2,006 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
Neural Dynamics in Parkinsonian Brain:The Boundary Between Synchronized and Nonsynchronized Dynamics
Synchronous oscillatory dynamics is frequently observed in the human brain.
We analyze the fine temporal structure of phase-locking in a realistic network
model and match it with the experimental data from parkinsonian patients. We
show that the experimentally observed intermittent synchrony can be generated
just by moderately increased coupling strength in the basal ganglia circuits
due to the lack of dopamine. Comparison of the experimental and modeling data
suggest that brain activity in Parkinson's disease resides in the large
boundary region between synchronized and nonsynchronized dynamics. Being on the
edge of synchrony may allow for easy formation of transient neuronal
assemblies
X-ray Binaries and Globular Clusters in Elliptical Galaxies
The X-ray emission from normal elliptical galaxies has two major components:
soft emission from diffuse gas and harder emission from populations of
accreting (low-mass) stellar X-ray binaries (LMXB). If LMXB populations are
tied to the field stellar populations in galaxies, their total X-ray
luminosities should be proportional to the optical luminosities of galaxies.
However, recent ASCA and Chandra X-ray observations show that the global
luminosities of LMXB components in ellipticals exhibit significant scatter at a
given optical luminosity. This scatter may reflect a range of evolutionary
stages among LMXB populations in ellipticals of different ages. If so, the
ratio of the global LMXB X-ray luminosity to the galactic optical luminosity,
L_LMXB/L_opt, may be used to determine when the bulk of stars were formed in
individual ellipticals. To test this, we compare variations in L_LMXB/L_opt for
LMXB populations in ellipticals to optically-derived estimates of stellar ages
in the same galaxies. We find no correlation, implying that L_LMXB/L_opt
variations are not good age indicators for ellipticals. Alternatively, LMXBs
may be formed primarily in globular clusters (through stellar tidal
interactions), rather than in the stellar fields of galaxies. Since elliptical
galaxies exhibit a wide range of globular cluster populations for a given
galaxian luminosity, this may induce a dispersion in the LMXB populations of
ellipticals with similar optical luminosities. Indeed, we find that
L_LMXB/L_opt ratios for LMXB populations are strongly correlated with the
specific globular cluster frequencies in elliptical galaxies. This suggests
that most LMXBs were formed in globular clusters.Comment: 5 pages, emulateapj5 style, 2 embedded EPS figures, to appear in ApJ
Letter
Asynchronous response of coupled pacemaker neurons
We study a network model of two conductance-based pacemaker neurons of
differing natural frequency, coupled with either mutual excitation or
inhibition, and receiving shared random inhibitory synaptic input. The networks
may phase-lock spike-to-spike for strong mutual coupling. But the shared input
can desynchronize the locked spike-pairs by selectively eliminating the lagging
spike or modulating its timing with respect to the leading spike depending on
their separation time window. Such loss of synchrony is also found in a large
network of sparsely coupled heterogeneous spiking neurons receiving shared
input.Comment: 11 pages, 4 figures. To appear in Phys. Rev. Let
Understanding Pound-Drever-Hall locking using voltage controlled radio-frequency oscillators: An undergraduate experiment
We have developed a senior undergraduate experiment that illustrates
frequency stabilization techniques using radio-frequency electronics. The
primary objective is to frequency stabilize a voltage controlled oscillator to
a cavity resonance at 800 MHz using the Pound-Drever-Hall method. This
technique is commonly applied to stabilize lasers at optical frequencies. By
using only radio-frequency equipment it is possible to systematically study
aspects of the technique more thoroughly, inexpensively, and free from eye
hazards. Students also learn about modular radio-frequency electronics and
basic feedback control loops. By varying the temperature of the resonator,
students can determine the thermal expansion coefficients of copper, aluminum,
and super invar.Comment: 9 pages, 10 figure
Women Farmers: Pulling Up Their Own Educational Boot Straps with Extension
Women comprise a rapidly growing segment in agriculture. In this article, we examine how a network of women farmers, Extension educators, and researchers responded to the significant increase in women farmers in one state by creating a membership organization that draws on the expertise and resources of the land-grant university and Extension in Pennsylvania to create educational events with networking opportunities. We report 4 years of evaluation data for 37 events indicating educational impact, expansion and enhancement of the network, and marketing strategies for Extension to improve participation of women
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
Cortical oxygenation suggests increased effort during cognitive inhibition in ecstasy polydrug users
Background: It is understood that 3,4-methylenedioxymethamphetamine (ecstasy) causes serotonin dysfunction and deficits in executive functioning. When investigating executive function, functional neuroimaging allows the physiological changes underlying these deficits to be investigated. The present study investigated behavioural and brain indices of inhibition in ecstasy-polydrug users. Methods: Twenty ecstasy-polydrug users and 20 drug-naïve participants completed an inhibitory control task (Random Letter Generation (RLG)) while prefrontal haemodynamic response was assessed using functional near infrared spectroscopy (fNIRS). Results: There were no group differences on background measures including sleep quality and mood state. There were also no behavioural differences between the two groups. However, ecstasy-polydrug users displayed significant increases in oxygenated haemoglobin (oxy-Hb) from baseline compared to controls at several voxels relating to areas of the inferior right medial prefrontal cortex, as well the right and left dorsolateral prefrontal cortex. Regression analysis revealed that recency of ecstasy use was a significant predictor of oxy-Hb increase at two voxels over the right hemisphere after controlling for alcohol and cannabis use indices. Conclusion: Ecstasy-polydrug users show increased neuronal activation in the prefrontal cortex compared to non-users. This is taken to be compensatory activation/recruitment of additional resources to attain similar performance levels on the task, which may be reversible with prolonged abstinence
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
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