548 research outputs found
Spectral Analysis and Metastable Absorption Measurements of High Pressure Capacitively and Inductively Coupled Radio-Frequency Argon Helium Discharges
Characterization of the excited state argon population in an argon-helium discharge is of crucial importance to the development of a continuous wave Discharge Assisted Noble Gas Laser. Experiments were conducted in the Richard W. Davis Advanced Laser Facility at AFRL on Kirtland Air Force Base, Albuquerque, NM. During these tests the capability to maintain a capactively-coupled and inductively-coupled radio-frequency discharge at pressures of 300 Torr was demonstrated. Optical Emission Spectroscopy experiments were conducted in order to examine the population distribution of the p-state in argon. The population distributions for these high-pressure and high-power cases were compared to a low-pressure, low-power pure argon discharge to examine the influence of helium on an argon discharge. Tuneable Diode Laser Absorption Spectroscopy experiments were also performed on the inductively coupled discharge to examine the column density of the s5 and s3 metastable states for different discharge parameters. The absorption data was fit to an approximated Voigt profile from which pressure broadening rates of 18 + or - 8 MHz/Torr and 22 ±2 MHz/Torr were calculated for the 801.48 nm p8 yields s5 and 794.82 nm p4 yields s3 transitions, respectively
Studies of Bacterial Branching Growth using Reaction-Diffusion Models for Colonial Development
Various bacterial strains exhibit colonial branching patterns during growth
on poor substrates. These patterns reflect bacterial cooperative
self-organization and cybernetic processes of communication, regulation and
control employed during colonial development. One method of modeling is the
continuous, or coupled reaction-diffusion approach, in which continuous time
evolution equations describe the bacterial density and the concentration of the
relevant chemical fields. In the context of branching growth, this idea has
been pursued by a number of groups. We present an additional model which
includes a lubrication fluid excreted by the bacteria. We also add fields of
chemotactic agents to the other models. We then present a critique of this
whole enterprise with focus on the models' potential for revealing new
biological features.Comment: 1 latex file, 40 gif/jpeg files (compressed into tar-gzip). Physica
A, in pres
Modeling branching and chiral colonial patterning of lubricating bacteria
In nature, microorganisms must often cope with hostile environmental
conditions. To do so they have developed sophisticated cooperative behavior and
intricate communication capabilities, such as: direct cell-cell physical
interactions via extra-membrane polymers, collective production of
extracellular "wetting" fluid for movement on hard surfaces, long range
chemical signaling such as quorum sensing and chemotactic (bias of movement
according to gradient of chemical agent) signaling, collective activation and
deactivation of genes and even exchange of genetic material. Utilizing these
capabilities, the colonies develop complex spatio-temporal patterns in response
to adverse growth conditions. We present a wealth of branching and chiral
patterns formed during colonial development of lubricating bacteria (bacteria
which produce a wetting layer of fluid for their movement). Invoking ideas from
pattern formation in non-living systems and using ``generic'' modeling we are
able to reveal novel survival strategies which account for the salient features
of the evolved patterns. Using the models, we demonstrate how communication
leads to self-organization via cooperative behavior of the cells. In this
regard, pattern formation in microorganisms can be viewed as the result of the
exchange of information between the micro-level (the individual cells) and the
macro-level (the colony). We mainly review known results, but include a new
model of chiral growth, which enables us to study the effect of chemotactic
signaling on the chiral growth. We also introduce a measure for weak chirality
and use this measure to compare the results of model simulations with
experimental observations.Comment: 50 pages, 24 images in 44 GIF/JPEG files, Proceedings of IMA
workshop: Pattern Formation and Morphogenesis (1998
Clique of functional hubs orchestrates population bursts in developmentally regulated neural networks
It has recently been discovered that single neuron stimulation can impact
network dynamics in immature and adult neuronal circuits. Here we report a
novel mechanism which can explain in neuronal circuits, at an early stage of
development, the peculiar role played by a few specific neurons in
promoting/arresting the population activity. For this purpose, we consider a
standard neuronal network model, with short-term synaptic plasticity, whose
population activity is characterized by bursting behavior. The addition of
developmentally inspired constraints and correlations in the distribution of
the neuronal connectivities and excitabilities leads to the emergence of
functional hub neurons, whose stimulation/deletion is critical for the network
activity. Functional hubs form a clique, where a precise sequential activation
of the neurons is essential to ignite collective events without any need for a
specific topological architecture. Unsupervised time-lagged firings of
supra-threshold cells, in connection with coordinated entrainments of
near-threshold neurons, are the key ingredients to orchestrateComment: 39 pages, 15 figures, to appear in PLOS Computational Biolog
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