1,720,969 research outputs found
Bidirectional Shift in the Cornu Ammonis 3 Pyramidal Dendritic Organization Following Brief Stress
Full text linkThe negative impact of chronic stress at the structure of apical dendrite branches of cornu ammonis 3 (CA3) pyramidal neurons is well established. However, there is no information available on the CA3 dendritic organization related to short-lasting stress, which suffices to produce long-term habituation or sensitization of anxiety behaviors and neuroendocrine responses. Here, we tested the effects evoked by brief stress on the arrangements of CA3 pyramidal neuron dendrites, and the activity-dependent properties of the commissural-associational (C/A) excitatory postsynaptic potentials (EPSPs). Adult male rats were socially defeated followed by 3 weeks without further treatment or as comparison exposed to a regimen of a social defeat every second day for the same time period. We assessed CA3 pyramidal neurons with somatic whole-cell recording and neurobiotin application in acute hippocampal slices. The results from morphometric analysis of post hoc reconstructions demonstrated that CA3 dendrites from repeatedly stressed rats were reduced in surface area and length selectively at the apical cone (70% of control, approximately 280 µm from the soma). Brief stress, however, produced a similar decrease in apical dendritic length (77% of control, approximately 400 µm from the soma), accompanied by an increased length (167% of control) and branch complexity at the basal cone. The structural changes of the dendrites significantly influenced signal propagation by shortening the onset latency of EPSPs and increasing input resistance (r=0.45, P<0.01), of which the first was significantly changed in repeatedly stressed animals. Both brief and repeated stress long-lastingly impaired long-term potentiation of C/A synapses to a similar degree (P<0.05). These data indicate that the geometric plasticity of CA3 dendrites is dissociated from repetition of aversive experiences. A double social conflict suffices to drive a dynamic reorganization, by site-selective elimination and de novo growth of dendrite branches over the course of weeks after the actual experience.
Biophysical Properties of Saltating Potentials in Myelinated Axons
No full textThe driving objective of this dissertation is to describe the essential biophysical structure, and consequent computational properties, of myelinated axons. In other words, if one were to engineer a brain, what elements should constitute the myelinated axon? Though important parts of the axo-glial biophysical structure are known, many ultrastructural aspects are often overlooked. One such aspect has the potential to fundamentally change the biophysical model for the operation of myelinated axons, including signal processing and energy consumption: conduction in the submyelin spaces. Myelin envelopes nearly all larger-diameter axons, where axons comprise the tracts by which neurons communicate with each other as well as a myriad of other cell types, both from within the central and peripheral nervous systems to those of other organs and muscles throughout the animal body.
The current biophysical model for myelination describes an axo-myelin structure devoid of intermediary conducting pathways (Chapter 1). Empirically, however, multiple spaces with the potential for ionic conduction exist between the axon and its myelin sheath, notably the periaxonal space, which runs along most of the length of myelinated regions. Is this submyelin space materially conductive? A series of experiments and computational modelling demonstrate the conductivity of not only the periaxonal, but also paranodal submyelin spaces (Chapter 2). In addition, these submyelin pathways are connected axially to the extracellular space, creating a double cable biophysical architecture for myelinated axons. A number of biophysical consequences are inferred from the resolved biophysical structure, such as the distinct effect of periaxonal or paranodal space sizes on conduction velocity and signal processing in general (Chapter 2). This approach is generalized and automated to resolve the biophysical properties of the entire neuron – particularly if myelinated – on a rapid and large scale (Chapter 3). Further, digital demyelination of optimized neuron models reveals a mechanism for the experimentally-observed intrinsic hyperexcitability of demyelinated axons (Chapter 3). Finally, the consequences of the double cable architecture of myelinated axons in health and disease are discussed (Chapter 4), describing the axonal plasticity, neuro-glial communication, homeostatic, and energy equilibrium roles submyelin spaces could play, and how this understanding may be harnessed to address disregulated conduction in myelinated axons, such as in degenerative myelin diseases
Colourful Depth : Adaptive Optics and Spectral Unmixing for Single-Molecule Localization Microscopy
No full textSingle-Molecule Localization Microscopy is a powerful imaging technique which enables the localization of individual molecules with a precision of a few nanometres. This technique relies on the blinking of individual fluorescent molecules. By recording these blinking events it is possible to reconstruct a single image with more detail compared to standard fluorescence microscopy. This recently developed imaging modality has let to numerous biological discoveries using thin cellular samples, but cannot be easily applied to tissue samples. When imaging inside these samples, the emitted light is disturbed by the sample, which hampers detection and localization. This optical aberration can be corrected by placing a deformable mirror in the collection path of the microscope, a technique called adaptive optics. It is possible to partially correct the aberration by adjusting the deformable mirror. In this thesis I compare different correction methods and quantify the improvement which can be achieved with adaptive optics for localization microscopy.
Another challenge for localization microscopy is the use of differently coloured fluorescent molecules, which are used to stain different structures inside samples. However, the types of fluorescent molecules with suitable blinking properties are similar in colour and are therefore difficult to distinguish. We demonstrate a novel un-mixing method, based on a recent proposed method, which uses photon statistics and is easier to implement on conventional microscopes. The research presented in this thesis enhances the usability of localization microscopy in tissue and improves multi-colour imaging to visualize different structures inside samples
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
- …
