300 research outputs found
Fatigue in U.S. Astronauts Onboard the International Space Station: Environmental Factors, Operational Impacts, and Implementation of Countermeasures
Since 2000, US astronauts have been supporting missions up to a six month duration on the International Space Station (ISS). Crewmembers have experienced fatigue for reasons similar to military deployments. Astronauts experience psychological stressors such as heavy workloads, extended duty periods, circadian misalignment, inadequate/ineffective sleep, and loss of the environmental cues of a gravity environment. Complicating the psychological stressors are environmental factors; distracting background noise, unexpected and variable mission schedules, unfavorable thermal control, elevated CO2 levels, and an unusual sleep environment with schedules that impinge on presleep periods. Physiological contributors to poor sleep and fatigue include a cephalad fluid shift and back pain. Restful sleep is further challenged due to a lack of gravityrelated proprioceptive cues and need for restraints. The term "space fog" has been used by astronauts to describe a phenomenon of forgetfulness, slowed reaction time and transient confusion while trying to complete tasks. There is a distinct temporal correlation with arrival on the Space Station and the onset of slowed cognitive skills and a spontaneous resolution that may take up to 6 weeks. The Genesis of this phenomenon may be chronic fatigue secondary to transitioning from a planar environment to a 360deg microgravity perspective. Recently, countermeasures to improve sleep duration and quality in astronauts on the ISS have been instituted with moderate degrees of success as measured by selfreaction time (psychomotor vigilance task testing), actigraphy, and subjective reports. Judicious use of stimulants and hypnotics, light therapy, controlled sleep periods and sleep shifting and reducing ambient CO2 levels are a few of the most promising countermeasures being used in space to improve sleep and reduce fatigue
Setting a VO2 Max Standard for NASA Astronauts During Spaceflight
Aerobic fitness is best measured by Maximal Aerobic Capacity or VO2 Max which is defined as a measure of oxygen utilization and transport. Increased Vo2 max indicates improved oxygen consumption during high level exercise and is widely accepted as a predictor of an individual's likelihood of successfully completing a demanding task. As such, agencies and organizations have adopted VO2 max as part of a comprehensive set of physical requirements. The purpose of this study is to review the literature and existing medical and occupational VO2 max data, to propose a VO2 max standard for NASA astronauts for training and spaceflight
A dynamical trichotomy for structured populations experiencing positive density-dependence in stochastic environments
Positive density-dependence occurs when individuals experience increased
survivorship, growth, or reproduction with increased population densities.
Mechanisms leading to these positive relationships include mate limitation,
saturating predation risk, and cooperative breeding and foraging. Individuals
within these populations may differ in age, size, or geographic location and
thereby structure these populations. Here, I study structured population models
accounting for positive density-dependence and environmental stochasticity i.e.
random fluctuations in the demographic rates of the population. Under an
accessibility assumption (roughly, stochastic fluctuations can lead to
populations getting small and large), these models are shown to exhibit a
dynamical trichotomy: (i) for all initial conditions, the population goes
asymptotically extinct with probability one, (ii) for all positive initial
conditions, the population persists and asymptotically exhibits unbounded
growth, and (iii) for all positive initial conditions, there is a positive
probability of asymptotic extinction and a complementary positive probability
of unbounded growth. The main results are illustrated with applications to
spatially structured populations with an Allee effect and age-structured
populations experiencing mate limitation
Stratified spatiotemporal chaos in anisotropic reaction-diffusion systems
Numerical simulations of two dimensional pattern formation in an anisotropic
bistable reaction-diffusion medium reveal a new dynamical state, stratified
spatiotemporal chaos, characterized by strong correlations along one of the
principal axes. Equations that describe the dependence of front motion on the
angle illustrate the mechanism leading to stratified chaos
Fast flowing populations are not well mixed
In evolutionary dynamics, well-mixed populations are almost always associated
with all-to-all interactions; mathematical models are based on complete graphs.
In most cases, these models do not predict fixation probabilities in groups of
individuals mixed by flows. We propose an analytical description in the
fast-flow limit. This approach is valid for processes with global and local
selection, and accurately predicts the suppression of selection as competition
becomes more local. It provides a modelling tool for biological or social
systems with individuals in motion.Comment: 19 pages, 8 figure
Actin-dependent vacuolar occupancy of the cell determines auxin-induced growth repression
The cytoskeleton is an early attribute of cellular life and its main components are composed of conserved proteins (Fletcher and Mullins, 2010). The actin cytoskeleton has a direct impact on cell size control in animal cells (Fletcher and Mullins, 2010; Faix et al., 1996), but its mechanistic contribution to cellular growth in plants remains largely elusive. Here, we reveal a role of actin in cell size regulation in plants. The actin cytoskeleton shows proximity to vacuoles, and the phytohormone auxin not only controls the organisation of actin filaments, but also impacts on vacuolar morphogenesis in an actin-dependent manner.
Pharmacological and genetic interference with the actin-myosin system abolishes the auxin effect on vacuoles and thus disrupts its negative influence on cellular growth. SEM-based 3D nanometre resolution imaging of the vacuoles revealed that auxin controls the constriction and luminal size of the vacuole. We show that this actin-dependent mechanism controls the relative cellular occupancy of the vacuole, thus proposing an unanticipated mechanism for cytosol homeostasis during cellular growth
Non-thermal response of YBCO thin films to picosecond THz pulses
The photoresponse of YBa2Cu3O7-d thin film microbridges with thicknesses
between 15 and 50 nm was studied in the optical and terahertz frequency range.
The voltage transients in response to short radiation pulses were recorded in
real time with a resolution of a few tens of picoseconds. The bridges were
excited by either femtosecond pulses at a wavelength of 0.8 \mu m or broadband
(0.1 - 1.5 THz) picosecond pulses of coherent synchrotron radiation. The
transients in response to optical radiation are qualitatively well explained in
the framework of the two-temperature model with a fast component in the
picosecond range and a bolometric nanosecond component whose decay time depends
on the film thickness. The transients in the THz regime showed no bolometric
component and had amplitudes up to three orders of magnitude larger than the
two-temperature model predicts. Additionally THz-field dependent transients in
the absence of DC bias were observed. We attribute the response in the THz
regime to a rearrangement of vortices caused by high-frequency currents
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