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At least 73 records · Page 4Linked to original sources
PubMed2002

Perception--action coupling model for human locomotor pointing.

How do humans achieve the precise positioning of the feet during walking, for example, to reach the first step of a stairway? We addressed this question at the visuomotor integration level. Based on the optical specification of the required adaptation, a dynamical system model of the visuomotor control of human locomotor pointing was devised for the positioning of a foot on a visible target on the floor during walking. Visuomotor integration consists of directly linking optical information to a motor command that specifically modulates step length in accordance with the ongoing dynamics of locomotor pattern generation. The adaptation of locomotion emerges from a perception-action coupling type of control based on temporal information rather than on feedforward planning of movements. The proposed model reproduces experimental results obtained for human locomotor pointing.

Foot↗
PubMed2006

Plasmon coupling in nanorod assemblies: optical absorption, discrete dipole approximation simulation, and exciton-coupling model.

The shape anisotropy of nanorods gives rise to two distinct orientational modes by which nanorods can be assembled, i.e., end-to-end and side-by-side, analogous to the well-known H and J aggregation in organic chromophores. Optical absorption spectra of gold nanorods have earlier been observed to show a red-shift of the longitudinal plasmon band for the end-to-end linkage of nanorods, resulting from the plasmon coupling between neighboring nanoparticles, similar to the assembly of gold nanospheres. We observe, however, that side-by-side linkage of nanorods in solution shows a blue-shift of the longitudinal plasmon band and a red-shift of the transverse plasmon band. Optical spectra calculated using the discrete dipole approximation method were used to simulate plasmon coupling in assembled nanorod dimers. The longitudinal plasmon band is found to shift to lower energies for end-to-end assembly, but a shift to higher energies is found for the side-by-side orientation, in agreement with the optical absorption experiments. The strength of plasmon coupling was seen to increase with decreasing internanorod distance and an increase in the number of interacting nanorods. For both side-by-side and end-to-end assemblies, the strength of the longitudinal plasmon coupling increases with increasing nanorod aspect ratio as a result of the increasing dipole moment of the longitudinal plasmon. For both the side-by-side and end-to-end orientation, the simulation of a dimer of nanorods having dissimilar aspect ratios showed a longitudinal plasmon resonance with both a blue-shifted and a red-shifted component, as a result of symmetry breaking. A similar result is observed for a pair of similar aspect ratio nanorods assembled in a nonparallel orientation. The internanorod plasmon coupling scheme concluded from the experimental results and simulations is found to be qualitatively consistent with the molecular exciton coupling theory, which has been used to describe the optical spectra of H and J aggregates of organic molecules. The coupled nanorod plasmons are also suggested to be electromagnetic analogues of molecular orbitals. Investigation of the plasmon coupling in assembled nanorods is important for the characterization of optical excitations and plasmon propagation in these nanostructures. The surface plasmon resonance shift resulting from nanorod assembly also offers a promising alternative for analyte-sensing assays.

Absorption↗
PubMed2005

Towards a spin coupling model for the Mn4 cluster in Photosystem II.

The X-band EPR spectra of the IR sensitive untreated PSII and of MeOH- and NH(3)-treated PSII from spinach in the S(2)-state are simulated with collinear and rhombic g- and Mn-hyperfine tensors. The obtained principal values indicate a 1Mn(III)3Mn(IV) composition for the Mn(4) cluster. The four isotropic components of the Mn-hyperfine tensors are found in good agreement with the previously published values determined from EPR and (55)Mn-ENDOR data. Assuming intrinsic isotropic components of the Mn-hyperfine interactions identical to those of the Mn-catalase, spin density values are calculated. A Y-shape 4J-coupling scheme is explored to reproduce the spin densities for the untreated PSII. All the required criteria such as a S=1/2 ground state with a low lying excited spin state (30 cm(-1)) and an easy conversion to a S=5/2 system responsible for the g=4.1 EPR signal are shown to be satisfied with four antiferromagnetic interactions lying between -290 and -130 cm(-1).

Ammonia↗
PubMed2004

Coupled modeling of cement/claystone interactions and radionuclide migration.

The interactions between cement and a clayey host-rock of an underground repository for intermediate-level radioactive waste are studied with the reactive transport code HYTEC for supporting performance assessment. Care is taken in using relevant time scales (100,000 years) and dimensions. Based on a literature review, three hypotheses are considered with respect to the mineralogical composition of the claystone and the neo-formed phases. In the long term, the pH is buffered for all hypotheses and important mineral transformations occur both in cement and the host-rock. The destruction of the primary minerals is localized close to the cement/claystone interface and is characterized by the precipitation of secondary phases with retention properties (illite, zeolite). However, beyond the zone of intense mineral transformations, the pore water chemistry is also disturbed over a dozen meters due to an attenuated but continuous flux of hydroxyl, potassium and calcium ions. Four interdependent mechanisms control the profile in the whole system: diffusion of the alkaline plume, mineralogical buffering, ion exchange and clogging of the pore space at the cement/claystone interface. The migration of a selected group of radionuclides (Cs, Ra, Tc and U) is explicitly integrated in the simulations of the strongly coupled system. Theoretical profiles of distribution coefficient (Kd) and solubility limit values are derived from the simulations, and their sensitivity with respect to the system evolution is estimated.

Aluminum Silicates↗
PubMed2004

Assessment of carotenoid bioavailability of whole foods using a Caco-2 cell culture model coupled with an in vitro digestion.

Epidemiological studies have shown that consumption of carotenoid-rich fruits and vegetables is associated with a reduced risk of developing chronic diseases. beta-Carotene, alpha-carotene, and beta-cryptoxanthin are precursors of vitamin A, a nutrient essential for human health. However, little is known about the bioavailability of carotenoids from whole foods. This study characterized the intestinal uptake performance of carotenoids using monolayers of differentiated Caco-2 human intestinal cells and mimicked human digestion to assess carotenoid absorption from carrots and corn. Results showed that Caco-2 cellular uptake of beta-carotene and zeaxanthin was higher than that of lutein. Uptake performances of pure carotenoids and carotenoids from whole foods by Caco-2 cells were both curvilinear, reaching saturated levels after 4 h of incubation. The time kinetics and dose response of carotenoid uptake presented a similar pattern in Caco-2 cells after plating for 2 and 14 days. Furthermore, the applicability of this new model was verified with whole grain corn, showing that cooked corn grain significantly enhanced carotenoid bioavailability. These results support the feasibility of the in vitro digestion cell model for assessing carotenoid absorption from whole foods as a suitable and cost-effective physiological alternative to current methodologies.

Biological Availability↗
PubMed2003

Imaging coexisting fluid domains in biomembrane models coupling curvature and line tension.

Lipid bilayer membranes--ubiquitous in biological systems and closely associated with cell function--exhibit rich shape-transition behaviour, including bud formation and vesicle fission. Membranes formed from multiple lipid components can laterally separate into coexisting liquid phases, or domains, with distinct compositions. This process, which may resemble raft formation in cell membranes, has been directly observed in giant unilamellar vesicles. Detailed theoretical frameworks link the elasticity of domains and their boundary properties to the shape adopted by membranes and the formation of particular domain patterns, but it has been difficult to experimentally probe and validate these theories. Here we show that high-resolution fluorescence imaging using two dyes preferentially labelling different fluid phases directly provides a correlation between domain composition and local membrane curvature. Using freely suspended membranes of giant unilamellar vesicles, we are able to optically resolve curvature and line tension interactions of circular, stripe and ring domains. We observe long-range domain ordering in the form of locally parallel stripes and hexagonal arrays of circular domains, curvature-dependent domain sorting, and membrane fission into separate vesicles at domain boundaries. By analysing our observations using available membrane theory, we are able to provide experimental estimates of boundary tension between fluid bilayer domains.

Cell Membrane↗
PubMed1996

Simulation of the rapid regeneration of the actin-myosin working stroke with a tight coupling model of muscle contraction.

A. F. Huxley's suggestion in Nature (1992) that a structural modification in the myosin head driven by phosphate release can explain the rapid regeneration of the working stroke, which follows the quick recovery elicited by a step release of moderate size (3-6 nm per half-sarcomere), has been tested with a theoretical model. It is assumed that, in the shortening muscle, cross-bridges can undergo their work producing interaction in two ways distinct for the biochemical state and for the amount of filament sliding allowed. During shortening at low speed, as well as after a shortening step of moderate size, phosphate release from the cross-bridge in the AM-ADP-P state promotes a 100 s-1 structural change which resets the myosin head in a configuration that allows for a new complete working stroke in the AM-ADP state. In this case the total sliding distance for interaction is about 15 nm. With the increase in shortening velocity a progressively larger fraction of interacting cross-bridges remains in the AM-ADP-P state throughout the working stroke and the sliding distance for interaction is about 11 nm. Reattachment of detached cross-bridges occurs at moderate rate whichever is the pathway from which they originate. The model predicts satisfactorily the time course of the rapid regeneration of the working stroke in double step experiments, but fails to simulate the transition to the steady state response in staircase experiments, the maximum power output during steady shortening and the decrease in rate of energy liberation at high shortening velocities. These results strengthen the conclusion of our previous modelling work where we demonstrated that the condition necessary to fit the mechanical and energetic properties of shortening muscle is to assume two pathways for cross-bridge cycling distinct for the kinetics of detachment and reattachment.

Actins↗
PubMed1991

Prediction of physical aging in controlled-release coatings: the application of the relaxation coupling model to glassy cellulose acetate.

The effect of physical aging on both the water transport properties and the mechanical properties of glassy cellulose acetate was investigated. Results indicate a reduction in the mechanical rate of relaxation as well as a reduction in the water permeability as the glass ages. A model which describes the low-frequency relaxation behavior of condensed, amorphous systems is used to quantitate the mechanical relaxation data. Systematic changes in key parameters from this model signify alterations in the microscopic or short-range structure as the glass physically ages. Predictions from this model correlate quite closely with the observed water permeability reductions and thus indicate that the transport properties of glassy polymers are dependent on the structure of the glass. This approach may provide further insight into the effects of nonequilibrium behavior on pharmaceutically important properties and may serve as a basis for predicting aging and permeability changes in controlled-release dosage forms.

Cellulose↗
PubMed2005

Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model.

Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called Dansgaard-Oeschger events, with a quasi-periodicity of approximately 1,470 years for the late glacial period. Various hypotheses have been suggested to explain these rapid temperature shifts, including internal oscillations in the climate system and external forcing, possibly from the Sun. But whereas pronounced solar cycles of approximately 87 and approximately 210 years are well known, a approximately 1,470-year solar cycle has not been detected. Here we show that an intermediate-complexity climate model with glacial climate conditions simulates rapid climate shifts similar to the Dansgaard-Oeschger events with a spacing of 1,470 years when forced by periodic freshwater input into the North Atlantic Ocean in cycles of approximately 87 and approximately 210 years. We attribute the robust 1,470-year response time to the superposition of the two shorter cycles, together with strongly nonlinear dynamics and the long characteristic timescale of the thermohaline circulation. For Holocene conditions, similar events do not occur. We conclude that the glacial 1,470-year climate cycles could have been triggered by solar forcing despite the absence of a 1,470-year solar cycle.

Journal Article↗