101 research outputs found
Noise-assisted spike propagation in myelinated neurons
We consider noise-assisted spike propagation in myelinated axons within a
multi-compartment stochastic Hodgkin-Huxley model. The noise originates from a
finite number of ion channels in each node of Ranvier. For the subthreshold
internodal electric coupling, we show that (i) intrinsic noise removes the
sharply defined threshold for spike propagation from node to node, and (ii)
there exists an optimum number of ion channels which allows for the most
efficient signal propagation and it corresponds to the actual physiological
values.Comment: 8 pages, 12 figures, accepted for publication in Phys. Rev.
Noise analysis of ion channels in non-space-clamped cables: estimates of channel parameters in olfactory cilia
Ion channels in the cilia of olfactory neurons are part of the transduction machinery of olfaction. Odorant stimuli have been shown to induce a biphasic current response, consisting of a cAMP-activated current and a Ca(2+)-activated Cl- current. We have developed a noise analysis method to study ion channels in leaky cables, such as the olfactory cilium, under non-space-clamp conditions. We performed steady-state noise analysis on ligand-induced currents in excised cilia, voltage-clamped at input and internally perfused with cAMP or Ca2+. The cAMP-activated channels analyzed by this method gave results similar to those of single-channel recordings (gamma = 8.3 pS). Single-channel currents have not yet been recorded for the Ca(2+)-activated Cl- channels. Using our noise analysis method, we estimate a unit conductance, gamma = 0.8 pS, for these channels. The density of channels was found to be approximately 70 channels/micron2 for both channel species
Instability of the Gravitational N-Body Problem in the Large-N Limit
We use a systolic N-body algorithm to evaluate the linear stability of the
gravitational N-body problem for N up to 1.3 x 10^5, two orders of magnitude
greater than in previous experiments. For the first time, a clear ~ln
N-dependence of the perturbation growth rate is seen. The e-folding time for N
= 10^5 is roughly 1/20 of a crossing time.Comment: Accepted for publication in The Astrophysical Journa
Stochastic resonance as a collective property of ion channel assemblies
By use of a stochastic generalization of the Hodgkin-Huxley model we
investigate both the phenomena of stochastic resonance (SR) and coherence
resonance (CR) in variable size patches of an excitable cell membrane. Our
focus is on the challenge how internal noise stemming from individual ion
channels does affect collective properties of the whole ensemble. We
investigate both an unperturbed situation with no applied stimuli and one in
which the membrane is stimulated externally by a periodic signal and additional
external noise. For the nondriven case, we demonstrate the existence of an
optimal size of the membrane patch for which the internal noise causes a most
regular spike activity. This phenomenon shall be termed intrinsic CR. In
presence of an applied periodic stimulus we demonstrate that the
signal-to-noise ratio (SNR) exhibits SR vs. decreasing patch size, or vs.
increasing internal noise strength, respectively. Moreover, we demonstrate that
conventional SR vs. the external noise intensity occurs only for sufficiently
large membrane patches, when the intensity of internal noise is below its
optimal level. Thus, biological SR seemingly is rooted in the collective
properties of large ion channel ensembles rather than in the individual
stochastic dynamics of single ion channels.Comment: 9 pages, 2 figure
A numerical comparison of theories of violent relaxation
Using N-body simulations with a large set of massless test particles we
compare the predictions of two theories of violent relaxation, the well known
Lynden-Bell theory and the more recent theory by Nakamura. We derive ``weaken''
versions of both theories in which we use the whole equilibrium coarse-grained
distribution function as a constraint instead of the total energy constraint.
We use these weaken theories to construct expressions for the conditional
probability that a test particle initially at the phase-space
coordinate would end-up in the 'th macro-cell at equilibrium. We show
that the logarithm of the ratio is
directly proportional to the initial phase-space density for the
Lynden-Bell theory and inversely proportional to for the Nakamura
theory. We then measure using a set of N-body simulations of a
system undergoing a gravitational collapse to check the validity of the two
theories of violent relaxation. We find that both theories are at odds with the
numerical results, qualitatively and quantitatively.Comment: Replaced with a revised version, which is now accepted to MNRAS.
LaTeX, 12 pages, 6 figure
Effect of channel block on the spiking activity of excitable membranes in a stochastic Hodgkin-Huxley model
The influence of intrinsic channel noise on the spontaneous spiking activity
of poisoned excitable membrane patches is studied by use of a stochastic
generalization of the Hodgkin-Huxley model. Internal noise stemming from the
stochastic dynamics of individual ion channels is known to affect the
collective properties of the whole ion channel cluster. For example, there
exists an optimal size of the membrane patch for which the internal noise alone
causes a regular spontaneous generation of action potentials. In addition to
varying the size of ion channel clusters, living organisms may adapt the
densities of ion channels in order to optimally regulate the spontaneous
spiking activity. The influence of channel block on the excitability of a
membrane patch of certain size is twofold: First, a variation of ion channel
densities primarily yields a change of the conductance level. Second, a
down-regulation of working ion channels always increases the channel noise.
While the former effect dominates in the case of sodium channel block resulting
in a reduced spiking activity, the latter enhances the generation of
spontaneous action potentials in the case of a tailored potassium channel
blocking. Moreover, by blocking some portion of either potassium or sodium ion
channels, it is possible to either increase or to decrease the regularity of
the spike train.Comment: 10 pages, 3 figures, published 200
Capacitance fluctuations causing channel noise reduction in stochastic Hodgkin-Huxley systems
Voltage-dependent ion channels determine the electric properties of axonal
cell membranes. They not only allow the passage of ions through the cell
membrane but also contribute to an additional charging of the cell membrane
resulting in the so-called capacitance loading. The switching of the channel
gates between an open and a closed configuration is intrinsically related to
the movement of gating charge within the cell membrane. At the beginning of an
action potential the transient gating current is opposite to the direction of
the current of sodium ions through the membrane. Therefore, the excitability is
expected to become reduced due to the influence of a gating current. Our
stochastic Hodgkin-Huxley like modeling takes into account both the channel
noise -- i.e. the fluctuations of the number of open ion channels -- and the
capacitance fluctuations that result from the dynamics of the gating charge. We
investigate the spiking dynamics of membrane patches of variable size and
analyze the statistics of the spontaneous spiking. As a main result, we find
that the gating currents yield a drastic reduction of the spontaneous spiking
rate for sufficiently large ion channel clusters. Consequently, this
demonstrates a prominent mechanism for channel noise reduction.Comment: 18 page
The role of input noise in transcriptional regulation
Even under constant external conditions, the expression levels of genes
fluctuate. Much emphasis has been placed on the components of this noise that
are due to randomness in transcription and translation; here we analyze the
role of noise associated with the inputs to transcriptional regulation, the
random arrival and binding of transcription factors to their target sites along
the genome. This noise sets a fundamental physical limit to the reliability of
genetic control, and has clear signatures, but we show that these are easily
obscured by experimental limitations and even by conventional methods for
plotting the variance vs. mean expression level. We argue that simple, global
models of noise dominated by transcription and translation are inconsistent
with the embedding of gene expression in a network of regulatory interactions.
Analysis of recent experiments on transcriptional control in the early
Drosophila embryo shows that these results are quantitatively consistent with
the predicted signatures of input noise, and we discuss the experiments needed
to test the importance of input noise more generally.Comment: 11 pages, 5 figures minor correction
P/2010A2 LINEAR - I: An impact in the Asteroid Main Belt
Comet P/2010A2 LINEAR is a good candidate for membership with the Main Belt
Comet family. It was observed with several telescopes (ESO NTT, La Silla;
Gemini North, Mauna Kea; UH 2.2m, Mauna Kea) from 14 Jan. until 19 Feb. 2010 in
order to characterize and monitor it and its very unusual dust tail, which
appears almost fully detached from the nucleus; the head of the tail includes
two narrow arcs forming a cross. The immediate surroundings of the nucleus were
found dust-free, which allowed an estimate of the nucleus radius of 80-90m. A
model of the thermal evolution indicates that such a small nucleus could not
maintain any ice content for more than a few million years on its current
orbit, ruling out ice sublimation dust ejection mechanism. Rotational spin-up
and electrostatic dust levitations were also rejected, leaving an impact with a
smaller body as the favoured hypothesis, and ruling out the cometary nature of
the object.
The impact is further supported by the analysis of the tail structure.
Finston-Probstein dynamical dust modelling indicates the tail was produced by a
single burst of dust emission. More advanced models, independently indicate
that this burst populated a hollow cone with a half-opening angle alpha~40degr
and with an ejection velocity v_max ~ 0.2m/s, where the small dust grains fill
the observed tail, while the arcs are foreshortened sections of the burst cone.
The dust grains in the tail are measured to have radii between a=1-20mm, with a
differential size distribution proportional to a^(-3.44 +/- 0.08). The dust
contained in the tail is estimated to at least 8x10^8kg, which would form a
sphere of 40m radius. Analysing these results in the framework of crater
physics, we conclude that a gravity-controlled crater would have grown up to
~100m radius, i.e. comparable to the size of the body. The non-disruption of
the body suggest this was an oblique impact.Comment: 15 pages, 11 figures, in pres
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