Statistical properties of fractures in damaged materials

We introduce a model for the dynamics of mud cracking in the limit of of extremely thin layers. In this model the growth of fracture proceeds by selecting the part of the material with the smallest (quenched) breaking threshold. In addition, weakening affects the area of the sample neighbour to the crack. Due to the simplicity of the model, it is possible to derive some analytical results. In particular, we find that the total time to break down the sample grows with the dimension L of the lattice as L^2 even though the percolating cluster has a non trivial fractal dimension. Furthermore, we obtain a formula for the mean weakening with time of the whole sample.Comment: 5 pages, 4 figures, to be published in Europhysics Letter

Stream Fishes Estimate Water Quality in Dayton-Montgomery County Park District Reserves

Author Institution: Department of Zoology, Miami University-MiddletownInformation on fish communities within seven streams and rivers in or near Dayton-Montgomery County Park District reserves was collected and analyzed to compile a preliminary species list for Park District waters and to estimate water quality of the streams. Fifty-two species of fishes were found in lotic habitats in or near the reserves, and more extensive sampling likely will reveal the presence of additional species. Index of Biotic Integrity (IBI) scoring of the fish communities indicated that water quality within the reserves ranged from fair to exceptional. It would appear that improvements of water quality and fish communities within the reserves may require changes in wastewater treatment outfalls and stream drainage land-use practices

The time to extinction for an SIS-household-epidemic model

We analyse a stochastic SIS epidemic amongst a finite population partitioned into households. Since the population is finite, the epidemic will eventually go extinct, i.e., have no more infectives in the population. We study the effects of population size and within household transmission upon the time to extinction. This is done through two approximations. The first approximation is suitable for all levels of within household transmission and is based upon an Ornstein-Uhlenbeck process approximation for the diseases fluctuations about an endemic level relying on a large population. The second approximation is suitable for high levels of within household transmission and approximates the number of infectious households by a simple homogeneously mixing SIS model with the households replaced by individuals. The analysis, supported by a simulation study, shows that the mean time to extinction is minimized by moderate levels of within household transmission

The Little Probe That Could! (A Story of Mission Impossible Engineering)

Starting with the Space Shuttle flight 4A (Nov. 30, 2000), the International Space Station (ISS) power system employs large, high voltage, solar arrays with the negative ground tied to chassis. An intense study by a NASA sponsored Tiger Team in the early ‘90s determined that this configuration leads to the structure being at a high negative potential relative to the local plasma (approximately 140v negative without any intervention) and, that at any potential greater than around 70v negative, the anodized aluminum structure and its components will undergo destructive arcing. A set of plasma contactor units (PCUs) was deployed to provide a conductive xenon plasma path for remitting electrons collected by the arrays and thus bring the potential closer to zero and mitigate the arcing danger. In late July 2000, the ISS program office at JSC issued an engineering change notice that directed the development of some means to independently assess the performance of the PCU’s, and to have hardware available for launch on STS-97 (ISS Flight 4A) the very mission scheduled to deliver and install the first set of large Station solar arrays on November 30th. This allowed only a mere 4.5 months to design, build, test, manifest, complete EVA training, and deliver for launch. NASA Glenn, NASA Johnson, and Design_Net Engineering formed a unique team to try to accomplish the directive. The subject of this paper is to describe the Floating Potential Probe (FPP) and the fast-track program approach used to quickly develop this autonomous system for measuring the electrical potential between the ISS and the surrounding space plasma. At the time, most people involved with the Floating Potential Probe (FPP) project believed that there was less than a 10% chance of successfully making it onboard Flight 4A and even less chance that it would work

Dendritic and axonal targeting patterns of a genetically-specified class of retinal ganglion cells that participate in image-forming circuits.

BackgroundThere are numerous functional types of retinal ganglion cells (RGCs), each participating in circuits that encode a specific aspect of the visual scene. This functional specificity is derived from distinct RGC morphologies and selective synapse formation with other retinal cell types; yet, how these properties are established during development remains unclear. Islet2 (Isl2) is a LIM-homeodomain transcription factor expressed in the developing retina, including approximately 40% of all RGCs, and has previously been implicated in the subtype specification of spinal motor neurons. Based on this, we hypothesized that Isl2+ RGCs represent a related subset that share a common function.ResultsWe morphologically and molecularly characterized Isl2+ RGCs using a transgenic mouse line that expresses GFP in the cell bodies, dendrites and axons of Isl2+ cells (Isl2-GFP). Isl2-GFP RGCs have distinct morphologies and dendritic stratification patterns within the inner plexiform layer and project to selective visual nuclei. Targeted filling of individual cells reveals that the majority of Isl2-GFP RGCs have dendrites that are monostratified in layer S3 of the IPL, suggesting they are not ON-OFF direction-selective ganglion cells. Molecular analysis shows that most alpha-RGCs, indicated by expression of SMI-32, are also Isl2-GFP RGCs. Isl2-GFP RGCs project to most retino-recipient nuclei during early development, but specifically innervate the dorsal lateral geniculate nucleus and superior colliculus (SC) at eye opening. Finally, we show that the segregation of Isl2+ and Isl2- RGC axons in the SC leads to the segregation of functional RGC types.ConclusionsTaken together, these data suggest that Isl2+ RGCs comprise a distinct class and support a role for Isl2 as an important component of a transcription factor code specifying functional visual circuits. Furthermore, this study describes a novel genetically-labeled mouse line that will be a valuable resource in future investigations of the molecular mechanisms of visual circuit formation

Changes in Farm Financial Conditions and Farming Practices in Ohio, 1986-1990

Highlights of a five year study of farm households are reported. Two facets of farm households, their financial condition and those farming practices affecting the environment, are analyzed. Results indicate improvements in farm household financial condition, changes to less soil erosive farming practices, but little adoption of low input farming systems

The Role of Surgery, Radiosurgery and Whole Brain Radiation Therapy in the Management of Patients with Metastatic Brain Tumors

Brain tumors constitute the most common intracranial tumor. Management of brain metastases has become increasingly complex as patients with brain metastases are living longer and more treatment options develop. The goal of this paper is to review the role of stereotactic radiosurgery (SRS), whole brain radiation therapy (WBRT), and surgery, in isolation and in combination, in the contemporary treatment of brain metastases. Surgery and SRS both offer management options that may help to optimize therapy in selected patients. WBRT is another option but can lead to late toxicity and suboptimal local control in longer term survivors. Improved prognostic indices will be critical for selecting the best therapies. Further prospective trials are necessary to continue to elucidate factors that will help triage patients to the proper brain-directed therapy for their cancer