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Cortical and spinal mechanisms of task failure of sustained submaximal fatiguing contractions
Get PDFIn this and the subsequent companion paper, results are presented that collectively seek to delineate the contribution that supraspinal circuits have in determining the time to task failure (TTF) of sustained submaximal contractions. The purpose of this study was to compare adjustments in supraspinal and spinal excitability taken concurrently throughout the performance of two different fatigue tasks with identical mechanical demands but different TTF (i.e., force-matching and position-matching tasks). On separate visits, ten healthy volunteers performed the force-matching or position-matching task at 15% of maximum strength with the elbow flexors to task failure. Single-pulse transcranial magnetic stimulation (TMS), paired-pulse TMS, paired cortico-cervicomedullary stimulation, and brachial plexus electrical stimulation were delivered in a 6-stimuli sequence at baseline and every 2–3 minutes throughout fatigue-task performance. Contrary to expectations, the force-matching task TTF was 42% shorter (17.5±7.9 min) than the position-matching task (26.9±15.11 min; p0.05). Therefore, failure occurred after a similar mean decline in motorneuron excitability developed (p0.10) and an index of upstream excitation of the motor cortex remained constant (p>0.40). Together, these results suggest that as fatigue develops prior to task failure, the increase in corticospinal excitability observed in relationship to the decrease in spinal excitability results from a combination of decreasing intracortical inhibition with constant levels of intracortical facilitation and upstream excitability that together eventually fail to provide the input to the motor cortex necessary for descending drive to overcome the spinal cord resistance, thereby contributing to task failure
An iterative and targeted sampling design informed by habitat suitability models for detecting focal plant species over extensive areas
Get PDFPrioritizing areas for management of non-native invasive plants is critical, as invasive plants can negatively impact plant community structure. Extensive and multi-jurisdictional inventories are essential to prioritize actions aimed at mitigating the impact of invasions and changes in disturbance regimes. However, previous work devoted little effort to devising sampling methods sufficient to assess the scope of multi-jurisdictional invasion over extensive areas. Here we describe a large-scale sampling design that used species occurrence data, habitat suitability models, and iterative and targeted sampling efforts to sample five species and satisfy two key management objectives: 1) detecting non-native invasive plants across previously unsampled gradients, and 2) characterizing the distribution of non-native invasive plants at landscape to regional scales. Habitat suitability models of five species were based on occurrence records and predictor variables derived from topography, precipitation, and remotely sensed data. We stratified and established field sampling locations according to predicted habitat suitability and phenological, substrate, and logistical constraints. Across previously unvisited areas, we detected at least one of our focal species on 77% of plots. In turn, we used detections from 2011 to improve habitat suitability models and sampling efforts in 2012, as well as additional spatial constraints to increase detections. These modifications resulted in a 96% detection rate at plots. The range of habitat suitability values that identified highly and less suitable habitats and their environmental conditions corresponded to field detections with mixed levels of agreement. Our study demonstrated that an iterative and targeted sampling framework can address sampling bias, reduce time costs, and increase detections. Other studies can extend the sampling framework to develop methods in other ecosystems to provide detection data. The sampling methods implemented here provide a meaningful tool when understanding the potential distribution and habitat of species over multi-jurisdictional and extensive areas is needed for achieving management objectives
Insights to genetic characterization tools for epidemiological tracking of Francisella tularensis in Sweden
Get PDFTularaemia, caused by the bacterium Francisella tularensis, is endemic in Sweden and is poorly understood. The aim of this study was to evaluate the effectiveness of three different genetic typing systems to link a genetic type to the source and place of tularemia infection in Sweden. Canonical single nucleotide polymorphisms (canSNPs), MLVA including five variable number of tandem repeat loci and PmeI-PFGE were tested on 127 F. tularensis positive specimens collected from Swedish case-patients. All three typing methods identified two major genetic groups with near-perfect agreement. Higher genetic resolution was obtained with canSNP and MLVA compared to PFGE; F. tularensis samples were first assigned into ten phylogroups based on canSNPs followed by 33 unique MLVA types. Phylogroups were geographically analysed to reveal complex phylogeographic patterns in Sweden. The extensive phylogenetic diversity found within individual counties posed a challenge to linking specific genetic types with specific geographic locations. Despite this, a single phylogroup (B.22), defined by a SNP marker specific to a lone Swedish sequenced strain, did link genetic type with a likely geographic place. This result suggests that SNP markers, highly specific to a particular reference genome, may be found most frequently among samples recovered from the same location where the reference genome originated. This insight compels us to consider whole-genome sequencing (WGS) as the appropriate tool for effectively linking specific genetic type to geography. Comparing the WGS of an unknown sample to WGS databases of archived Swedish strains maximizes the likelihood of revealing those rare geographically informative SNPs
Physical properties of near-Earth asteroid 2011 MD
Get PDFWe report on observations of near-Earth asteroid 2011 MD with the Spitzer Space Telescope. We have spent 19.9 hr of observing time with channel 2 (4.5 μm) of the Infrared Array Camera and detected the target within the 2σ positional uncertainty ellipse. Using an asteroid thermophysical model and a model of nongravitational forces acting upon the object, we constrain the physical properties of 2011 MD, based on the measured flux density and available astrometry data. We estimate 2011 MD to be (6) m in diameter with a geometric albedo of 0.3 (uncertainties are 1σ). We find the asteroid's most probable bulk density to be (1.1) g cm–3, which implies a total mass of (50-350) t and a macroporosity of ≥65%, assuming a material bulk density typical of non-primitive meteorite materials. A high degree of macroporosity suggests that 2011 MD is a rubble-pile asteroid, the rotation of which is more likely to be retrograde than prograde
"Teaching with fire: (Re) igniting the passion for teaching" An assessment of the 3rd annual teaching day
Get PDFThe NAU Teaching Academy and the staff of the Faculty Professional Development Program convened the Third Annual
NAU Teaching Day from 11:30 a.m. to 3:00 p.m. for over 200 per-registered participants. The visiting speaker was Dr. John Pollard, a faculty member at the University of Arizona, who talked about his work on teaching students to think within their discipline (“thinking chemically” in his case). Following his presentation, a series of round table discussions was convened, each focusing on a different topical area related to university teaching
Nitrogen inputs and losses in response to chronic CO2 exposure in a subtropical oak woodland
Get PDFRising atmospheric CO2 concentrations may alter the nitrogen (N) content of ecosystems by changing N inputs and N losses, but responses vary in field experiments, possibly because multiple mechanisms are at play. We measured N fixation and N losses in a subtropical oak woodland exposed to 11 years of elevated atmospheric CO2 concentrations. We also explored the role of herbivory, carbon limitation, and competition for light or nutrients in shaping the response of N fixation to elevated CO2. Elevated CO2 did not significantly alter gaseous N losses, but lower recovery and deeper distribution in the soil of a long-term 15N tracer indicated that elevated CO2 increased leaching losses. Elevated CO2 had no effect on nonsymbiotic N fixation, and had a transient effect on symbiotic N fixation by the dominant legume. Elevated CO2 tended to reduce soil and plant concentrations of iron, molybdenum, phosphorus, and vanadium, nutrients essential for N fixation. Competition for nutrients and herbivory likely contributed to the declining response of N fixation to elevated CO2. These results indicate that positive responses of N fixation to elevated CO2 may be transient and that chronic exposure to elevated CO2 can increase N leaching. Models that assume increased fixation or reduced N losses with elevated CO2 may overestimate future N accumulation in the biosphere
“TNOs are Cool”: A survey of the trans-Neptunian region: XI. A Herschel-PACS view of 16 Centaurs⋆
Get PDFContext. Centaurs are the transitional population between trans-Neptunian objects (TNOs) and Jupiter-family comets. Their physical properties provide an insight into TNO properties, but only under restricted conditions since Centaurs are closer to the Sun and Earth. For this reason it is possible to access the smaller ones, which is more difficult to do with the TNO population. Aims. The goal of this work is to characterize a set of 16 Centaurs in terms of their size, albedo, and thermal properties. We study the correlations, for a more extended sample obtained from the literature, of diameter, albedo, orbital parameters, and spectral slopes. Methods. We performed three-band photometric observations using Herschel-PACS and used a consistent method for the data reduction and aperture photometry of this sample to obtain monochromatic flux densities at 70, 100, and 160 μm. Additionally, we used Spitzer-MIPS flux densities at 24 and 70 μm when available. We also included in our Centaur sample scattered disk objects, a dynamical family of TNOs, using results previously published by our team, and some Centaurs observed only with the Spitzer-MIPS instrument. Results. We have determined new radiometric sizes and albedos of 16 Centaurs. The first conclusion is that the albedos of Centaur objects are not correlated with their orbital parameters. Similarly, there is no correlation between diameter and orbital parameters. Most of the objects in our sample are dark (pv< 7%) and most of them are small (D< 120 km). However, there is no correlation between albedo and diameter, in particular for the group of small objects as albedo values are homogeneously distributed between 4 to 16%. The correlation with the color of the objects showed that red objects are all small (mean diameter 65 km), while the gray ones span a wide range of sizes (mean diameter 120 km). Moreover, the gray objects tend to be darker, with a mean albedo of 5.6%, compared with a mean of 8.5% (ranging from 5 to 15%) for the red objects
The discovery of cometary activity in near-Earth asteroid (3552) Don Quixote
Get PDFThe near-Earth object (NEO) population, which mainly consists of fragments from collisions between asteroids in the main asteroid belt, is thought to include contributions from short-period comets as well. One of the most promising NEO candidates for a cometary origin is near-Earth asteroid (3552) Don Quixote, which has never been reported to show activity. Here we present the discovery of cometary activity in Don Quixote based on thermal-infrared observations made with the Spitzer Space Telescope in its 3.6 and 4.5 μm bands. Our observations clearly show the presence of a coma and a tail in the 4.5 μm but not in the 3.6 μm band, which is consistent with molecular band emission from CO2. Thermal modeling of the combined photometric data on Don Quixote reveals a diameter of 18.4 km and an albedo of , which confirms Don Quixote to be the third-largest known NEO. We derive an upper limit on the dust production rate of 1.9 kg s–1 and derive a CO2 gas production rate of (1.1 ± 0.1) × 1026 molecules s–1. Spitzer Infrared Spectrograph spectroscopic observations indicate the presence of fine-grained silicates, perhaps pyroxene rich, on the surface of Don Quixote. Our discovery suggests that CO2 can be present in near-Earth space over a long time. The presence of CO2 might also explain that Don Quixote's cometary nature remained hidden for nearly three decades
Estimated stocks of circumpolar permafrost carbon with quantified uncertainty ranges and identified data gaps
Get PDFSoils and other unconsolidated deposits in the northern circumpolar permafrost region store large amounts of soil organic carbon (SOC). This SOC is potentially vulnerable to remobilization following soil warming and permafrost thaw, but SOC stock estimates were poorly constrained and quantitative error estimates were lacking. This study presents revised estimates of permafrost SOC stocks, including quantitative uncertainty estimates, in the 0-3m depth range in soils as well as for sediments deeper than 3m in deltaic deposits of major rivers and in the Yedoma region of Siberia and Alaska. Revised estimates are based on significantly larger databases compared to previous studies. Despite this there is evidence of significant remaining regional data gaps. Estimates remain particularly poorly constrained for soils in the High Arctic region and physiographic regions with thin sedimentary overburden (mountains, highlands and plateaus) as well as for deposits below 3m depth in deltas and the Yedoma region. While some components of the revised SOC stocks are similar in magnitude to those previously reported for this region, there are substantial differences in other components, including the fraction of perennially frozen SOC. Upscaled based on regional soil maps, estimated permafrost region SOC stocks are 217 +/- 12 and 472 +/- 27 Pg for the 0-0.3 and 0-1 m soil depths, respectively (+/- 95% confidence intervals). Storage of SOC in 0-3m of soils is estimated to 1035 +/- 150 Pg. Of this, 34 +/- 16 PgC is stored in poorly developed soils of the High Arctic. Based on generalized calculations, storage of SOC below 3m of surface soils in deltaic alluvium of major Arctic rivers is estimated as 91 +/- 52 Pg. In the Yedoma region, estimated SOC stocks below 3mdepth are 181 +/- 54 Pg, of which 74 +/- 20 Pg is stored in intact Yedoma (late Pleistocene ice-and organic-rich silty sediments) with the remainder in refrozen thermokarst deposits. Total estimated SOC storage for the permafrost region is similar to 1300 Pg with an uncertainty range of similar to 1100 to 1500 Pg. Of this, similar to 500 Pg is in non-permafrost soils, seasonally thawed in the active layer or in deeper taliks, while similar to 800 Pg is perennially frozen. This represents a substantial similar to 300 Pg lowering of the estimated perennially frozen SOC stock compared to previous estimates
Carbon cycle uncertainty in the Alaskan Arctic
Get PDFClimate change is leading to a disproportionately large warming in the high northern latitudes, but the magnitude and sign of the future carbon balance of the Arctic are highly uncertain. Using 40 terrestrial biosphere models for the Alaskan Arctic from four recent model intercomparison projects – NACP (North American Carbon Program) site and regional syntheses, TRENDY (Trends in net land atmosphere carbon exchanges), and WETCHIMP (Wetland and Wetland CH4 Inter-comparison of Models Project) – we provide a baseline of terrestrial carbon cycle uncertainty, defined as the multi-model standard deviation (σ) for each quantity that follows. Mean annual absolute uncertainty was largest for soil carbon (14.0 ± 9.2 kg C m−2), then gross primary production (GPP) (0.22 ± 0.50 kg C m−2 yr−1), ecosystem respiration (Re) (0.23 ± 0.38 kg C m−2 yr−1), net primary production (NPP) (0.14 ± 0.33 kg C m−2 yr−1), autotrophic respiration (Ra) (0.09 ± 0.20 kg C m−2 yr−1), heterotrophic respiration (Rh) (0.14 ± 0.20 kg C m−2 yr−1), net ecosystem exchange (NEE) (−0.01 ± 0.19 kg C m−2 yr−1), and CH4 flux (2.52 ± 4.02 g CH4 m−2 yr−1). There were no consistent spatial patterns in the larger Alaskan Arctic and boreal regional carbon stocks and fluxes, with some models showing NEE for Alaska as a strong carbon sink, others as a strong carbon source, while still others as carbon neutral. Finally, AmeriFlux data are used at two sites in the Alaskan Arctic to evaluate the regional patterns; observed seasonal NEE was captured within multi-model uncertainty. This assessment of carbon cycle uncertainties may be used as a baseline for the improvement of experimental and modeling activities, as well as a reference for future trajectories in carbon cycling with climate change in the Alaskan Arctic and larger boreal region