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At least 199 records · Page 11Linked to original sources
PubMed1998

Model Analysis of Spatial Patterns in Mountain Pine Beetle Outbreaks.

The mountain pine beetle [MPB, Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)] is an aggressive bark beetle, one that typically needs to kill host trees in order to successfully reproduce. This ecological adaptation has resulted in an organism that is both economically important and ecologically significant. Even though significant resources have been expended on MPB research, and a great deal of knowledge exists regarding individual aspects of MPB ecology, some of the most basic questions regarding outbreaks remain unanswered. In our opinion, one reason for the lack of synthesis and predictive power is the inadequate treatment of spatial dynamics in outbreak theories. This paper explicitly addresses the role of spatial dynamics in the precipitation and propagation of MPB outbreaks. We first describe a spatially dynamic model of the MPB/forest interaction that includes chemical ecology, spatial redistribution of beetles, attack, and resulting host mortality. The model is a system of 6 coupled, partial differential equations with 7 state variables and 20 parameters. It represents an attempt to capture the relatively complex predator/prey interaction between MPB and host trees by including the minimum phenomenological descriptions necessary for ecological credibility. This system of equations describes the temporal dynamics of: beetle attraction as a function of pheromone concentration; the change in numbers of flying and nesting beetles; tree resistance/susceptibility; and tree recovery from attack. Spatial dynamics are modeled by fluxes due to gradients in pheromones and kairomones, and the random redistribution of beetles in absence of semiochemicals. We then use the parameterized model to explore three issues central to the ecology of MPB/forest interaction. The first of these is in response to the need for objective ways to compare patterns of successful beetle attacks as they evolve in space. Simulation results indicate that at endemic levels, the pattern of successful attacks are determined almost exclusively by the underlying distribution of susceptible host trees (environmental determinism). As an outbreak develops, the pattern of successfully attacked trees switches to one that is dynamically driven by the self-generated semiochemical landscape (dynamic determinism). This switch from an environmentally determined spatial pattern to a dynamically driven pattern is the hallmark of an outbreak. We discuss the application of a spatial correlation coefficient that can be used to differentiate between the spatial distribution of killed trees in endemic and outbreak phases. The second issue we address through simulation is synchrony in adult emergence. Synchronous adult emergence is critical for the mass attack strategy necessary for overcoming tree defenses. Results from these simulations indicate that the degree of synchrony in adult emergence can have important consequences for assessing the risk of an outbreak. The final issue we investigate through simulation is the effect of spatial pattern of nurse trees (those successfully attacked the previous year) on outbreak potential. Simulations indicated that the spatial proximity of nurse trees was an important determinant of subsequent successful attacks. We conclude with a discussion of the general implications of our simulation experiments. Copyright 1998 Academic Press.

Journal Article↗
PubMed1996

Response of Bacillus subtilis spores to dehydration and UV irradiation at extremely low temperatures.

Spores of Bacillus subtilis have been exposed to the conditions of extreme dehydration (argon/silica gel; simulated space vacuum) for up to 12 weeks at 298 K and 80 K in the dark. The inactivation has been correlated with the production of DNA-double strand-breaks. The temperature-dependence of the rate constants for inactivation or production of DNA-double strand-breaks is surprisingly low. Controls kept in the frozen state at 250 K for the same period of time showed no sign of deterioration. In another series of experiments the spores have been UV irradiated (253.7 nm) at 298 K, 200 K and 80 K after exposure to dehydrating conditions for 3 days. Fluence-effect relationships for inactivation, production of DNA-double strand-breaks and DNA-protein cross-links are presented. The corresponding F37-values for inactivation and production of DNA lesions are significantly increased only at 80 K (factor of 4 to 5). The data indicate that the low temperatures that prevail in the outer parts of the Solar System or at the nightside of Mars or the Moon are not sufficiently low to crucially inhibit inactivation by dehydration. Our data place further constraints on the panspermia hypothesis.

Bacillus subtilis↗
PubMed1994

EVA design: lessons learned.

Extravehicular Activities (EVAs) are very demanding and specialized space flight activities. There are many aspects to consider in the design of hardware, tools, and procedures to be used on an EVA mission. To help minimize costs and optimize the EVA productivity, experience shows that astronauts should become involved early in the design process.

Astronauts↗
PubMed1994

EVA: European plans.

The current status of European EVA (extravehicular activity) plans is reviewed. The major difference to already existing EVA scenarios in U.S.A. and Soviet Union consists in the adoption of a higher suit pressure, namely 500 hPa. The results of a study concerned with the physiological consequences of this adoption are presented, including recommendations for protective procedures and their necessary experimental validation. A certain discrepancy between laboratory experimental decompression data and EVA operational results is discussed, leading to the identification of several items which may influence space decompression. Microgravity and the influence of the space suit itself are most likely factors in the explanation of this discrepancy, and both experimental procedures and technological developments are proposed to clarify their role for the future design of EVA procedures.

Aerospace Medicine↗
PubMed1995

EVA-related activities in Japan.

Recognizing the importance and fertility of the extra vehicular activity technologies in future space development and utilization, some related research activities have been conducted in Japan, though it has not committed itself to developing its own manned space flight capability. The purpose of this paper is to introduce and evaluate these EVA-related activities and to indicate the next step for Japan to get into the era of manned space activities including EVA in reality.

Carbon Dioxide↗
PubMedDate not supplied

Ensuring of long operation life of the orbiting station EVA space suit.

Russia has gained a lot of experience in operating the space suits (SS) during the extravehicular activities (EVA) by the crews of SALYUT-6, SALYUT-7 and MIR orbiting stations. A total of 21 Orlan-type space suits of various models were operated onboard the orbiting stations (OS) during almost 20 years period. Some of these space suits served up to 3 years in orbit. The paper reviews special features of long SS operation (without return to the Earth) onboard an orbiting station as well as the problems associated with SS repeated use by several crews. An analysis of measures to support solving of the problems of SS long stay and reliable operation onboard the orbiting station is made: selection of a corresponding SS type and separate elements design; selection of the materials; routine and preventive maintenance; development tests. The advantages of the space suit of a semi-rigid type for solving the above problems are shown. The paper includes a short analysis of space suits' operation onboard the Russian orbiting station MIR, and some restuts of inspection of the Orlan-DMA space suit returned to the Earth from orbit by STS-79 alter long operation in orbit. Recommendations on further improvement of the space suits for EVA operations in the International Space Station (ISS) are given.

Decompression Sickness↗
PubMedDate not supplied

Evaluation of safety of hypobaric decompressions and EVA from positions of probabilistic theory.

Formation and subsequent evolution of gas bubbles in blood and tissues of subjects exposed to decompression are casual processes in their nature. Such character of bubbling processes in a body predetermines probabilistic character of decompression sickness (DCS) incidence in divers, aviators and astronauts. Our original probabilistic theory of decompression safety is based on stochastic models of these processes and on the concept of critical volume of a free gas phase in body tissues. From positions of this theory, the probability of DCS incidence during single-stage decompressions and during hypobaric decompressions under EVA in particular, is defined by the distribution of possible values of nucleation efficiency in "pain" tissues and by its critical significance depended on the parameters of a concrete decompression. In the present study the following is shown: 1) the dimensionless index of critical nucleation efficiency for "pain" body tissues is a more adequate index of decompression stress in comparison with Tissue Ratio, TR; 2) a priory the decompression under EVA performed according to the Russian protocol is more safe than decompression under EVA performed in accordance with the U.S. protocol; 3) the Russian space suit operated at a higher pressure and having a higher "rigidity" induces a stronger inhibition of mechanisms of cavitation and gas bubbles formation in tissues of a subject located in it, and by that provides a more considerable reduction of the DCS risk during real EVA performance.

Aerospace Medicine↗
PubMed1998

Postnatal development under conditions of simulated weightlessness and space flight.

The adaptability of the developing nervous system to environmental influences and the mechanisms underlying this plasticity has recently become a subject of interest in space neuroscience. Ground studies on neonatal rats using the tail suspension model of weightlessness have shown that the force of gravity clearly influences the events underlying the postnatal development of motor function. These effects depend on the age of the animal, duration of the perturbation and the motor function studied. A nine-day flight study has shown that a dam and neonates can develop under conditions of space flight. The motor function of the flight animals after landing was consistent with that seen in the tail suspension studies, being marked by limb joint extension. However, there were expected differences due to: (1) the unloading of the vestibular system in flight, which did not occur in the ground-based experiments; (2) differences between flight and suspension durations; and (3) the inability to evaluate motor function during the flight. The next step is to conduct experiments in space with the flexibility and rigor that is now limited to ground studies: an opportunity offered by the International Space Station.

Animals↗
PubMed2001

Three-dimensional structure of the histidine-containing phosphocarrier protein (HPr) from Enterococcus faecalis in solution.

The histidine-containing phosphocarrier protein (HPr) transfers a phosphate group between components of the prokaryotic phosphoenolpyruvate-dependent phosphotransferase system (PTS), which is finally used to phosphorylate the carbohydrate transported by the PTS through the cell membrane. Recently it has also been found to act as an intermediate in the signaling cascade that regulates transcription of genes related to the carbohydrate-response system. Both functions involve phosphorylation/dephosphorylation reactions, but at different sites. Using multidimensional (1)H-NMR spectroscopy and angular space simulated annealing calculations, we determined the structure of HPr from Enterococcus faecalis in aqueous solution using 1469 distance and 44 angle constraints derived from homonuclear NMR data. It has a similar overall fold to that found in HPrs from other organisms. Four beta strands, A, B, C, D, encompassing residues 2-7, 32-37, 40-42 and 60-66, form an antiparallel beta sheet lying opposite the two antiparallel alpha helices, a and c (residues 16-26 and 70-83). A short alpha helix, b, from residues 47-53 is also observed. The pairwise root mean square displacement for the backbone heavy atoms of the mean of the 16 NMR structures to the crystal structure is 0.164 nm. In contrast with the crystalline state, in which a torsion angle strain in the active-center loop has been described [Jia, Z., Vandonselaar, M., Quail, J.W. & Delbaere, L.T.J. (1993) Nature (London) 361, 94-97], in the solution structure, the active-site His15 rests on top of helix a, and the phosphorylation site N(delta 1) of the histidine ring is oriented towards the surface, making it easily accessible to the solvent. Back calculation of the 2D NOESY NMR spectra from both the NMR and X-ray structures shows that the active-center structure derived by X-ray crystallography is not compatible with experimental data recorded in solution. The observed torsional strain must either be a crystallization artefact or represents a conformational state that exists only to a small extent in solution.

Bacterial Proteins↗
PubMed2005

Basaloid squamous carcinoma of the anal canal with an adenoid cystic pattern: histologic and immunohistochemical reappraisal of an unusual variant.

Two cases of a distinctive variety of basaloid squamous carcinoma (BSC) of the anal canal are described. Both occurred in female patients who presented with bleeding per rectum. Histologic evaluation of the tumors showed lobules and aggregates of medium-sized basaloid cells with distinctive peripheral palisading and focal areas of central, comedo-necrosis. Accompanying dysplasia of the overlying squamous mucosa was absent. However, the microscopic pattern was dominated by the presence of eosinophilic, hyaline, paucicellular basement membrane-like material around and within tumor nests. This appearance together with microcystic spaces simulated that of an adenoid cystic carcinoma. Immunohistochemistry of the tumors revealed the following profile: CK7, CK5/CK6, 34betaE12 positive, CK14 focally positive but CK20 negative. The following were all negative: EMA, CEA, smooth muscle and muscle-specific actin, calponin, and S-100. The tumor cells exhibited diffuse nuclear positivity with p63. The eosinophilic basement membrane hyaline material was positive for collagen type IV and also for laminin. BSC of the anal canal with an adenoid cystic pattern is an infrequently encountered and reported variant, although it is seen more often in the aerodigestive tract. There may be an increased propensity for BSC with an adenoid cystic pattern to metastasize to the liver, but the number of cases encountered are too small to be definitive. The histologic differential diagnosis is true salivary gland-type adenoid cystic carcinoma and basal cell adenocarcinoma. Immunohistochemistry and awareness of this unusual pattern of BSC will facilitate the correct diagnosis being reached.

Anal Canal↗
PubMed2005

Diffuse-reflection boundary conditions for a thermal lattice Boltzmann model in two dimensions: evidence of temperature jump and slip velocity in microchannels.

We discuss the implementation of diffuse reflection boundary conditions in a thermal lattice Boltzmann model for which the upwind finite difference scheme is used to solve the set of evolution equations recovered after discretization of the velocity space. Simulation of heat transport between two parallel walls at rest shows evidence of temperature jumps at the walls that increase with Knudsen number. When the walls move in opposite directions with speeds +/- u(W), fluid velocity slip is observed at the walls, together with temperature jumps.

Journal Article↗
PubMed1993

Resolution enhancement of biomagnetic images using the method of alternating protections.

Resolution of biomagnetic images using the technique of the alternating projections is proposed. Our image reconstruction procedure is divided in two steps. First, the biomagnetic inverse problem is solved by use of the projection theorem to reconstruct an initial image of the current distribution from a given magnetic field profile. Although the current distribution thus obtained has poor resolution, it can resemble the original shape of the current distribution. The second step improves the resolution of the reconstructed image by using the method of alternating projections. The procedure assumes that images can be represented by line like elements and involves finding the line like elements based on the initial image and projecting back onto the original solution space. Simulation studies were performed on a set of parallel conductors and a shape of the conductors in the form of letters, UWB@. All conductors were of line like thickness. Restored images closely resemble the original shape of the conductors.

Humans↗
PubMed1998

Electromyographic study of the contractile and electrical properties of the human triceps surae muscle in a simulated microgravity environment.

1. The effects of 7 days of simulated space flight, achieved with the technique of 'dry' water immersion, on human triceps surae muscle function have been investigated. 2. The maximal voluntary contraction (MVC) was reduced by 33.8 % (P < 0.01) while the electrically evoked maximal tetanic contraction force (Po) decreased by 8.2 % (P > 0.05). This suggests that most of the force loss is due to a reduction in motor drive. 3. The decrease in Po was associated with a small increase in maximal rates of tension development (7.2 %). The twitch tension (Pt) was not significantly changed and the Pt : Po ratio was decreased by 8.7 % after immersion. 4. A standard fatigue test, consisting of sixty 1 s intermittent isometric contractions (50 impulses s-1) separated by 1 s rest decreased tetanic force to approximately 60 % of initial values, but force reduction was not significantly different before and after immersion: the fatigue index was 36.2 +/- 5.4 % before and 38.6 +/- 2.8 % after immersion (P > 0.05). Whilst there were similar changes in mechanical output between control and disused muscles, there were differences in the pattern of electrical activity.

Adult↗
PubMed1967

Statolon-induced resistance of mice to mengovirus.

Mice receiving statolon intraperitoneally were 1,000 times more resistant to intraperitoneal challenge with mengovirus than were untreated controls. Protection was afforded when statolon was administered 1 day before or 1 day after intraperitoneal inoculation with the virus. No therapeutic effect was observed when treatment with statolon was delayed for 2 days or more after virus infection. Exposure of mice to a simulated space cabin environment did not increase their susceptibility to the lethal effects of mengovirus infection or eliminate the protective effect of statolon.

Animals↗
PubMed1996

Restoration of plasma volume after 16 days of head-down tilt induced by a single bout of maximal exercise.

Seven healthy men performed maximal exercise 24 h before the end of 16 days exposure to 6 degrees head-down tilt (HDT) to test the hypothesis that such an exercise technique could restore plasma volume (PV) at the end of a simulated space mission. Exercise consisted of supine cycling with graded work rates increasing by 16 W/min to volitional fatigue and required an average of 16 min. The experimental protocol was a standard cross-over design in which the order of treatment (exercise or control) was counterbalanced across all seven subjects. PV, fluid intake (ad libitum), urine output, renal function, and hormones associated with fluid homeostasis were measured before HDT, 24 h before the end of HDT just prior to exercise, and at the end of HDT 24 h after exercise. HDT reduced PV by 16% in both control and exercise conditions. Maximal exercise completely restored plasma volume within 24 h to 3.9 +/- 3.2% of pre-HDT levels despite continued HDT. Compared with control, exercise induced a 660-ml larger positive fluid balance because of greater fluid intake and reduced urine volume during the 24 h after exercise. These results suggest that one bout of maximal leg exercise before return from 16 days of spaceflight may be completely effective in stimulating thirst and restoring plasma volume to preflight levels.

Adult↗
PubMed2002

Reaching to grasp with a multi-jointed arm. I. Computational model.

The generation of goal-directed movements requires the solution of many difficult computational problems. Among these are transformations from extrinsic to intrinsic reference frames, specifying solution paths, removing under-specification due to excess degrees of freedom and path multiplicity, constraint satisfaction, and error correction. There are no current motor-control computational models that address these issues in the context of realistic arm movement with redundant degrees of freedom. In this paper, we conjecture there is a geometric stage between sensory input and physical execution. The geometric stage determines movement trajectories independently of forces. It uses a gradient technique that relies on the metric of the space of postures to resolve endpoint path selection, posture-change specification, error correction, and multiple constraint satisfaction on-line without preplanning. The model is instantiated in an arm with seven degrees of freedom that moves in three-dimensional space. Simulated orientation-matching movements are compared with actual human movement data to assess the validity of several of the model's behavioral predictions.

Algorithms↗
PubMedDate not supplied

Calf muscle area and strength changes after five weeks of horizontal bed rest.

Nine male volunteers participated in a 10 week metabolic study in which subjects underwent 5 weeks of ambulatory control and 5 weeks of complete horizontal bed rest. Bed rest is a model commonly used to simulate space flight. The changes in muscle area and strength of the calf dorsiflexors and plantar flexors were measured before and after bed rest using magnetic resonance imaging (MRI) and a Cybex II dynamometer. The muscle area of the plantar flexors (gastrocnemius and soleus) decreased 12%, whereas the muscle area of the dorsiflexors was not significantly decreased. The maximal muscle strength of the plantar flexors decreased 26%; the muscle strength of the dorsiflexors was not significantly decreased. These results, which demonstrate differential muscle atrophy and a larger loss in strength relative to muscle area, have important implications in the development of exercise counter-measures to be implemented during space flight. The results also have implications for patients who have severe orthopaedic disorders and must be bed rested for long periods of time, and for persons who are voluntarily inactive (a large number of the elderly).

Adult↗