68,600 research outputs found

    Identification of Hindbrain Neural Substrates for Motor Initiation in the hatchling Xenopus laevis Tadpole

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    Animal survival profoundly depends on the ability to detect stimuli in the environment, process them and respond accordingly. In this respect, motor responses to a sensory stimulation evolved into a variety of coordinated movements, which involve the control of brain centres over spinal locomotor circuits. The hatchling Xenopus tadpole, even in its embryonic stage, is able to detect external sensory information and to swim away if the stimulus is considered noxious. To do so, the tadpole relies on well-known ascending sensory pathway, which carries the sensory information to the brain. When the stimulus is strong enough, descending interneurons are activated, leading to the excitation of spinal CPG neurons, which causes the undulatory movement of swimming. However, the activation of descending interneurons that marks the initiation of motor response appears after a long delay from the sensory stimulation. Furthermore, the long-latency response is variable in time, as observed in the slow-summating excitation measured in descending interneurons. These two features, i.e. long-latency and variability, cannot be explained by the firing time and pattern of the ascending sensory pathway of the Xenopus tadpole. Therefore, a novel neuronal population has been proposed to lie in the hindbrain of the tadpole, and being able to 'hold' the sensory information, thus accounting for the long and variable delay of swim initiation. In this work, the role of the hindbrain in the maintenance of the long and variable response to trunk skin stimulation is investigated in the Xenopustadpole at developmental stage 37/38. A multifaceted approach has been used to unravel the neuronal mechanisms underlying the delayed motor response, including behavioural experiments, electrophysiology analysis of fictive swimming, hindbrain extracellular recordings and imaging experiments. Two novel neuronal populations have been identified in the tadpole's hindbrain, which exhibit activation patterns compatible with the role of delaying the excitation of the spinal locomotor circuit. Future work on cellular properties and synaptic connections of these newly discovered populations might shed light on the mechanism of descending control active at embryonic stage. Identifying supraspinal neuronal populations in an embryonic organism could aid in understanding mechanisms of descending motor control in more complex vertebrates

    Optimizing the lateral beamforming step for filtered-delay multiply and sum beamforming to improve active contour segmentation using ultrafast ultrasound imaging

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    As an alternative to delay-and-sum beamforming, a novel beamforming technique called filtered-delay multiply and sum (FDMAS) was introduced recently to improve ultrasound B-mode image quality. Although a considerable amount of work has been performed to evaluate FDMAS performance, no study has yet focused on the beamforming step size, , in the lateral direction. Accordingly, the performance of FDMAS was evaluated in this study by fine-tuning to find its optimal value and improve boundary definition when balloon snake active contour (BSAC) segmentation was applied to a B-mode image in ultrafast imaging. To demonstrate the effect of altering in the lateral direction on FDMAS, measurements were performed on point targets, a tissue-mimicking phantom and in vivo carotid artery, by using the ultrasound array research platform II equipped with one 128-element linear array transducer, which was excited by 2-cycle sinusoidal signals. With 9-angle compounding, results showed that the lateral resolution (LR) of the point target was improved by 67.9% and 81.2%, when measured at −6 dB and −20 dB respectively, when was reduced from to . Meanwhile the image contrast ratio (CR) measured on the CIRS phantom was improved by 10.38 dB at the same reduction and the same number of compounding angles. The enhanced FDMAS results with lower side lobes and less clutter noise in the anechoic regions provides a means to improve boundary definition on a B-mode image when BSAC segmentation is applied

    Tunable polaritons of spiral nanowire metamaterials

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    The tunable spiral nanowire metamaterial design at optical frequency is presented, and the surface polaritons are theoretically studied. It was found that the dispersions of the polaritons could be tuned by varying physical dimensions of the spiral nanowire metamaterial. This geometry is unique. Doing so, one may dynamically control the properties of surface polaritons. In addition, the Ferrell–Berreman modes can be excited that is impossible with the regular nanowire metamaterials having the circular cross-section of the nanowires. Herein, the presence of Ferrell–Berreman branches is confirmed by the performed analysis of the metamaterial band structure. It is worthwhile noting, that existence of Ferrell–Berreman modes is possible without epsilon-near-zero (ENZ) regime. The design of devices where Ferrell–Berreman modes can be exploited for practical applications ranging from plasmonic sensing to imaging and absorption enhancement is possible because of the propagation constant revealing subtle microscopic resonances

    A Review of Fibre Reinforced Polymer Structures

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    This paper reviews Fibre Reinforced Polymer (FRP) composites in Civil Engineering applications. Three FRP types are used in Structural Engineering: FRP profiles for new construction, FRP rebars and FRP strengthening systems. Basic materials (fibres and resins), manufacturing processes and material properties are discussed. The focus of the paper is on all-FRP new-build structures and their joints. All-FRP structures use pultruded FRP profiles. Their connections and joints use bolting, bonding or a combination of both. For plate-to-pate connections, effects of geometry, fibre direction, type and rate of loading, bolt torque and bolt hole clearance, and washers on failure modes and strength are reviewed. FRP beam-columns joints are also reviewed. The joints are divided into five categories: web cleated, web and flange cleated, high strength, plate bolted and box profile joints. The effect of both static and cyclic loading on joints is studied. The joints’ failure modes are also discussed

    Quantitative analysis of phase transitions in two-dimensional XY models using persistent homology

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    We use persistent homology and persistence images as an observable of three different variants of the two-dimensional XY model in order to identify and study their phase transitions. We examine models with the classical XY action, a topological lattice action, and an action with an additional nematic term. In particular, we introduce a new way of computing the persistent homology of lattice spin model configurations and, by considering the fluctuations in the output of logistic regression and k-nearest neighbours models trained on persistence images, we develop a methodology to extract estimates of the critical temperature and the critical exponent of the correlation length. We put particular emphasis on finite-size scaling behaviour and producing estimates with quantifiable error. For each model we successfully identify its phase transition(s) and are able to get an accurate determination of the critical temperatures and critical exponents of the correlation length

    Examining the Potential for Isotope Analyses of Carbon, Nitrogen, and Sulphur in Burned Bone from Experimental and Archaeological Contexts.

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    The aim of this project was to determine whether isotope analyses of carbon, nitrogen and sulphur can be conducted on collagen extracted from burned bone. This project was conducted in two phases: a controlled heating experiment and an archaeological application. The controlled heating experiment used cow (Bos taurus) bone to test the temperature thresholds for the conservation of δ13C, δ15N, and δ34S values. These samples were also used to test the efficacy of Fourier Transform Infrared spectroscopy (FTIR) and colour analysis, for determining the burning intensities experienced by bone burned in unknown conditions. The experiment showed that δ13C values were relatively unchanged up to 400°C (<2‰ variation), while δ15N values were relatively stable up to 200°C (0.5‰ variation). Values of δ34S were also relatively stable up to 200°C (1.4‰ variation). Colour change and FTIR data were well correlated with the change in isotope ratios. Models estimating burning intensities were created from the FTIR data. For the archaeological application, samples were selected from two early Anglo-Saxon cemetery sites: Elsham and Cleatham. Samples were selected from both inhumed and cremated individuals. Among the inhumed individuals δ13C values suggested a C3 terrestrial diet and δ15N values suggested protein derived largely from terrestrial herbivores, as expected for the early Anglo-Saxon period. However, δ34S values suggested the consumption of freshwater resources and that this consumption was related to both the age and sex of the individual. The experimental data shows that there is potential for isotope analyses of cremated remains, as during the cremation process heat exposures are not uniform across the body. The samples selected for the archaeological application, however, were not successful. Bone samples heated in controlled conditions produced viable collagen for isotope analysis; however, there are several differences between experiments conducted in a muffle furnace and open-air pyre cremation that need to be investigated further. Additionally, the influence of taphonomy on collagen survival in burned bone needs to be quantified. Finally, methods of sample selection need to be improved to find bone samples from archaeologically cremated remains that are most likely to retain viable collagen. While there is significant research that must be conducted before this research can be widely applied there are a multitude of cultures that practised cremation throughout history and around the world that could be investigated through the analyses proposed in this project
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