538 research outputs found
Investigations of potential mechanisms underlying spinal cord injury-induced polyuria.
Spinal cord injury (SCI) results in neurological impairments including motor, sensory, and autonomic dysfunction. These neurological deficits result in a litany of complications apart from muscular paralysis, including bladder, bowel, cardiovascular, and sexual function. SCI-induced polyuria (the overproduction/passage of urine) remains understudied, and therefore mechanisms behind it are largely unknown and require extensive investigation for potential targeted therapies to improve quality of life. The objective of this dissertation was to investigate potential mechanisms of SCI-induced polyuria and explore potential therapies to improve quality of life in the SCI population. Metabolic cages, Western blot, enzyme-linked immunoassay, and immunostaining were first used to determine the timing of fluctuations in biomarkers associated with SCI-induced polyuria, including arginine vasopressin (AVP), atrial natriuretic peptide (ANP), vasopressin 2 receptor (V2R), natriuretic peptide receptor A (NPRA), and epithelial sodium channel (ENaC). Next, to identify which neural substrates induce polyuria with a T9-level SCI, a higher level (T3) contusion above the local sympathetic supply to the kidneys were also examined. Lastly, the effect of anantin (NPRA antagonist) on SCI-induced polyuria was explored, in addition to utilizing an established treadmill activity-based recovery training (ABRT) program. There were significant alterations of multiple biomarkers after SCI, beginning at 7 days post injury (dpi), in addition to a lower number of AVP-labeled neurons in the hypothalamus. By 7 dpi, continuing through 6 weeks post-SCI, T3 contused rats showed a significant increase in 24-hour void volume as well as significant changes in ANP and AVP like the T9 injury. There was also a significant decrease in AVP-labelled cells in the suprachiasmatic nucleus post-T9 and T3 contusion relative to controls. A reduction in void volume was found for rats having ABRT but not anantin treatment. A significant decrease in mean arterial pressure was measured in all animal groups lasting chronically, and there was a significant increase in serum potassium at 14 dpi in addition to a significant decrease in serum sodium at the chronic time point. Together, these studies provide a detailed account of systemic responses to SCI that are associated with SCI-induced polyuria and fluid homeostasis
Modeling temporal fluctuations in avalanching systems
We demonstrate how to model the toppling activity in avalanching systems by
stochastic differential equations (SDEs). The theory is developed as a
generalization of the classical mean field approach to sandpile dynamics by
formulating it as a generalization of Itoh's SDE. This equation contains a
fractional Gaussian noise term representing the branching of an avalanche into
small active clusters, and a drift term reflecting the tendency for small
avalanches to grow and large avalanches to be constricted by the finite system
size. If one defines avalanching to take place when the toppling activity
exceeds a certain threshold the stochastic model allows us to compute the
avalanche exponents in the continum limit as functions of the Hurst exponent of
the noise. The results are found to agree well with numerical simulations in
the Bak-Tang-Wiesenfeld and Zhang sandpile models. The stochastic model also
provides a method for computing the probability density functions of the
fluctuations in the toppling activity itself. We show that the sandpiles do not
belong to the class of phenomena giving rise to universal non-Gaussian
probability density functions for the global activity. Moreover, we demonstrate
essential differences between the fluctuations of total kinetic energy in a
two-dimensional turbulence simulation and the toppling activity in sandpiles.Comment: 14 pages, 11 figure
Renormalization group theory for finite-size scaling in extreme statistics
We present a renormalization group (RG) approach to explain universal
features of extreme statistics, applied here to independent, identically
distributed variables. The outlines of the theory have been described in a
previous Letter, the main result being that finite-size shape corrections to
the limit distribution can be obtained from a linearization of the RG
transformation near a fixed point, leading to the computation of stable
perturbations as eigenfunctions. Here we show details of the RG theory which
exhibit remarkable similarities to the RG known in statistical physics. Besides
the fixed points explaining universality, and the least stable eigendirections
accounting for convergence rates and shape corrections, the similarities
include marginally stable perturbations which turn out to be generic for the
Fisher-Tippett-Gumbel class. Distribution functions containing unstable
perturbations are also considered. We find that, after a transitory divergence,
they return to the universal fixed line at the same or at a different point
depending on the type of perturbation.Comment: 15 pages, 8 figures, to appear in Phys. Rev.
Anisotropy studies with multiscale autocorrelation function
We present a novel method, based on a multiscale approach, for detecting
anisotropy signatures in the arrival direction distribution of the highest
energy cosmic rays. This method is catalog independent, i.e. it does not depend
on the choice of a particular catalog of candidate sources, and it provides a
good discrimination power even in presence of contaminating isotropic
background. We present applications to simulated data sets of events
corresponding to plausible scenarios for events detected, in the last decades,
by world-wide surface detector-based observatories for charged particles.Comment: 8 pages, 4 figures, proceed. of conferenc
Role of disorder in the size-scaling of material strength
We study the sample size dependence of the strength of disordered materials
with a flaw, by numerical simulations of lattice models for fracture. We find a
crossover between a regime controlled by the fluctuations due to disorder and
another controlled by stress-concentrations, ruled by continuum fracture
mechanics. The results are formulated in terms of a scaling law involving a
statistical fracture process zone. Its existence and scaling properties are
only revealed by sampling over many configurations of the disorder. The scaling
law is in good agreement with experimental results obtained from notched paper
samples.Comment: 4 pages 5 figure
Diffusion of Tagged Particle in an Exclusion Process
We study the diffusion of tagged hard core interacting particles under the
influence of an external force field. Using the Jepsen line we map this many
particle problem onto a single particle one. We obtain general equations for
the distribution and the mean square displacement of the tagged
center particle valid for rather general external force fields and initial
conditions. A wide range of physical behaviors emerge which are very different
than the classical single file sub-diffusion $ \sim t^{1/2}$ found
for uniformly distributed particles in an infinite space and in the absence of
force fields. For symmetric initial conditions and potential fields we find
$ = {{\cal R} (1 - {\cal R})\over 2 N {\it r} ^2} $ where $2 N$ is
the (large) number of particles in the system, ${\cal R}$ is a single particle
reflection coefficient obtained from the single particle Green function and
initial conditions, and $r$ its derivative. We show that this equation is
related to the mathematical theory of order statistics and it can be used to
find even when the motion between collision events is not Brownian
(e.g. it might be ballistic, or anomalous diffusion). As an example we derive
the Percus relation for non Gaussian diffusion
Level Density of a Bose Gas and Extreme Value Statistics
We establish a connection between the level density of a gas of
non-interacting bosons and the theory of extreme value statistics. Depending on
the exponent that characterizes the growth of the underlying single-particle
spectrum, we show that at a given excitation energy the limiting distribution
function for the number of excited particles follows the three universal
distribution laws of extreme value statistics, namely Gumbel, Weibull and
Fr\'echet. Implications of this result, as well as general properties of the
level density at different energies, are discussed.Comment: 4 pages, no figure
Roughness correction to the Casimir force at short separations: Contact distance and extreme value statistics
So far there has been no reliable method to calculate the Casimir force at
separations comparable to the root-mean-square of the height fluctuations of
the surfaces. Statistical analysis of rough gold samples has revealed the
presence of peaks considerably higher than the root-mean-square roughness.
These peaks redefine the minimum separation distance between the bodies and can
be described by extreme value statistics. Here we show that the contribution of
the high peaks to the Casimir force can be calculated with a pairwise additive
summation, while the contribution of asperities with normal height can be
evaluated perturbatively. This method provides a reliable estimate of the
Casimir force at short distances, and it solves the significant, so far
unexplained discrepancy between measurements of the Casimir force between rough
surfaces and the results of perturbation theory. Furthermore, we illustrate the
importance of our results in a technologically relevant situation.Comment: 29 pages, 11 figures, to appear in Phys. Rev.
Chemical fracture and distribution of extreme values
When a corrosive solution reaches the limits of a solid sample, a chemical
fracture occurs. An analytical theory for the probability of this chemical
fracture is proposed and confirmed by extensive numerical experiments on a two
dimensional model. This theory follows from the general probability theory of
extreme events given by Gumbel. The analytic law differs from the Weibull law
commonly used to describe mechanical failures for brittle materials. However a
three parameters fit with the Weibull law gives good results, confirming the
empirical value of this kind of analysis.Comment: 7 pages, 5 figures, to appear in Europhysics Letter
On the Role of Global Warming on the Statistics of Record-Breaking Temperatures
We theoretically study long-term trends in the statistics of record-breaking
daily temperatures and validate these predictions using Monte Carlo simulations
and data from the city of Philadelphia, for which 126 years of daily
temperature data is available. Using extreme statistics, we derive the number
and the magnitude of record temperature events, based on the observed Gaussian
daily temperatures distribution in Philadelphia, as a function of the number of
elapsed years from the start of the data. We further consider the case of
global warming, where the mean temperature systematically increases with time.
We argue that the current warming rate is insufficient to measurably influence
the frequency of record temperature events over the time range of the
observations, a conclusion that is supported by numerical simulations and the
Philadelphia temperature data.Comment: 11 pages, 6 figures, 2-column revtex4 format. For submission to
Journal of Climate. Revised version has some new results and some errors
corrected. Reformatted for Journal of Climate. Second revision has an added
reference. In the third revision one sentence that explains the simulations
is reworded for clarity. New revision 10/3/06 has considerable additions and
new results. Revision on 11/8/06 contains a number of minor corrections and
is the version that will appear in Phys. Rev.
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