH4.8时,纤维的膨胀达到最小值,而织物的尺寸则随着溶液酸性的增加而减小. 相似文献
957.
958.
B.Wünnemann 《科学通报(英文版)》2002,47(23):1946-1956
Comprehensive field investigations and laboratory analyses show that palaeolakes, including fresh- mesohaline water Megalake Tengger and other semi-con- nected, isolated water bodies, during late Pleistocene covered an area of more than 20000 km2, which is more than half of the Tengger Desert in NW China. Stratigraphic correlation and chronological evidence indicate that before ca. 42000 aBP the area was more arid. The palaeolakes started to develop around 40000 uncal. 14C aBP but until 37000 14C aBP their scope was limited. High water levels established from 35000 14C aBP lasted until 22000 14CaBP. Lake levels regressed between 22000 and 20000 14C aBP but transgressed from 20000 to 18600 14C aBP. Subsequently, water level declined further and the Megalake Tengger finally desiccated at around 18000 14C aBP. Megalake Tengger possessed a fresh-mesohaline water property, implying that the regional precipitation increased significantly. During the period of Megalake Tengger, the climate was warmer-humid than present. The annual rainfall was 250 to 350 mm more than that of today and the temperature was 1.5 to 3.0℃ higher. 相似文献
959.
Superoxide activates mitochondrial uncoupling proteins. 总被引:64,自引:0,他引:64
Karim S Echtay Damien Roussel Julie St-Pierre Mika B Jekabsons Susana Cadenas Jeff A Stuart James A Harper Stephen J Roebuck Alastair Morrison Susan Pickering John C Clapham Martin D Brand 《Nature》2002,415(6867):96-99
Uncoupling protein 1 (UCP1) diverts energy from ATP synthesis to thermogenesis in the mitochondria of brown adipose tissue by catalysing a regulated leak of protons across the inner membrane. The functions of its homologues, UCP2 and UCP3, in other tissues are debated. UCP2 and UCP3 are present at much lower abundance than UCP1, and the uncoupling with which they are associated is not significantly thermogenic. Mild uncoupling would, however, decrease the mitochondrial production of reactive oxygen species, which are important mediators of oxidative damage. Here we show that superoxide increases mitochondrial proton conductance through effects on UCP1, UCP2 and UCP3. Superoxide-induced uncoupling requires fatty acids and is inhibited by purine nucleotides. It correlates with the tissue expression of UCPs, appears in mitochondria from yeast expressing UCP1, and is absent in skeletal muscle mitochondria from UCP3 knockout mice. Our findings indicate that the interaction of superoxide with UCPs may be a mechanism for decreasing the concentrations of reactive oxygen species inside mitochondria. 相似文献
960.
Determination of the Altyn Tagh strike-slip fault basin and its relationship with mountains 总被引:8,自引:0,他引:8
Li Haibing Yang Jingsui Shi Rendeng Wu Cailai P. Tapponnier Wan Yusheng Zhang Jianxin Meng Fanchong 《科学通报(英文版)》2002,47(7):572-577
A special extended basin topography is developed in the middle segment of the Altyn Tagh Fault Zone. The ratio of its length to width is over 50. The long boundaries at the two sides of the basin are controlled by the straight normal faults with strike-slip component. Within the basin, the Cenozoic strata are spread. The Altyn Tagh main fault goes through the basin, and a series of strike-slip topography was formed within the basin. The reverse thrust structures were formed at the two sides of the center of the basin, thus making the geological bodies composed of old metamorphic rocks at the two sides of the basin extrude vertically,and forming the extended massif (mountain) at the sides of the basin and parallel to the basin. This special topography was called the strike-slip fault basin. The giant extended strike-slip fault basin began to form during Pliocene, and its topography was basically formed during the late Pleistocene. It is the special topography formed during the strike-slip deformation process of the Altyn Tagh Fault Zone and under both transpression and uplifting. 相似文献