Cartan's spiral staircase in physics and, in particular, in the gauge theory of dislocations

In 1922, Cartan introduced in differential geometry, besides the Riemannian curvature, the new concept of torsion. He visualized a homogeneous and isotropic distribution of torsion in three dimensions (3d) by the "helical staircase", which he constructed by starting from a 3d Euclidean space and by defining a new connection via helical motions. We describe this geometric procedure in detail and define the corresponding connection and the torsion. The interdisciplinary nature of this subject is already evident from Cartan's discussion, since he argued - but never proved - that the helical staircase should correspond to a continuum with constant pressure and constant internal torque. We discuss where in physics the helical staircase is realized: (i) In the continuum mechanics of Cosserat media, (ii) in (fairly speculative) 3d theories of gravity, namely a) in 3d Einstein-Cartan gravity - this is Cartan's case of constant pressure and constant intrinsic torque - and b) in 3d Poincare gauge theory with the Mielke-Baekler Lagrangian, and, eventually, (iii) in the gauge field theory of dislocations of Lazar et al., as we prove for the first time by arranging a suitable distribution of screw dislocations. Our main emphasis is on the discussion of dislocation field theory.Comment: 31 pages, 8 figure

Altered Transition Between Agonist-and Antagonist-Favoring States of Μ-Opioid Receptor in Brain Membranes with Modified Microviscosity

In unmodified synaptosomal brain membranes the presence of NaCl inhibited the binding to Μ receptors of the tritiated opioid agonists etorphine, Tyr-D-Ala-Gly-(Me)Phe-Gly-ol, and sufentanil by 53, 43, and 37%, respectively, and increased that of the antagonist [ 3 H]naltrexone by 54%. On the other hand, in membranes whose microviscosity was increased by incorporation of cholesteryl hemi-succinate (CHS) the effects of sodium on opioid agonist and antagonist binding were abolished and strongly reduced, respectively. Furthermore, in the modified membranes the ability of sodium to protect the opioid receptor from inactivation by the sulfhydryl-reactive agent N -ethyl-maleimide (NEM) was diminished. In CHS-treated membranes whose elevated microviscosity was reduced by the incorporation of oleic acid, the effectiveness of sodium in modulating opioid binding and attenuating receptor inactivation by NEM was restored. The results implicate membrane microviscosity in the mechanism by which sodium modulates the conversion between agonist-and antagonist-favoring states of Μ opioid receptor.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65931/1/j.1471-4159.1993.tb03630.x.pd

Altered Microviscosity at Brain Membrane Surface Induces Distinct and Reversible Inhibition of Opioid Receptor Binding

In synaptosomal membranes from rat and monkey brain cortex, the addition of petroselenic (18:1, cis -Δ 6 ) acid, oleic (18:1, m-Δ 9 ) acid, and vaccenic (18:1, cis -Δ 11 ) acid or their corresponding methyl esters at 0.5 Μmol/mg of membrane protein caused a similar 7–10% decrease in the microviscosity of the membrane core, whereas at the membrane surface the microviscosity was reduced 5–7% by the fatty acids but only 1% by their methyl esters. Concomi-tantly, the fatty acids, but not the methyl esters, inhibited the specific binding of the tritiated Μ-, Δ-, and K-opioids Tyr-D-Ala-Gly-(Me)Phe-Gly-ol (DAMGO), [D-Pen 2 ,D-Pen 5 ]-enkephalin (DPDPE), and U69,593, respectively. As shown with oleic acid, the sensitivity of opioid receptor binding toward inhibition by fatty acids was in the order Δ > ΜΚ k , whereby the binding of [ 3 H]DPDPE was abolished, but significant inhibition of [ 3 H]U69,593 binding, determined in membranes from monkey brain, required membrane modification with a twofold higher fatty acid concentration. Except for the unchanged K D of [ 3 H]U69,593, the inhibition by oleic acid involved both the B max and affinity of opioid binding. Cholesteryl hemisuccinate (0.5–3 Μmol/mg of protein), added to membranes previously modified by fatty acids, reversed the fluidization caused by the latter compounds and restored inhibited Μ-, Δ-, and k -opioid binding toward control values. In particular, the B max of [ 3 H]-DPDPE binding completely recovered after being undetectable. The results implicate membrane surface fluidity in the modulation of opioid receptor binding, reveal distinct sensitivity of Δ, Μ, and K receptors toward that modulation, and identify unsaturated fatty acids and cholesterol as possible endogenous regulators of opioid receptor function.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/66380/1/j.1471-4159.1992.tb08432.x.pd

Kontrolirano izgaranje vodika i kisika u plamenu pri visokom tlaku

In this article the results of experiments of technology of controlled combustion of hydrogen-oxygen flame are presented during the special conditions in high-pressure chamber. The measurement of achieved pressures and temperatures at the controlled process of combustion of hydrogen and oxygen flame was realised as temperature measurement in calibration zone, temperature measurement at the front face of the head, temperature measurement in the pressure chamber and pressure measurement in the pressure chamber.U ovom članku prikazani su rezultati eksperimenata tehnološki kontroliranog izgaranja mješavine vodika i kisika u plamenu tijekom posebnih uvjeta u visokotlačnoj komori. Mjerenja ostvarenih tlakova i temperatura pri kontroliranom procesu izgaranja vodika i kisika u plamenu realizirana su kao mjerenje temperature u zoni kalibracije, mjerenje temperature na prednjoj strani glave, mjerenje temperature u tlačnoj komori i mjerenje tlaka u tlačnoj komori

Submesoscale Turbulence over a Topographic Slope

Regions of the ocean near continental slopes are linked to significant vertical velocities caused by advection over a sloping bottom, frictional processes and diffusion. Oceanic motions at submesoscales are also characterized by enhanced vertical velocities, as compared to mesoscale motions, due to greater contributions from ageostrophic flows. These enhanced vertical velocities can make an important contribution to turbulent fluxes. Sloping topography may also induce large-scale potential vorticity gradients by modifying the slope of interior isopycnal surfaces. Potential vorticity gradients, in turn, may feed back on mesoscale stirring and the generation of submesoscale features. In this study, we explore the impact of sloping topography on the characteristics of submesoscale motions. We conduct high-resolution (1 km × 1 km) simulations of a wind-driven frontal current over an idealized continental shelf and slope. We explore changes in the magnitude, skewness and spectra of surface vorticity and vertical velocity across different configurations of the topographic slope and wind-forcing orientations. All of these properties are strongly modulated by the background topography. Furthermore, submesoscale characteristics exhibit spatial variability across the continental shelf and slope. We find that changes in the statistical properties of submesoscale motions are linked to mesoscale stirring responding to differences in the interior potential vorticity distributions, which are set by frictional processes at the ocean surface and over the sloping bottom. Improved parameterizations of submesoscale motions over topography may be needed to simulate the spatial variability of these features in coarser-resolution models, and are likely to be important to represent vertical nutrient fluxes in coastal waters

Performance analysis of cables with attached tuned-inerter-dampers

Cables are structural elements designed to bear tensile forces and experience vibration problems due to their slenderness and low mass. In the field of civil engineering, they are mostly used in bridges where the vibrations are mainly induced by wind, rain, traffic and earthquakes. This paper proposes the use of a tuned-inerter-damper (TID) system, mounted on cables to suppress unwanted vibrations. These are to be attached transversally to the cable, in the vicinity of the support, connected between the deck and the cable. The potential advantage of using a TID system consists in the high apparent mass that can be produced by the inerter. Our analysis showed that the modal damping ratio obtained is much higher than in the case of traditional dampers or tuned mass dampers, leading to an improved overall response. An optimal tuning methodology is also discussed. Numerical results are shown with a cable subjected to both free and forced vibrations and the TID performance is improved when compared with equivalent dampers

Untangling perceptual memory: Hysteresis and adaptation map into separate cortical networks

Perception is an active inferential process in which prior knowledge is combined with sensory input, the result of which determines the contents of awareness. Accordingly, previous experience is known to help the brain "decide" what to perceive. However, a critical aspect that has not been addressed is that previous experience can exert 2 opposing effects on perception: An attractive effect, sensitizing the brain to perceive the same again (hysteresis), or a repulsive effect, making it more likely to perceive something else (adaptation). We used functional magnetic resonance imaging and modeling to elucidate how the brain entertains these 2 opposing processes, and what determines the direction of such experience-dependent perceptual effects. We found that although affecting our perception concurrently, hysteresis and adaptation map into distinct cortical networks: a widespread network of higher-order visual and fronto-parietal areas was involved in perceptual stabilization, while adaptation was confined to early visual areas. This areal and hierarchical segregation may explain how the brain maintains the balance between exploiting redundancies and staying sensitive to new information. We provide a Bayesian model that accounts for the coexistence of hysteresis and adaptation by separating their causes into 2 distinct terms: Hysteresis alters the prior, whereas adaptation changes the sensory evidence (the likelihood function)