3,587 research outputs found
Photoinduced Removal of Nifedipine Reveals Mechanisms of Calcium Antagonist Action on Single Heart Cells
The currents through voltage-activated calcium channels in heart cell membranes are suppressed by dihydropyridine calcium antagonists such as nifedipine. Nifedipine is photolabile, and the reduction of current amplitude by this drug can be reversed within a few milliseconds after a 1-ms light flash. The blockade by nifedipine and its removal by flashes were studied in isolated myocytes from neonatal rat heart using the whole-cell clamp method. The results suggest that nifedipine interacts with closed, open, and inactivated calcium channels. It is likely that at the normal resting potential of cardiac cells, the suppression of current amplitude arises because nifedipine binds to and stabilizes channels in the resting, closed state. Inhibition is enhanced at depolarized membrane potentials, where interaction with inactivated channels may also become important. Additional block of open channels is suggested when currents are carried by Ba^(2+) but is not indicated with Ca^(2+) currents. Numerical simulations reproduce the experimental observations with molecular dissociation constants on the order of 10^(-7) M for closed and open channels and 10^(-8) M for inactivated channels
Zero field spin splitting in AlSb/InAs/AlSb quantum wells induced by surface proximity effects
InAs quantum well heterostructures are of considerable interest for
mesoscopic device applications such as scanning probe and magnetic recording
sensors, which require the channel to be close to the surface. Here we report
on magnetotransport measurements of AlSb/InAs/AlSb Hall bars at a shallow depth
of 20 nm. Analysis of the observed Shubnikov-de Haas oscillations and modeling
show that spin splitting energies in excess of 2.3 meV occur at zero magnetic
field. We conclude that the spin-splitting results from the Rashba effect due
to the band bending in the quantum well. This is caused by substantial electron
transfer from the surface to the quantum well and becomes significant when the
quantum well is located near the surface.Comment: 14 pages, 2 figures. (To be published in APL
WORD-MAKING POTENTIALITIES OF ENGLISH
The purpose of this thesis is to examine the various methods of word-formation in the English language, to ascertain which methods are active in the post-World War II of today, and decide, if possible, which methods are coming into increasing use and which methods are temporarily out of favor.
For this study materials over a seventeen -year period 1939-1956 were consulted, and then the war words which seemed to find no permanent place in the language were discarded. Many of the war words were retained, however, if the seemed to fill a continuing need.
Of course, any judgement concerning what is, or what is not, a useful word is a personal evaluation, although an attempt to justify this decision by application of linguistic principles has been made
The Local Economic Growth Impact of Broadband Infrastructure 1998 to 2008
This dissertation presents estimates of the relationship between early investment in broadband infrastructure and a number of local economic indicators using a data set of communities (by zip code) across the U.S. Data is matched from the FCC (Form 477) on broadband infrastructure availability with demographic and other socio-economic data from the U.S. Population Censuses and Business Trends Surveys. Spatial econometric techniques are utilized. Even after controlling for community-level factors known to influence broadband availability and economic activity, it was found that between 1998 and 2008, communities in which broadband was available by 1999, compared to those that did not, experienced a greater difference in the growth of 1) rents, 2) salaries, 3) employment, and 4) overall establishments. In addition, broadband contributed to the share of different industry structures lending support to the GPT hypothesis. This research replicates and extends Lehr et al. (2005
The Local Economic Growth Impact of Broadband Infrastructure 1998 to 2008
This dissertation presents estimates of the relationship between early investment in broadband infrastructure and a number of local economic indicators using a data set of communities (by zip code) across the U.S. Data is matched from the FCC (Form 477) on broadband infrastructure availability with demographic and other socio-economic data from the U.S. Population Censuses and Business Trends Surveys. Spatial econometric techniques are utilized. Even after controlling for community-level factors known to influence broadband availability and economic activity, it was found that between 1998 and 2008, communities in which broadband was available by 1999, compared to those that did not, experienced a greater difference in the growth of 1) rents, 2) salaries, 3) employment, and 4) overall establishments. In addition, broadband contributed to the share of different industry structures lending support to the GPT hypothesis. This research replicates and extends Lehr et al. (2005
Emergent structured transition from variation to repetition in a biologically-plausible model of learning in basal ganglia.
Often, when animals encounter an unexpected sensory event, they transition from executing a variety of movements to repeating the movement(s) that may have caused the event. According to a recent theory of action discovery (Redgrave and Gurney, 2006), repetition allows the animal to represent those movements, and the outcome, as an action for later recruitment. The transition from variation to repetition often follows a non-random, structured, pattern. While the structure of the pattern can be explained by sophisticated cognitive mechanisms, simpler mechanisms based on dopaminergic modulation of basal ganglia (BG) activity are thought to underlie action discovery (Redgrave and Gurney, 2006). In this paper we ask the question: can simple BG-mediated mechanisms account for a structured transition from variation to repetition, or are more sophisticated cognitive mechanisms always necessary? To address this question, we present a computational model of BG-mediated biasing of behavior. In our model, unlike most other models of BG function, the BG biases behavior through modulation of cortical response to excitation; many possible movements are represented by the cortical area; and excitation to the cortical area is topographically-organized. We subject the model to simple reaching tasks, inspired by behavioral studies, in which a location to which to reach must be selected. Locations within a target area elicit a reinforcement signal. A structured transition from variation to repetition emerges from simple BG-mediated biasing of cortical response to excitation. We show how the structured pattern influences behavior in simple and complicated tasks. We also present analyses that describe the structured transition from variation to repetition due to BG-mediated biasing and from biasing that would be expected from a type of cognitive biasing, allowing us to compare behavior resulting from these types of biasing and make connections with future behavioral experiments
Programming a hillslope water movement model on the MPP
A physically based numerical model was developed of heat and moisture flow within a hillslope on a parallel architecture computer, as a precursor to a model of a complete catchment. Moisture flow within a catchment includes evaporation, overland flow, flow in unsaturated soil, and flow in saturated soil. Because of the empirical evidence that moisture flow in unsaturated soil is mainly in the vertical direction, flow in the unsaturated zone can be modeled as a series of one dimensional columns. This initial version of the hillslope model includes evaporation and a single column of one dimensional unsaturated zone flow. This case has already been solved on an IBM 3081 computer and is now being applied to the massively parallel processor architecture so as to make the extension to the one dimensional case easier and to check the problems and benefits of using a parallel architecture machine
The transition zone as a host for recycled volatiles: Evidence from nitrogen and carbon isotopes in ultra-deep diamonds from Monastery and Jagersfontein (South Africa)
Sublithospheric (ultra-deep) diamonds provide a unique window into the deepest parts of Earth's mantle, which otherwise remain inaccessible. Here, we report the first combined C- and N-isotopic data for diamonds from the Monastery and Jagersfontein kimberlites that sample the deep asthenosphere and transition zone beneath the Kaapvaal Craton, in the mid Cretaceous, to investigate the nature of mantle fluids at these depths and the constraints they provide on the deep volatile cycle.
Both diamond suites exhibit very light δ13C values (down to − 26‰) and heavy δ15N (up to + 10.3‰), with nitrogen abundances generally below 70 at. ppm but varying up to very high concentrations (2520 at. ppm) in rare cases. Combined, these signatures are consistent with derivation from subducted crustal materials. Both suites exhibit variable nitrogen aggregation states from 25 to 100% B defects. Internal growth structures, revealed in cathodoluminescence (CL) images, vary from faintly layered, through distinct cores to concentric growth patterns with intermittent evidence for dissolution and regular octahedral growth layers in places.
Modelling the internal co-variations in δ13C-δ15N-N revealed that diamonds grew from diverse C-H-O-N fluids involving both oxidised and reduced carbon species. The diversity of the modelled diamond-forming fluids highlights the complexity of the volatile sources and the likely heterogeneity of the deep asthenosphere and transition zone. We propose that the Monastery and Jagersfontein diamonds form in subducted slabs, where carbon is converted into either oxidised or reduced species during fluid-aided dissolution of subducted carbon before being re-precipitated as diamond. The common occurrence of recycled C and N isotopic signatures in super-deep diamonds world-wide indicates that a significant amount of carbon and nitrogen is recycled back to the deep asthenosphere and transition zone via subducting slabs, and that the transition zone may be dominated by recycled C and N
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