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  • Articles  (1,352)
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  • Physics  (1,352)
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  • Articles  (1,352)
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  • 1
    Electronic Resource
    Electronic Resource
    Reduction of conductance-based neuron models (1992)
    Springer
    Biological cybernetics 66 (1992), S. 381-387 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract We present a scheme for systematically reducing the number of differential equations required for biophysically realistic neuron models. The techniques are general, are designed to be applicable to a large set of such models and retain in the reduced system as high a degree of fidelity to the original system as possible. As examples, we provide reductions of the Hodgkin-Huxley system and the A-current model of Connor et al. (1977).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00197717
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  • 2
    Electronic Resource
    Electronic Resource
    Spike initiation and propagation on axons with slow inward currents (1993)
    Springer
    Biological cybernetics 68 (1993), S. 209-214 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract We investigate spike initiation and propagation in a model axon that has a slow regenerative conductance as well as the usual Hodgkin-Huxley type sodium and potassium conductances. We study the role of slow conductance in producing repetitive firing, compute the dispersion relation for an axon with an additional slow conductance, and show that under appropriate conditions such an axon can produce a traveling zone of secondary spike initiation. This study illustrates some of the complex dynamics shown by excitable membranes with fast and slow conductances.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00224853
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  • 3
    Electronic Resource
    Electronic Resource
    Binding by temporal structure in multiple feature domains of an oscillatory neuronal network (1994)
    Springer
    Biological cybernetics 70 (1994), S. 397-405 
    Details
    ISSN: 1432-0770
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Computer Science , Physics
    Notes: Abstract. An important step in visual processing is the segregation of objects in a visual scene from one another and from the embedding background. According to current theories of visual neuroscience, the different features of a particular object are represented by cells which are spatially distributed across multiple visual areas in the brain. The segregation of an object therefore requires the unique identification and integration of the pertaining cells which have to be “bound” into one assembly coding for the object in question. Several authors have suggested that such a binding of cells could be achieved by the selective synchronization of temporally structured responses of the neurons activated by features of the same stimulus. This concept has recently gained support by the observation of stimulus-dependent oscillatory activity in the visual system of the cat, pigeon and monkey. Furthermore, experimental evidence has been found for the formation and segregation of synchronously active cell assemblies representing different stimuli in the visual field. In this study, we investigate temporally structured activity in networks with single and multiple feature domains. As a first step, we examine the formation and segregation of cell assemblies by synchronizing and desynchronizing connections within a single feature module. We then demonstrate that distributed assemblies can be appropriately bound in a network comprising three modules selective for stimulus disparity, orientation and colour, respectively. In this context, we address the principal problem of segregating assemblies representing spatially overlapping stimuli in a distributed architecture. Using synchronizing as well as desynchronizing mechanisms, our simulations demonstrate that the binding problem can be solved by temporally correlated responses of cells which are distributed across multiple feature modules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s004220050043
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  • 4
    Electronic Resource
    Electronic Resource
    Effect of in situ post-deposition annealing on the formation of α-In2Se3 thin films grown by elemental evaporation (1992)
    Springer
    Applied physics 54 (1992), S. 293-299 
    Details
    ISSN: 1432-0630
    Keywords: 68.55 ; 73.60F ; 78.65J ; 81.15E
    Source: Springer Online Journal Archives 1860-2000
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Abstract Stoichiometric polycrystalline In2Se3 thin films have been grown by elemental evaporation on both glass and quartz substrates. The compositions are examined by DAN fluorimetry and X-ray photoelectron spectroscopy (XPS). Structure of the films are characterized by X-ray diffraction. The structure of this α-form of thin films have been determined to be hexagonal. Optimization of the preparative conditions employed for elemental evaporation, helped in preparing monophasic films by the suppression of other phases to a very minor extent. Influence of annealing conditions on the stoichiometry of the films are investigated in detail.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00323853
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  • 5
    Electronic Resource
    Electronic Resource
    Calmodulin-like activity in a mineralising tissue: The rat molar tooth germ (1981)
    Springer
    Calcified tissue international 33 (1981), S. 545-548 
    Details
    ISSN: 1432-0827
    Keywords: calmodulin ; calcium ; mineralisation ; tooth germ
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Medicine , Physics
    Notes: Summary Calmodulin, a calcium binding protein, has been implicated in the regulation of many calcium-dependent biological processes. Since calcium has an important role in hard tissue genesis, both at intra- and extracellular levels, we anticipate that calcium binding proteins may modulate this process. The present study investigated a mineralising tissue, the rat molar tooth germ, to determine the presence of calmodulin-like activity. A heat-treated cell-free extract of tooth germs provided enhancement of Ca2+-dependent Mg2+-ATPase and 3′:5′-nucleotide phosphodiesterase activity. No enhancement occurred in the absence of calcium or in the presence of trifluoperazine. SDS-polyacrylamide gel electrophoresis of this extract revealed a protein band of approximately 18,000 mol. wt. These findings indicate the presence of calmodulin-like activity in rat molar tooth germs and support the proposal that calcium and calcium binding proteins, in particular calmodulin, have a major regulatory role in the biology of mineralising tissues.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF02409487
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  • 6
    Electronic Resource
    Electronic Resource
    X-ray diffraction study of a Langmuir monolayer of C21H43OH (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 2257-2270 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have studied the structure of a monolayer of C21H43OH on water, in the region near close packing, by grazing incidence in-plane x-ray diffraction. For all temperatures studied the isotherms in the πa plane show a kink, signaling a phase transition. Along an isotherm, and for pressures above the kink, we observe that the transverse structure factor has one peak which has constant position, width, and intensity; below the kink the diffraction peak shifts to smaller scattering vector (larger separation) and the amplitude decays as the surface pressure decreases, but the width of the peak remains constant. We rationalize these observations in terms of the influence on the transverse structure factor of gauche configurations in the amphiphile tails, with the kink representing the point at which the last of the gauche configurations is squeezed out of the chain. Along an isobar which is at higher pressure than the kink pressures of all isotherms crossed, the transverse structure factor has a single peak above a transition temperature and two peaks below that temperature; for π=30 dyn/cm the transition temperature is in the range 16.3〈T〈21.3 °C. We interpret this observation, by comparison with the properties of the lamellar crystalline n-paraffins, as a hexagonal-to-pseudohexagonal structural transition analogous to the crystal rotator II-to-rotator I transition. Our results imply that the hydrocarbon tails of the amphiphile molecules dominate the properties of the monolayer.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455068
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  • 7
    Electronic Resource
    Electronic Resource
    Kinetics of a structural phase transition in Langmuir monolayers studied using x-ray diffraction (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 2393-2397 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Frequently, time-dependent effects are seen in monolayers of amphiphilic molecules (Langmuir films) when they are compressed, so that the pressure after some time is different from that recorded immediately after compression. We have identified for the first time a microscopic relaxation mechanism in monolayers of heneicosanol (C21H43OH): namely, a transition from a uniaxially distorted ("pseudohexagonal'') structural, formed upon compression, to an undistorted hexagonal structure. For T〉20 °C we observe only an apparently hexagonal phase, while at T=5 °C we observe only an apparently stable pseudohexagonal phase. When 10≤T≤20 °C, the monolayer structure changes with time from pseudohexagonal to hexagonal. The rate at which this transformation occurs is strongly temperature dependent. We propose that the observed temperature dependence is determined by the rate of nucleation of a hexagonal phase from a metastable shear-induced structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456653
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  • 8
    Electronic Resource
    Electronic Resource
    Time-dependent, four-point density correlation function description of dynamical heterogeneity and decoupling in supercooled liquids (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 509-512 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Dynamical heterogeneity and the decoupling of diffusion and relaxation in a supercooled liquid is investigated via a time-dependent, four-point density correlation function. We show that the main contribution to the corresponding generalized susceptibility χ4(t) in a molecular dynamics simulation of a Lennard-Jones liquid arises from spatial correlations between temporarily localized ("caged") particles. By comparing χ4(t) with a generalized susceptibility χM(t) related to a correlation function for the squared particle displacements, we demonstrate a connection between dynamical heterogeneity and the decoupling of relaxation and diffusion. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480541
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  • 9
    Electronic Resource
    Electronic Resource
    Crossover to potential energy landscape dominated dynamics in a model glass-forming liquid (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 9834-9840 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An equilibrated model glass-forming liquid is studied by mapping successive configurations produced by molecular dynamics simulation onto a time series of inherent structures (local minima in the potential energy). Using this "inherent dynamics" approach we find direct numerical evidence for the long held view that below a crossover temperature, Tx, the liquid's dynamics can be separated into (i) vibrations around inherent structures and (ii) transitions between inherent structures [M. Goldstein, J. Chem. Phys. 51, 3728 (1969)], i.e., the dynamics become "dominated" by the potential energy landscape. In agreement with previous proposals, we find that Tx is within the vicinity of the mode-coupling critical temperature Tc. We further find that near Tx, transitions between inherent structures occur via cooperative, stringlike rearrangements of groups of particles moving distances substantially smaller than the average interparticle distance. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481621
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  • 10
    Electronic Resource
    Electronic Resource
    Laboratory multiple-crystal X-ray topography and reciprocal-space mapping of protein crystals: influence of impurities on crystal perfection (2001)
    Copenhagen : International Union of Crystallography (IUCr)
    Acta crystallographica 57 (2001), S. 840-846 
    Details
    ISSN: 1399-0047
    Source: Crystallography Journals Online : IUCR Backfile Archive 1948-2001
    Topics: Chemistry and Pharmacology , Geosciences , Physics
    Notes: Double-axis multiple-crystal X-ray topography, rocking-curve measurements and triple-axis reciprocal-space mapping have been combined to characterize protein crystals using a laboratory source. Crystals of lysozyme and lysozyme crystals doped with acetylated lysozyme impurities were examined. It was shown that the incorporation of acetylated lysozyme into crystals of lysozyme induces mosaic domains that are responsible for the broadening and/or splitting of rocking curves and diffraction-space maps along the direction normal to the reciprocal-lattice vector, while the overall elastic lattice strain of the impurity-doped crystals does not appear to be appreciable in high angular resolution reciprocal-space maps. Multiple-crystal monochromatic X-ray topography, which is highly sensitive to lattice distortions, was used to reveal the spatial distribution of mosaic domains in crystals which correlates with the diffraction features in reciprocal space. Discussions of the influence of acetylated lysozyme on crystal perfection are given in terms of our observations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1107/S0907444901005832
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