632 research outputs found
An axon initial segment is required for temporal precision in action potential encoding by neuronal populations
Central neurons initiate action potentials (APs) in the axon initial segment
(AIS), a compartment characterized by a high concentration of voltage-dependent
ion channels and specialized cytoskeletal anchoring proteins arranged in a
regular nanoscale pattern. Although the AIS was a key evolutionary innovation
in neurons, the functional benefits it confers are not clear. Using a mutation
of the AIS cytoskeletal protein \beta IV-spectrin, we here establish an in
vitro model of neurons with a perturbed AIS architecture that retains nanoscale
order but loses the ability to maintain a high NaV density. Combining
experiments and simulations we show that a high NaV density in the AIS is not
required for axonal AP initiation; it is however crucial for a high bandwidth
of information encoding and AP timing precision. Our results provide the first
experimental demonstration of axonal AP initiation without high axonal channel
density and suggest that increasing the bandwidth of the neuronal code and
hence the computational efficiency of network function was a major benefit of
the evolution of the AIS.Comment: Title adjusted, no other change
Electrodynamic coupling of electric dipole emitters to a fluctuating mode density within a nano-cavity
We investigate the impact of rotational diffusion on the electrodynamic
coupling of fluorescent dye molecules (oscillating electric dipoles) to a
tunable planar metallic nanocavity. Fast rotational diffusion of the molecules
leads to a rapidly fluctuating mode density of the electromagnetic field along
the molecules' dipole axis, which significantly changes their coupling to the
field as compared to the opposite limit of fixed dipole orientation. We derive
a theoretical treatment of the problem and present experimental results for
rhodamine 6G molecules in cavities filled with low and high viscosity liquids.
The derived theory and presented experimental method is a powerful tool for
determining absolute quantum yield values of fluorescence.Comment: 5 pages, 3 figures, accepted by Physical Review Letter
Self-assembly of the simple cubic lattice with an isotropic potential
Conventional wisdom presumes that low-coordinated crystal ground states
require directional interactions. Using our recently introduced optimization
procedure to achieve self-assembly of targeted structures (Phys. Rev. Lett. 95,
228301 (2005), Phys. Rev. E 73, 011406 (2006)), we present an isotropic pair
potential for a three-dimensional many-particle system whose classical
ground state is the low-coordinated simple cubic (SC) lattice. This result is
part of an ongoing pursuit by the authors to develop analytical and
computational tools to solve statistical-mechanical inverse problems for the
purpose of achieving targeted self-assembly. The purpose of these methods is to
design interparticle interactions that cause self-assembly of technologically
important target structures for applications in photonics, catalysis,
separation, sensors and electronics. We also show that standard approximate
integral-equation theories of the liquid state that utilize pair correlation
function information cannot be used in the reverse mode to predict the correct
simple cubic potential. We report in passing optimized isotropic potentials
that yield the body-centered cubic and simple hexagonal lattices, which provide
other examples of non-close-packed structures that can be assembled using
isotropic pair interactions.Comment: 16 pages, 12 figures. Accepted for publication in Physical Review
Influence of static electric fields on an optical ion trap
We recently reported on a proof-of-principle experiment demonstrating optical
trapping of an ion in a single-beam dipole trap superimposed by a static
electric potential [Nat. Photonics 4, 772--775 (2010)]. Here, we first discuss
the experimental procedures focussing on the influence and consequences of the
static electric potential. These potentials can easily prevent successful
optical trapping, if their configuration is not chosen carefully. Afterwards,
we analyse the dipole trap experiments with different analytic models, in which
different approximations are applied. According to these models the
experimental results agree with recoil heating as the relevant heating effect.
In addition, a Monte-Carlo simulation has been developed to refine the
analysis. It reveals a large impact of the static electric potential on the
dipole trap experiments in general. While it supports the results of the
analytic models for the parameters used in the experiments, the analytic models
cease their validity for significantly different parameters. Finally, we
propose technical improvements for future realizations of experiments with
optically trapped ions.Comment: 16 pages, 16 figure
The Relationship between Brachycephalic Head Features in Modern Persian Cats and Dysmorphologies of the Skull and Internal Hydrocephalus
Background: Cat breeders observed a frequent occurrence of internal hydrocephalus in Persian cats with extreme brachycephalic head morphology. Objective: To investigate a possible relationship among the grade of brachycephaly, ventricular dilatation, and skull dysmorphologies in Persian cats. Animals: 92 Persian-, 10 Domestic shorthair cats. Methods: The grade of brachycephaly was determined on skull models based on CT datasets. Cranial measurements were examined with regard to a possible correlation with relative ventricular volume, and cranial capacity. Persians with high (peke-face Persians) and lower grades of brachycephaly (doll-face Persians) were investigated for the presence of skull dysmorphologies.
Results: The mean cranial index of the peke-face Persians (0.97 ± 0.14) was significantly higher than the mean cranial index of doll-face Persians (0.66 ± 0.04; P < 0.001). Peke-face Persians had a lower relative nasal bone length (0.15 ± 0.04) compared to doll-face (0.29 ± 0.08; P < 0.001). The endocranial volume was significantly lower in doll-face than peke-face Persians (89.6 ± 1.27% versus 91.76 ± 2.07%; P < 0.001). The cranial index was significantly correlated with this variable (Spearman´s r: 0.7; P < 0.0001).
Mean ventricle: Brain ratio of the peke-face group (0.159 ± 0.14) was significantly higher compared to doll-face Persians (0.015 ± 0.01; P < 0.001). Conclusion and Clinical Relevance: High grades of brachycephaly are also associated with malformations of the calvarial and facial bones as well as dental malformations. As these dysmorphologies can affect animal welfare, the selection for extreme forms of brachycephaly in Persian cats should be reconsidered
Surface sticking and lateral diffusion of lipids in supported bilayers
The diffusion of fluorescently labeled lipids in supported bilayers is studied using two different methods: Z-scan fluorescence correlation spectroscopy (z-scan FCS) and two-focus fluorescence correlation spectroscopy (2f-FCS). It is found that the data can be fitted consistently only when taking into account partial sticking of the labeled lipids to the supporting glass surface. A kinetic reaction-diffusion model is developed and applied to the data. We find a very slow sticking rate which, however, when neglected, leads to strongly varying estimates of the free diffusion coefficient. The study reveals a strong sensitivity of FCS on even slight binding/unbinding kinetics of the labeled molecules, which has significance for related diffusion measurements in cellular lipid membranes
Recommended from our members
The statistics of single molecule detection: An overview
An overview of our recent results in modeling single molecule detection in fluid flow is presented. Our mathematical approach is based on a path integral representation. The model accounts for all experimental details, such as light collection, laser excitation, hydrodynamics and diffusion, and molecular photophysics. Special attention is paid to multiple molecule crossings through the detection volume. Numerical realization of the theory is discussed. Measurements of burst size distributions in single B-phycoerythrin molecule detection experiments are presented and compared with theoretical predictions
Absolute quantum yield measurements of fluorescent proteins using a plasmonic nanocavity
One of the key photophysical properties of fluorescent proteins that is most difficult to measure is the quantum yield. It describes how efficiently a fluorophore converts absorbed light into fluorescence. Its measurement using conventional methods become particularly problematic when it is unknown how many of the proposedly fluorescent molecules of a sample are indeed fluorescent (for example due to incomplete maturation, or the presence of photophysical dark states). Here, we use a plasmonic nanocavity-based method to measure absolute quantum yield values of commonly used fluorescent proteins. The method is calibration-free, does not require knowledge about maturation or potential dark states, and works on minute amounts of sample. The insensitivity of the nanocavity-based method to the presence of non-luminescent species allowed us to measure precisely the quantum yield of photo-switchable proteins in their on-state and to analyze the origin of the residual fluorescence of protein ensembles switched to the dark state
Charge and spin distributions in GaMnAs/GaAs Ferromagnetic Multilayers
A self-consistent electronic structure calculation based on the
Luttinger-Kohn model is performed on GaMnAs/GaAs multilayers. The Diluted
Magnetic Semiconductor layers are assumed to be metallic and ferromagnetic. The
high Mn concentration (considered as 5% in our calculation) makes it possible
to assume the density of magnetic moments as a continuous distribution, when
treating the magnetic interaction between holes and the localized moment on the
Mn(++) sites. Our calculation shows the distribution of heavy holes and light
holes in the structure. A strong spin-polarization is observed, and the charge
is concentrated mostly on the GaMnAs layers, due to heavy and light holes with
their total angular momentum aligned anti-parallel to the average
magnetization. The charge and spin distributions are analyzed in terms of their
dependence on the number of multilayers, the widths of the GaMnAs and GaAs
layers, and the width of lateral GaAs layers at the borders of the structure.Comment: 12 pages,7 figure
Gap modification of atomically thin boron nitride by phonon mediated interactions
A theory is presented for the modification of bandgaps in atomically thin
boron nitride (BN) by attractive interactions mediated through phonons in a
polarizable substrate, or in the BN plane. Gap equations are solved, and gap
enhancements are found to range up to 70% for dimensionless electron-phonon
coupling \lambda=1, indicating that a proportion of the measured BN bandgap may
have a phonon origin
- …