6,031 research outputs found
The State of the Circumstellar Medium Surrounding Gamma-Ray Burst Sources and its Effect on the Afterglow Appearance
We present a numerical investigation of the contribution of the presupernova
ejecta of Wolf-Rayet stars to the environment surrounding gamma-ray bursts
(GRBs), and describe how this external matter can affect the observable
afterglow characteristics. An implicit hydrodynamic calculation for massive
stellar evolution is used here to provide the inner boundary conditions for an
explicit hydrodynamical code to model the circumstellar gas dynamics. The
resulting properties of the circumstellar medium are then used to calculate the
deceleration of a relativistic, gas-dynamic jet and the corresponding afterglow
light curve produced as the shock wave propagates through the shocked-wind
medium. We find that variations in the stellar wind drive instabilities that
may produce radial filaments in the shocked-wind region. These comet-like tails
of clumps could give rise to strong temporal variations in the early afterglow
lightcurve. Afterglows may be expected to differ widely among themselves,
depending on the angular anisotropy of the jet and the properties of the
stellar progenitor; a wide diversity of behaviors may be the rule, rather than
the exception.Comment: 17 pages, 7 figures, ApJ in pres
Investigation of acceptor levels and hole scattering mechanisms in p-gallium selenide by means of transport measurements under pressure
The effect of pressure on acceptor levels and hole scattering mechanisms in
p-GaSe is investigated through Hall effect and resistivity measurements under
quasi-hydrostatic conditions up to 4 GPa. The pressure dependence of the hole
concentration is interpreted through a carrier statistics equation with a
single (nitrogen) or double (tin) acceptor whose ionization energies decrease
under pressure due to the dielectric constant increase. The pressure effect on
the hole mobility is also accounted for by considering the pressure
dependencies of both the phonon frequencies and the hole-phonon coupling
constants involved in the scattering rates.Comment: 13 pages, Latex, 4 ps figures. to appear in High Pressure Research 69
(1997
Relevance of death receptors in nervous system: role in the pathogenesis of neurodegenerative diseases and targets for therapy
L'apoptosi és un procés fisiològic que controla el nombre de
cèl·lules en organismes superiors. L'apoptosi està estrictament
regulada i s'ha vist que està implicada en la patogènesi d'algunes
malalties del sistema nerviós. En aquest sentit, un excés
de mort cel·lular contribueix a les malalties neurodegeneratives,
mentre que, el seu dèficit és una de les raons del desenvolupament
de tumors. El punt principal de regulació del procés
apoptòtic és l'activació de les caspases, cisteïna-proteases
que tenen especificitat pels residus aspàrtic. Les caspases es
poden activar per dos mecanismes principals: (1) alliberament
de citocrom C dels mitocondris alterats al citoplasma i (2) l'activació
dels receptors de la membrana anomenats receptors
de mort (DR, de l'anglès death receptor). Aquests receptors
s'han caracteritzat extensament en el sistema immunitari, mentre
que en el sistema nerviós les seves funcions són encara
desconegudes. El present article se centra en el paper dels DR
en la patogènesi de malalties neurodegeneratives i suggereix el
seu potencial des del punt de vista terapèutic. També es descriuen
diverses molècules intracel·lulars caracteritzades per la
seva habilitat en la modulació dels DR. Entre elles, presentem
dues noves proteïnes lifeguard i FAIM que s'expressen específicament
al sistema nerviós.Apoptosis is a strictly controlled, physiological process by
which the number of cells in metazoan organisms is regulated.
Recently, it has been shown that apoptosis is involved in the
pathogenesis of certain nervous system diseases. Excess cell
death is thought to contribute to neurodegenerative disorders
while defects in apoptosis lead to the development of neoplasias.
Regulation of apoptosis primarily occurs through the activation
of caspases, cysteine proteases that specifically cleave
aspartic acid residues. Caspases are activated by two mechanisms:
(1) release of cytochrome C from mitochondria to the
cytoplasm and (2) activation of plasma membrane death receptors
(DRs). These latter proteins have been widely characterized
in the immune system, whereas in the nervous system
their function remains elusive.
In this article we focus on the role of DRs in the pathogenesis
of neurodegenerative diseases and on the potential of these
proteins as therapeutic targets. We also discuss several intracellular
molecules that modulate DR activation. Among these,
we introduce two novel proteins, Lifeguard and FAIM, which
are specifically expressed in the nervous system
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