共查询到20条相似文献，搜索用时 31 毫秒
1.
Taylor FW Diner DJ Elson LS Hanner MS McCleese DJ Martonchik JV Reichley PE Houghton JT Delderfield J Schofield JT Bradley SE Ingersoll AP 《Science (New York, N.Y.)》1979,203(4382):779-781
Orbiter infrared measurements of the Venus atmosphere in the 60- to 140-kilometer region show very small diurnal temperature differences near the cloud tops, increasing somewhat at higher levels. The seasonal (that is, equator to pole) contrasts are an order of magnitude larger, and the temperatures unexpectedly increase with increasing latitude below 80 kilometers. An isothermal layer at least two scale heights in vertical extent is found near the 100-kilometer altitude, where the temperature is about 175 K. Structure is present in the cloud temperature maps on a range of spatial scales. The most striking is at high latitude, where contrasts of nearly 50 K are observed between a cold circumpolar band and the region near the pole itself. 相似文献
2.
笔者将激光技术应用于森林勘测,研制出具有可见光导向、激光测距和坡面直读水平距三项主要功能的新型测量工具。本仪由发射系统和接收系统两部分组成。发射系统发射出红色激光束,在森林环境中设置出既定方位的可见方向线,作业人员可按照红色光斑的指示砍伐测线。激光束通过半楔形透镜,分为具有一定夹角的两束光,在接收杆上直接读取实际距离值。在坡面上测量时,若接收杆垂直于水平面,读数为斜距。若接收杆垂直于坡面,读数则为水平距。利用遮光罩或定角器可以保证接收杆垂直于坡面,发明了在坡面上测距不经斜距换算而直接获得水平距的新技术。 相似文献
3.
As the interface between hippocampus and neocortex, the entorhinal cortex is likely to play a pivotal role in memory. To determine how information is represented in this area, we measured spatial modulation of neural activity in layers of medial entorhinal cortex projecting to the hippocampus. Close to the postrhinal-entorhinal border, entorhinal neurons had stable and discrete multipeaked place fields, predicting the rat's location as accurately as place cells in the hippocampus. Precise positional modulation was not observed more ventromedially in the entorhinal cortex or upstream in the postrhinal cortex, suggesting that sensory input is transformed into durable allocentric spatial representations internally in the dorsocaudal medial entorhinal cortex. 相似文献
4.
Hippocampal place cells are a model system of how the brain constructs cognitive representations and of how these representations support complex behavior, learning, and memory. There is, however, a lack of detailed knowledge about the properties of hippocampal afferents. We recorded multiple single units from the hippocampus and the medial and lateral entorhinal areas of behaving rats. Although many medial entorhinal neurons had highly specific place fields, lateral entorhinal neurons displayed weak spatial specificity. This finding demonstrates a fundamental dissociation between the information conveyed to the hippocampus by its major input streams, with spatial information represented by the medial and nonspatial information represented by the lateral entorhinal cortex. 相似文献
5.
南疆铁路全线开通以后,对南疆铁路沿线地区社会经济发展产生积极的影响,推进了该地区城镇化进程。该文以南疆铁路沿线地区的县级以上城镇为研究对象,先利用人均GDP和工业总产值来概述该地区各城镇经济发展现状;然后利用城市人口规模来表征南疆铁路沿线5地州总共37个市、县的规模等级进行分类和分析;最后,借助统计分析软件STATISTICA,选取2007年统计年鉴所提供的南疆铁路沿线37个市、县的10项比较符合描述该地区社会经济发展情况的指标进行聚类分析,对该地区的社会经济状况增长极进行分类及分析。相关数据及研究表明：该地区社会经济发展的总体水平比较低,多数地区的工业产值在国民经济当中所占的比例很小,产业结构不完善;城镇规模小,对周围地区社会经济发展的辐射带动作用不明显;以农业为主的地方增长极类型的城镇占多数。发展多种形式的加工业和服务业,完善产业结构,发挥中心城市对周围小城镇的辐射带动作用,是全面发展该区社会经济的关键所在。 相似文献
6.
从经济规模和金融集聚规模两个方面构建金融集聚竞争力评价指标体系，运用因子分析法和威尔逊最大熵原理对珠三角经济圈内城市的金融集聚竞争力和辐射域进行综合评价.实证结果表明，珠三角城市之间已经形成了互相辐射的网络格局，同时检验了所构建的金融集聚评价指标体系的科学性.本文创造性地将珠三角城市划分为一个具有三层金融增长极的经济发展圈，这使得珠三角经济圈具有双辐射的示范效果. 相似文献
7.
A detailed record of a field reversal has been obtained from the natural remanent magnetization of the Tatoosh intrusion in Mount Rainier National Park, Washington. The reversal took place at 14.7 +/- 1 million years and is interpreted to be from reverse to normal. A decrease in the intensity of the field of about an order of magnitude occurs immediately before the reversal, while its orientation remains substantially unchanged. The onset of the reversal is marked by abrupt swinging of the virtual geomagnetic pole along an arc of a great circle. During the reversal the pole traces a path across the Pacific. In the last stage of the process recorded in the sections, the succession of virtual geomagnetic poles is very similar to those generated by secular variation in the recent past. Although the cooling rate of the intrusion is not sufficiently well known to permit a useful calculation of the duration of the reversal process, an estimate based on the length of the supposed secular variation cycles gives 1 to 4 x 103 years for the reversal of field direction and approximately 1 x 104 years for the time scale of the intensity changes. 相似文献
8.
通过遥感反模型、综合估算模型、生物地球化学循环模型3个模型探讨了土地用途转换效应评价模型与方法。结果表明,遥感反演模型以植被第一生产力模型为代表,能够在较大尺度上监测与土地用途转换密切联系的生态系统生产力变化;综合估算模型以农业生产力估算系统（ESAP）为代表,适用于评价土地用途转换导致的农业生产力变化;生物地球化学循环模型以CENTURY为代表,一般服务于模拟生态系统内部C、N、P与S营养元素的变化规律;土地用途转换效应评价模型为评价土地用途转换效应,深入分析土地用途带来的生态环境问题提供了工具,利用土地用途转换效应评价模型测算出的区域尺度土地用途转换效应对于合理制定土地利用管理政策具有指导价值。 相似文献
9.
Peltier WR 《Science (New York, N.Y.)》1988,240(4854):895-901
Recent analyses of long time scale secular variations of sea level, based on tide gauge observations, have established that sea level is apparently rising at a globally averaged rate somewhat in excess of 1 millimeter per year. It has been suggested that the nonsteric component of this secular rate might be explicable in terms of ongoing mass loss from the small ice sheets and glaciers of the world. Satellite laser ranging and very long baseline interferometry data may be used to deliver strong constraints on this important scenario because of the information that these systems provide on variations of the length of day and of the position of the rotation pole with respect to the earth's surface geography. These data demonstrate that the hypothesis of mass loss is plausible if the Barents Sea was covered by a substantial ice sheet at the last maximum of the current ice age 18,000 years ago. 相似文献
10.
基于多尺度Fourier描述符的二维轮廓曲线匹配 总被引：2，自引：1，他引：1
周术诚 《福建农林大学学报(自然科学版)》2009,38(2)
根据破碎物体碎片拼接的需求,提出一种多尺度Fourier描述符的二维曲线匹配方法.利用不同尺度的Gaussian函数对轮廓曲线进行滤波后,以Fourier变换的系数作为曲线的特征矢量,通过比较2条轮廓曲线特征矢量来判断轮廓曲线的相似性,实现曲线匹配和碎片拼接.结果表明该算法具有准确性、鲁棒性和容错性. 相似文献
11.
Cytomembrane differentiation in the endoplasmic reticulum-Golgi apparatus-vesicle complex 总被引：23，自引：0，他引：23
Diversity of cytomembrane types is confirmed in hyphae of the fungus Pythium ultimum by electron microscopy. A transition in membrane morphology across stacks of dictyosome cisternae (from endoplasmic reticulumlike at one pole to plasma membrane-like at the opposite pole) suggests that dictyosomes of the Golgi apparatus are sites of membrane interconversion. 相似文献
12.
以1 hm2马尾松次生林为试验样地,调查全部植株不同维度的生物学参数,采用相邻格子法对比分析了多维度下种群属性参数的空间格局类型、聚集强度及其复杂性。结果表明,马尾松单株属性参数的变异系数表现为三维参数(植冠体积、地上生物量、叶生物量和枝生物量)二维参数(冠幅面积)一维参数(株高、胸径、冠幅直径)的趋势。株数的空间格局具有强烈的尺度依赖性,即随尺度增大其格局类型由均匀分布变为聚集分布;其他属性参数始终为聚集分布,但聚集强度均随尺度增加而增大。各参数的变异系数随尺度增大均显著减小,但变异复杂程度不完全相同,其中植冠体积变异复杂性最高,而地上生物量最低。可见,仅株数一个参数并不能全面反映种群空间格局特征,需结合其他属性参数来共同表征。 相似文献
13.
The dynamics of free calcium in dendritic spines in response to repetitive synaptic input 总被引：10，自引：0，他引：10
Increased levels of intracellular calcium at either pre- or postsynaptic sites are thought to precede changes in synaptic strength. Thus, to induce long-term potentiation in the hippocampus, periods of intense synaptic stimulation would have to transiently raise the levels of cytosolic calcium at postsynaptic sites--dendritic spines in the majority of cases. Since direct experimental verification of this hypothesis is not possible at present, calcium levels have been studied by numerically solving the appropriate electro-diffusion equations for two different postsynaptic structures. Under the assumption that voltage-dependent calcium channels are present on dendritic spines, free intracellular calcium in spines can reach micromolar levels after as few as seven spikes in 20 milliseconds. Moreover, a short, but high-frequency, burst of presynaptic activity is more effective in raising levels of calcium and especially of the calcium-calmodulin complex than sustained low-frequency activity. This behavior is different from that seen at the soma of a typical vertebrate neuron. 相似文献
14.
SNAP-25在成年期甲减大鼠海马内表达变化及甲状腺素治疗效应 总被引：1，自引：1，他引：0
观察成年期甲状腺功能减退症(简称甲减)大鼠海马内突触相关蛋白SNAP-25(synaptosomal associated protein of 25 kD)改变及不同剂量甲状腺素治疗后的恢复状况,探讨甲减脑损伤及恢复可能的分子机制。结果显示,甲减大鼠血清T3、T4水平显著低于正常对照组(P<0.05),背侧海马突触小体内SNAP-25的表达水平显著高于正常对照组(P<0.05);常规剂量替代治疗组大鼠血清T3、T4恢复至正常水平,SNAP-25表达水平与甲减组比较未见明显改变(P>0.05);经大剂量替代治疗后血清T3、T4高于正常;SNAP-25的表达恢复到正常水平(P<0.05)。这些结果提示,成年期甲减大鼠海马内SNAP-25表达增加,甲状腺素治疗能使其恢复,大剂量替代治疗使SNAP-25表达恢复至正常水平。 相似文献
15.
16.
Luz D Berry DL Piccioni G Drossart P Politi R Wilson CF Erard S Nuccilli F 《Science (New York, N.Y.)》2011,332(6029):577-580
Initial images of Venus's south pole by the Venus Express mission have shown the presence of a bright, highly variable vortex, similar to that at the planet's north pole. Using high-resolution infrared measurements of polar winds from the Venus Express Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) instrument, we show the vortex to have a constantly varying internal structure, with a center of rotation displaced from the geographic south pole by ~3 degrees of latitude and that drifts around the pole with a period of 5 to 10 Earth days. This is indicative of a nonsymmetric and varying precession of the polar atmospheric circulation with respect to the planetary axis. 相似文献
17.
Robertson DS Carter WE Tapley BD Schutz BE Eanes RJ 《Science (New York, N.Y.)》1985,229(4719):1259-1261
An important bound on the accuracy of modern techniques for monitoring polar motion is established by intercomparison of measurement series from two different observing techniques, very long baseline interferometry and satellite laser ranging. The root-mean-square differences between the estimates of the pole position from both techniques are shown to be only 2 milliseconds of arc (about 6 centimeters at one Earth radius). In the absence of common systematic errors, these differences bound the total errors in both sets of estimates. An initial investigation did not reveal any clear signature in the pole position that seems to be associated with major earthquakes. Continued measurements at this level of accuracy hold promise for resolving long-standing arguments over such questions as the nature of the excitation mechanism required to maintain the motion of the pole. 相似文献
18.
油橄榄叶提取物对铅中毒小鼠大脑海马组织抗氧化酶及NO与NOS的影响 总被引：1，自引：0，他引：1
王昱 《甘肃农业大学学报》2012,47(2):21-24,33
为探讨油橄榄叶提取物(OLE)对铅中毒小鼠大脑海马组织超氧化物歧化酶(SOD)、过氧化氢酶(CAT)、一氧化氮合酶(NOS)活性及丙二醛(MDA)、一氧化氮(NO)含量的影响,选100只健康小鼠,每日灌胃醋酸铅溶液的同时灌胃不同剂量的OLE进行治疗,连续用药30d,检测海马组织SOD、CAT、NOS活性及MDA和NO含量的变化.结果表明:与模型对照组相比,小鼠灌胃OLE后海马组织SOD、CAT、NO、NOS水平均升高,MDA含量降低.说明油橄榄叶提取物对铅中毒小鼠有一定的疗效,能减轻铅中毒引起的海马脂质过氧化损伤,提高NOS、NO的水平. 相似文献
19.
利用免疫组化技术检测年龄对SAMP8小鼠海马突触前蛋白complexin1/2表达的影响。结果显示,与4.5月龄相比,13月龄组齿状回多形细胞层和外分子层及CA1区放射层的complexin1/2相对含量均显著下降,8月龄组后两者也下降。此外13月龄组CA3透明层complexin1/2低于表达较8月龄组。这些结果提示在SAMP8鼠海马complexin1/2含量随年龄增加而呈现环路特异性下降。 相似文献
20.
【目的】 研究外源性γ-氨基丁酸（GABA）抵抗仔猪氧化应激的作用效果,及海马神经元GABA受体调节凋亡信号通路在其中可能的介导作用,为GABA作为动物应激调节剂的应用提供科学依据。【方法】 基于成功构建的仔猪活体和大鼠海马神经元氧化应激模型,考察外源性GABA对仔猪血清及海马组织中氧化/抗氧化相关指标、仔猪日增重、脑海马GABA受体以及大鼠海马神经元中GABA受体及凋亡信号通路相关基因表达水平的影响。【结果】 低、中、高浓度的GABA灌喂组（LD+OS; MD+OS;HD+OS）仔猪血清的MDA含量均极显著低于氧化应激（OS）组(P<0.01),而GSH水平均极显著高于OS组(P<0.01),同时HD+OS 组仔猪血清T-AOC水平极显著高于OS组和对照组(P<0.01);且高浓度(100 mg·kg-1 BW)GABA降低仔猪血清MDA含量和增加GSH水平的幅度均高于低浓度(20 mg·kg-1 BW)和中浓度(60 mg·kg-1 BW)GABA。因此,后续研究仅考察100 mg·kg-1 BW GABA的作用效果及其抗氧化应激的机制。OS组仔猪0-7、8-14和0-28日龄的日增重极显著低于对照组(P<0.01),而HD+OS组0-7、8-14和0-28日龄仔猪的日增重极显著高于OS组(P<0.01);OS组仔猪15-28日龄的日增重显著低于对照组(P<0.05),HD+OS组15-28日龄的日增重极显著高于OS组(P<0.01)。以上结果表明,100 mg·kg-1 BW GABA的灌喂极显著地增加了仔猪的日增重。对照组、OS组和HD+OS组在前、中、后期的腹泻率均无显著差异(P>0.05)。氧化应激组海马组织的MDA含量极显著高于对照组和HD+OS组 (P<0.01),而T-AOC和GSH的水平均极显著低于另外两组(P<0.01),表明GABA能提高仔猪海马组织抗氧化能力。HD+OS组的海马组织GABAA和GABAB受体水平均极显著高于对照组和氧化应激组(P<0.01),表明GABA提高了海马组织GABAA和GABAB的水平。氧化应激组脑海马Bcl-2蛋白水平显著低于对照组(P<0.05),Bax和Caspase-3蛋白水平极显著高于对照组(P<0.01)。HD+OS组的Bcl-2蛋白水平极显著高于氧化应激组（P<0.01）,Bax蛋白水平极显著低于氧化应激组（P<0.01）,Caspase-3蛋白水平显著低于氧化应激组（P<0.05）。与此一致的是,氧化应激组、GABA+OS+Picrotoxin组和GABA+OS+CGP54626组大鼠海马神经元的Bax和Caspase-3蛋白水平均显著高于对照组和GABA+OS组（P<0.05）,表明GABA缓解了氧化应激状态下海马神经元的损伤,而GABA受体抑制剂的添加阻挡了GABA的抗应激损伤作用。【结论】 GABA降低了仔猪海马的氧化应激水平,而GABA抗应激的作用机制可能与其降低凋亡蛋白基因的表达相关,而GABAA和GABAB受体介导了该过程。 相似文献