203 research outputs found
Dynamical systems analysis of spike-adding mechanisms in transient bursts
Transient bursting behaviour of excitable cells, such as neurons, is a common feature observed experimentally, but theoretically, it is not well understood. We analyse a five-dimensional simplified model of after-depolarisation that exhibits transient bursting behaviour when perturbed with a short current injection. Using one-parameter continuation of the perturbed orbit segment formulated as a well-posed boundary value problem, we show that the spike-adding mechanism is a canard-like transition that has a different character from known mechanisms for periodic burst solutions. The biophysical basis of the model gives a natural time-scale separation, which allows us to explain the spike-adding mechanism using geometric singular perturbation theory, but it does not involve actual bifurcations as for periodic bursts. We show that unstable sheets of the critical manifold, formed by saddle equilibria of the system that only exist in a singular limit, are responsible for the spike-adding transition; the transition is organised by the slow flow on the critical manifold near folds of this manifold. Our analysis shows that the orbit segment during the spike-adding transition includes a fast transition between two unstable sheets of the slow manifold that are of saddle type. We also discuss a different parameter regime where the presence of additional saddle equilibria of the full system alters the spike-adding mechanism
Coral calcification under daily oxygen saturation and pH dynamics reveals the important role of oxygen
Coral reefs are essential to many nations, and are currently in global decline. Although climate models predict decreases in seawater pH (~0.3 units) and oxygen saturation (~5 percentage points), these are exceeded by the current daily pH and oxygen fluctuations on many reefs (pH 7.8–8.7 and 27–241% O2 saturation). We investigated the effect of oxygen and pH fluctuations on coral calcification in the laboratory using the model species Acropora millepora. Light calcification rates were greatly enhanced (+178%) by increased seawater pH, but only at normoxia; hyperoxia completely negated this positive effect. Dark calcification rates were significantly inhibited (51–75%) at hypoxia, whereas pH had no effect. Our preliminary results suggest that within the current oxygen and pH range, oxygen has substantial control over coral growth, whereas the role of pH is limited. This has implications for reef formation in this era of rapid climate change, which is accompanied by a decrease in seawater oxygen saturation owing to higher water temperatures and coastal eutrophication
Sensitivity analysis to explain the excitability in a pyramidal neuron with application to Alzheimer’s disease
The detection of cluster magnetic fields via radio source depolarisation
It has been well established that galaxy clusters have magnetic fields. The
exact properties and origin of these magnetic fields are still uncertain even
though these fields play a key role in many astrophysical processes. Various
attempts have been made to derive the magnetic field strength and structure of
nearby galaxy clusters using Faraday rotation of extended cluster radio
sources. This approach needs to make various assumptions that could be
circumvented when using background radio sources. However, because the number
of polarised radio sources behind clusters is low, at the moment such a study
can only be done statistically. In this paper, we investigate the
depolarisation of radio sources inside and behind clusters in a sample of 124
massive clusters at observed with the Karl G. Jansky Very Large Array.
We detect a clear depolarisation trend with the cluster impact parameter, with
sources at smaller projected distances to the cluster centre showing more
depolarisation. By combining the radio observations with ancillary X-ray data
from Chandra, we compare the observed depolarisation with expectations from
cluster magnetic field models using individual cluster density profiles. The
best-fitting models have a central magnetic field strength of G with
power-law indices between and . We find no strong difference in the
depolarisation trend between sources embedded in clusters and background
sources located at similar projected radii, although the central region of
clusters is still poorly probed by background sources. We also examine the
depolarisation trend as a function of cluster properties such as the dynamical
state, mass, and redshift. Our findings show that the statistical
depolarisation of radio sources is a good probe of cluster magnetic field
parameters. [abridged]Comment: Replaced with Published version (A&A). 32 pages, 34 figure
Cytochrome oxidase subunit VI of Trypanosoma brucei is imported without a cleaved presequence and is developmentally regulated at both RNA and protein levels
Mitochondrial respiration in the African trypanosome undergoes dramatic developmental stage regulation. This requires co-ordinated control of components encoded by both the nuclear genome and the kinetoplast, the unusual mitochondrial genome of these parasites. As a model for understanding the co-ordination of these genomes, we have examined the regulation and mitochondrial import of a nuclear-encoded component of the cytochrome oxidase complex, cytochrome oxidase subunit VI (COXVI). By generating transgenic trypanosomes expressing intact or mutant forms of this protein, we demonstrate that COXVI is not imported using a conventional cleaved presequence and show that sequences at the N-terminus of the protein are necessary for correct mitochondrial sorting. Analyses of endogenous and transgenic COXVI mRNA and protein expression in parasites undergoing developmental stage differentiation demonstrates a temporal order of control involving regulation in the abundance of, first, mRNA and then protein. This represents the first dissection of the regulation and import of a nuclear-encoded protein into the cytochrome oxidase complex in these organisms, which were among the earliest eukaryotes to possess a mitochondrion
Warmer klimaat slecht voor veen in Friesland
Enkele jaren geleden is voor het westelijk veenweidegebied aangetoond dat een warmer klimaat leidt tot snellere afbraak van veenbodems. Nu zijn er ook gegevens over de Friese veenweiden. Als het echt opwarmt is in Friesland halverwege deze eeuw op veel plekken het veen praktisch verdwenen
A MeerKAT-meets-LOFAR study of Abell 1413: a moderately disturbed non-cool-core cluster hosting a kpc 'mini'-halo
Many relaxed cool-core clusters host diffuse radio emission on scales of
hundreds of kiloparsecs: mini-haloes. However, the mechanism responsible for
generating them, as well as their connection with central active galactic
nuclei, is elusive and many questions related to their physical properties and
origins remain unanswered. This paper presents new radio observations of the
galaxy cluster Abell 1413 performed with MeerKAT (L-band; 872 to 1712 MHz) and
LOFAR HBA (120 to 168 MHz) as part of a statistical and homogeneous census of
mini-haloes. Abell 1413 is unique among mini-halo clusters as it is a
moderately-disturbed non-cool-core cluster. Our study reveals an asymmetric
mini-halo up to 584 kpc in size at 1283 MHz, twice as large as first reported
at similar frequencies. The spectral index is flatter than previously reported,
with an integrated value of , shows significant
spatial variation, and a tentative radial steepening. We studied the
point-to-point X-ray/radio surface brightness correlation to investigate the
thermal/non-thermal connection: our results show a strong connection between
these components, with a super-linear slope of at 1283 MHz
and at 145 MHz. We also explore the X-ray surface
brightness/radio spectral index correlation, finding a slope of . Both investigations support the evidence of spectral steepening.
Finally, in the context of understanding the particle acceleration mechanism,
we present a simple theoretical model which demonstrates that hybrid scenarios
- secondary electrons (re-)accelerated by turbulence - reproduce a super-linear
correlation slope.Comment: 19 pages, 11 figures, 4 tables. Accepted for publication in MNRA
A giant radio bridge connecting two clusters in Abell 1758
Collisions between galaxy clusters dissipate enormous amounts of energy in
the intra-cluster medium (ICM) through turbulence and shocks. In the process,
Mpc-scale diffuse synchrotron emission in form of radio halos and relics can
form. However, little is known about the very early phase of the collision. We
used deep radio observations from 53 MHz to 1.5 GHz to study the pre-merging
galaxy clusters A1758N and A1758S that are Mpc apart. We confirm the
presence of a giant bridge of radio emission connecting the two systems that
was reported only tentatively in our earlier work. This is the second
large-scale radio bridge observed to date in a cluster pair. The bridge is
clearly visible in the LOFAR image at 144 MHz and tentatively detected at 53
MHz. Its mean radio emissivity is more than one order of magnitude lower than
that of the radio halos in A1758N and A1758S. Interestingly, the radio and
X-ray emissions of the bridge are correlated. Our results indicate that
non-thermal phenomena in the ICM can be generated also in the region of
compressed gas in-between infalling systems.Comment: 6 pages, 4 figures, 1 table. Accepted for publication in MNRAS
Letter
A giant radio bridge connecting two galaxy clusters in Abell 1758
Collisions between galaxy clusters dissipate enormous amounts of energy in the intracluster medium (ICM) through turbulence and shocks. In the process, Mpc-scale diffuse synchrotron emission in form of radio haloes and relics can form. However, little is known about the very early phase of the collision. We used deep radio observations from 53 MHz to 1.5 GHz to study the pre-merging galaxy clusters A1758N and A1758S that are ∼2 Mpc apart. We confirm the presence of a giant bridge of radio emission connecting the two systems that was reported only tentatively in our earlier work. This is the second large-scale radio bridge observed to date in a cluster pair. The bridge is clearly visible in the LOFAR image at 144 MHz and tentatively detected at 53 MHz. Its mean radio emissivity is more than one order of magnitude lower than that of the radio haloes in A1758N and A1758S. Interestingly, the radio and X-ray emissions of the bridge are correlated. Our results indicate that non-thermal phenomena in the ICM can be generated also in the region of compressed gas in-between infalling systems
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