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21.
23.
The ascendent theory that unifies the Earth Sciences is Plate Tectonics,a regime which is unique to Earth.Other convectively active,solid silicate bodies of our... 相似文献
24.
Experience-dependent plasticity of dendritic spines in the developing rat barrel cortex in vivo 总被引:14,自引:0,他引:14
Do changes in neuronal structure underlie cortical plasticity? Here we used time-lapse two-photon microscopy of pyramidal neurons in layer 2/3 of developing rat barrel cortex to image the structural dynamics of dendritic spines and filopodia. We found that these protrusions were highly motile: spines and filopodia appeared, disappeared or changed shape over tens of minutes. To test whether sensory experience drives this motility we trimmed whiskers one to three days before imaging. Sensory deprivation markedly (approximately 40%) reduced protrusive motility in deprived regions of the barrel cortex during a critical period around postnatal days (P)11-13, but had no effect in younger (P8-10) or older (P14-16) animals. Unexpectedly, whisker trimming did not change the density, length or shape of spines and filopodia. However, sensory deprivation during the critical period degraded the tuning of layer 2/3 receptive fields. Thus sensory experience drives structural plasticity in dendrites, which may underlie the reorganization of neural circuits. 相似文献
25.
Two federally listed fishes, the Foskett speckled dace and Warner sucker, are endemic to Warner Basin in south central Oregon. The Foskett speckled dace is native only to a single spring in Coleman Valley. A nearby spring was stocked with dace in 1979 and 1980, and now provides a second population. The present numbers of dace probably are at their highest levels since settlement of the region. The Warner sucker historically occurred throughout much of the Warner Valley, but its distribution and abundance have been reduced by construction of reservoirs and irrigation dams and the introduction of predatory game fishes. Lentic habitats have become dominated by introduced fishes, particularly white crappie, black crappie, and brown bullhead. The largest remaining population of Warner suckers occurs in Hart Lake, where successful reproduction was documented but there is no evidence of recruitment to the adult population. 相似文献
26.
Weaver HA Stern SA Mutchler MJ Steffl AJ Buie MW Merline WJ Spencer JR Young EF Young LA 《Nature》2006,439(7079):943-945
Pluto's first known satellite, Charon, was discovered in 1978. It has a diameter (approximately 1,200 km) about half that of Pluto, which makes it larger, relative to its primary, than any other moon in the Solar System. Previous searches for other satellites around Pluto have been unsuccessful, but they were not sensitive to objects less, similar150 km in diameter and there are no fundamental reasons why Pluto should not have more satellites. Here we report the discovery of two additional moons around Pluto, provisionally designated S/2005 P 1 (hereafter P1) and S/2005 P 2 (hereafter P2), which makes Pluto the first Kuiper belt object known to have multiple satellites. These new satellites are much smaller than Charon, with estimates of P1's diameter ranging from 60 km to 165 km, depending on the surface reflectivity; P2 is about 20 per cent smaller than P1. Although definitive orbits cannot be derived, both new satellites appear to be moving in circular orbits in the same orbital plane as Charon, with orbital periods of approximately 38 days (P1) and approximately 25 days (P2). 相似文献
27.
The low temperature EPR spectrum of a quickly reacted mixture of oxyhemoglobin and phenylhydrazine was studied. With the use of a computer, the spectral contribution of methemoglobin in the region of g = 2 was subtracted. The remaining spectrum was that of an axial free radical (g perpendicular = 2.00, g parallel = 2.06) having the magnetic parameters of superoxide anion. In the presence of superoxide dismutase, this axial radical was not seen, confirming that superoxide anion is indeed generated by the reaction. 相似文献
28.
The induction of the hermaphrodite vulva and the migration of the sex myoblasts in the nematode Caenorhabditis elegans are both controlled by intercellular signalling. The gonadal anchor cell induces formation of the vulva from nearby hypodermal cells, and a set of somatic gonadal cells attract the migrating sex myoblasts to their final positions. Many genes required for vulval induction have been identified, including the let-23 receptor tyrosine kinase gene and the let-60 ras gene. We report here the identification and characterization of a new gene, sem-5 (sem, sex muscle abnormal), that acts both in vulval induction and in sex myoblast migration. On the basis of its DNA sequence, sem-5 encodes a novel 228-amino-acid protein which consists almost entirely of one SH2 (SH, src homology region) and two SH3 domains. SH2 and SH3 domains are present in many signalling proteins regulated by receptor and non-receptor tyrosine kinases. Mutations that impair sem-5 activity alter residues that are highly conserved among different SH2 and SH3 domains. Our results indicate that the sem-5 gene encodes a novel protein that functions in at least two distinct cell-signalling processes. 相似文献
29.
F K Jouffroy W L Jungers J T Stern 《Comptes rendus des séances de l'Académie des sciences. Série D, Sciences naturelles》1979,288(21):1627-1630
The vastus lateralis muscle is of primary importance in leaping inasmuch as it initiates the jumps; on the contrary the activity of the vastus intermedius does not increase during jumping, but it is the only one to be active in quadrupedal resting postures. 相似文献
30.
P. Stern 《Cellular and molecular life sciences : CMLS》1971,27(5):537-538
Summary As is known, 6-HD destroys the sympathetic trunc when given to new-born mice. In these mice (weighing 18 g) skin homo-transplantations were made and it was found that the homografts strikingly survived. 相似文献