920 research outputs found
Investigating demyelination, efficient remyelination and remyelination failure in organotypic cerebellar slice cultures:Workflow and practical tips
Healthy myelin is essential for proper brain function. When the myelin sheath is damaged, fast saltatory impulse conduction is lost and neuronal axons become vulnerable to degeneration. Thus, regeneration of the myelin sheath by encouraging oligodendrocyte lineage cells to remyelinate the denuded axons is a promising therapeutic target for demyelinating diseases such as multiple sclerosis. Ex vivo organotypic cerebellar slice cultures are a useful model to study developmental myelination, demyelination, remyelination and remyelination failure. In these cultures, the cerebellum's three-dimensional architecture and various cell populations remain largely intact, providing a realistic and relatively cost-efficient model that can be easily manipulated by the addition of viral vectors, pharmaceuticals or (transgenic) cells to augment or replace resident cell populations. Moreover, slice cultures can be treated with lysolecithin or polyinosinic:polycytidylic acid to induce demyelination and mimic efficient as well as inefficient remyelination. It can be challenging to set up slice cultures for the first time, as in our experience, seemingly minor differences in technique and materials can make a great difference to the quality of the cultures. Therefore, this report provides an in-depth description for the generation and maintenance of ex vivo organotypic cerebellar cultures for demyelination-remyelination studies with a focus on practical tips for scientists that are new to this technique
Time-dependent effects of psychosocial stress on the contextualization of neutral memories
Memories about stressful experiences need to be both specific and generalizable to adequately guide future behavior. Memory strength is influenced by emotional significance, and contextualization (i.e., encoding experiences with their contextual details) enables selective context-dependent retrieval and protects against overgeneralization. The current randomized-controlled study investigated how the early and late phase of the endogenous stress response affects the contextualization of neutral and negative information. One hundred healthy male participants were randomly divided into three experimental groups that performed encoding either 1) without stress (control), 2) immediately after acute stress (early) or 3) two hours after acute stress (late). Stress was induced via the Trier Social Stress Test and salivary alpha-amylase and cortisol levels were measured throughout the experiment. In the Memory Contextualization Task, neutral and angry faces (items) were depicted against unique context pictures during encoding. During testing 24 h later, context-dependent recognition memory of the items was assessed by presenting these in either congruent or incongruent contexts (relative to encoding). Multilevel analyses revealed that neutral information was more contextualized when encoding took place two hours after psychosocial stress, than immediately after the stressor. Results suggest that the late effects in the unique, time-dependent sequence of a healthy endogenous stress response, could complement reduced contextualization immediately after stress. The contextualization of negative information was not influenced by psychosocial stress, as opposed to earlier reported effects of exogenous hydrocortisone administration. An imbalance between the early and late effects of the endogenous stress response could increase vulnerability for stress-related psychopathology.</p
Electrostatic potential in a superconductor
The electrostatic potential in a superconductor is studied. To this end
Bardeen's extension of the Ginzburg-Landau theory to low temperatures is used
to derive three Ginzburg-Landau equations - the Maxwell equation for the vector
potential, the Schroedinger equation for the wave function and the Poisson
equation for the electrostatic potential. The electrostatic and the
thermodynamic potential compensate each other to a great extent resulting into
an effective potential acting on the superconducting condensate. For the
Abrikosov vortex lattice in Niobium, numerical solutions are presented and the
different contributions to the electrostatic potential and the related charge
distribution are discussed.Comment: 19 pages, 11 figure
No Time-Dependent Effects of Psychosocial Stress on Fear Contextualization and Generalization:A Randomized-Controlled Study With Healthy Participants
The formation of context-dependent fear memories (fear contextualization) can aid the recognition of danger in new, similar, situations. Overgeneralization of fear is often seen as hallmark of anxiety and trauma-related disorders. In this randomized-controlled study, we investigated whether exposure to a psychosocial stressor influences retention of fear contextualization and generalization in a time-dependent manner. The Trier Social Stress Test was used to induce psychosocial stress. Healthy male participants (n = 117) were randomly divided into three experimental groups that were subjected to the acquisition phase of the Fear Generalization Task: (1) without stress, (2) immediately after acute stress, or (3) 2 h after acute stress. In this task, a male with neutral facial expression (conditioned stimuli) was depicted in two different contexts that modulated the conditioned stimuli–unconditioned stimuli (=shock) association (threat, safe). Salivary alpha-amylase and cortisol levels were measured throughout the experiment. After a 24-h delay, context-dependency of fear memory was investigated with an unannounced memory test consisting of the threat and safe contexts alternated with a novel context (the generalization context). Multilevel analyses revealed that participants showed increased fear-potentiated startle responses to the conditioned stimuli in the threat compared to the safe context, at the end of the acquisition phase, indicating adequate fear contextualization. Directly after acquisition, there were no time-dependent effects of psychosocial stress on fear contextualization. Context-dependency of fear memories was retained 24 h later, as fear-potentiated startle responding was modulated by context (threat > safe or novel). At that time, the context-dependency of fear memories was also not influenced by the early or late effects of the endogenous stress response during acquisition. These results with experimental stress deviate in some aspects from those earlier obtained with exogenous hydrocortisone administration, suggesting a distinct role for stress mediators other than cortisol
Failed theories of superconductivity
Almost half a century passed between the discovery of superconductivity by
Kamerlingh Onnes and the theoretical explanation of the phenomenon by Bardeen,
Cooper and Schrieffer. During the intervening years the brightest minds in
theoretical physics tried and failed to develop a microscopic understanding of
the effect. A summary of some of those unsuccessful attempts to understand
superconductivity not only demonstrates the extraordinary achievement made by
formulating the BCS theory, but also illustrates that mistakes are a natural
and healthy part of the scientific discourse, and that inapplicable, even
incorrect theories can turn out to be interesting and inspiring.Comment: 14 pages, 3 figures (typos fixed), to appear in: Bardeen Cooper and
Schrieffer: 50 YEARS, edited by Leon N Cooper and Dmitri Feldma
Optical BCS conductivity at imaginary frequencies and dispersion energies of superconductors
We present an efficient expression for the analytic continuation to arbitrary
complex frequencies of the complex optical and AC conductivity of a homogeneous
superconductor with arbitrary mean free path. Knowledge of this quantity is
fundamental in the calculation of thermodynamic potentials and dispersion
energies involving type-I superconducting bodies. When considered for imaginary
frequencies, our formula evaluates faster than previous schemes involving
Kramers--Kronig transforms. A number of applications illustrates its
efficiency: a simplified low-frequency expansion of the conductivity, the
electromagnetic bulk self-energy due to longitudinal plasma oscillations, and
the Casimir free energy of a superconducting cavity.Comment: 20 pages, 7 figures, calculation of Casimir energy adde
Spin flip lifetimes in superconducting atom chips: BCS versus Eliashberg theory
We investigate theoretically the magnetic spin-flip transitions of neutral
atoms trapped near a superconducting slab. Our calculations are based on a
quantum-theoretical treatment of electromagnetic radiation near dielectric and
metallic bodies. Specific results are given for rubidium atoms near a niobium
superconductor. At the low frequencies typical of the atomic transitions, we
find that BCS theory greatly overestimates coherence effects, which are much
less pronounced when quasiparticle lifetime effects are included through
Eliashberg theory. At 4.2 K, the typical atomic spin lifetime is found to be
larger than a thousand seconds, even for atom-superconductor distances of one
micrometer. This constitutes a large enhancement in comparison with normal
metals.Comment: 10 pages, 4 figure
The concept of correlated density and its application
The correlated density appears in many physical systems ranging from dense
interacting gases up to Fermi liquids which develop a coherent state at low
temperatures, the superconductivity. One consequence of the correlated density
is the Bernoulli potential in superconductors which compensates forces from
dielectric currents. This Bernoulli potential allows to access material
parameters. Though within the surface potential these contributions are largely
canceled, the bulk measurements with NMR can access this potential. Recent
experiments are explained and new ones suggested. The underlying quantum
statistical theory in nonequilibrium is the nonlocal kinetic theory developed
earlier.Comment: 14 pages, CMT30 proceeding
Scattering of first and second sound waves by quantum vorticity in superfluid Helium
We study the scattering of first and second sound waves by quantum vorticity
in superfluid Helium using two-fluid hydrodynamics. The vorticity of the
superfluid component and the sound interact because of the nonlinear character
of these equations. Explicit expressions for the scattered pressure and
temperature are worked out in a first Born approximation, and care is exercised
in delimiting the range of validity of the assumptions needed for this
approximation to hold. An incident second sound wave will partly convert into
first sound, and an incident first sound wave will partly convert into second
sound. General considerations show that most incident first sound converts into
second sound, but not the other way around. These considerations are validated
using a vortex dipole as an explicitely worked out example.Comment: 24 pages, Latex, to appear in Journal of Low Temperature Physic
Kinetic Inductance and Penetration Depth of Thin Superconducting Films Measured by THz Pulse Spectroscopy
We measure the transmission of THz pulses through thin films of YBCO at
temperatures between 10K and 300K. The pulses possess a useable bandwidth
extending from 0.1 -- 1.5 THz (3.3 cm^-1 -- 50 cm^-1). Below T_c we observe
pulse reshaping caused by the kinetic inductance of the superconducting charge
carriers. From transmission data, we extract values of the London penetration
depth as a function of temperature, and find that it agrees well with a
functional form (\lambda(0)/\lambda(T))^2 = 1 - (T/T_c)^{\alpha}, where
\lambda(0) = 148 nm, and \alpha = 2. *****Figures available upon request*****Comment: 7 Pages, LaTe
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