587 research outputs found
Dissimilar bouncy walkers
We consider the dynamics of a one-dimensional system consisting of dissimilar
hardcore interacting (bouncy) random walkers. The walkers' (diffusing
particles') friction constants xi_n, where n labels different bouncy walkers,
are drawn from a distribution rho(xi_n). We provide an approximate analytic
solution to this recent single-file problem by combining harmonization and
effective medium techniques. Two classes of systems are identified: when
rho(xi_n) is heavy-tailed, rho(xi_n)=A xi_n^(-1-\alpha) (0<alpha<1) for large
xi_n, we identify a new universality class in which density relaxations,
characterized by the dynamic structure factor S(Q,t), follows a Mittag-Leffler
relaxation, and the the mean square displacement of a tracer particle (MSD)
grows as t^delta with time t, where delta=alpha/(1+\alpha). If instead rho is
light-tailedsuch that the mean friction constant exist, S(Q,t) decays
exponentially and the MSD scales as t^(1/2). We also derive tracer particle
force response relations. All results are corroborated by simulations and
explained in a simplified model.Comment: 11 pages, to appear in Journal of Chemical Physic
Aging dynamics in interacting many-body systems
Low-dimensional, complex systems are often characterized by logarithmically
slow dynamics. We study the generic motion of a labeled particle in an ensemble
of identical diffusing particles with hardcore interactions in a strongly
disordered, one-dimensional environment. Each particle in this single file is
trapped for a random waiting time with power law distribution
, such that the values are
independent, local quantities for all particles. From scaling arguments and
simulations, we find that for the scale-free waiting time case ,
the tracer particle dynamics is ultra-slow with a logarithmic mean square
displacement (MSD) . This extreme
slowing down compared to regular single file motion is due to the high likelihood that the labeled
particle keeps encountering strongly immobilized neighbors. For the case
we observe the MSD scaling , where we recover Harris law
.Comment: 5 pages, 4 figure
Diabetes Mellitus as a Model of Psychosomatic and Somatopsychic Interrelationships
The article reviews research on the problem of interrelationship between different physical and psychosocial factors in type 1 diabetes mellitus (DM1). The authors consider methodological principles of health-related quality of life (HRQoL) assessment in DM1 patients and stress the need for an integrated biopsychosocial approach to the management of the disease. DM1 is a chronic metabolic disease with an absolute requirement for insulin replacement therapy. The stress inducing nature of DM1 is associated with its unexpected and dramatic manifestation in juvenile years, life-threatening nature of severe hypo-/hyperglycaemias and long-term complications, with the burden of diabetes self-management, threat of work disability, employment and career problems etc. These features of DM1 increase the likelihood of the development of anxiety and depressive disorders, which, in turn, may negatively influence the course of diabetes and in particular, diabetes self-care. This necessitates early diagnosis of emotional and behavioral disturbances in DM1 using self-report instruments as well as clinical assessment. Evidence suggests that active problem-focused coping behavior and adequate social support promote adherence to diabetes regimes and may act as a buffer against negative effects of the disease on HRQoL in DM1 patients. The core element in the HRQoL structure is personal disease picture (as opposed by objective clinical picture) – the cognitive-affectivebehavioral complex reflecting the patient’s personal perception of the disease. Examination of the personal disease picture and attitude towards the ailment in DM1 patients may help to improve understanding of the mechanisms of poor adjustment. Problems in disease adjustment can be detected also by diabetes-specific HRQoL assessment. The measures of HRQoL can be applied as screening instruments useful in increasing the effectiveness of patient-provider interactions and diabetes care.El artículo revisa la investigación del problema de la interrelación entre diferentes factores físicos y psicosociales en la diabetes melitus tipo 1 (DM1). Los autores consideran los principios de la evaluación de la calidad de vida relacionada con la salud (CVRS) en pacientes de DM1 y ponen de manifiesto la necesidad de un abordaje biopsicosocial integrado del manejo de la enfermedad. La DM1 es una enfermedad metabólica crónica con una necesidad absoluta de terapia de reemplazo de insulina. La naturaleza estresante de la DM1 se asocia con su aparición inesperada y dramática durante los años de juventud, la naturaleza peligrosa de las hipo e hiperglucemias y las complicaciones a largo plazo, con la carga del auto-cuidado de la diabetes, la amenaza de discapacidad laboral, problemas de empleo y carrera, etc. Estas características de la DM1 aumentan la probabilidad de desarrollar trastornos de ansiedad y depresivos que, a su vez, pueden afectar negativamente el curso de la diabetes y, en particular, el auto-cuidado de la diabetes. Esto requiere un diagnóstico temprano de los trastornos emocionales y comportamentales en los enfermos de DM1, empleando instrumentos de auto informe además de la evaluación clínica. La evidencia sugiere que el comportamiento de afrontamiento activo centrado en los problemas y el apoyo social adecuado promocionan la adherencia a los regímenes diabéticos y pueden amortiguar los efectos negativos de la enfermedad en la CVRS en los pacientes con DM1. El elemento esencial en la estructura de la CVRS es el cuadro clínico personal (en contraste con el cuadro clínico objetivo)— el complejo cognitivo-afectivo-conductual que refleja la percepción personal que tiene el paciente de su enfermedad. El examen del cuadro de enfermedad personal y la actitud hacia la enfermedad en los pacientes con DM1 puede mejorar la comprensión de los mecanismos de desadaptación. Los problemas en la adaptación a la enfermedad también pueden detectarse por medio de la evaluación de la CVRS específica para la diabetes. Las medidas de CVRS pueden aplicarse como instrumentos de criba útiles para incrementar la efectividad de las interacciones paciente-cuidador y del cuidado de la diabetes
Single-file dynamics with different diffusion constants
We investigate the single-file dynamics of a tagged particle in a system
consisting of N hardcore interacting particles (the particles cannot pass each
other) which are diffusing in a one-dimensional system where the particles have
different diffusion constants. For the two particle case an exact result for
the conditional probability density function (PDF) is obtained for arbitrary
initial particle positions and all times. The two-particle PDF is used to
obtain the tagged particle PDF. For the general N-particle case (N large) we
perform stochastic simulations using our new computationally efficient
stochastic simulation technique based on the Gillespie algorithm. We find that
the mean square displacement for a tagged particle scales as the square root of
time (as for identical particles) for long times, with a prefactor which
depends on the diffusion constants for the particles; these results are in
excellent agreement with very recent analytic predictions in the mathematics
literature.Comment: 9 pages, 5 figures. Journal of Chemical Physics (in press
Single-file diffusion with non-thermal initial conditions
Single-file diffusion is a theoretically challenging many-body problem where
the calculation of even the simplest observables, e.g. mean square
displacement, for a tracer particle requires a heavy mathematical machinery.
There is therefore a need for simple approaches which predict qualitatively
correct behaviours. Here we put forward one such method which we use to
investigate the influence of non-thermal initial conditions on the dynamics of
a tracer particle. With our new approach we reproduce, up to scaling, several
known asymptotic results for the tracer particle mean square displacement.Comment: 4 pages, 1 figur
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A Drift-Diffusion Model of Interval Timing in the Peak Procedure
Drift-diffusion models (DDMs) are a popular framework for explaining response times in decision-making tasks. Recently, the DDM architecture has been used to model interval timing. The Time-adaptive DDM (TDDM) is a physiologically plausible mechanism that adapts in real-time to different time intervals while preserving timescale invariance. One key open question is how the TDDM could deal with situations where reward is omitted, as in the peak procedure—a benchmark in the timing literature. When reward is omitted, there is a consistent pattern of correlations between the times at which animals start and stop responding. Here we develop a formulation of the TDDM’s stationary properties that allows for the derivation of such correlations analytically. Using this simplified formulation we show that a TDDM with two thresholds–one to mark the start of responding and another the stop–can reproduce the same pattern of correlations observed in the data, as long as the start threshold is allowed to be noisy. We confirm this by running simulations with the standard TDDM formulation and show that the simplified formulation approximates well the full model under steady-state conditions. Moreover, we show that this simplified version of the TDDM is formally equivalent to Scalar Expectancy Theory (SET) under stationary behaviours, the most prominent theory of interval timing. This equivalence establishes the TDDM as a more complete drift-diffusion based theory with SET as a special case under steady-state conditions
Turns in transmembrane helices – determination of the minimal length of a “helical hairpin” and derivation of a fine-grained turn propensity scale
We have recently reported a first experimental turn propensity scale for transmembrane helices. This scale was derived from measurements of how efficiently a given residue placed in the middle of a 40 residue poly(Leu) stretch induces the formation of a "helical hairpin" with two rather than one transmembrane segment. We have now extended these studies, and have determined the minimum length of a poly(Leu) stretch compatible with the formation of a helical hairpin. We have also derived a more fine-grained turn propensity scale by (i) introducing each of the 20 amino acid residues into the middle of the shortest poly(Leu) stretch compatible with helical hairpin formation, and (ii) introducing pairs of residues in the middle of the 40 residue poly(Leu) stretch. The new turn propensities are consistent with the amino acid frequencies found in short hairpin loops in membrane proteins of known 3D structure
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