7,263 research outputs found
Molecular basis of gap junctional communication in the CNS of the leech Hirudo medicinalis
Gap junctions are intercellular channels that allow the passage of ions and small molecules between cells. In the nervous system, gap junctions mediate electrical coupling between neurons. Despite sharing a common topology and similar physiology, two unrelated gap junction protein families exist in the animal kingdom. Vertebrate gap junctions are formed by members of the connexin family, whereas invertebrate gap junctions are composed of innexin proteins. Here we report the cloning of two innexins from the leech Hirudo medicinalis. These innexins show a differential expression in the leech CNS: Hm-inx1 is expressed by every neuron in the CNS but not in glia, whereas Hm-inx2 is expressed in glia but not neurons. Heterologous expression in the paired Xenopus oocyte system demonstrated that both innexins are able to form functional homotypic gap junctions. Hm-inx1 forms channels that are not strongly gated. In contrast, Hm-inx2 forms channels that are highly voltage-dependent; these channels demonstrate properties resembling those of a double rectifier. In addition, Hm-inx1 and Hm-inx2 are able to cooperate to form heterotypic gap junctions in Xenopus oocytes. The behavior of these channels is primarily that predicted from the properties of the constituent hemichannels but also demonstrates evidence of an interaction between the two. This work represents the first demonstration of a functional gap junction protein from a Lophotrochozoan animal and supports the hypothesis that connexin-based communication is restricted to the deuterostome clade
Coherence-Preserving Quantum Bits
Real quantum systems couple to their environment and lose their intrinsic
quantum nature through the process known as decoherence. Here we present a
method for minimizing decoherence by making it energetically unfavorable. We
present a Hamiltonian made up solely of two-body interactions between four
two-level systems (qubits) which has a two-fold degenerate ground state. This
degenerate ground state has the property that any decoherence process acting on
an individual physical qubit must supply energy from the bath to the system.
Quantum information can be encoded into the degeneracy of the ground state and
such coherence-preserving qubits will then be robust to local decoherence at
low bath temperatures. We show how this quantum information can be universally
manipulated and indicate how this approach may be applied to a quantum dot
quantum computer.Comment: 5 pages, 1 figur
The Optimal Single Copy Measurement for the Hidden Subgroup Problem
The optimization of measurements for the state distinction problem has
recently been applied to the theory of quantum algorithms with considerable
successes, including efficient new quantum algorithms for the non-abelian
hidden subgroup problem. Previous work has identified the optimal single copy
measurement for the hidden subgroup problem over abelian groups as well as for
the non-abelian problem in the setting where the subgroups are restricted to be
all conjugate to each other. Here we describe the optimal single copy
measurement for the hidden subgroup problem when all of the subgroups of the
group are given with equal a priori probability. The optimal measurement is
seen to be a hybrid of the two previously discovered single copy optimal
measurements for the hidden subgroup problem.Comment: 8 pages. Error in main proof fixe
Transcriptional control of behaviour: engrailed knockout changes cockroach escape trajectories
The cerci of the cockroach are covered with identified sensory hairs that detect air movements. The sensory neurons that innervate these hairs synapse with giant interneurons in the terminal ganglion that in turn synapse with interneurons and leg motor neurons in thoracic ganglia. This neural circuit mediates the animal's escape behavior. The transcription factor Engrailed (En) is expressed only in the medially born sensory neurons, which suggested that it could work as a positional determinant of sensory neuron identity. Previously, we used double-stranded RNA interference to abolish En expression and found that the axonal arborization and synaptic outputs of an identified En-positive sensory neuron changed so that it came to resemble a nearby En-negative cell, which was itself unaffected. We thus demonstrated directly that En controls synaptic choice, as well as axon projections. Is escape behavior affected as a result of this miswiring? We showed recently that adult cockroaches keep each escape unpredictable by running along one of a set of preferred escape trajectories (ETs) at fixed angles from the direction of the threatening stimulus. The probability of selecting a particular ET is influenced by wind direction. In this present study, we show that early instar juvenile cockroaches also use those same ETs. En knock-out significantly perturbs the animals' perception of posterior wind, altering the choice of ETs to one more appropriate for anterior wind. This is the first time that it has been shown that knock-out of a transcription factor controlling synaptic connectivity can alter the perception of a directional stimulus
Weak Gravitational Flexion
Flexion is the significant third-order weak gravitational lensing effect
responsible for the weakly skewed and arc-like appearance of lensed galaxies.
Here we demonstrate how flexion measurements can be used to measure galaxy halo
density profiles and large-scale structure on non-linear scales, via
galaxy-galaxy lensing, dark matter mapping and cosmic flexion correlation
functions. We describe the origin of gravitational flexion, and discuss its
four components, two of which are first described here. We also introduce an
efficient complex formalism for all orders of lensing distortion. We proceed to
examine the flexion predictions for galaxy-galaxy lensing, examining isothermal
sphere and Navarro, Frenk & White (NFW) profiles and both circularly symmetric
and elliptical cases. We show that in combination with shear we can precisely
measure galaxy masses and NFW halo concentrations. We also show how flexion
measurements can be used to reconstruct mass maps in 2-D projection on the sky,
and in 3-D in combination with redshift data. Finally, we examine the
predictions for cosmic flexion, including convergence-flexion
cross-correlations, and find that the signal is an effective probe of structure
on non-linear scales.Comment: 17 pages, including 12 figures, submitted to MNRA
Probing the presence of a single or binary black hole in the globular cluster NGC 6752 with pulsar dynamics
The five millisecond pulsars that inhabit NGC 6752 display locations or
accelerations that are quite unusual compared to all other pulsars known in
globular clusters. In particular PSR-A, a binary pulsar, lives in the cluster
halo, while PSR-B and PSR-E, located in the core, show remarkably high negative
spin derivatives. This is suggestive that some uncommon dynamical process is at
play in the cluster core that we attribute to the presence of a massive
perturber. We here investigate whether a single intermediate-mass black hole,
lying on the extrapolation of the Mass versus Sigma relation observed in galaxy
spheroids, or a less massive binary consisting of two black holes could play
the requested role. To this purpose we simulated binary-binary encounters
involving PSR-A, its companion star, and the black hole(s). Various scenarios
are discussed in detail. In our close 4-body encounters, a black hole-black
hole binary may attract on a long-term stable orbit a millisecond pulsar.
Timing measurements on the captured satellite-pulsar, member of a hierarchical
triplet, could unambiguously unveil the presence of a black hole(s) in the core
of a globular cluster.Comment: 13 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
Teen Fathers: Implications for School Counselors
Historically researchers have focused on teen mothers, while neglecting teen fathers. Teen fathers are portrayed negatively and are often described as using teen girl only for sex and abandoning her when she tells him she is pregnant. Six teen fathers were interviewed in an urban school district in the southeastern region of the United States. The teen fathers expressed their needs, stressors, and involvement with their children. Suggestions for high school counselors working with this at-risk population are discussed as well as suggestions for future research
Finite automata for caching in matrix product algorithms
A diagram is introduced for visualizing matrix product states which makes
transparent a connection between matrix product factorizations of states and
operators, and complex weighted finite state automata. It is then shown how one
can proceed in the opposite direction: writing an automaton that ``generates''
an operator gives one an immediate matrix product factorization of it. Matrix
product factorizations have the advantage of reducing the cost of computing
expectation values by facilitating caching of intermediate calculations. Thus
our connection to complex weighted finite state automata yields insight into
what allows for efficient caching in matrix product algorithms. Finally, these
techniques are generalized to the case of multiple dimensions.Comment: 18 pages, 19 figures, LaTeX; numerous improvements have been made to
the manuscript in response to referee feedbac
Density mapping with weak lensing and phase information
The available probes of the large scale structure in the Universe have
distinct properties: galaxies are a high resolution but biased tracer of mass,
while weak lensing avoids such biases but, due to low signal-to-noise ratio,
has poor resolution. We investigate reconstructing the projected density field
using the complementarity of weak lensing and galaxy positions. We propose a
maximum-probability reconstruction of the 2D lensing convergence with a
likelihood term for shear data and a prior on the Fourier phases constructed
from the galaxy positions. By considering only the phases of the galaxy field,
we evade the unknown value of the bias and allow it to be calibrated by lensing
on a mode-by-mode basis. By applying this method to a realistic simulated
galaxy shear catalogue, we find that a weak prior on phases provides a good
quality reconstruction down to scales beyond l=1000, far into the noise domain
of the lensing signal alone.Comment: 11 pages, 9 figures, published in MNRA
Mapping the inner regions of the polar disk galaxy NGC4650A with MUSE
[abridged] The polar disk galaxy NGC4650A was observed during the
commissioning of the MUSE at the ESO VLT to obtain the first 2D map of the
velocity and velocity dispersion for both stars and gas. The new MUSE data
allow the analysis of the structure and kinematics towards the central regions
of NGC4650A, where the two components co-exist. These regions were unexplored
by the previous long-slit literature data available for this galaxy. The
extended view of NGC~4650A given by the MUSE data is a galaxy made of two
perpendicular disks that remain distinct and drive the kinematics right into
the very centre of this object. In order to match this observed structure for
NGC4650A, we constructed a multicomponent mass model made by the combined
projection of two disks. By comparing the observations with the 2D kinematics
derived from the model, we found that the modelled mass distribution in these
two disks can, on average, account for the complex kinematics revealed by the
MUSE data, also in the central regions of the galaxy where the two components
coexist. This result is a strong constraint on the dynamics and formation
history of this galaxy; it further supports the idea that polar disk galaxies
like NGC~4650A were formed through the accretion of material that has different
angular momentum.Comment: 14 pages, 10 figures; accepted for publication in Astronomy &
Astrophysic
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