448 research outputs found
The Frog Vestibular System as a Model for Lesion-Induced Plasticity: Basic Neural Principles and Implications for Posture Control
Studies of behavioral consequences after unilateral labyrinthectomy have a long tradition in the quest of determining rules and limitations of the central nervous system (CNS) to exert plastic changes that assist the recuperation from the loss of sensory inputs. Frogs were among the first animal models to illustrate general principles of regenerative capacity and reorganizational neural flexibility after a vestibular lesion. The continuous successful use of the latter animals is in part based on the easy access and identifiability of nerve branches to inner ear organs for surgical intervention, the possibility to employ whole brain preparations for in vitro studies and the limited degree of freedom of postural reflexes for quantification of behavioral impairments and subsequent improvements. Major discoveries that increased the knowledge of post-lesional reactive mechanisms in the CNS include alterations in vestibular commissural signal processing and activation of cooperative changes in excitatory and inhibitory inputs to disfacilitated neurons. Moreover, the observed increase of synaptic efficacy in propriospinal circuits illustrates the importance of limb proprioceptive inputs for postural recovery. Accumulated evidence suggests that the lesion-induced neural plasticity is not a goal-directed process that aims toward a meaningful restoration of vestibular reflexes but rather attempts a survival of those neurons that have lost their excitatory inputs. Accordingly, the reaction mechanism causes an improvement of some components but also a deterioration of other aspects as seen by spatio-temporally inappropriate vestibulo-motor responses, similar to the consequences of plasticity processes in various sensory systems and species. The generality of the findings indicate that frogs continue to form a highly amenable vertebrate model system for exploring molecular and physiological events during cellular and network reorganization after a loss of vestibular function
Power laws in microrheology experiments on living cells: comparative analysis and modelling
We compare and synthesize the results of two microrheological experiments on
the cytoskeleton of single cells. In the first one, the creep function J(t) of
a cell stretched between two glass plates is measured after applying a constant
force step. In the second one, a micrometric bead specifically bound to
transmembrane receptors is driven by an oscillating optical trap, and the
viscoelastic coefficient is retrieved. Both and
exhibit power law behavior: and , with the same exponent
. This power law behavior is very robust ; is
distributed over a narrow range, and shows almost no dependance on the cell
type, on the nature of the protein complex which transmits the mechanical
stress, nor on the typical length scale of the experiment. On the contrary, the
prefactors and appear very sensitive to these parameters. Whereas
the exponents are normally distributed over the cell population, the
prefactors and follow a log-normal repartition. These results are
compared with other data published in the litterature. We propose a global
interpretation, based on a semi-phenomenological model, which involves a broad
distribution of relaxation times in the system. The model predicts the power
law behavior and the statistical repartition of the mechanical parameters, as
experimentally observed for the cells. Moreover, it leads to an estimate of the
largest response time in the cytoskeletal network: s.Comment: 47 pages, 14 figures // v2: PDF file is now Acrobat Reader 4 (and up)
compatible // v3: Minor typos corrected - The presentation of the model have
been substantially rewritten (p. 17-18), in order to give more details -
Enhanced description of protocols // v4: Minor corrections in the text : the
immersion angles are estimated and not measured // v5: Minor typos corrected.
Two references were clarifie
Dual function of thalamic low-vigilance state oscillations: Rhythm-regulation and plasticity
During inattentive wakefulness and non-rapid eye movement (NREM) sleep, the neocortex and thalamus cooperatively engage in rhythmic activities that are exquisitely reflected in the electroencephalogram as distinctive rhythms spanning a range of frequencies from <1 Hz slow waves to 13 Hz alpha waves. In the thalamus, these diverse activities emerge through the interaction of cell-intrinsic mechanisms and local and long-range synaptic inputs. One crucial feature, however, unifies thalamic oscillations of different frequencies: repetitive burst firing driven by voltage-dependent Ca(2+) spikes. Recent evidence reveals that thalamic Ca(2+) spikes are inextricably linked to global somatodendritic Ca(2+) transients and are essential for several forms of thalamic plasticity. Thus, we propose herein that alongside their rhythm-regulation function, thalamic oscillations of low-vigilance states have a plasticity function that, through modifications of synaptic strength and cellular excitability in local neuronal assemblies, can shape ongoing oscillations during inattention and NREM sleep and may potentially reconfigure thalamic networks for faithful information processing during attentive wakefulness
Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using high
resolution spectra obtained at ESO VLT and NTT telescopes. For the first time
the abundances are derived, where possible, from three optical multiplets:
Mult. 1, 6, and 8. By combining our own measurements with data in the
literature we assemble a sample of 253 stars in the metallicity range -3.2 <
[Fe/H] < +0.5. Two important features, which could hardly be detected in
smaller samples, are obvious from this large sample: 1) a sizeable scatter in
[S/Fe] ratios around [Fe/H] ~ -1 ; 2) at low metallicities we observe stars
with [S/Fe] ~ 0.4, as well as stars with higher [S/Fe] ratios. The latter do
not seem to be kinematically different from the former ones. Whether the latter
finding stems from a distinct population of metal-poor stars or simply from an
increased scatter in sulphur abundances remains an open question.Comment: accepted by A&
Sulphur and zinc abundances in Galactic stars and damped Lyman-alpha systems
High resolution spectra of 34 halo population dwarf and subgiant stars have
been obtained with VLT/UVES and used to derive sulphur abundances from the
8694.0, 8694.6 A and 9212.9, 9237.5 A SI lines. In addition, iron abundances
have been determined from 19 FeII lines and zinc abundances from the 4722.2,
4810.5 ZnI lines. The abundances are based on a classical 1D, LTE model
atmosphere analysis, but effects of 3D hydrodynamical modelling on the [S/Fe],
[Zn/Fe] and [S/Zn] ratios are shown to be small. We find that most halo stars
with metallicities in the range -3.2 < [Fe/H] < -0.8 have a near-constant
[S/Fe] = +0.3; a least square fit to [S/Fe] vs. {Fe/H] shows a slope of only
-0.04 +/- 0.01. Among halo stars with -1.2 < [Fe/H] < -0.8 the majority have
[S/Fe] ~ +0.3, but two stars (previously shown to have low [alpha/Fe] ratios)
have [S/Fe] ~ 0. For disk stars with [Fe/H] > -1, [S/Fe] decreases with
increasing [Fe/H]. Hence, sulphur behaves like other typical alpha-capture
elements, Mg, Si and Ca. Zinc, on the other hand, traces iron over three orders
of magnitude in [Fe/H], although there is some evidence for a small systematic
Zn overabundance ([Zn/Fe] ~ +0.1) among metal-poor disk stars and for halo
stars with [Fe/H] < -2.0. Recent measurements of S and Zn in ten damped
Ly-alpha systems (DLAs) with redshifts between 1.9 and 3.4 and zinc abundances
in the range -2.1 < [Zn/H] < -0.15 show an offset relative to the [S/Zn] -
[Zn/H] relation in Galactic stars. Possible reasons for this offset are
discussed, including low and intermittent star formation rates in DLAs.Comment: Accepted for publication in A&A. 16pages; 9 figure
V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity
This paper reports the detection of a heavy element enriched RV Tauri
variable with an abundance pattern that differs significantly from a standard
s-process enriched object: V453 Oph. Based on optical high-resolution spectra,
we determined that this object of low intrinsic metallicity ([Fe/H] = -2.2) has
a mild, but significant, enrichment ([s/Fe] ~ +0.5) of heavy elements for which
the distribution points to slow neutron capture nucleosynthesis. This result is
strengthened by a comparative analysis to the non-enriched RV Tauri star DS Aqr
([s/Fe] = 0.0). Although V453 Oph is the first RV Tauri star showing a strong
s-process signature, it is NOT accompanied by C enhancement, challenging our
current nucleosynthetic models of post-AGB stars that predict a simultaneous
enrichment in C and s-process elements. The low N abundance excludes CN cycling
as being responsible for the low C abundance. We explore three different
scenarios to explain the heavy element distribution in this evolved object: an
enrichment of the parental cloud, an accretion scenario in which the chemical
patterns were acquired by mass transfer in a binary system and an intrinsic
enrichment by dredge-up.Comment: Accepted for publication in A&
Intersection rules, dynamics and symmetries
We consider theories containing gravity, at most one dilaton and form field
strengths. We show that the existence of particular BPS solutions of
intersecting extremal closed branes select the theories, which upon dimensional
reduction to three dimensions possess a simple simply laced Lie group symmetry
G. Furthermore these theories can be fully reconstructed from the dynamics of
such branes and of their openings. Amongst such theories are the effective
actions of the bosonic sector of M-theory and of the bosonic string. The BPS
intersecting brane solutions form representations of a subgroup of the group of
Weyl reflections and outer automorphisms of the triple Kac-Moody extension G+++
of the G algebra, which cannot be embedded in the overextended Kac-Moody
subalgebra G++ characterising the cosmological Kasner solutions.Comment: Latex 30 pages, 3 figure
Analysis of 26 Barium Stars I. Abundances
We present a detailed analysis of 26 barium stars, including dwarf barium
stars, providing their atmospheric parameters (Teff, log g, [Fe/H], vt) and
elemental abundances. We aim at deriving gravities and luminosity classes of
the sample stars, in particular to confirm the existence of dwarf barium stars.
Accurate abundances of chemical elements were derived. Abundance ratios between
nucleosynthetic processes, by using Eu and Ba as representatives of the r- and
s-processes are presented. High-resolution spectra with the FEROS spectrograph
at the ESO-1.5m Telescope, and photometric data with Fotrap at the Zeiss
telescope at the LNA were obtained. The atmospheric parameters were derived in
an iterative way, with temperatures obtained from colour-temperature
calibrations. The abundances were derived using spectrum synthesis for Li, Na,
Al, alpha-, iron peak, s- and r-elements atomic lines, and C and N molecular
lines. Atmospheric parameters in the range 4300 < Teff < 6500, -1.2 < [Fe/H] <
0.0 and 1.4 < log g < 4.6 were derived, confirming that our sample contains
giants, subgiants and dwarfs. The abundance results obtained for Li, Al, Na,
alpha- and iron peak elements for the sample stars show that they are
compatible with the values found in the literature for normal disk stars in the
same range of metallicities. Enhancements of C, N and heavy elements relative
to Fe, that characterise barium stars, were derived and showed that [X/Ba] vs.
[Ba/H] and [X/Ba] vs. [Fe/H] present different behaviour as compared to [X/Eu]
vs. [Eu/H] and [X/Eu] vs. [Fe/H], reflecting the different nucleosynthetic
sites for the s- and r-processes.Comment: 32 pages including 18 figures, accepted to A&
Frequent questions and responses on the 2022 lipoprotein(a) consensus statement of the European Atherosclerosis Society
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