Wide Range Of Tensions Research Articles

Mechanical Principles Governing the Shapes of Dendritic Spines.

Dendritic spines are small, bulbous protrusions along the dendrites of neurons and are sites of excitatory postsynaptic activity. The morphology of spines has been implicated in their function in synaptic plasticity and their shapes have been well-characterized, but the potential mechanics underlying their shape development and maintenance have not yet been fully understood. In this work, we explore the mechanical principles that could underlie specific shapes using a minimal biophysical model of membrane-actin interactions. Using this model, we first identify the possible force regimes that give rise to the classic spine shapes—stubby, filopodia, thin, and mushroom-shaped spines. We also use this model to investigate how the spine neck might be stabilized using periodic rings of actin or associated proteins. Finally, we use this model to predict that the cooperation between force generation and ring structures can regulate the energy landscape of spine shapes across a wide range of tensions. Thus, our study provides insights into how mechanical aspects of actin-mediated force generation and tension can play critical roles in spine shape maintenance.

The Politics of Postcolonial Erasure in Sarajevo

To observers of contemporary Bosnia-Herzegovina, the years 2013–2016 have been marked by heightened public discontent and the emergence of civil protests. While early protests surrounded sites unmistakably pertaining to the country's socialist legacy, such as privatized factories, more recent dissatisfaction is being articulated around sites that point to Bosnia's past as a colony of the Hapsburg Empire. This essay argues it is not accidental that these colonial markers have become instigators of protest and dissent articulated through the memory of the recent Socialist past. Under the claim of cultural heritage protection, former colonial sites in Bosnia-Herzegovina and throughout the Balkans are unearthed tangibly to mark new realities of postcoloniality. The resurrection of Hapsburg heritage in Bosnia is instructive as it helps illuminate the wide range of tensions and contradictions inherent in the move from colonial Europe to a (post)colonial European Union. Far from wiping the slate clean and offering a fresh start, as EU actors often claim, this process is paradoxically reinstalling institutions that were once the embodiment of colonial expansion. In arguing for the expansion of postcolonial critique to include the Balkans, this essay examines the political, economic, and symbolic ways in which Hapsburg colonial sites and institutions are restored into public visibility as registers of Sarajevo's European futures.

A polarized view on DNA under tension

In the past decades, sensitive fluorescence microscopy techniques have contributed significantly to our understanding of the dynamics of DNA. The specific labeling of DNA using intercalating dyes has allowed for quantitative measurement of the thermal fluctuations the polymers undergo. On the other hand, recent advances in single-molecule manipulation techniques have unraveled the mechanical and elastic properties of this intricate polymer. Here, we have combined these two approaches to study the conformational dynamics of DNA under a wide range of tensions. Using polarized fluorescence microscopy in conjunction with optical-tweezers-based manipulation of YOYO-intercalated DNA, we controllably align the YOYO dyes using DNA tension, enabling us to disentangle the rapid dynamics of the dyes from that of the DNA itself. With unprecedented control of the DNA alignment, we resolve an inconsistency in reports about the tilted orientation of intercalated dyes. We find that intercalated dyes are on average oriented perpendicular to the long axis of the DNA, yet undergo fast dynamics on the time scale of absorption and fluorescence emission. In the overstretching transition of double-stranded DNA, we do not observe changes in orientation or orientational dynamics of the dyes. Only beyond the overstretching transition, a considerable depolarization is observed, presumably caused by an average tilting of the DNA base pairs. Our combined approach thus contributes to the elucidation of unique features of the molecular dynamics of DNA.

Effect of entropy on the nucleation of cavitation bubbles in water under tension.

Water can exist in a metastable liquid state under tension for long times before the system relaxes into the vapor via cavitation, i.e., bubble nucleation. Microscopic information on the cavitation process can be extracted from experimental data by the use of the nucleation theorem, which relates measured cavitation rates to the size of the critical bubble. To apply the nucleation theorem to experiments performed along an isochoric path, for instance, in cavitation experiments in mineral inclusions, knowledge of the bubble entropy is required. Using computer simulations, we compute the entropy of bubbles in water as a function of their volume over a wide range of tensions from free energy calculations. We find that the bubble entropy is an important contribution to the free energy that significantly lowers the barrier to bubble nucleation, thereby facilitating cavitation. Furthermore, the bubble entropy per surface area depends on the curvature of the liquid-vapor interface, decreasing approximately linearly with its mean curvature over the studied range of bubble volumes. At room temperature, the entropy of a flat liquid-vapor interface at ambient pressure is very similar to that of critical bubbles over a wide range of tensions, which justifies the use of the former as an approximation when interpreting data from experiments. Based on our simulation results, we obtain an estimate for the volume of the critical bubble from experimentally measured cavitation rates.

Tension is required but not sufficient for focal adhesion maturation without a stress fiber template

Focal adhesion composition and size are modulated in a myosin II-dependent maturation process that controls adhesion, migration, and matrix remodeling. As myosin II activity drives stress fiber assembly and enhanced tension at adhesions simultaneously, the extent to which adhesion maturation is driven by tension or altered actin architecture is unknown. We show that perturbations to formin and α-actinin 1 activity selectively inhibited stress fiber assembly at adhesions but retained a contractile lamella that generated large tension on adhesions. Despite relatively unperturbed adhesion dynamics and force transmission, impaired stress fiber assembly impeded focal adhesion compositional maturation and fibronectin remodeling. Finally, we show that compositional maturation of focal adhesions could occur even when myosin II-dependent cellular tension was reduced by 80%. We propose that stress fiber assembly at the adhesion site serves as a structural template that facilitates adhesion maturation over a wide range of tensions. This work identifies the essential role of lamellar actin architecture in adhesion maturation.

Local constraints on cosmic string loops from photometry and pulsar timing

We constrain the cosmological density of cosmic string loops using two observational signatures---gravitational microlensing and the Kaiser-Stebbins effect. Photometry from RXTE and CoRoT space missions and pulsar timing from Parkes Pulsar Timing Array, Arecibo and Green Bank radio telescopes allow us to probe cosmic strings in a wide range of tensions $G\ensuremath{\mu}/{c}^{2}={10}^{\ensuremath{-}16}\textdiv{}{10}^{\ensuremath{-}10}$. We find that pulsar timing data provide the most stringent constraints on the abundance of light strings at the level ${\ensuremath{\Omega}}_{s}\ensuremath{\sim}{10}^{\ensuremath{-}3}$. Future observational facilities such as the Square Kilometer Array will allow one to improve these constraints by orders of magnitude.

Quasar variability limits on cosmological density of cosmic strings

We put robust upper limits on the average cosmological density ${\ensuremath{\Omega}}_{s}$ of cosmic strings based on the variability properties of a large homogeneous sample of Sloan Digital Sky Survey quasars. We search for an excess of characteristic variations of quasar brightness that are associated with string lensing and use the observed distribution of these variations to constrain the density of strings. The limits obtained do not invoke any clustering of strings. They apply to both open segments and closed loops of strings, usefully extend over a wide range of tensions ${10}^{\ensuremath{-}13}<G\ensuremath{\mu}/{c}^{2}<{10}^{\ensuremath{-}9}$, and reach down to the level of ${\ensuremath{\Omega}}_{s}=0.01$ and below. Further progress in this direction will depend on better understanding of quasar intrinsic variability rather than a mere increase in the volume of data.

Determination of the viscoelasticity of polyamide yarns over a wide tension range

A method of determining the deformation characteristics of polycaproamide complex yarns over a wide range of extension (4–8%) by short-term measurement of the parameters of the relaxation process has been developed. A modernization of the determining nonlinear viscoelasticity equation has been carried out, and a procedure based on it has been developed for the calculated prediction of stretching diagrams over a wide range of extension at an assigned rate. It is also recommended to use the procedure developed for the mathematical description of more complex deformation regimes.

Stiffness of cat soleus muscle and tendon during activation of part of muscle.

Experiments have been carried out on the soleus muscle and its tendon in the anesthetized cat. Measurements of isometric tension and muscle stiffness were made during contraction of whole or part of the muscle in response to stimulation of ventral root filaments. In an attempt to determine the distribution of tension in different portions of the tendon during activation of only part of the muscle, the free tendon of insertion was split longitudinally into two halves and a strain gauge attached to each piece. From a large number of measurements, it was found that the mean fraction of tension recorded in one-half of the tendon remained about the same, over a wide range of tensions. However, the scatter of values, which increased as the portion of muscle contracting was reduced, was greater than expected if muscle fibers were randomly distributed throughout the muscle. Measurements of muscle and tendon stiffness were made from length and tension changes during stretch of the actively contracting muscle. Ventral root stimulation that engaged 20% or more of the muscle yielded a value for tendon compliance (0.09 mm/N), which was the same as for stimulating the whole muscle. This result suggested that for contraction of portions as small as 20% of the muscle, fibers were effectively attached to the whole tendon, indicating that tendinous attachments of individual muscle fibers ran independent of one another over only a short distance and were bound together over most of their remaining course. It was concluded that groups of muscle fibers selected by stimulation of ventral root filaments are not entirely randomly distributed throughout the muscle. However, for groups representing larger fractions of the total tension, (greater than 20%) the distribution is uniform enough and the connections between their tendinous attachments firm enough for the force applied by such a group to act through a tendon compliance, which is the same as that seen by the whole muscle.

Tension minima and activation in frog twitch muscle fibres

Responses to restimulation at different times during isometric relaxation were recorded from single muscle fibres at 15 degrees C. The reactivation latency was constant over a wide range of tensions but the time (tmin) between restimulation and the subsequent tension minimum always varied by several milliseconds. Variation in tmin was directly correlated with the rate of relaxation at the time of restimulation and was inversely related to tension throughout most of the phase of relaxation. The results are discussed in relation to excitation-contraction coupling and it is shown that the tension minima did not collectively represent the time course of activation of the sarcomere.

THE TECHNOLOGY OF THE PRODUCTION OF FALSE-TWIST TEXTURED YARNS

Experimental work in the production of false-twist textured yarns using a versatile apparatus is described. Measurements of tension are made at various stages of the process on two systems using different methods of feeding the yarn. The distribution of twist throughout the set-up is discussed. Some measurements of tension are also made on yarns given a simple thermal treatment only. Yarns have been made at a wide range of tensions, twists, and temperatures and are compared in terms of their visual appearance, breaking load and breaking extension, residual shrinkage, stretch, recovery behaviour, specific volume and change in denier on processing.

RELATIONS BETWEEN PRE- AND POST-ANAEROBIC OXYGEN CONSUMPTION AND OXYGEN TENSION IN SOME FRESH WATER SNAILS

1. The respiration of four species of fresh water snails was somewhat dependent on the oxygen tension below 21 per cent oxygen with only minor differences among the various species.2. The degree of dependency was influenced little, if any, by alteration of the metabolic rate of Australorbis glabratus, but some changes were obtained in the cases of Helisoma duryi and Lymnaea stagnalis.3. All species showed a long lasting respiratory rebound after 16 hours anaerobiosis. The post-anaerobic respiration was more dependent on the oxygen tension than the pre-anaerobic respiration if referred to the normal rate shown at 21 per cent oxygen. But if the post-anaerobic rates were compared with the rates sustained pre-anaerobically at an identical oxygen tension, an approximately equal percentage increase was observed over a wide range of tensions.4. The implications of these observations and auxiliary observations dealing with the rate of the heart-beat under various conditions are discussed insofar as they shed light on the mechanism of the pre- and post-anaerobic respiration. It is concluded that diffusion alone cannot be the sole limiting factor that reduces the over-all oxygen consumption when the tension is lowered below a critical point, and the idea is discussed that the oxygen tension as such may be more important for certain cellular processes than usually assumed.

Relations between Roll-force, Torque, and the Applied Tensions in Strip-rolling

Expressions are derived for the roll-force P, and torque G, in strip-rolling under tension, as functions of the pressure distribution on the rolls and of the radius of the deformed arc of contact. By consideration of various artificial distributions of pressure the ratio G/P is shown to depend only slightly on the precise shape of the friction hill, and a linear relation is deduced between G/P and T/P, where T is the difference of the front and back tensions. This is confirmed by experiments on annealed mild-steel strip and it is found also that T/P is approximately equal to the angular arc-of-contact when the torque is zero (Steckel rolling). A more detailed investigation of the effects of tension, based on the simplest possible hypothesis, indicates that a back tension, Tb, increases the torque per roll by , while a front tension, T f, decreases it by , where r is the fractional reduction in strip thickness and R is the undeformed roll-radius. The theory indicates also that the roll-force decreases linearly with both front and back tensions. Experimental data confirm the theoretical predictions of linearity, and show that the formula for the effect on torque is accurate over a wide range of tensions.

OBSERVATIONS ON THE RESPIRATION OF TRYPANOSOMA CRUZI IN CULTURE

1. The oxygen consumption of cultural forms of Trypanosoma cruzi decreases in intensity with increasing age of the cultures; no correlation with any other factor studied could be established. 2. The respiratory quotient was high for the first 10 days, i.e. as long as the population increased; with the onset of a decline in numbers, the R.Q. began to drop. It is believed that the flagellates consume in the beginning predominantly sugar and later predominantly protein. Observations on the pH of the cultures bear out this view. 3. The oxygen consumption was independent of the oxygen tension over a wide range of tensions. 4. The oxygen consumption increased in the temperature range 13 degrees to 40 degrees C., while a temperature of 44 degrees C. proved to be lethal. Upon application of Arrhenius' equation, two straight lines, intersecting at about 28 degrees C., resulted. The micro values were 23,980 and 5275 for the lower and higher temperature range respectively. 5. Of the oxidase inhibitors tested, strong inhibition of the oxygen consumption was achieved with azide, cyanide, and hydrogen sulfide. Pyrophosphate had no influence at all. There is some probability that cytochrome oxidase is the chief oxidase present. 6. The strongest inhibitory influence due to dehydrogenase inhibitors was observed with propyl carbamate and high concentrations of ethyl carbamate. 7. A small fraction of the oxygen consumption, about 10 per cent, may be due to substances with sulfhydryl groups, as indicated by a slight but distinct inhibition due to dilute iodoacetate and to arsenite.