754 research outputs found
Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost
Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming
Modeling rhythmic patterns in the hippocampus
We investigate different dynamical regimes of neuronal network in the CA3
area of the hippocampus. The proposed neuronal circuit includes two fast- and
two slowly-spiking cells which are interconnected by means of dynamical
synapses. On the individual level, each neuron is modeled by FitzHugh-Nagumo
equations. Three basic rhythmic patterns are observed: gamma-rhythm in which
the fast neurons are uniformly spiking, theta-rhythm in which the individual
spikes are separated by quiet epochs, and theta/gamma rhythm with repeated
patches of spikes. We analyze the influence of asymmetry of synaptic strengths
on the synchronization in the network and demonstrate that strong asymmetry
reduces the variety of available dynamical states. The model network exhibits
multistability; this results in occurrence of hysteresis in dependence on the
conductances of individual connections. We show that switching between
different rhythmic patterns in the network depends on the degree of
synchronization between the slow cells.Comment: 10 pages, 9 figure
Thermal Impact on Spiking Properties in Hodgkin-Huxley Neuron with Synaptic Stimulus
The effect of environmental temperature on neuronal spiking behaviors is
investigated by numerically simulating the temperature dependence of spiking
threshold of the Hodgkin-Huxley neuron subject to synaptic stimulus. We find
that the spiking threshold exhibits a global minimum in a "comfortable
temperature" range where spike initiation needs weakest synaptic strength,
indicating the occurrence of optimal use of synaptic transmission in neural
system. We further explore the biophysical origin of this phenomenon in ion
channel gating kinetics and also discuss its possible biological relevance in
information processing in neural systems.Comment: 10 pages, 4 figure
Magnetic Field-Induced Condensation of Triplons in Han Purple Pigment BaCuSiO
Besides being an ancient pigment, BaCuSiO is a quasi-2D magnetic
insulator with a gapped spin dimer ground state. The application of strong
magnetic fields closes this gap creating a gas of bosonic spin triplet
excitations called triplons. The topology of the spin lattice makes
BaCuSiO an ideal candidate for studying the Bose-Einstein condensation
of triplons as a function of the external magnetic field, which acts as a
chemical potential. In agreement with quantum Monte Carlo numerical
simulations, we observe a distinct lambda-anomaly in the specific heat together
with a maximum in the magnetic susceptibility upon cooling down to liquid
Helium temperatures.Comment: published on August 20, 200
Emergent global oscillations in heterogeneous excitable media: The example of pancreatic beta cells
Using the standard van der Pol-FitzHugh-Nagumo excitable medium model I
demonstrate a novel generic mechanism, diversity, that provokes the emergence
of global oscillations from individually quiescent elements in heterogeneous
excitable media. This mechanism may be operating in the mammalian pancreas,
where excitable beta cells, quiescent when isolated, are found to oscillate
when coupled despite the absence of a pacemaker region.Comment: See home page http://lec.ugr.es/~julya
Numerical Solution of Differential Equations by the Parker-Sochacki Method
A tutorial is presented which demonstrates the theory and usage of the
Parker-Sochacki method of numerically solving systems of differential
equations. Solutions are demonstrated for the case of projectile motion in air,
and for the classical Newtonian N-body problem with mutual gravitational
attraction.Comment: Added in July 2010: This tutorial has been posted since 1998 on a
university web site, but has now been cited and praised in one or more
refereed journals. I am therefore submitting it to the Cornell arXiv so that
it may be read in response to its citations. See "Spiking neural network
simulation: numerical integration with the Parker-Sochacki method:" J. Comput
Neurosci, Robert D. Stewart & Wyeth Bair and
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2717378
New conditional symmetries and exact solutions of nonlinear reaction-diffusion-convection equations. II
In the first part of this paper math-ph/0612078, a complete description of
Q-conditional symmetries for two classes of reaction-diffusion-convection
equations with power diffusivities is derived. It was shown that all the known
results for reaction-diffusion equations with power diffusivities follow as
particular cases from those obtained in math-ph/0612078 but not vise versa. In
the second part the symmetries obtained in are successfully applied for
constructing exact solutions of the relevant equations. In the particular case,
new exact solutions of nonlinear reaction-diffusion-convection (RDC) equations
arising in application and their natural generalizations are found
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