15,717 research outputs found
Ultrastructural Characteristics of Red Maple (Acer Rubrum L.) Wood
The anatomy of red maple (Acer rubrum L.) was examined using the transmission electron microscope. Direct carbon replicas and ultrathin sections of inner and outer sapwood and inner and outer heartwood were prepared. In cross-sectional view sapwood intervessel pit membranes appear thin; in surface views of air-dried and extracted samples of the second sapwood ring and inner sapwood, openings in the intervessel pit membranes are visible. Intervessel pit membranes are permeated with extractives in the heartwood. Vessel-ray parenchyma pits have been described as similar to intervessel pits; but differences in shape, apertures, and pit membranes were detected in this study. The ray parenchyma cells appear different in ultrastructural details from those in species that have been studied with the transmission electron microscope as they do not have well-defined protective layers in the sapwood when they are adjacent to vessels, plasmodesmata channels are not apparent in the parenchyma-parenchyma pits, and there are no pits to the intercellular spaces in the rays. Red maple is an unusual hardwood as it has longitudinal intercellular spaces adjacent to the fibers; the appearance of these spaces is similar to that of the longitudinal intercellular spaces in members of the conifer family, Araucariaceae
Phase field modeling of electrochemistry II: Kinetics
The kinetic behavior of a phase field model of electrochemistry is explored
for advancing (electrodeposition) and receding (electrodissolution) conditions
in one dimension. We described the equilibrium behavior of this model in [J. E.
Guyer, W. J. Boettinger, J.A. Warren, and G. B. McFadden, ``Phase field
modeling of electrochemistry I: Equilibrium'', cond-mat/0308173]. We examine
the relationship between the parameters of the phase field method and the more
typical parameters of electrochemistry. We demonstrate ohmic conduction in the
electrode and ionic conduction in the electrolyte. We find that, despite making
simple, linear dynamic postulates, we obtain the nonlinear relationship between
current and overpotential predicted by the classical ``Butler-Volmer'' equation
and observed in electrochemical experiments. The charge distribution in the
interfacial double layer changes with the passage of current and, at
sufficiently high currents, we find that the diffusion limited deposition of a
more noble cation leads to alloy deposition with less noble species.Comment: v3: To be published in Phys. Rev. E v2: Attempt to work around
turnpage bug. Replaced color Fig. 4a with grayscale 13 pages, 7 figures in 10
files, REVTeX 4, SIunits.sty, follows cond-mat/030817
Hidden Dirac Monopoles
Dirac showed that the existence of one magnetic pole in the universe could
offer an explanation of the discrete nature of the electric charge. Magnetic
poles appear naturally in most grand unified theories. Their discovery would be
of greatest importance for particle physics and cosmology. The intense
experimental search carried thus far has not met with success. I proposed a
universe with magnetic poles which are not observed free because they hide in
deeply bound monopole--anti-monopole states named monopolium. I discuss the
realization of this proposal and its consistency with known cosmological
features. I furthermore analyze its implications and the experimental
signatures that confirm the scenario.Comment: Comments: 15 pages, 3 figure
Uncertainty Relations for Positive Operator Valued Measures
How much unavoidable randomness is generated by a Positive Operator Valued
Measure (POVM)? We address this question using two complementary approaches.
First we study the variance of a real variable associated to the POVM outcomes.
In this context we introduce an uncertainty operator which measures how much
additional noise is introduced by carrying out a POVM rather than a von Neumann
measurement. We illustrate this first approach by studying the variances of
joint estimates of \sigma_x and \sigma_z for spin 1/2 particles. We show that
for unbiased measurements the sum of these variances is lower bounded by 1. In
our second approach we study the entropy of the POVM outcomes. In particular we
try to establish lower bounds on the entropy of the POVM outcomes. We
illustrate this second approach by examples.Comment: 5 pages, minor modifications and clarification
New two-sided bound on the isotropic Lorentz-violating parameter of modified Maxwell theory
There is a unique Lorentz-violating modification of the Maxwell theory of
photons, which maintains gauge invariance, CPT, and renormalizability.
Restricting the modified-Maxwell theory to the isotropic sector and adding a
standard spin-one-half Dirac particle p^\pm with minimal coupling to the
nonstandard photon \widetilde{\gamma}, the resulting
modified-quantum-electrodynamics model involves a single dimensionless
"deformation parameter," \widetilde{\kappa}_{tr}. The exact tree-level decay
rates for two processes have been calculated: vacuum Cherenkov radiation p^\pm
\to p^\pm \widetilde{\gamma} for the case of positive \widetilde{\kappa}_{tr}
and photon decay \widetilde{\gamma} \to p^+ p^- for the case of negative
\widetilde{\kappa}_{tr}. From the inferred absence of these decays for a
particular high-quality ultrahigh-energy-cosmic-ray event detected at the
Pierre Auger Observatory and an excess of TeV gamma-ray events observed by the
High Energy Stereoscopic System telescopes, a two-sided bound on
\widetilde{\kappa}_{tr} is obtained, which improves by eight orders of
magnitude upon the best direct laboratory bound. The implications of this
result are briefly discussed.Comment: 18 pages, v5: published version in preprint styl
Corticosterone Regulates Both Naturally Occurring and Cocaine‐Induced Dopamine Signaling by Selectively Decreasing Dopamine Uptake
Stressful and aversive events promote maladaptive reward‐seeking behaviors such as drug addiction by acting, in part, on the mesolimbic dopamine system. Using animal models, data from our laboratory and others show that stress and cocaine can interact to produce a synergistic effect on reward circuitry. This effect is also observed when the stress hormone corticosterone is administered directly into the nucleus accumbens (NAc), indicating that glucocorticoids act locally in dopamine terminal regions to enhance cocaine\u27s effects on dopamine signaling. However, prior studies in behaving animals have not provided mechanistic insight. Using fast‐scan cyclic voltammetry, we examined the effect of systemic corticosterone on spontaneous dopamine release events (transients) in the NAc core and shell in behaving rats. A physiologically relevant systemic injection of corticosterone (2 mg/kg i.p.) induced an increase in dopamine transient amplitude and duration (both voltammetric measures sensitive to decreases in dopamine clearance), but had no effect on the frequency of transient release events. This effect was compounded by cocaine (2.5 mg/kg i.p.). However, a second experiment indicated that the same injection of corticosterone had no detectable effect on the dopaminergic encoding of a palatable natural reward (saccharin). Taken together, these results suggest that corticosterone interferes with naturally occurring dopamine uptake locally, and this effect is a critical determinant of dopamine concentration specifically in situations in which the dopamine transporter is pharmacologically blocked by cocaine
Apparent horizons in simplicial Brill wave initial data
We construct initial data for a particular class of Brill wave metrics using
Regge calculus, and compare the results to a corresponding continuum solution,
finding excellent agreement. We then search for trapped surfaces in both sets
of initial data, and provide an independent verification of the existence of an
apparent horizon once a critical gravitational wave amplitude is passed. Our
estimate of this critical value, using both the Regge and continuum solutions,
supports other recent findings.Comment: 7 pages, 6 EPS figures, LaTeX 2e. Submitted to Class. Quant. Gra
Factorized domain wall partition functions in trigonometric vertex models
We obtain factorized domain wall partition functions for two sets of
trigonometric vertex models: 1. The N-state Deguchi-Akutsu models, for N = {2,
3, 4} (and conjecture the result for all N >= 5), and 2. The sl(r+1|s+1)
Perk-Schultz models, for {r, s = \N}, where (given the symmetries of these
models) the result is independent of {r, s}.Comment: 12 page
Corticosterone Acts in the Nucleus Accumbens to Enhance Dopamine Signaling and Potentiate Reinstatement of Cocaine Seeking
Stressful life events are important contributors to relapse in recovering cocaine addicts, but the mechanisms by which they influence motivational systems are poorly understood. Studies suggest that stress may “set the stage” for relapse by increasing the sensitivity of brain reward circuits to drug-associated stimuli. We examined the effects of stress and corticosterone on behavioral and neurochemical responses of rats to a cocaine prime after cocaine self-administration and extinction. Exposure of rats to acute electric footshock stress did not by itself reinstate drug-seeking behavior but potentiated reinstatement in response to a subthreshold dose of cocaine. This effect of stress was not observed in adrenalectomized animals, and was reproduced in nonstressed animals by administration of corticosterone at a dose that reproduced stress-induced plasma levels. Pretreatment with the glucocorticoid receptor antagonist RU38486 did not block the corticosterone effect. Corticosterone potentiated cocaine-induced increases in extracellular dopamine in the nucleus accumbens (NAc), and pharmacological blockade of NAc dopamine receptors blocked corticosterone-induced potentiation of reinstatement. Intra-accumbens administration of corticosterone reproduced the behavioral effects of stress and systemic corticosterone. Corticosterone treatment acutely decreased NAc dopamine clearance measured by fast-scan cyclic voltammetry, suggesting that inhibition of uptake2-mediated dopamine clearance may underlie corticosterone effects. Consistent with this hypothesis, intra-accumbens administration of the uptake2 inhibitor normetanephrine potentiated cocaine-induced reinstatement. Expression of organic cation transporter 3, a corticosterone-sensitive uptake2 transporter, was detected on NAc neurons. These findings reveal a novel mechanism by which stress hormones can rapidly regulate dopamine signaling and contribute to the impact of stress on drug intake
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