741 research outputs found
Roughness experiments of viscous fluid interfaces in disordered Hele-Shaw cells
Se presenta un trabajo experimental sobre el crecimiento de interficies rugosas en medios desordenados. El experimento consiste en una celda de Hele-Shaw en cuya placa inferior se han distribuido una serie de obstáculos de cobre que hacen la función de desorden congelado. Un aceite de silicona es inyectado por uno de los extremos de la celda desplazando el aire inicialmente presente. El frente es estable por contraste de viscosidad pero se hace rugoso debido a la presencia del desorden. Los experimentos realizados se pueden clasificar en dos grupos principales según el tipo de mecanismo utilizado para la inyección del aceite: A- Velocidad constante y B- Presión constante.
Para el Caso A- se han realizado multitud de experimentos variando la velocidad del fluido, el espaciado entre placas en la celda de Hele-Shaw y la configuración del desorden. Se ha observado que cuando la velocidad promedio del aceite es alta y/o la longitud de correlación del desorden en la dirección de crecimiento es baja (desorden débil), entonces el escalamiento de las fluctuaciones de la interfície sigue el escalamiento dinámico de Family-Vicsek, con un exponente de crecimiento de beta=0,50 independiente de la velocidad promedio de la interface, espaciado entre placas y configuración del desorden. También se ha estudiado del exponente de rugosidad, encontrando dos regímenes, alfa-1 a escalas pequeñas y alfa-2 a escalas grandes, con unos valores que dependen de la velocidad promedio, el espaciado entre placas y la configuración del desorden. Para velocidades grandes se obtienen un valor límite de alfa 1=1,3 y alfa 2=0 independientemente de los parámetros experimentales. También se ha caracterizado el punto de crossover entre alfa-1 y alfa-2, el cual escala con la velocidad con un exponente 0.5, de acuerdo con predicciones teóricas
Roughness experiments of viscous fluid interfaces in disordered Hele-Shaw cells
[spa] Se presenta un trabajo experimental sobre el crecimiento de interficies rugosas en medios desordenados. El experimento consiste en una celda de Hele-Shaw en cuya placa inferior se han distribuido una serie de obstáculos de cobre que hacen la función de desorden congelado. Un aceite de silicona es inyectado por uno de los extremos de la celda desplazando el aire inicialmente presente. El frente es estable por contraste de viscosidad pero se hace rugoso debido a la presencia del desorden. Los experimentos realizados se pueden clasificar en dos grupos principales según el tipo de mecanismo utilizado para la inyección del aceite: A- Velocidad constante y B- Presión constante. Para el Caso A- se han realizado multitud de experimentos variando la velocidad del fluido, el espaciado entre placas en la celda de Hele-Shaw y la configuración del desorden. Se ha observado que cuando la velocidad promedio del aceite es alta y/o la longitud de correlación del desorden en la dirección de crecimiento es baja (desorden débil), entonces el escalamiento de las fluctuaciones de la interfície sigue el escalamiento dinámico de Family-Vicsek, con un exponente de crecimiento de beta=0,50 independiente de la velocidad promedio de la interface, espaciado entre placas y configuración del desorden. También se ha estudiado del exponente de rugosidad, encontrando dos regímenes, alfa-1 a escalas pequeñas y alfa-2 a escalas grandes, con unos valores que dependen de la velocidad promedio, el espaciado entre placas y la configuración del desorden. Para velocidades grandes se obtienen un valor límite de alfa 1=1,3 y alfa 2=0 independientemente de los parámetros experimentales. También se ha caracterizado el punto de crossover entre alfa-1 y alfa-2, el cual escala con la velocidad con un exponente 0.5, de acuerdo con predicciones teóricas
Neuronal Cultures: Exploring Biophysics, Complex Systems, and Medicine in a Dish
Neuronal cultures are one of the most important experimental models in modern interdisciplinary neuroscience, allowing to investigate in a control environment the emergence of complex behavior from an ensemble of interconnected neurons. Here, I review the research that we have conducted at the neurophysics laboratory at the University of Barcelona over the last 15 years, describing first the neuronal cultures that we prepare and the associated tools to acquire and analyze data, to next delve into the different research projects in which we actively participated to progress in the understanding of open questions, extend neuroscience research on new paradigms, and advance the treatment of neurological disorders. I finish the review by discussing the drawbacks and limitations of neuronal cultures, particularly in the context of brain-like models and biomedicine
Model-free reconstruction of neuronal network connectivity from calcium imaging signals
A systematic assessment of global neural network connectivity through direct
electrophysiological assays has remained technically unfeasible even in
dissociated neuronal cultures. We introduce an improved algorithmic approach
based on Transfer Entropy to reconstruct approximations to network structural
connectivities from network activity monitored through calcium fluorescence
imaging. Based on information theory, our method requires no prior assumptions
on the statistics of neuronal firing and neuronal connections. The performance
of our algorithm is benchmarked on surrogate time-series of calcium
fluorescence generated by the simulated dynamics of a network with known
ground-truth topology. We find that the effective network topology revealed by
Transfer Entropy depends qualitatively on the time-dependent dynamic state of
the network (e.g., bursting or non-bursting). We thus demonstrate how
conditioning with respect to the global mean activity improves the performance
of our method. [...] Compared to other reconstruction strategies such as
cross-correlation or Granger Causality methods, our method based on improved
Transfer Entropy is remarkably more accurate. In particular, it provides a good
reconstruction of the network clustering coefficient, allowing to discriminate
between weakly or strongly clustered topologies, whereas on the other hand an
approach based on cross-correlations would invariantly detect artificially high
levels of clustering. Finally, we present the applicability of our method to
real recordings of in vitro cortical cultures. We demonstrate that these
networks are characterized by an elevated level of clustering compared to a
random graph (although not extreme) and by a markedly non-local connectivity.Comment: 54 pages, 8 figures (+9 supplementary figures), 1 table; submitted
for publicatio
Messenger RNA Fluctuations and Regulatory RNAs Shape the Dynamics of Negative Feedback Loop
Single cell experiments of simple regulatory networks can markedly differ
from cell population experiments. Such differences arise from stochastic events
in individual cells that are averaged out in cell populations. For instance,
while individual cells may show sustained oscillations in the concentrations of
some proteins, such oscillations may appear damped in the population average.
In this paper we investigate the role of RNA stochastic fluctuations as a
leading force to produce a sustained excitatory behavior at the single cell
level. Opposed to some previous models, we build a fully stochastic model of a
negative feedback loop that explicitly takes into account the RNA stochastic
dynamics. We find that messenger RNA random fluctuations can be amplified
during translation and produce sustained pulses of protein expression.
Motivated by the recent appreciation of the importance of non--coding
regulatory RNAs in post--transcription regulation, we also consider the
possibility that a regulatory RNA transcript could bind to the messenger RNA
and repress translation. Our findings show that the regulatory transcript helps
reduce gene expression variability both at the single cell level and at the
cell population level.Comment: 87.18.Vf --> Systems biology 87.10.Mn --> Stochastic models in
biological systems 87.18.Tt --> Noise in biological systems
http://www.ncbi.nlm.nih.gov/pubmed/20365787
http://www.weizmann.ac.il/complex/tlusty/papers/PhysRevE2010.pd
Grafted human pluripotent stem cell-derived cortical neurons integrate into adult human cortical neural circuitry
Several neurodegenerative diseases cause loss of cortical neurons, leading to sensory, motor, and cognitive impairments. Studies in different animal models have raised the possibility that transplantation of human cortical neuronal progenitors, generated from pluripotent stem cells, might be developed into a novel therapeutic strategy for disorders affecting cerebral cortex. For example, we have shown that human longterm neuroepithelial-like stem (lt-NES) cell-derived cortical neurons, produced from induced pluripotent stem cells and transplanted into stroke-injured adult rat cortex, improve neurological deficits and establish both afferent and efferent morphological and functional connections with host cortical neurons. So far, all studies with human pluripotent stem cell-derived neurons have been carried out using xenotransplantation in animal models. Whether these neurons can integrate also into adult human brain circuitry is unknown. Here, we show that cortically fated lt-NES cells, which are able to form functional synaptic networks in cell culture, differentiate to mature, layer-specific cortical neurons when transplanted ex vivo onto organotypic cultures of adult human cortex. The grafted neurons are functional and establish both afferent and efferent synapses with adult human cortical neurons in the slices as evidenced by immuno-electron microscopy, rabies virus retrograde monosynaptic tracing, and whole-cell patch-clamp recordings. Our findings provide the first evidence that pluripotent stem cell-derived neurons can integrate into adult host neural networks also in a human-to-human grafting situation, thereby supporting their potential future clinical use to promote recovery by neuronal replacement in the patient's diseased brain
- …