445 research outputs found
Helical Tubes in Crowded Environments
When placed in a crowded environment, a semi-flexible tube is forced to fold
so as to make a more compact shape. One compact shape that often arises in
nature is the tight helix, especially when the tube thickness is of comparable
size to the tube length. In this paper we use an excluded volume effect to
model the effects of crowding. This gives us a measure of compactness for
configurations of the tube, which we use to look at structures of the
semi-flexible tube that minimize the excluded volume. We focus most of our
attention on the helix and which helical geometries are most compact. We found
that helices of specific pitch to radius ratio 2.512 to be optimally compact.
This is the same geometry that minimizes the global curvature of the curve
defining the tube. We further investigate the effects of adding a bending
energy or multiple tubes to begin to explore the more complete space of
possible geometries a tube could form.Comment: 10 page
The thalamic low-threshold Ca2+ potential: a key determinant of the local and global dynamics of the slow (<1 Hz) sleep oscillation in thalamocortical networks
During non-rapid eye movement sleep and certain types of anaesthesia, neurons in the neocortex and thalamus exhibit a distinctive slow (<1âHz) oscillation that consists of alternating UP and DOWN membrane potential states and which correlates with a pronounced slow (<1âHz) rhythm in the electroencephalogram. While several studies have claimed that the slow oscillation is generated exclusively in neocortical networks and then transmitted to other brain areas, substantial evidence exists to suggest that the full expression of the slow oscillation in an intact thalamocortical (TC) network requires the balanced interaction of oscillator systems in both the neocortex and thalamus. Within such a scenario, we have previously argued that the powerful low-threshold Ca2+ potential (LTCP)-mediated burst of action potentials that initiates the UP states in individual TC neurons may be a vital signal for instigating UP states in related cortical areas. To investigate these issues we constructed a computational model of the TC network which encompasses the important known aspects of the slow oscillation that have been garnered from earlier in vivo and in vitro experiments. Using this model we confirm that the overall expression of the slow oscillation is intricately reliant on intact connections between the thalamus and the cortex. In particular, we demonstrate that UP state-related LTCP-mediated bursts in TC neurons are proficient in triggering synchronous UP states in cortical networks, thereby bringing about a synchronous slow oscillation in the whole network. The importance of LTCP-mediated action potential bursts in the slow oscillation is also underlined by the observation that their associated dendritic Ca2+ signals are the only ones that inform corticothalamic synapses of the TC neuron output, since they, but not those elicited by tonic action potential firing, reach the distal dendritic sites where these synapses are located
Education and articulation: Laclau and Mouffeâs radical democracy in school
This paper outlines a theory of radical democratic education by addressing a key concept in Laclau and Mouffeâs Hegemony and Socialist Strategy: articulation. Through their concept of articulation, Laclau and Mouffe attempt to liberate Gramsciâs theory of hegemony from Marxist economism, and adapt it to a political sphere inhabited by a plurality of struggles and agents none of which is predominant. However, while for Gramsci the political process of hegemony formation has an explicit educational dimension, Laclau and Mouffe ignore this dimension altogether. My discussion starts with elaborating the concept of articulation and analysing it in terms of three dimensions: performance, connection and transformation. I then address the role of education in Gramsciâs politics, in which the figure of the intellectual is central, and argue that radical democratic education requires renouncing that figure. In the final section, I offer a theory of such education, in which both teacher and students articulate their political differences and identities
Ground-state properties of tubelike flexible polymers
In this work we investigate structural properties of native states of a
simple model for short flexible homopolymers, where the steric influence of
monomeric side chains is effectively introduced by a thickness constraint. This
geometric constraint is implemented through the concept of the global radius of
curvature and affects the conformational topology of ground-state structures. A
systematic analysis allows for a thickness-dependent classification of the
dominant ground-state topologies. It turns out that helical structures,
strands, rings, and coils are natural, intrinsic geometries of such tubelike
objects
Deformed Coordinate-Space Hartree-Fock-Bogoliubov Approach to Weakly Bound Nuclei and Large Deformations
The coordinate space formulation of the Hartree-Fock-Bogoliubov (HFB) method
enables self-consistent treatment of mean-field and pairing in weakly bound
systems whose properties are affected by the particle continuum space. Of
particular interest are neutron-rich, deformed drip-line nuclei which can
exhibit novel properties associated with neutron skin. To describe such systems
theoretically, we developed an accurate 2D lattice Skyrme-HFB solver {\hfbax}
based on B-splines. Compared to previous implementations, we made a number of
improvements aimed at boosting the solver's performance. These include:
explicit imposition of axiality and space inversion, use of the modified
Broyden's method to solve self-consistent equations, and a partial
parallelization of the code. {\hfbax} has been benchmarked against other HFB
solvers, both spherical and deformed, and the accuracy of the B-spline
expansion was tested by employing the multiresolution wavelet method.
Illustrative calculations are carried out for stable and weakly bound nuclei at
spherical and very deformed shapes, including constrained fission pathways. In
addition to providing new physics insights, {\hfbax} can serve as a useful tool
to assess the reliability and applicability of coordinate-space and
configuration-space HFB solvers, both existing and in development.Comment: 12 pages,7 figs, submitted to Phys. Rev.
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