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1.
Seasonal recruitment of hippocampal neurons in adult free-ranging black-capped chickadees. 总被引:12,自引:1,他引:11 下载免费PDF全文
A Barnea F Nottebohm 《Proceedings of the National Academy of Sciences of the United States of America》1994,91(23):11217-11221
Neurogenesis and neuronal replacement in a population of wild free-ranging animals are described. Our subjects were adult black-capped chickadees (Parus atricapillus). Neuronal birth dates were determined by a single systemic injection of [3H]thymidine, followed by release of the bird and its recapture 6 or more weeks later. Newly formed neurons appeared in the hippocampal complex during all times of year, but with a marked peak in the fall (October). New neurons were also added to the hippocampal complex of captive chickadees, but at levels only half as high as seen in the wild birds. Neurons born at different times of the year lived for a few months and then disappeared. We suggest that the neurons added are part of a process of neuronal replacement and that they are important for the acquisition of new spatial memories, a need that is particularly acute in the fall. Hormonal and experiential factors may determine the rate of neuronal replacement. 相似文献
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In humans, learning to produce correct visually guided movements to adapt to new sensorimotor conditions requires the formation of an internal model that represents the new transformation between visual input and the required motor command. When the new environment requires adaptation to directional errors, learning generalizes poorly to untrained locations and directions, indicating that such learning is local. Here we replicated these behavioral findings in rhesus monkeys using a visuomotor rotation task and simultaneously recorded neuronal activity. Specific changes in activity were observed only in a subpopulation of cells in the motor cortex with directional properties corresponding to the locally learned rotation. These changes adhered to the dynamics of behavior during learning and persisted between learning and relearning of the same rotation. These findings suggest a neural mechanism for the locality of newly acquired sensorimotor tasks and provide electrophysiological evidence for their retention in working memory. 相似文献
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Monte Carlo methods have been used to simulate the scattering of x rays in polystyrene and water phantoms. In particular, the ratio of the scattered to total x-ray fluence (scatter fraction) has been calculated for monoenergetic x-ray beams in the energy region relevant to diagnostic radiology and nuclear medicine (30-660 keV). Simulations have been made for representative values of the pertinent geometrical factors; phantom thickness from 5 to 21 cm, x-ray beam diameters of 10 and 25 cm, and scatterer-to-image-plane separations from 0 to 20 cm. As a function of x-ray energy, the scatter fraction was found to vary slowly between 30 and 100 keV, and to decrease between 100 and 660 keV. The present results were generated with a special transport code which included the effects of special geometries and the response of the x-ray detector. With the inclusion of these effects, the results resolved inconsistencies and showed good agreement with previous measured and calculated data. 相似文献
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The complex relationship among myocardial contractility, preload, afterload, and coronary autoregulation was studied using both analytical and numerical methods. To study autoregulation and coronary reserve changes in response to changes in cardiac oxygen consumption and in arterial pressure generation, a new variable was introduced: myocardial resistance to oxygen flow (
). This variable was defined as the ratio of the coronary driving pressure to left-ventricular oxygen uptake. High values for this variable indicate small consumption relative to the generated aortic pressure. Conditions which produce the highest obtainable value for
are considered as optimal. An expression relating
to ventricular hemodynamic variables was developed and studied using a mathematical model of the cardiovascular system. The model included a mechanism of local autoregulation based on the assumption that, in steady state, the amount of oxygen consumed equals the amount extracted from coronary blood. Heart rate, peripheral resistance, end-diastolic volume, and myocardial contractility were varied while the coronary circulation was adjusted to meet ventricular oxygen consumption at each state. The model predicts that, for each state of the circulation, there is an optimal level of cardiac contractility for which the coronary reserve is maximized. 相似文献
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Daniela Jakubowicz Oren Froy Bo Ahrén Mona Boaz Zohar Landau Yosefa Bar-Dayan Tali Ganz Maayan Barnea Julio Wainstein 《Diabetologia》2014,57(9):1807-1811