220 research outputs found
Light responses and light adaptation in rat retinal rods at different temperatures
Rod responses to brief pulses of light were recorded as electroretinogram (ERG) mass potentials
across isolated, aspartate-superfused rat retinas at different temperatures and intensities of
steady background light. The objective was to clarify to what extent differences in sensitivity,
response kineticsandlight adaptationbetweenmammalianandamphibianrods can be explained
by temperature and outer-segment size without assuming functional differences in the phototransduction
molecules. Corresponding information for amphibian rods from the literature
was supplemented by new recordings from toad retina. All light intensities were expressed as
photoisomerizations per rod (Rhâ). In the rat retina, an estimated34%of incident photons at the
wavelength of peak sensitivity caused isomerizations in rods, as the (hexagonally packed) outer
segments measured 1.7 ÎŒmĂ22 ÎŒm and had specific absorbance of 0.016 ÎŒmâ1 on average.
Fractional sensitivity (S) in darkness increased with cooling in a similar manner in rat and toad
rods, but the rat function as a whole was displaced to a ca 0.7 log unit higher sensitivity level.
This difference can be fully explained by the smaller dimensions of rat rod outer segments,
since the same rate of phosphodiesterase (PDE) activation by activated rhodopsin will produce
a faster drop in cGMP concentration, hence a larger response in rat than in toad. In the range
15â25âŠC, the waveformand absolute time scale of dark-adapted dim-flash photoresponses at any
given temperature were similar in rat and toad, although the overall temperature dependence
of the time to peak (tp) was somewhat steeper in rat (Q10 â 4 versus 2â3). Light adaptation
was similar in rat and amphibian rods when measured at the same temperature. The mean
background intensity that depressed S by 1 log unit at 12âŠC was in the range 20â50 Rhâ sâ1 in
both, compared with ca 4500 Rhâ sâ1 in rat rods at 36âŠC. We conclude that it is not necessary
to assume major differences in the functional properties of the phototransduction molecules to
account for the differences in response properties of mammalian and amphibian rods
Radio emission of SN1993J. The complete picture: II. Simultaneous fit of expansion and radio light curves
We report on a simultaneous modelling of the expansion and radio light curves
of SN1993J. We have developed a simulation code capable of generating synthetic
expansion and radio light curves of supernovae by taking into consideration the
evolution of the expanding shock, magnetic fields, and relativistic electrons,
as well as the finite sensitivity of the interferometric arrays used in the
observations. Our software successfully fits all the available radio data of SN
1993J with an standard emission model for supernovae extended with some
physical considerations, as an evolution in the opacity of the ejecta material,
a radial drop of the magnetic fields inside the radiating region, and a
changing radial density profile of the circumstellar medium beyond day 3100
after explosion.Comment: 12 pages, 12 figures, accepted for publication in A&
Chromophore supply rate-limits mammalian photoreceptor dark adaptation
Efficient regeneration of visual pigment following its destruction by light is critical for the function of mammalian photoreceptors. Here, we show that misexpression of a subset of cone genes in the rd7 mouse hybrid rods enables them to access the normally cone-specific retina visual cycle. The rapid supply of chromophore by the retina visual cycle dramatically accelerated the mouse rod dark adaptation. At the same time, the competition between rods and cones for retina-derived chromophore slowed cone dark adaptation, indicating that the cone specificity of the retina visual cycle is key for rapid cone dark adaptation. Our findings demonstrate that mammalian photoreceptor dark adaptation is dominated by the supply of chromophore. Misexpression of cone genes in rods may represent a novel approach to treating visual disorders associated with mutations of visual cycle proteins or with reduced retinal pigment epithelium function due to aging
Coronal emission from the shocked circumstellar ring of SN 1987A
High resolution spectra with UVES/VLT of SN 1987A from December 2000 until
November 2005 show a number of high ionization lines from gas with velocities
of roughly 350 km/s, emerging from the shocked gas formed by the ejecta-ring
collision. These include coronal lines from [Fe X], [Fe XI] and [Fe XIV] which
have increased by a factor of about 20 during the observed period. The
evolution of the lines is similar to that of the soft X-rays, indicating that
they arise in the same component. The line ratios are consistent with those
expected from radiative shocks with velocity 310-390 km/s, corresponding to a
shock temperature of (1.6-2.5) x 10^6 K. A fraction of the coronal emission
may, however, originate in higher velocity adiabatic shocks.Comment: 11 pages, 10 figures, accepted for publication in A&
Modeling the X-ray emission of SN 1993J
We investigate the effects of radiative shocks on the observed X-ray emission
from the Type II supernova SN 1993J. To this end, the X-ray emission is modeled
as a result of the interaction between the supernova ejecta and a dense
circumstellar medium at an age of 8 years. The circumstances under which the
reverse shock is radiative are discussed and the observed X-ray emission is
analyzed using the numerical code described in Nymark et al. (2006). We argue
that the original analysis of the X-ray observations suffered from the lack of
self-consistent models for cooling shocks with high density and velocity,
leading to questionable conclusions about the temperatures and elemental
abundances. We reanalyze the spectra with our numerical model, and discuss the
expected spectra for different explosion models for the progenitors. We find
that the spectra of SN 1993J are compatible with a CNO-enriched composition and
that the X-ray flux is dominated by the reverse shock.Comment: 12 pages, 7 figures, 3 tables. Accepted for publication in A&
X-ray emission from radiative shocks in Type II supernovae
The X-ray emission from the circumstellar interaction in Type II supernovae
with a dense circumstellar medium is calculated. In Type IIL and Type IIn
supernovae mass loss rates are generally high enough for the region behind the
reverse shock to be radiative, producing strong radiation, particularly in
X-rays. We present a model for the emission from the cooling region in the case
of a radiative reverse shock. Under the assumption of a stationary flow, a
hydrodynamic model is combined with time dependent ionization balance and
multilevel calculations. The applicability of the steady state approximation is
discussed for various values of the ejecta density gradient and different sets
of chemical composition. We show how the emerging spectrum depends strongly on
the reverse shock velocity and the composition of the shocked gas. We discuss
differences between a spectrum produced by this model and a single-temperature
spectrum. Large differences for especially the line emission are found, which
seriously can affect abundance estimates. We also illustrate the effects of
absorption in the cool shocked ejecta. The applicability of our model for
various types of supernovae is discussed.Comment: 25 pages, 15 figures, 4 tables. Accepted for publication in A&
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