183 research outputs found
An axiomatic approach to electromagnetic and gravitational radiation reaction of particles in curved spacetime
The problem of determining the electromagnetic and gravitational
``self-force'' on a particle in a curved spacetime is investigated using an
axiomatic approach. In the electromagnetic case, our key postulate is a
``comparison axiom'', which states that whenever two particles of the same
charge have the same magnitude of acceleration, the difference in their
self-force is given by the ordinary Lorentz force of the difference in their
(suitably compared) electromagnetic fields. We thereby derive an expression for
the electromagnetic self-force which agrees with that of DeWitt and Brehme as
corrected by Hobbs. Despite several important differences, our analysis of the
gravitational self-force proceeds in close parallel with the electromagnetic
case. In the gravitational case, our final expression for the (reduced order)
equations of motion shows that the deviation from geodesic motion arises
entirely from a ``tail term'', in agreement with recent results of Mino et al.
Throughout the paper, we take the view that ``point particles'' do not make
sense as fundamental objects, but that ``point particle equations of motion''
do make sense as means of encoding information about the motion of an extended
body in the limit where not only the size but also the charge and mass of the
body go to zero at a suitable rate. Plausibility arguments for the validity of
our comparison axiom are given by considering the limiting behavior of the
self-force on extended bodies.Comment: 37 pages, LaTeX with style package RevTeX 3.
Frontal traumatic brain injury in rats causes long-lasting impairments in impulse control that are differentially sensitive to pharmacotherapeutics and associated with chronic neuroinflammation.
Traumatic brain injury (TBI) affects millions yearly, and is increasingly associated with chronic neuropsychiatric symptoms. We assessed the long-term effects of different bilateral frontal controlled cortical impact injury severities (mild, moderate, severe) on the five-choice serial reaction time task, a paradigm with relatively independent measurements of attention, motor impulsivity and motivation. Moderately- and severely-injured animals exhibited impairments across all cognitive domains that were still evident 14 weeks post-injury, while mild-injured animals only demonstrated persistent deficits in impulse control. However, recovery of function varied considerably between subjects such that some showed no impairment (âTBI-resilientâ), some demonstrated initial deficits that recovered (âTBI-vulnerableâ) and some never recovered (âchronically-impairedâ). Three clinically-relevant treatments for impulsecontrol or TBI, amphetamine, atomoxetine, and amantadine, were assessed for efficacy in treating injury-induced deficits. Susceptibility to TBI affected the response to pharmacological challenge with amphetamine. Whereas sham and TBI-resilient animals showed characteristic impairments in impulse control at higher doses, amphetamine had the opposite effect in chronically-impaired rats, improving task performance. In contrast, atomoxetine and amantadine reduced premature responding but increased omissions, suggesting psychomotor slowing. Analysis of brain tissue revealed that generalized neuroinflammation was associated with impulsivity even when accounting for the degree of brain damage. This is one of the first studies to characterize psychiatric-like symptoms in experimental TBI. Our data highlight the importance of testing pharmacotherapies in TBI models in order to predict efficacy, and suggest that neuroinflammation may represent a treatment target for impulse control problems following injury
Angular momentum exchange during secular migration of two-planet systems
We investigate the secular dynamics of two-planet coplanar systems evolving
under mutual gravitational interactions and dissipative forces. We consider two
mechanisms responsible for the planetary migration: star-planet (or
planet-satellite) tidal interactions and interactions of a planet with a
gaseous disc. We show that each migration mechanism is characterized by a
specific law of orbital angular momentum exchange. Calculating stationary
solutions of the conservative secular problem and taking into account the
orbital angular momentum leakage, we trace the evolutionary routes followed by
the planet pairs during the migration process. This procedure allows us to
recover the dynamical history of two-planet systems and constrain parameters of
the involved physical processes.Comment: 20 pages, 9 figures, accepted for publication in Celestial Mechanics
and Dynamical Astronomy (special issue on Exoplanets
Desempenho de Lecanicillium lecanii em meios de cultura contendo vitaminas e concentraçÔes de extrato de levedura
Parùmetros ruminais e desaparecimento da FDN da forragem em bovinos suplementados em pastagem na estação das åguas
Cultivation of juvenile fat snook (Centropomus parallelus Poey, 1860) fed probiotic in laboratory conditions
Herpetofauna da årea do Igarapé Esperança na Reserva Extrativista Riozinho da Liberdade, Acre - Brasil
Post-acute COVID-19 neuropsychiatric symptoms are not associated with ongoing nervous system injury
A proportion of patients infected with severe acute respiratory syndrome coronavirus 2 experience a range of neuropsychiatric symptoms months after infection, including cognitive deficits, depression and anxiety. The mechanisms underpinning such symptoms remain elusive. Recent research has demonstrated that nervous system injury can occur during COVID-19. Whether ongoing neural injury in the months after COVID-19 accounts for the ongoing or emergent neuropsychiatric symptoms is unclear. Within a large prospective cohort study of adult survivors who were hospitalized for severe acute respiratory syndrome coronavirus 2 infection, we analysed plasma markers of nervous system injury and astrocytic activation, measured 6 months post-infection: neurofilament light, glial fibrillary acidic protein and total tau protein. We assessed whether these markers were associated with the severity of the acute COVID-19 illness and with post-acute neuropsychiatric symptoms (as measured by the Patient Health Questionnaire for depression, the General Anxiety Disorder assessment for anxiety, the Montreal Cognitive Assessment for objective cognitive deficit and the cognitive items of the Patient Symptom Questionnaire for subjective cognitive deficit) at 6 months and 1 year post-hospital discharge from COVID-19. No robust associations were found between markers of nervous system injury and severity of acute COVID-19 (except for an association of small effect size between duration of admission and neurofilament light) nor with post-acute neuropsychiatric symptoms. These results suggest that ongoing neuropsychiatric symptoms are not due to ongoing neural injury
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