1,309 research outputs found
Gain Control With A-Type Potassium Current: IA As A Switch Between Divisive And Subtractive Inhibition
Neurons process and convey information by transforming barrages of synaptic inputs into spiking activity. Synaptic inhibition typically suppresses the output firing activity of a neuron, and is commonly classified as having a subtractive or divisive effect on a neuron’s output firing activity. Subtractive inhibition can narrow the range of inputs that evoke spiking activity by eliminating responses to non-preferred inputs. Divisive inhibition is a form of gain control: it modifies firing rates while preserving the range of inputs that evoke firing activity. Since these two “modes” of inhibition have distinct impacts on neural coding, it is important to understand the biophysical mechanisms that distinguish these response profiles. In this study, we use simulations and mathematical analysis of a neuron model to find the specific conditions (parameter sets) for which inhibitory inputs have subtractive or divisive effects. Significantly, we identify a novel role for the A-type Potassium current (IA). In our model, this fast-activating, slowly-inactivating outward current acts as a switch between subtractive and divisive inhibition. In particular, if IA is strong (large maximal conductance) and fast (activates on a time-scale similar to spike initiation), then inhibition has a subtractive effect on neural firing. In contrast, if IA is weak or insufficiently fast-activating, then inhibition has a divisive effect on neural firing. We explain these findings using dynamical systems methods (plane analysis and fast-slow dissection) to define how a spike threshold condition depends on synaptic inputs and IA. Our findings suggest that neurons can “self-regulate” the gain control effects of inhibition via combinations of synaptic plasticity and/or modulation of the conductance and kinetics of A-type Potassium channels. This novel role for IA would add flexibility to neurons and networks, and may relate to recent observations of divisive inhibitory effects on neurons in the nucleus of the solitary tract
Gain control with A-type potassium current: IA as a switch between divisive and subtractive inhibition
Neurons process information by transforming barrages of synaptic inputs into
spiking activity. Synaptic inhibition suppresses the output firing activity of
a neuron, and is commonly classified as having a subtractive or divisive effect
on a neuron's output firing activity. Subtractive inhibition can narrow the
range of inputs that evoke spiking activity by eliminating responses to
non-preferred inputs. Divisive inhibition is a form of gain control: it
modifies firing rates while preserving the range of inputs that evoke firing
activity. Since these two "modes" of inhibition have distinct impacts on neural
coding, it is important to understand the biophysical mechanisms that
distinguish these response profiles.
We use simulations and mathematical analysis of a neuron model to find the
specific conditions for which inhibitory inputs have subtractive or divisive
effects. We identify a novel role for the A-type Potassium current (IA). In our
model, this fast-activating, slowly- inactivating outward current acts as a
switch between subtractive and divisive inhibition. If IA is strong (large
maximal conductance) and fast (activates on a time-scale similar to spike
initiation), then inhibition has a subtractive effect on neural firing. In
contrast, if IA is weak or insufficiently fast-activating, then inhibition has
a divisive effect on neural firing. We explain these findings using dynamical
systems methods to define how a spike threshold condition depends on synaptic
inputs and IA.
Our findings suggest that neurons can "self-regulate" the gain control
effects of inhibition via combinations of synaptic plasticity and/or modulation
of the conductance and kinetics of A-type Potassium channels. This novel role
for IA would add flexibility to neurons and networks, and may relate to recent
observations of divisive inhibitory effects on neurons in the nucleus of the
solitary tract.Comment: 20 pages, 11 figure
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Double Neutron Star Systems and Natal Neutron Star Kicks
We study the four double neutron star systems found in the Galactic disk in
terms of the orbital characteristics of their immediate progenitors and the
natal kicks imparted to neutron stars. Analysis of the effect of the second
supernova explosion on the orbital dynamics, combined with recent results from
simulations of rapid accretion onto neutron stars lead us to conclude that the
observed systems could not have been formed had the explosion been symmetric.
Their formation becomes possible if kicks are imparted to the radio-pulsar
companions at birth. We identify the constraints imposed on the immediate
progenitors of the observed double neutron stars and calculate the ranges
within which their binary characteristics (orbital separations and masses of
the exploding stars) are restricted. We also study the dependence of these
limits on the magnitude of the kick velocity and the time elapsed since the
second explosion. For each of the double neutron stars, we derive a minimum
kick magnitude required for their formation, and for the two systems in close
orbits we find it to exceed 200km/s. Lower limits are also set to the
center-of-mass velocities of double neutron stars, and we find them to be
consistent with the current proper motion observations.Comment: 25 pages, 6 figs (9 parts), 4 tables, AASTeX, Accepted in Ap
Comparison of surface and column measurements of aerosol scattering properties over the western North Atlantic Ocean at Bermuda
Light scattering by size-resolved aerosols in near-surface air at Tudor
Hill, Bermuda, was measured between January and June 2009. Vertical
distributions of aerosol backscattering and column-averaged aerosol optical
properties were characterized in parallel with a micro-pulse lidar (MPL) and
an automated sun–sky radiometer. Comparisons were made between extensive
aerosol parameters in the column, such as the lidar-retrieved extinction at
400 m and the aerosol optical depth (AOD), and scattering was measured with a
surface nephelometer. Comparisons were also made for intensive parameters
such as the Ångström exponent and calculations using
AERONET(Aerosol Robotic Network)-derived
aerosol physical parameters (size distribution, index of refraction) and Mie
theory, and the ratio of submicron scattering to total scattering for size-segregated
nephelometer measurements. In these comparisons the <i>r</i><sup>2</sup> was
generally around 0.50. Data were also evaluated based on back trajectories.
The correlation between surface scattering and lidar extinction was highest
for flows when the surface scattering was dominated by smaller particles and
the flow had a longer footprint over land then over the ocean. The
correlation of AOD with surface scatter was similar for all flow regimes.
There was also no clear dependence of the atmospheric lapse rate, as
determined from a nearby radiosonde station, on flow regime. The
Ångström exponent for most flow regimes was 0.9–1.0, but for the
case of air originating from North America, but with significant time over
the ocean, the Ångström exponent was 0.57 ± 0.18. The submicron
fraction of aerosol near the surface (<i>R</i><sub>sub-surf</sub>) was significantly
greater for the flows from land (0.66 ± 0.11) than for the flows which
spent more time over the ocean (0.40 ± 0.05). When comparing
<i>R</i><sub>sub-surf</sub> and the column-integrated submicron scattering fraction,
<i>R</i><sub>sub-col</sub>, the correlation was similar, <i>r</i><sup>2</sup> = 0.50, but
<i>R</i><sub>sub-surf</sub> was generally less than <i>R</i><sub>sub-col</sub>, indicating more large
particles contributing to light scattering at the surface, contrary to
conditions over continents and for polluted continental transport over the
ocean. In general, though, the marginal correlations indicate that the column
optical properties are weakly correlated with the surface optical
measurements. Thus, if it is desired to associate aerosol chemical/physical
properties with their optical properties, it is best to use optical and
chemical/physical measurements with both collected at the surface or both
collected in the column
Lysosomes in iron metabolism, ageing and apoptosis
The lysosomal compartment is essential for a variety of cellular functions, including the normal turnover of most long-lived proteins and all organelles. The compartment consists of numerous acidic vesicles (pH ∼4 to 5) that constantly fuse and divide. It receives a large number of hydrolases (∼50) from the trans-Golgi network, and substrates from both the cells’ outside (heterophagy) and inside (autophagy). Many macromolecules contain iron that gives rise to an iron-rich environment in lysosomes that recently have degraded such macromolecules. Iron-rich lysosomes are sensitive to oxidative stress, while ‘resting’ lysosomes, which have not recently participated in autophagic events, are not. The magnitude of oxidative stress determines the degree of lysosomal destabilization and, consequently, whether arrested growth, reparative autophagy, apoptosis, or necrosis will follow. Heterophagy is the first step in the process by which immunocompetent cells modify antigens and produce antibodies, while exocytosis of lysosomal enzymes may promote tumor invasion, angiogenesis, and metastasis. Apart from being an essential turnover process, autophagy is also a mechanism by which cells will be able to sustain temporary starvation and rid themselves of intracellular organisms that have invaded, although some pathogens have evolved mechanisms to prevent their destruction. Mutated lysosomal enzymes are the underlying cause of a number of lysosomal storage diseases involving the accumulation of materials that would be the substrate for the corresponding hydrolases, were they not defective. The normal, low-level diffusion of hydrogen peroxide into iron-rich lysosomes causes the slow formation of lipofuscin in long-lived postmitotic cells, where it occupies a substantial part of the lysosomal compartment at the end of the life span. This seems to result in the diversion of newly produced lysosomal enzymes away from autophagosomes, leading to the accumulation of malfunctioning mitochondria and proteins with consequent cellular dysfunction. If autophagy were a perfect turnover process, postmitotic ageing and several age-related neurodegenerative diseases would, perhaps, not take place
The landscape of gifted and talented education in England and Wales: How are teachers implementing policy?
This is an Author's Accepted Manuscript of an article published in Research Papers in Education, 27(2), 167-186, 2012, copyright Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/02671522.2010.509514.This paper explores the evidence relating to how primary schools are responding to the ‘gifted and talented’ initiative in England and Wales. A questionnaire survey which invited both closed and open-ended responses was carried out with a national sample of primary schools. The survey indicated an increasing proportion of coordinators, compared with a survey carried out in 1996, were identifying their gifted and talented children as well as having associated school policies. However, the survey also highlighted a number of issues which need addressing if the initiative is to achieve its objective of providing the best possible educational opportunities for children. For example, it was found that a significant number of practitioners were not aware of the existence of the National Quality Standards for gifted and talented education, provided by the UK government in 2007, and the subject-specific criteria provided by the UK’s Curriculum Authority for identification and provision have been largely ignored. The process of identifying children to be placed on the ‘gifted and talented’ register seems haphazard and based on pragmatic reasons. Analysis of teachers’ responses also revealed a range of views and theoretical positioning held by them, which have implications for classroom practice. As the ‘gifted and talented’ initiative in the UK is entering a second decade, and yet more significant changes in policy are introduced, pertinent questions need to be raised and given consideration
Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295
We have discovered both intermediate-order gravity mode and low-order
pressure mode pulsation in the same star, HD 209295. It is therefore both a
Gamma Doradus and a Delta Scuti star, which makes it the first pulsating star
to be a member of two classes.
The star is a single-lined spectroscopic binary with an orbital period of
3.10575 d and an eccentricity of 0.352. Weak pulsational signals are found in
both the radial velocity and line-profile variations, allowing us to show that
the two highest-amplitude Gamma Doradus pulsation modes are consistent with l=1
and |m|=1.
In our 280 h of BVI multi-site photometry we detected ten frequencies in the
light variations, one in the Delta Scuti regime and nine in the Gamma Doradus
domain. Five of the Gamma Doradus frequencies are exact integer multiples of
the orbital frequency. This observation leads us to suspect they are tidally
excited. Results of theoretical modeling (stability analysis, tidal excitation)
were consistent with the observations.
We could not detect the secondary component of the system in infrared
photometry, suggesting that it may not be a main-sequence star. Archival data
of HD 209295 show a strong ultraviolet excess, the origin of which is not
known. The orbit of the primary is consistent with a secondary mass of M > 1.04
Msun indicative of a neutron star or a white dwarf companion.Comment: 18 pages, 18 figures, accepted for publication in MNRAS, shortened
abstrac
Resonance of coefficient of variation induced by rebound currents for stochastic inhibitory inputs
Three-dimensional coherent X-ray diffraction imaging of a ceramic nanofoam: determination of structural deformation mechanisms
Ultra-low density polymers, metals, and ceramic nanofoams are valued for
their high strength-to-weight ratio, high surface area and insulating
properties ascribed to their structural geometry. We obtain the labrynthine
internal structure of a tantalum oxide nanofoam by X-ray diffractive imaging.
Finite element analysis from the structure reveals mechanical properties
consistent with bulk samples and with a diffusion limited cluster aggregation
model, while excess mass on the nodes discounts the dangling fragments
hypothesis of percolation theory.Comment: 8 pages, 5 figures, 30 reference
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