1,134 research outputs found
Cessation of Nightly Voluntary Wheel Running Activity Following Exposure to a Mouse Model of Posttraumatic Stress
Regular physical activity (PA) is well known to positively impact physical and mental health outcomes. In our work to examine cardiovascular benefits of PA in a mouse model of posttraumatic stress, we stumbled upon the reciprocal relationship between PA and stress exposure, wherein stress significantly reduced healthy levels of routine PA. The aim of the present studies was to define the parameters of our paradigm. C67BL/6J male mice were divided into four groups (n=8/group): sedentary/control, voluntary running/control, sedentary/stress, and voluntary running/stress. Voluntary running groups were given unlimited access to a running wheel for 9 weeks. Stress groups were then exposed to a 5-day resident-intruder social stress that models human posttraumatic stress. Running behavior essentially ceased following stress. Habituation to stress occurred, as running distance increased by the 5th day of stress but remained significantly low. A separate study examined a single exposure to resident-intruder social stress. Plasma corticosterone significantly increased while nightly running dropped significantly but returned to normal by the 3rd night post-stress. These studies show the sensitivity of habitual running behavior to stress exposure and suggest the utility of this mouse model in exploring the means by which stress negatively impacts routine PA
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Translation of Anticancer Efficacy From Nonclinical Models to the Clinic
Mouse cancer models have provided critical insights into tumor biology; however, clinical translation of these findings has been challenging. This perspective posits that factors impacting on successful translation start with limitations in capturing human cancer pathophysiology and end with challenges in generating robust translatable preclinical end points. A comprehensive approach that considers clinically relevant mouse models with both an integrated biomarker strategy and a complementary modeling and simulation effort will strengthen the current oncology drug development paradigm
Dislocation core field. I. Modeling in anisotropic linear elasticity theory
Aside from the Volterra field, dislocations create a core field, which can be
modeled in linear anisotropic elasticity theory with force and dislocation
dipoles. We derive an expression of the elastic energy of a dislocation taking
full account of its core field and show that no cross term exists between the
Volterra and the core fields. We also obtain the contribution of the core field
to the dislocation interaction energy with an external stress, thus showing
that dislocation can interact with a pressure. The additional force that
derives from this core field contribution is proportional to the gradient of
the applied stress. Such a supplementary force on dislocations may be important
in high stress gradient regions, such as close to a crack tip or in a
dislocation pile-up
Ground Vibrational State SiO Emission in the VLA BAaDE Survey
Using a subsample of the Bulge Asymmetries and Dynamical Evolution (BAaDE)
survey of stellar SiO masers, we explore the prevalence and characteristics of
SiO emission. We identify 90 detections of maser, thermal,
or composite SiO emission out of approximately 13,000
candidate spectra from the NSF's Karl G. Jansky Very Large Array (VLA). We find
that the detected sources are likely asymptotic giant branch (AGB) stars
belonging to a bright, foreground Milky Way stellar disk population. For the 32
sources showing thermal components, we extract values for outflow velocity by
fitting thermal line profiles. We find a range of circumstellar envelope
expansion velocities, and compare to previously recorded OH and CO expansion
velocities. This preliminary survey is already the largest study of stellar
ground-vibrational-state SiO masers to date, and will be expanded to include
the entire VLA BAaDE dataset when data reduction for the 18,988 target sources
is completed.Comment: 23 pages, 8 figures, to be published in The Astronomical Journa
Dynamic ductile to brittle transition in a one-dimensional model of viscoplasticity
We study two closely related, nonlinear models of a viscoplastic solid. These
models capture essential features of plasticity over a wide range of strain
rates and applied stresses. They exhibit inelastic strain relaxation and steady
flow above a well defined yield stress. In this paper, we describe a first step
in exploring the implications of these models for theories of fracture and
related phenomena. We consider a one dimensional problem of decohesion from a
substrate of a membrane that obeys the viscoplastic constitutive equations that
we have constructed. We find that, quite generally, when the yield stress
becomes smaller than some threshold value, the energy required for steady
decohesion becomes a non-monotonic function of the decohesion speed. As a
consequence, steady state decohesion at certain speeds becomes unstable. We
believe that these results are relevant to understanding the ductile to brittle
transition as well as fracture stability.Comment: 10 pages, REVTeX, 12 postscript figure
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. The flares were sometimes intense, had rapid rise and
decay phases, and occurred late relative to the ``prompt'' phase. Some
remarkable flares are observed with fluence comparable to the prompt GRB
fluence. Many GRBs have several flares, which are sometimes overlapping. Short,
intense, repetitive, and late flaring can be most easily understood within the
context of the standard fireball model with the internal engine that powers the
prompt GRB emission in an active state at late times. However, other models for
flares have been proposed. Flare origin can be investigated by comparing the
flare spectra to that of the afterglow and the initial prompt emission. In this
work, we have analyzed all significant X-ray flares from the first 110 GRBs
observed by Swift. From this sample 33 GRBs were found to have significant
X-ray flares, with 77 flares that were detected above the 3 level. In
addition to temporal analysis presented in a companion paper, a variety of
spectral models have been fit to each flare. In some cases, we find that the
spectral fits favor a Band function model, which is more akin to the prompt
emission than to that of an afterglow. We find that the average fluence of the
flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is
approximately a factor of ten below the average prompt GRB fluence. These
results, when combined with those presented in the companion paper on temporal
properties of flares, supports the hypothesis that most X-ray flares are
late-time activity of the internal engine that spawned the initial GRB; not an
afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table
Heterogeneous chlorine activation on stratospheric aerosols and clouds in the Arctic polar vortex
Chlorine activation in the Arctic is investigated by examining different parameterizations for uptake coefficients on stratospheric aerosols, high-resolution in-situ measurements and vortex-wide satellite observations. The parameterizations for heterogeneous chemistry on liquid aerosols are most sensitive to temperature with the reaction rates doubling for every 1 K increase in temperature. However, differences between the currently available parameterizations are negligible. For Nitric Acid Trihydrate particles (NAT) the major factors of uncertainty are the number density of nucleated particles and different parameterizations for heterogeneous chemistry. These two factors induce an uncertainty that covers several orders of magnitude on the reaction rate. Nonetheless, since predicted reaction rates on liquid aerosols always exceed those on NAT, the overall uncertainty for chlorine activation is small. In-situ observations of ClO<sub>x</sub> from Arctic winters in 2005 and 2010 are used to evaluate the heterogeneous chemistry parameterizations. The conditions for these measurements proved to be very different between those two winters with HCl being the limiting reacting partner for the 2005 measurements and ClONO<sub>2</sub> for the 2010 measurements. Modeled levels of chlorine activation are in very good agreement with the in-situ observations and the surface area provided by Polar Stratospheric Clouds (PSCs) has only a limited impact on modeled chlorine activation. This indicates that the parameterizations give a good representation of the processes in the atmosphere. Back-trajectories started on the location of the observations in 2005 indicate temperatures on the threshold for PSC formation, hence the surface area is mainly provided by the background aerosol. Still, the model shows additional chlorine activation during this time-frame, providing cautionary evidence for chlorine activation even in the absence of PSCs. Vortex-averaged satellite observations by the MLS instrument also show no definite connection between chlorine activation and PSC formation. The inter -and intra-annual variability of vortex-average HCl and HNO<sub>3</sub> based on MLS observations is examined for the Arctic winters 2004/2005 to 2010/2011. These observations show that removal of HCl and HNO<sub>3</sub> from the gas-phase are not correlated. HNO<sub>3</sub> loss exhibits great inter-annual variability depending on prevailing temperatures while HCl loss is continuous through December without considerable inter- or intra-annual variability. Only the recovery of HCl in late winter depends on the level of denitrification. Hence, the occurrence of HNO<sub>3</sub> containing PSC particles does not seem to have a significant effect on the speed of initial chlorine activation on a vortex-wide scale
Left Ventricle Biomechanics of Low-Flow, Low-Gradient Aortic Stenosis: A Patient-Specific Computational Model
This study aimed to create an imaging-derived patient-specific computational model of low-flow, low-gradient (LFLG) aortic stenosis (AS) to obtain biomechanics data about the left ventricle. LFLG AS is now a commonly recognized sub-type of aortic stenosis. There remains much controversy over its management, and investigation into ventricular biomechanics may elucidate pathophysiology and better identify patients for valve replacement. ECG-gated cardiac computed tomography images from a patient with LFLG AS were obtained to provide patient-specific geometry for the computational model. Surfaces of the left atrium, left ventricle (LV), and outflow track were segmented. A previously validated multi-scale, multi-physics computational human heart model was adapted to the patient-specific geometry, yielding a model consisting of 91,000 solid elements. This model was coupled to a virtual circulatory system and calibrated to clinically measured parameters from echocardiography and cardiac catheterization data. The simulation replicated key physiologic parameters within 10% of their clinically measured values. Global LV systolic myocardial stress was 7.1 ± 1.8 kPa. Mean stress of the basal, middle, and apical segments were 7.7 ± 1.8 kPa, 9.1 ± 3.8 kPa, and 6.4 ± 0.4 kPa, respectively. This is the first patient-specific computational model of LFLG AS based on clinical imaging. Low myocardial stress correlated with low ejection fraction and eccentric LV remodeling. Further studies are needed to understand how alterations in LV biomechanics correlates with clinical outcomes of AS
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