19,378 research outputs found
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
Species Limits and Phylogeography of North American Cricket Frogs (Acris: Hylidae)
Cricket frogs are widely distributed across the eastern United States and two species, the northern cricket frog (Acris crepitans) and the southern cricket frog (A. gryllus) are currently recognized. We generated a phylogenetic hypothesis for Acris using fragments of nuclear and mitochondrial genes in separate and combined phylogenetic analyses. We also used distance methods and fixation indices to evaluate species limits within the genus and the validity of currently recognized subspecies of A. crepitans. The distributions of existing A. crepitans subspecies, defined by morphology and call types, do not match the distributions of evolutionary lineages recovered using our genetic data. We discuss a scenario of call evolution to explain this disparity. We also recovered distinct phylogeographic groups within A. crepitans and A. gryllus that are congruent with other codistributed taxa. Under a lineage-based species concept, we recognize Acris blanchardi as a distinct species. The importance of this revised taxonomy is discussed in light of the dramatic declines in A. blanchardi across the northern and western portions of its range
Suppression of Kelvon-induced decay of quantized vortices in oblate Bose-Einstein Condensates
We study the Kelvin mode excitations on a vortex line in a three-dimensional
trapped Bose-Einstein condensate at finite temperature. Our stochastic
Gross-Pitaevskii simulations show that the activation of these modes can be
suppressed by tightening the confinement along the direction of the vortex
line, leading to a strong suppression in the vortex decay rate as the system
enters a regime of two-dimensional vortex dynamics. As the system approaches
the condensation transition temperature we find that the vortex decay rate is
strongly sensitive to dimensionality and temperature, observing a large
enhancement for quasi-two-dimensional traps. Three-dimensional simulations of
the recent vortex dipole decay experiment of Neely et al. [Phys. Rev. Lett.
104, 160401 (2010)] confirm two-dimensional vortex dynamics, and predict a
dipole lifetime consistent with experimental observations and suppression of
Kelvon-induced vortex decay in highly oblate condensates.Comment: 8 pages, 8 figure
Axistationary perfect fluids -- a tetrad approach
Stationary axisymmetric perfect fluid space-times are investigated using the
curvature description of geometries. Attention is focused on space-times with a
vanishing electric part of the Weyl tensor. It is shown that the only
incompressible axistationary magnetic perfect fluid is the interior
Schwarzschild solution. The existence of a rigidly rotating perfect fluid,
generalizing the interior Schwarzschild metric is proven. Theorems are stated
on Petrov types and electric/magnetic Weyl tensors.Comment: 12 page
Spin domain formation in spinor Bose-Einstein condensation
The spatial structure of the spinor Bose-Einstein condensates with the spin
degrees of freedom is analyzed based on the generalized Gross-Pitaevskii
equation (GP) in the light of the present spin domain experiment on m_F=\pm 1,
and 0 of the hyperfine state F=1 of ^{23}Na atom gases. The GP solutions in
three- and one-spatial dimensional cases reproduce the observed spin domain
structures, revealing the length scale associated with the existence of the
weak interaction of the spin-spin channel, other than the ordinary coherence
length related to the density-density channel. The obtained domain structure in
GP is compared with the result in Thomas-Fermi approximation. The former
solution is found to better describe the observed features than the latter.Comment: 9 pages, 14 figure
Austral seabirds: challenges and opportunities for research and conservation
This paper draws together the themes of papers on procellariiform biology contained within this special issue of the Papers and Proceedings ofthe Royal Society ofTasmania which is a tribute to Irynej Skira. The role of these birds as generalised biomonitors of marine ecosystem health as well as their important interactions with commercial fisheries and human societies are major considerations. Seabird conservation
faces challenges from introduced pest loss of habitat to urban development, marine pollution and climate and oceanographic changes. Studies are hampered by the difficulties of dealing with birds in remote areas (that are therefore expensive to study), a paucity of funds and problems of overlapping national and international jurisdictions in their individual home ranges or on their migration routes. There is a real need for long-term studies ofseabirds because they have delayed breeding systems, a slow rate ofreproduction, long life-spans and high adult survival. Such knowledge is important in understanding their population dynamics, the effects of changing climate, the impacts of commercial fishing, pollution, breeding habitat loss and the harvesting of chicks. The value of the long-term studies of Shorttailed Shearwaters, Puffinus tenuirostris on Fisher Island, Tasmania, is considered in the light of the importance of research investment in basic curiosity-driven research. Neglect of long-term studies and the diminishing role of local scientific journals in the dissemination of baseline data on which to build generalities are the result of changing priorities of government and other funding agencies towards shortterm output-based assessment models. Our present inability to answer the basic question of what determines seabird population abundance and distribution highlights a lack of fundamental population data in Austral seabird research. This must be addressed so that a mixture of comparative, experimental and modelling studies can be mobilised to complement descriptive studies
Elastic and inelastic collisions of 6Li in magnetic and optical traps
We use a full coupled channels method to calculate collisional properties of
magnetically or optically trapped ultracold 6Li. The magnetic field dependence
of the s-wave scattering lengths of several mixtures of hyperfine states are
determined, as are the decay rates due to exchange collisions. In one case, we
find Feshbach resonances at B=0.08 T and B=1.98 T. We show that the exact
coupled channels calculation is well approximated over the entire range of
magnetic fields by a simple analytical calculation.Comment: 4 pages revtex including 4 figures, submitted to PR
Dynamic and Energetic Stabilization of Persistent Currents in Bose-Einstein Condensates
We study conditions under which vortices in a highly oblate harmonically
trapped Bose-Einstein condensate (BEC) can be stabilized due to pinning by a
blue-detuned Gaussian laser beam, with particular emphasis on the potentially
destabilizing effects of laser beam positioning within the BEC. Our approach
involves theoretical and numerical exploration of dynamically and energetically
stable pinning of vortices with winding number up to , in correspondence
with experimental observations. Stable pinning is quantified theoretically via
Bogoliubov-de Gennes excitation spectrum computations and confirmed via direct
numerical simulations for a range of conditions similar to those of
experimental observations. The theoretical and numerical results indicate that
the pinned winding number, or equivalently the winding number of the superfluid
current about the laser beam, decays as a laser beam of fixed intensity moves
away from the BEC center. Our theoretical analysis helps explain previous
experimental observations, and helps define limits of stable vortex pinning for
future experiments involving vortex manipulation by laser beams.Comment: 8 pages 5 figure
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