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On the optical detection of the physical state of excitable membranes
Get PDFPhase transitions in biological systems are controversially discussed. As the origin of nonlinearities, they have been suggested responsible for cellular functions including nerve pulse propagation.
In pure lipid interfaces characteristic functions, such as permeability, are modulated during a transition. These relations could have drastic implications for cells because their membranes are lipid-based. However, evidence for these transitions in cellular membranes of excitable cells - which are involved in the transmission of nerve pulses - has not been provided so far.
Within this thesis thermodynamic phase states in lipid-based interfaces are characterized based on the use of a fluorescent dye (Atto488-DPPE) as local state reporter. Optical state diagrams of artificial, lipid interfaces are recorded, and state dependent kinetics investigated.
Upon the straightforward application of this method to single, neuronal cells, a nonlinearity in the optical response is detected within the cellular membrane and identified as phase transition.
The transition is extraordinary sharp (1°C) and sensitive to pH variations in the extracellular buffer.
The existence of distinct physical phase states in cellular membranes and their highly nonlinear characteristic provides strong evidence that the membrane state is indeed crucial for excitability and conduction of nerve pulses. The results further underline that the membrane state has the potential to modify cell functionality in general as it is subject to modulation by physiologically important parameters such as pH.Phasenübergänge in biologischen Systemen sind Gegenstand zahlreicher Diskussion. Als Ursprung von Nichtlinearitäten werden sie für zelluläre Funktionen einschließlich der Ausbreitung von Nervenimpulsen verantwortlich gemacht. In reinen Lipidmembranen werden charakteristische Funktionen, wie Permeabilität, während eines Phasenübergangs moduliert.
Diese Zusammenhänge könnten drastische Auswirkungen auf Zellen haben, da ihre Membranen lipidbasiert sind. Ein Nachweis für Phasenübergänge in Zellmembranen erregbarer Zellen - die an der Weiterleitung von Nervenimpulsen beteiligt sind - wurde bisher jedoch nicht erbracht.
In dieser Arbeit wird ein optischer Ansatz zur Charakterisierung thermodynamischer Phasenzustände in lipidbasierten Grenzflächen angewandt. Er basiert auf der Verwendung von Fluoreszenzfarbstoffen als lokale Reporter für Phasenzustände. Optische Zustandsdiagramme von Lipidgrenzflächen werden aufgenommen und zustandsabhängige Kinetiken innerhalb der Lipidmembranen untersucht.
Die Anwendung dieser Methodik auf einzelne neuronale Zellen zeigt eine charakteristische Nichtlinearität in der optischen Antwort des Farbstoffs (Atto488-DPPE), der auf Grundlage der Charakterisierung von Lipidmembranen als Phasenumwandlung identifiziert wird. Diese Übergänge sind außergewöhnlich scharf (1°C) und sind sensitiv auf Änderungen im pH. Der hier erbrachte Nachweis der hochgradigen Nichtlinearität liefert die Grundlage dafür, dass der Membranzustand für die Erregbarkeit und Weiterleitung von Nervenimpulsen entscheidend sein kann
Toward neuromorphic computing using longitudinal pulses in a fluid near phase transition
Get PDFLongitudinal waves propagate information about the stimulus in multiple dimensions, including the medium density and pressure. Pulses that reversibly cross a phase transition have a nonlinear response that resembles properties of neuronal signaling. This multidimensionality suggests that longitudinal pulses may be harnessed for in-materio computation, mimicking biological or artificial neural algorithms. To explore a feedforward physical neural network using longitudinal pulses, we demonstrate the implementation of (1) a complete set of logic gates, (2) classification of data, and (3) regression of a mathematical function. Our results illustrate the potential of harnessing nonlinear longitudinal waves—common in a plethora of materials—for the purpose of computation
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
It sounds like an action potential: unification of electrical, chemical and mechanical aspects of acoustic pulses in lipids
No full textSimilarities between action potentials and acoustic pulses in a van der Waals fluid
Get PDFAn action potential is typically described as a purely electrical change that propagates along the membrane of excitable cells. However, recent experiments have demonstrated that non-linear acoustic pulses that propagate along lipid interfaces and traverse the melting transition, share many similar properties with action potentials. Despite the striking experimental similarities, a comprehensive theoretical study of acoustic pulses in lipid systems is still lacking. Here we demonstrate that an idealized description of an interface near phase transition captures many properties of acoustic pulses in lipid monolayers, as well as action potentials in living cells. The possibility that action potentials may better be described as acoustic pulses in soft interfaces near phase transition is illustrated by the following similar properties: correspondence of time and velocity scales, qualitative pulse shape, sigmoidal response to stimulation amplitude (an ‘all-or-none’ behavior), appearance in multiple observables (particularly, an adiabatic change of temperature), excitation by many types of stimulations, as well as annihilation upon collision. An implication of this work is that crucial functional information of the cell may be overlooked by focusing only on electrical measurements
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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