1,372 research outputs found
Stochastic population growth in spatially heterogeneous environments
Classical ecological theory predicts that environmental stochasticity
increases extinction risk by reducing the average per-capita growth rate of
populations. To understand the interactive effects of environmental
stochasticity, spatial heterogeneity, and dispersal on population growth, we
study the following model for population abundances in patches: the
conditional law of given is such that when is small the
conditional mean of is approximately , where and are the abundance and per
capita growth rate in the -th patch respectivly, and is the
dispersal rate from the -th to the -th patch, and the conditional
covariance of and is approximately . We show for such a spatially extended population that if
is the total population abundance, then ,
the vector of patch proportions, converges in law to a random vector
as , and the stochastic growth rate equals the space-time average per-capita growth rate
\sum_i\mu_i\E[Y_\infty^i] experienced by the population minus half of the
space-time average temporal variation \E[\sum_{i,j}\sigma_{ij}Y_\infty^i
Y_\infty^j] experienced by the population. We derive analytic results for the
law of , find which choice of the dispersal mechanism produces an
optimal stochastic growth rate for a freely dispersing population, and
investigate the effect on the stochastic growth rate of constraints on
dispersal rates. Our results provide fundamental insights into "ideal free"
movement in the face of uncertainty, the persistence of coupled sink
populations, the evolution of dispersal rates, and the single large or several
small (SLOSS) debate in conservation biology.Comment: 47 pages, 4 figure
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
Keeping vigil over the profession: a grounded theory of the context of nurse anaesthesia practice
<p>Abstract</p> <p>Background</p> <p>Nurse anaesthetists in the US have faced continued, repeated challenges to their profession. Regardless, they have met these challenges and have established themselves as major anaesthesia care providers. In this paper we address the research question: How do certified registered nurse anaesthetists (CRNAs) manage the socio-political context in which they provide care for their patients?</p> <p>Methods</p> <p>Grounded theory was used to explore how nurse anaesthetists protect and promote their profession. Purposive, snowball, and theoretical sampling was used and data were collected through participant observation and interviews conducted at a conference of the professional association, an educational program, by telephone, email exchanges, and time spent in operating rooms and an outpatient surgical clinic. Analysis included coding at increasingly abstract levels and constant comparison.</p> <p>Results</p> <p>The basic social process identified was Keeping Vigil Over the Profession, which explains how nurse anaesthetists protect and promote their profession. It is comprised of three contextual categories: Establishing Public Credibility through regulatory and educational standards, Political Vigilance and taking action in governmental and policy arenas, and Tending the Flock through a continuous information loop between local and administrative/political levels.</p> <p>Conclusions</p> <p>From our study of the context of nurse anaesthesia practice, it is clear that CRNAs are dedicated to protecting their ability to provide high quality patient care by maintaining constant vigilance over their profession.</p
DNA-based Self-Assembly of Chiral Plasmonic Nanostructures with Tailored Optical Response
Surface plasmon resonances generated in metallic nanostructures can be
utilized to tailor electromagnetic fields. The precise spatial arrangement of
such structures can result in surprising optical properties that are not found
in any naturally occurring material. Here, the designed activity emerges from
collective effects of singular components equipped with limited individual
functionality. Top-down fabrication of plasmonic materials with a predesigned
optical response in the visible range by conventional lithographic methods has
remained challenging due to their limited resolution, the complexity of
scaling, and the difficulty to extend these techniques to three-dimensional
architectures. Molecular self-assembly provides an alternative route to create
such materials which is not bound by the above limitations. We demonstrate how
the DNA origami method can be used to produce plasmonic materials with a
tailored optical response at visible wavelengths. Harnessing the assembly power
of 3D DNA origami, we arranged metal nanoparticles with a spatial accuracy of 2
nm into nanoscale helices. The helical structures assemble in solution in a
massively parallel fashion and with near quantitative yields. As a designed
optical response, we generated giant circular dichroism and optical rotary
dispersion in the visible range that originates from the collective
plasmon-plasmon interactions within the nanohelices. We also show that the
optical response can be tuned through the visible spectrum by changing the
composition of the metal nanoparticles. The observed effects are independent of
the direction of the incident light and can be switched by design between left-
and right-handed orientation. Our work demonstrates the production of complex
bulk materials from precisely designed nanoscopic assemblies and highlights the
potential of DNA self-assembly for the fabrication of plasmonic nanostructures.Comment: 5 pages, 4 figure
Hypercapnic cerebral edema presenting in a woman with asthma: a case report
<p>Abstract</p> <p>Introduction</p> <p>Common causes of non-traumatic acute cerebral edema include malignant hypertension, hyponatremia, anoxia, and cerebral vascular accident. The computed tomographic images and data obtained during care of the patient described in this case report provide evidence that hypercarbia can cause increased intracranial pressure and coma without permanent brain injury. Partial pressure of carbon dioxide evaluation for coma is essential to provide faster diagnosis and therapeutic correction in certain common critical disease states. We present the case of a patient in a coma associated with cerebral edema during a typical asthma exacerbation with hypercapnic respiratory failure.</p> <p>Case presentation</p> <p>An obese 63-year-old African American woman with asthma presented to our hospital with facial swelling and shortness of breath. Immediately following intubation for hypercapnic respiratory failure, she was noted to have a dilated, unresponsive right pupil. An emergent computed tomographic head scan revealed that she had increased intracranial pressure. A neurosurgeon agreed with the computed tomography interpretation and recommended no surgical intervention. The patient's respiratory acidosis was corrected with ventilatory management over several hours in the intensive care unit. Nine and one-half hours later a follow-up head computed tomographic scan was read as normal without cerebral edema. At 12 hours, the patient's right pupil was 5 mm in diameter and reactive. By 24 hours, her pupils were symmetrically equal and reactive. Her symptoms had improved, and she was extubated. A brain magnetic resonance imaging scan revealed no abnormalities.</p> <p>Conclusion</p> <p>Alteration of consciousness related to hypercapnia during respiratory failure is not generally thought to be related to cerebral edema. Respiratory acidosis resulting from hypercarbia is known to produce carbon dioxide narcosis and coma, but no current treatment algorithm suggests that rapid hypercapnia correction can be critical to neurologic outcome. To the best of our knowledge, our case is a unique example of the physiological changes that may occur in relation to arterial carbon dioxide concentration in the normal brain in the setting of typical hypercapnic respiratory failure. Correction of respiratory acidosis reversed the neurologic symptoms and physiology causing cerebral edema and coma in our patient. Rare similar cases have been sporadically reported in the medical literature, typically in children. Our case is also unusual in that rapid deterioration and clinical status were directly observed on simultaneous computed tomographic scans. Had this patient been found unresponsive, or had she had brief respiratory or cardiac arrest, the scan could have been interpreted as global anoxic injury leading to a different therapeutic course.</p
Measuring Dissociation Rate Constants of Protein Complexes through Subunit Exchange: Experimental Design and Theoretical Modeling
Protein complexes are dynamic macromolecules that constantly dissociate into, and simultaneously are assembled from, free subunits. Dissociation rate constants, koff, provide structural and functional information on protein complexes. However, because all existing methods for measuring koff require high-quality purification and specific modifications of protein complexes, dissociation kinetics has only been studied for a small set of model complexes. Here, we propose a new method, called Metabolically-labeled Affinity-tagged Subunit Exchange (MASE), to measure koff using metabolic stable isotope labeling, affinity purification and mass spectrometry. MASE is based on a subunit exchange process between an unlabeled affinity-tagged variant and a metabolically-labeled untagged variant of a complex. The subunit exchange process was modeled theoretically for a heterodimeric complex. The results showed that koff determines, and hence can be estimated from, the observed rate of subunit exchange. This study provided the theoretical foundation for future experiments that can validate and apply the MASE method
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