Showing 141 - 150 of 733 Items
Wave transmission in time- and space-variant helicoidal phononic crystals
Date: 2014-11-04
Creator: F. Li, C. Chong, J. Yang, P. G. Kevrekidis, C., Daraio
Access: Open access
- We present a dynamically tunable mechanism of wave transmission in one-dimensional helicoidal phononic crystals in a shape similar to DNA structures. These helicoidal architectures allow slanted nonlinear contact among cylindrical constituents, and the relative torsional movements can dynamically tune the contact stiffness between neighboring cylinders. This results in cross-talking between in-plane torsional and out-of-plane longitudinal waves. We numerically demonstrate their versatile wave mixing and controllable dispersion behavior in both wavenumber and frequency domains. Based on this principle, a suggestion toward an acoustic configuration bearing parallels to a transistor is further proposed, in which longitudinal waves can be switched on and off through torsional waves.
The letter of the law: Administrative discretion and Obama's domestic unilateralism
Date: 2014-04-01
Creator: Andrew Rudalevige
Access: Open access
- In his 2014 State of the Union address Barack Obama pledged to act without Congress on a variety of fronts, following up his "we can't wait" campaign of unilateralism before the 2012 election. The partisan furor this engendered tended to obscure the longstanding efforts of presidents to "faithfully execute" the law in a manner that aligns with their policy preferences. This paper examines the broad logic of those efforts, and delineates five areas where the Obama administration has been particularly aggressive: in its (1) recess appointments; (2) refusal to defend federal law (notably, the Defense of Marriage Act) in court; (3) use of prosecutorial discretion in declining to pursue violations of immigration and drug laws; (4) use of waivers; and (5) its utilization of the regulatory process to interpret the meaning of statutes, as with the Clean Air Act and the Affordable Care Act. Presidents do have flexibility in many cases; but this ends where they seek to alter the plain "letter of the law.".
Sea star inspired crawling and bouncing
Date: 2020-01-01
Creator: Sina Heydari, Amy Johnson, Olaf Ellers, Matthew J. McHenry, Eva, Kanso
Access: Open access
- The oral surface of sea stars is lined with arrays of tube feet that enable them to achieve highly controlled locomotion on various terrains. The activity of the tube feet is orchestrated by a nervous system that is distributed throughout the body without a central brain. How such a distributed nervous system produces a coordinated locomotion is yet to be understood. We develop mathematical models of the biomechanics of the tube feet and the sea star body. In the model, the feet are coupled mechanically through their structural connection to a rigid body. We formulate hierarchical control laws that capture salient features of the sea star nervous system. Namely, at the tube foot level, the power and recovery strokes follow a state-dependent feedback controller. At the system level, a directionality command is communicated through the nervous system to all tube feet. We study the locomotion gaits afforded by this hierarchical control model. We find that these minimally coupled tube feet coordinate to generate robust forward locomotion, reminiscent of the crawling motion of sea stars, on various terrains and for heterogeneous tube feet parameters and initial conditions. Our model also predicts a transition from crawling to bouncing consistently with recent experiments. We conclude by commenting on the implications of these findings for understanding the neuromechanics of sea stars and their potential application to autonomous robotic systems.
The validity of the kdv approximation in case of resonances arising from periodic media
Date: 2011-11-15
Creator: Christopher Chong, Guido Schneider
Access: Open access
- It is the purpose of this short note to discuss some aspects of the validity question concerning the Korteweg-de Vries (KdV) approximation for periodic media. For a homogeneous model possessing the same resonance structure as it arises in periodic media we prove the validity of the KdV approximation with the help of energy estimates. © 2011 Elsevier Inc.
Recommendations for obtaining unbiased chlorophyll estimates from in situ chlorophyll fluorometers: A global analysis of WET Labs ECO sensors
Date: 2017-06-01
Creator: Collin Roesler, Julia Uitz, Hervé Claustre, Emmanuel Boss, Xiaogang, Xing, Emanuele Organelli, Nathan Briggs, Annick Bricaud, Catherine Schmechtig, Antoine Poteau, Fabrizio D'Ortenzio, Josephine Ras, Susan Drapeau, Nils Haëntjens, Marie Barbieux
Access: Open access
- Chlorophyll fluorometers provide the largest in situ global data set for estimating phytoplankton biomass because of their ease of use, size, power consumption, and relatively low price. While in situ chlorophyll a (Chl) fluorescence is proxy for Chl a concentration, and hence phytoplankton biomass, there exist large natural variations in the relationship between in situ fluorescence and extracted Chl a concentration. Despite this large natural variability, we present here a global validation data set for the WET Labs Environmental Characterization Optics (ECO) series chlorophyll fluorometers that suggests a factor of 2 overestimation in the factory calibrated Chl a estimates for this specific manufacturer and series of sensors. We base these results on paired High Pressure Liquid Chromatography (HPLC) and in situ fluorescence match ups for which non-photochemically quenched fluorescence observations were removed. Additionally, we examined matchups between the factory-calibrated in situ fluorescence and estimates of chlorophyll concentration determined from in situ radiometry, absorption line height, NASA's standard ocean color algorithm as well as laboratory calibrations with phytoplankton monocultures spanning diverse species that support the factor of 2 bias. We therefore recommend the factor of 2 global bias correction be applied for the WET Labs ECO sensors, at the user level, to improve the global accuracy of chlorophyll concentration estimates and products derived from them. We recommend that other fluorometer makes and models should likewise undergo global analyses to identify potential bias in factory calibration.
Analytical representation of a black hole puncture solution
Date: 2007-03-13
Creator: Thomas W. Baumgarte, Stephen G. Naculich
Access: Open access
- The "moving-puncture" technique has led to dramatic advancements in the numerical simulations of binary black holes. Hannam et al. have recently demonstrated that, for suitable gauge conditions commonly employed in moving-puncture simulations, the evolution of a single black hole leads to a well-known, time-independent, maximal slicing of Schwarzschild spacetime. They construct the corresponding solution in isotropic coordinates numerically and demonstrate its usefulness, for example, for testing and calibrating numerical codes that employ moving-puncture techniques. In this brief report we point out that this solution can also be constructed analytically, making it even more useful as a test case for numerical codes. © 2007 The American Physical Society.
Three members of a peptide family are differentially distributed and elicit differential state-dependent responses in a pattern generator-effector system
Date: 2018-05-01
Creator: Patsy S. Dickinson, Matthew K. Armstrong, Evyn S. Dickinson, Rebecca Fernandez, Alexandra, Miller, Sovannarath Pong, Brian W. Powers, Alixander Pupo-Wiss, Meredith E. Stanhope, Patrick J. Walsh, Teerawat Wiwatpanit, Andrew E. Christie
Access: Open access
- C-type allatostatins (AST-Cs) are pleiotropic neuropeptides that are broadly conserved within arthropods; the presence of three AST-C isoforms, encoded by paralog genes, is common. However, these peptides are hypothesized to act through a single receptor, thereby exerting similar bioactivities within each species. We investigated this hypothesis in the American lobster, Homarus americanus, mapping the distributions of AST-C isoforms within relevant regions of the nervous system and digestive tract, and comparing their modulatory influences on the cardiac neuromuscular system. Immunohistochemistry showed that in the pericardial organ, a neuroendocrine release site, AST-C I and/or III and AST-C II are contained within distinct populations of release terminals. Moreover, AST-C I/III-like immunoreactivity was seen in midgut epithelial endocrine cells and the cardiac ganglion (CG), whereas AST-C II-like immunoreactivity was not seen in these tissues. These data suggest that AST-C I and/or III can modulate the CG both locally and hormonally; AST-C II likely acts on the CG solely as a hormonal modulator. Physiological studies demonstrated that all three AST-C isoforms can exert differential effects, including both increases and decreases, on contraction amplitude and frequency when perfused through the heart. However, in contrast to many state-dependent modulatory changes, the changes in contraction amplitude and frequency elicited by the AST-Cs were not functions of the baseline parameters. The responses to AST-C I and III, neither of which is COOH-terminally amidated, are more similar to one another than they are to the responses elicited by AST-C II, which is COOH-terminally amidated. These results suggest that the three AST-C isoforms are differentially distributed in the lobster nervous system/midgut and can elicit distinct behaviors from the cardiac neuromuscular system, with particular structural features, e.g., COOH-terminal amidation, likely important in determining the effects of the peptides. NEW & NOTEWORTHY Multiple isoforms of many peptides exert similar effects on neural circuits. In this study we show that each of the three isoforms of C-type allatostatin (AST-C) can exert differential effects, including both increases and decreases in contraction amplitude and frequency, on the lobster cardiac neuromuscular system. The distribution of effects elicited by the nonamidated isoforms AST-C I and III are more similar to one another than to the effects of the amidated AST-C II.
Related neuropeptides use different balances of unitary mechanisms to modulate the cardiac neuromuscular system in the American lobster, Homarus americanus
Date: 2015-01-01
Creator: Patsy S. Dickinson, Andrew Calkins, Jake S. Stevens
Access: Open access
- To produce flexible outputs, neural networks controlling rhythmic motor behaviors can be modulated at multiple levels, including the pattern generator itself, sensory feedback, and the response of the muscle to a given pattern of motor output. We examined the role of two related neuropeptides, GYSDRNYLRFamide (GYS) and SGRNFLRFamide (SGRN), in modulating the neurogenic lobster heartbeat, which is controlled by the cardiac ganglion (CG). When perfused though an isolated whole heart at low concentrations, both peptides elicited increases in contraction amplitude and frequency. At higher concentrations, both peptides continued to elicit increases in contraction amplitude, but GYS caused a decrease in contraction frequency, while SGRN did not alter frequency. To determine the sites at which these peptides induce their effects, we examined the effects of the peptides on the periphery and on the isolated CG. When we removed the CG and stimulated the motor nerve with constant bursts of stimuli, both GYS and SGRN increased contraction amplitude, indicating that each peptide modulates the muscle or the neuromuscular junction. When applied to the isolated CG, neither peptide altered burst frequency at low peptide concentrations; at higher concentrations, SGRN decreased burst frequency, whereas GYS continued to have no effect on frequency. Together, these data suggest that the two peptides elicit some of their effects using different mechanisms; in particular, given the known feedback pathways within this system, the importance of the negative (nitric oxide) relative to the positive (stretch) feedback pathways may differ in the presence of the two peptides.
Prediction of a neuropeptidome for the eyestalk ganglia of the lobster Homarus americanus using a tissue-specific de novo assembled transcriptome
Date: 2017-03-01
Creator: Andrew E. Christie, Vittoria Roncalli, Matthew C. Cieslak, Micah G. Pascual, Andy, Yu, Tess J. Lameyer, Meredith E. Stanhope, Patsy S. Dickinson
Access: Open access
- In silico transcriptome mining is a powerful tool for crustacean peptidome prediction. Using homology-based BLAST searches and a simple bioinformatics workflow, large peptidomes have recently been predicted for a variety of crustaceans, including the lobster, Homarus americanus. Interestingly, no in silico studies have been conducted on the eyestalk ganglia (lamina ganglionaris, medulla externa, medulla interna and medulla terminalis) of the lobster, although the eyestalk is the location of a major neuroendocrine complex, i.e., the X-organ-sinus gland system. Here, an H. americanus eyestalk ganglia-specific transcriptome was produced using the de novo assembler Trinity. This transcriptome was generated from 130,973,220 Illumina reads and consists of 147,542 unique contigs. Eighty-nine neuropeptide-encoding transcripts were identified from this dataset, allowing for the deduction of 62 distinct pre/preprohormones. Two hundred sixty-two neuropeptides were predicted from this set of precursors; the peptides include members of the adipokinetic hormone-corazonin-like peptide, allatostatin A, allatostatin B, allatostatin C, bursicon α, CCHamide, corazonin, crustacean cardioactive peptide, crustacean hyperglycemic hormone (CHH), CHH precursor-related peptide, diuretic hormone 31, diuretic hormone 44, eclosion hormone, elevenin, FMRFamide-like peptide, glycoprotein hormone α2, glycoprotein hormone β5, GSEFLamide, intocin, leucokinin, molt-inhibiting hormone, myosuppressin, neuroparsin, neuropeptide F, orcokinin, orcomyotropin, pigment dispersing hormone, proctolin, pyrokinin, red pigment concentrating hormone, RYamide, short neuropeptide F, SIFamide, sulfakinin, tachykinin-related peptide and trissin families. The predicted peptides expand the H. americanus eyestalk ganglia neuropeptidome approximately 7-fold, and include 78 peptides new to the lobster. The transcriptome and predicted neuropeptidome described here provide new resources for investigating peptidergic signaling within/from the lobster eyestalk ganglia.
Distinct or shared actions of peptide family isoforms: II. Multiple pyrokinins exert similar effects in the lobster stomatogastric nervous system
Date: 2015-09-01
Creator: Patsy S. Dickinson, Sienna C. Kurland, Xuan Qu, Brett O. Parker, Anirudh, Sreekrishnan, Molly A. Kwiatkowski, Alex H. Williams, Alexandra B. Ysasi, Andrew E. Christie
Access: Open access
- Many neuropeptides are members of peptide families, with multiple structurally similar isoforms frequently found even within a single species. This raises the question of whether the individual peptides serve common or distinct functions. In the accompanying paper, we found high isoform specificity in the responses of the lobster (Homarus americanus) cardiac neuromuscular system to members of the pyrokinin peptide family: only one of five crustacean isoforms showed any bioactivity in the cardiac system. Because previous studies in other species had found little isoform specificity in pyrokinin actions, we examined the effects of the same five crustacean pyrokinins on the lobster stomatogastric nervous system (STNS). In contrast to our findings in the cardiac system, the effects of the five pyrokinin isoforms on the STNS were indistinguishable: they all activated or enhanced the gastric mill motor pattern, but did not alter the pyloric pattern. These results, in combination with those from the cardiac ganglion, suggest that members of a peptide family in the same species can be both isoform specific and highly promiscuous in their modulatory capacity. The mechanisms that underlie these differences in specificity have not yet been elucidated; one possible explanation, which has yet to be tested, is the presence and differential distribution of multiple receptors for members of this peptide family.