474 research outputs found
Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease
Because regional blood flow increases in association with the increased metabolic demand generated by localised increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer’s disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders
Cytochrome P450 oxidoreductase participates in nitric oxide consumption by rat brain
In low nanomolar concentrations, NO (nitric oxide) functions as a transmitter in brain and other tissues, whereas near-micromolar NO concentrations are associated with toxicity and cell death. Control of the NO concentration, therefore, is critical for proper brain function, but, although its synthesis pathway is well-characterized, the major route of breakdown of NO in brain is unclear. Previous observations indicate that brain cells actively consume NO at a high rate. The mechanism of this consumption was pursued in the present study. NO consumption by a preparation of central glial cells was abolished by cell lysis and recovered by addition of NADPH. NADPH-dependent consumption of NO localized to cell membranes and was inhibited by proteinase K, indicating the involvement of a membrane-bound protein. Purification of this activity yielded CYPOR (cytochrome P450 oxidoreductase). Antibodies against CYPOR inhibited NO consumption by brain membranes and the amount of CYPOR in several cell types correlated with their rate of NO consumption. NO was also consumed by purified CYPOR but this activity was found to depend on the presence of the vitamin E analogue Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), included in the buffer as a precaution against inadvertent NO consumption by lipid peroxidation. In contrast, NO consumption by brain membranes was independent of Trolox. Hence, it appears that, during the purification process, CYPOR becomes separated from a partner needed for NO consumption. Cytochrome P450 inhibitors inhibited NO consumption by brain membranes, making these proteins likely candidates
Effective Neutrino Mass Operators: A Guide to Model Building
Effective operators relevant for generating small Majorana masses for the
neutrinos in the Standard Model will be considered. These operators serve as a
useful guide for building models of neutrino mass. Some of these operators are
represented by familiar models in the literature, and others lead to
interesting new models. The number of relevant operators will be drastically
reduced if neutrinoless double beta decays are observed in current experiments.Comment: LaTeX, 4 pages; contribution to the NuFact'01 Workshop Proceeding
Classification of Effective Neutrino Mass Operators
We present a classification of SU(3) x SU(2) x U(1) gauge invariant \Delta L
= 2 (L being lepton number) effective operators relevant for generating small
Majorana neutrino masses. Operators of dimension up to 11 have been included in
our analysis. This approach enables us to systematically identify interesting
neutrino mass models. It is shown that many of the well-known models fall into
this classification. In addition, a number of new models are proposed and their
neutrino phenomenology is outlined. Of particular interest is a large class of
models in which neutrinoless double beta decays arise at a lower order compared
to the neutrino mass, making these decays accessible to the current round of
experiments.Comment: 34 pages in RevTeX with 18 figure
Schroedingers equation with gauge coupling derived from a continuity equation
We consider a statistical ensemble of particles of mass m, which can be
described by a probability density \rho and a probability current \vec{j} of
the form \rho \nabla S/m. The continuity equation for \rho and \vec{j} implies
a first differential equation for the basic variables \rho and S. We further
assume that this system may be described by a linear differential equation for
a complex state variable \chi. Using this assumptions and the simplest possible
Ansatz \chi(\rho,S) Schroedingers equation for a particle of mass m in an
external potential V(q,t) is deduced. All calculations are performed for a
single spatial dimension (variable q) Using a second Ansatz \chi(\rho,S,q,t)
which allows for an explict q,t-dependence of \chi, one obtains a generalized
Schroedinger equation with an unusual external influence described by a
time-dependent Planck constant. All other modifications of Schroeodingers
equation obtained within this Ansatz may be eliminated by means of a gauge
transformation. Thus, this second Ansatz may be considered as a generalized
gauging procedure. Finally, making a third Ansatz, which allows for an
non-unique external q,t-dependence of \chi, one obtains Schroedingers equation
with electromagnetic potentials \vec{A}, \phi in the familiar gauge coupling
form. A possible source of the non-uniqueness is pointed out.Comment: 25 pages, no figure
Quasars and their host galaxies
This review attempts to describe developments in the fields of quasar and
quasar host galaxies in the past five. In this time period, the Sloan and 2dF
quasar surveys have added several tens of thousands of quasars, with Sloan
quasars being found to z>6. Obscured, or partially obscured quasars have begun
to be found in significant numbers. Black hole mass estimates for quasars, and
our confidence in them, have improved significantly, allowing a start on
relating quasar properties such as radio jet power to fundamental parameters of
the quasar such as black hole mass and accretion rate. Quasar host galaxy
studies have allowed us to find and characterize the host galaxies of quasars
to z>2. Despite these developments, many questions remain unresolved, in
particular the origin of the close relationship between black hole mass and
galaxy bulge mass/velocity dispersion seen in local galaxies.Comment: Review article, to appear in Astrophysics Update
Supersymmetric Seesaw without Singlet Neutrinos: Neutrino Masses and Lepton-Flavour Violation
We consider the supersymmetric seesaw mechanism induced by the exchange of
heavy SU(2)_W triplet states, rather than `right-handed' neutrino singlets, to
generate neutrino masses. We show that in this scenario the neutrino flavour
structure tested at low-energy in the atmospheric and solar neutrino
experiments is directly inherited from the neutrino Yukawa couplings to the
triplets. This allows us to predict the ratio of the tau --> mu gamma (or tau
--> e gamma) and mu --> e gamma decay rates in terms of the low-energy neutrino
parameters. Moreover, once the model is embedded in a grand unified model,
quark-flavour violation can be linked to lepton-flavour violation.Comment: 26 LaTeX pages, 10 postscript figures, uses epsfig and axodraw.
Comments and references adde
Study and suppression of the microstructural anisotropy generated during the consolidation of a carbonyl iron powder by field-assisted hot pressing
Published OnlineA spherical carbonyl iron powder was consolidated by the field-assisted hot pressing technique using graphite tools at two different temperatures, both above the austenitizing temperature. The microstructures obtained exhibited a compositional gradient in carbon along the consolidated material. Thus, the outer rim of the cylindrical samples was composed of cementite and pearlite that gradually turned to pearlite, leading to a fully ferritic microstructure at the core of the sample. The increase in the temperature has led to a higher introduction of carbon within the sample. The interposition of a thin tungsten foil between the graphite die/punches and the powders has significantly reduced the diffusion of the carbon through the iron matrix and has suppressed the microstructural anisotropy.Publicad
Search for lepton-flavor violation at HERA
A search for lepton-flavor-violating interactions and has been performed with the ZEUS detector using the entire HERA I
data sample, corresponding to an integrated luminosity of 130 pb^{-1}. The data
were taken at center-of-mass energies, , of 300 and 318 GeV. No
evidence of lepton-flavor violation was found, and constraints were derived on
leptoquarks (LQs) that could mediate such interactions. For LQ masses below
, limits were set on , where
is the coupling of the LQ to an electron and a
first-generation quark , and is the branching ratio of
the LQ to the final-state lepton ( or ) and a quark . For
LQ masses much larger than , limits were set on the four-fermion
interaction term for LQs that couple to an electron and a quark
and to a lepton and a quark , where and are
quark generation indices. Some of the limits are also applicable to
lepton-flavor-violating processes mediated by squarks in -Parity-violating
supersymmetric models. In some cases, especially when a higher-generation quark
is involved and for the process , the ZEUS limits are the most
stringent to date.Comment: 37 pages, 10 figures, Accepted by EPJC. References and 1 figure (Fig.
6) adde
Multijet production in neutral current deep inelastic scattering at HERA and determination of alpha_s
Multijet production rates in neutral current deep inelastic scattering have
been measured in the range of exchanged boson virtualities 10 < Q2 < 5000 GeV2.
The data were taken at the ep collider HERA with centre-of-mass energy sqrt(s)
= 318 GeV using the ZEUS detector and correspond to an integrated luminosity of
82.2 pb-1. Jets were identified in the Breit frame using the k_T cluster
algorithm in the longitudinally invariant inclusive mode. Measurements of
differential dijet and trijet cross sections are presented as functions of jet
transverse energy E_{T,B}{jet}, pseudorapidity eta_{LAB}{jet} and Q2 with
E_{T,B}{jet} > 5 GeV and -1 < eta_{LAB}{jet} < 2.5. Next-to-leading-order QCD
calculations describe the data well. The value of the strong coupling constant
alpha_s(M_Z), determined from the ratio of the trijet to dijet cross sections,
is alpha_s(M_Z) = 0.1179 pm 0.0013(stat.) {+0.0028}_{-0.0046}(exp.)
{+0.0064}_{-0.0046}(th.)Comment: 22 pages, 5 figure
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