15,147 research outputs found
Tangle-bearing neurons survive despite disruption of membrane integrity in a mouse model of tauopathy
Neurofibrillary tangles (NFTs) are associated with neuronal loss and correlate with cognitive impairment in Alzheimer disease, but how NFTs relate to neuronal death is not clear. We studied cell death in Tg4510 mice that reversibly express P301L mutant human tau and accumulate NFTs using in vivo multiphoton imaging of neurofibrillary pathology, propidium iodide (PI) incorporation into cells, caspase activation and DNA labeling. We first observed that in live mice a minority of neurons was labeled with the caspase probe or with PI fluorescence. These markers of cell stress were localized in the same cells and appeared to be specifically within NFT-bearing neurons. Contrary to expectations, the PI-stained neurons did not die over a day of observation; the presence of Hoechst-positive nuclei in them on the subsequent day indicated that the NFT-associated membrane disruption suggested by PI staining and caspase activation do not lead to acute death of neurons in this tauopathy model. This unique combination of in vivo multiphoton imaging with markers of cell death and pathologic alteration is a powerful tool for investigating neuronal damage associated with neurofibrillary pathology
A Bit-String Model for Biological Aging
We present a simple model for biological aging. We studied it through
computer simulations and we have found this model to reflect some features of
real populations.Comment: LaTeX file, 4 PS figures include
Time evolution of the Partridge-Barton Model
The time evolution of the Partridge-Barton model in the presence of the
pleiotropic constraint and deleterious somatic mutations is exactly solved for
arbitrary fecundity in the context of a matricial formalism. Analytical
expressions for the time dependence of the mean survival probabilities are
derived. Using the fact that the asymptotic behavior for large time is
controlled by the largest matrix eigenvalue, we obtain the steady state values
for the mean survival probabilities and the Malthusian growth exponent. The
mean age of the population exhibits a power law decayment. Some Monte
Carlo simulations were also performed and they corroborated our theoretical
results.Comment: 10 pages, Latex, 1 postscript figure, published in Phys. Rev. E 61,
5664 (2000
New results for the two neutrino double beta decay in deformed nuclei with angular momentum projected basis
Four nuclei which are proved to be emitters (Ge,
Se, Nd, U), and four suspected, due to the corresponding
Q-values, to have this property (Nd, Sm, Gd,
Th), were treated within a proton-neutron quasiparticle random phase
approximation (pnQRPA) with a projected spherical single particle basis. The
advantage of the present procedure over the ones using a deformed Woods Saxon
or Nilsson single particle basis is that the actual pnQRPA states have a
definite angular momentum while all the others provide states having only K as
a good quantum number. The model Hamiltonian involves a mean field term
yielding the projected single particle states, a pairing interaction for alike
nucleons and a dipole-dipole proton-neutron interaction in both the
particle-hole (ph) and particle-particle (pp) channels. The effect of nuclear
deformation on the single beta strength distribution as well as on the double
beta Gamow-Teller transition amplitude (M) is analyzed. The
results are compared with the existent data and with the results from a
different approach, in terms of the process half life T. The case of
different deformations for mother and daughter nuclei is also presented.Comment: 45 pages, 13 figure
Ecosystem respiration: Drivers of daily variability and background respiration in lakes around the globe
We assembled data from a global network of automated lake observatories to test hypotheses regarding the drivers of ecosystem metabolism. We estimated daily rates of respiration and gross primary production (GPP) for up to a full year in each lake, via maximum likelihood fits of a free‐water metabolism model to continuous high‐frequency measurements of dissolved oxygen concentrations. Uncertainties were determined by a bootstrap analysis, allowing lake‐days with poorly constrained rate estimates to be down‐weighted in subsequent analyses. GPP and respiration varied considerably among lakes and at seasonal and daily timescales. Mean annual GPP and respiration ranged from 0.1 to 5.0 mg O2 L−1 d−1 and were positively related to total phosphorus but not dissolved organic carbon concentration. Within lakes, significant day‐to‐day differences in respiration were common despite large uncertainties in estimated rates on some lake‐days. Daily variation in GPP explained 5% to 85% of the daily variation in respiration after temperature correction. Respiration was tightly coupled to GPP at a daily scale in oligotrophic and dystrophic lakes, and more weakly coupled in mesotrophic and eutrophic lakes. Background respiration ranged from 0.017 to 2.1 mg O2 L−1 d−1 and was positively related to indicators of recalcitrant allochthonous and autochthonous organic matter loads, but was not clearly related to an indicator of the quality of allochthonous organic matter inputs
Exact Solution of an Evolutionary Model without Ageing
We introduce an age-structured asexual population model containing all the
relevant features of evolutionary ageing theories. Beneficial as well as
deleterious mutations, heredity and arbitrary fecundity are present and managed
by natural selection. An exact solution without ageing is found. We show that
fertility is associated with generalized forms of the Fibonacci sequence, while
mutations and natural selection are merged into an integral equation which is
solved by Fourier series. Average survival probabilities and Malthusian growth
exponents are calculated indicating that the system may exhibit mutational
meltdown. The relevance of the model in the context of fissile reproduction
groups as many protozoa and coelenterates is discussed.Comment: LaTeX file, 15 pages, 2 ps figures, to appear in Phys. Rev.
Mach's principle: Exact frame-dragging via gravitomagnetism in perturbed Friedmann-Robertson-Walker universes with
We show that the dragging of the axis directions of local inertial frames by
a weighted average of the energy currents in the universe is exact for all
linear perturbations of any Friedmann-Robertson-Walker (FRW) universe with K =
(+1, -1, 0) and of Einstein's static closed universe. This includes FRW
universes which are arbitrarily close to the Milne Universe, which is empty,
and to the de Sitter universe. Hence the postulate formulated by E. Mach about
the physical cause for the time-evolution of the axis directions of inertial
frames is shown to hold in cosmological General Relativity for linear
perturbations. The time-evolution of axis directions of local inertial frames
(relative to given local fiducial axes) is given experimentally by the
precession angular velocity of gyroscopes, which in turn is given by the
operational definition of the gravitomagnetic field. The gravitomagnetic field
is caused by cosmological energy currents via the momentum constraint. This
equation for cosmological gravitomagnetism is analogous to Ampere's law, but it
holds also for time-dependent situtations. In the solution for an open universe
the 1/r^2-force of Ampere is replaced by a Yukawa force which is of identical
form for FRW backgrounds with The scale of the exponential
cutoff is the H-dot radius, where H is the Hubble rate, and dot is the
derivative with respect to cosmic time. Analogous results hold for energy
currents in a closed FRW universe, K = +1, and in Einstein's closed static
universe.Comment: 23 pages, no figures. Final published version. Additional material in
Secs. I.A, I.J, III, V.H. Additional reference
Relevance of initial and final conditions for the Fluctuation Relation in Markov processes
Numerical observations on a Markov chain and on the continuous Markov process
performed by a granular tracer show that the ``usual'' fluctuation relation for
a given observable is not verified for finite (but arbitrarily large) times.
This suggests that some terms which are usually expected to be negligible, i.e.
``border terms'' dependent only on initial and final states, in fact cannot be
neglected. Furthermore, the Markov chain and the granular tracer behave in a
quite similar fashion.Comment: 23 pages, 5 figures, submitted to JSTA
Multi-Terminal Superconducting Phase Qubit
Mesoscopic multi-terminal Josephson junctions are novel devices that provide
weak coupling between several bulk superconductors through a common normal
layer. Because of the nonlocal coupling of the superconducting banks, a current
flow between two of the terminals can induce a phase difference and/or current
flow in the other terminals. This "phase dragging" effect is used in designing
a new type of superconducting phase qubit, the basic element of a quantum
computer. Time-reversal symmetry breaking can be achieved by inserting a
pi-phase shifter into the flux loop. Logical operations are done by applying
currents. This removes the necessity for local external magnetic fields to
achieve bistability or controllable operations.Comment: 7 pages, 3 figure
Performance Characterization of ESA's Tropospheric Delay Calibration System for Advanced Radio Science Experiments
Media propagation noises are amongst the main error sources of radiometric observables for deep space missions, with fluctuations of the tropospheric excess path length representing a relevant contributor to the Doppler noise budget. Microwave radiometers currently represent the most accurate instruments for the estimation of the tropospheric delay and delay rate along a slant direction. A prototype of a tropospheric delay calibration system (TDCS), using a 14 channel Ka/V band microwave radiometer, has been developed under a European Space Agency contract and installed at the deep space ground station in Malargüe, Argentina, in February 2019. After its commissioning, the TDCS has been involved in an extensive testbed campaign by recording a total of 44 tracking passes of the Gaia spacecraft, which were used to perform an orbit determination analysis. This work presents the first statistical characterization of the end-to-end performance of the TDCS prototype in an operational scenario. The results show that using TDCS-based calibrations instead of the standard GNSS-based calibrations leads to a significant reduction of the residual Doppler noise and instability
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