313 research outputs found
The coupling between Ca2+ channels and the exocytotic Ca2+ sensor at hair cell ribbon synapses varies tonotopically along the mature cochlea.
The cochlea processes auditory signals over a wide range of frequencies and intensities. However, the transfer characteristics at hair cell ribbon synapses are still poorly understood at different frequency locations along the cochlea. Using recordings from mature gerbils, we report here a surprisingly strong block of exocytosis by the slow Ca(2+) buffer EGTA (10 mM) in basal hair cells tuned to high frequencies (âŒ30 kHz). In addition, using recordings from gerbil, mouse and bullfrog auditory organs, we find that the spatial coupling between Ca(2+) influx and exocytosis changes from nanodomain in low-frequency tuned hair cells (âŒ2 kHz). Hair cell synapses have thus developed remarkable frequency-dependent tuning of exocytosis: accurate low-latency encoding of onset and offset of sound intensity in the cochlea's base and submillisecond encoding of membrane receptor potential fluctuations in the apex for precise phase-locking to sound signals. We also found that synaptic vesicle pool recovery from depletion was sensitive to high concentrations of EGTA, suggesting that intracellular Ca(2+) buffers play an important role in vesicle recruitment in both low- and high-frequency hair cells. In conclusion, our results indicate that microdomain coupling plays an important role in the exocytosis of high-frequency hair cells, and suggest a novel hypothesis for why these cells are more susceptible to sound-induced damage than low-frequency cells; high-frequency IHCs must have a low Ca(2+) buffer capacity in order to sustain exocytosis, thus making them more prone to Ca(2+)-induced cytotoxicity. SIGNIFICANCE STATEMENT: In the inner ear, sensory hair cells signal reception of sound. They do this by converting the sound induced movement of their hair bundles present at the top of these cells, into an electrical current. This current depolarizes the hair cell and triggers the calcium-induced release of the neurotransmitter glutamate that activates the postsynaptic auditory fibres. The speed and precision of this process enables the brain to perceive the vital components of sound such as frequency and intensity. We show that the coupling strength between calcium channels and the exocytosis calcium sensor at inner hair cell synapses changes along the mammalian cochlea such that the timing and/or intensity of sound is encoded with high precision
Critical Phenomena in Continuous Dimension
We present a calculation of critical phenomena directly in continuous
dimension d employing an exact renormalization group equation for the effective
average action. For an Ising-type scalar field theory we calculate the critical
exponents nu(d) and eta(d) both from a lowest--order and a complete
first--order derivative expansion of the effective average action. In
particular, this can be used to study critical behavior as a function of
dimensionality at fixed temperature.Comment: 5 pages, 1 figure, PLB version, references adde
Extranatural Inflation
We present a new model of inflation in which the inflaton is the extra
component of a gauge field in a 5d theory compactified on a circle. The chief
merit of this model is that the potential comes only from non-local effects so
that its flatness is not spoiled by higher dimensional operators or quantum
gravity corrections. The model predicts a red spectrum (n ~ 0.96) and a
significant production of gravitational waves (r ~ 0.11). We also comment on
the relevance of this idea to quintessence.Comment: 4 pages. Minor corrections and references added. Accepted for PR
New Form of the T-Duality Due to the Stability of a Compact Dimension
We study behaviors of a compact dimension and the -duality, in the
presence of the wrapped closed bosonic strings. When the closed strings
interact and form another system of strings, the radius of compactification
increases. This modifies the -duality, which we call it as -duality-like.
Some effects of the -duality-like will be studied.Comment: 12 pages, Latex, no figur
On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions
Within a hydrodynamical approach we investigate the sensitivity of single
inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three
different equations of state of nuclear matter. Two of the equations of state
are based on lattice QCD results and include a phase transition to a
quark-gluon plasma. The third equation of state has been extracted from the
microscopic transport code RQMD under the assumption of complete local
thermalization. All three equations of state provide reasonable fits to data
taken by the NA44 and NA49 Collaborations. The initial conditions before the
evolution of the fireballs and the space-time evolution pictures differ
dramatically for the three equations of state when the same freeze-out
temperature is used in all calculations. However, the softest of the equations
of state results in transverse mass spectra that are too steep in the central
rapidity region. We conclude that the transverse particle momenta are
determined by the effective softness of the equation of state during the
fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of
the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm
The extent of strangeness in equilibration in quark-gluon plasma
The evolution and production of strangeness from chemically equilibrating and
transversely expanding quark gluon plasma which may be formed in the wake of
relativistic heavy ion collisions is studied with initial conditions obtained
from the Self Screened Parton Cascade (SSPC) model. The extent of partonic
equilibration increases almost linearly with the square of the initial energy
density, which can then be scaled with number of participants.Comment: 4 pages including three figures, talk given at ICPAQGP'01, Jaipur,
India, to appear in Pramana - Journal of Physics, Indian Academy of Scienc
Heterogeneous Ca2+ influx along the adult calyx of held: A structural and computational study
The calyx of Held is a morphologically complex nerve terminal containing hundreds to thousands of active zones. The calyx must support high rates of transient, sound-evoked vesicular release superimposed on a background of sustained release, due to the high spontaneous rates of some afferent fibers. One means of distributing vesicle release in space and time is to have heterogeneous release probabilities (Pr) at distinct active zones, which has been observed at several CNS synapses including the calyx of Held. Pr may be modulated by vesicle proximity to Ca2+ channels, by Ca2+ buffers, by changes in phosphorylation state of proteins involved in the release process, or by local variations in Ca2+ influx. In this study, we explore the idea that the complex geometry of the calyx also contributes to heterogeneous Pr by impeding equal propagation of action potentials through all calyx compartments. Given the difficulty of probing ion channel distribution and recording from adult calyces, we undertook a structural and modeling approach based on computerized reconstructions of calyces labeled in adult cats. We were thus able to manipulate placement of conductances and test their effects on Ca2+ concentration in all regions of the calyx following an evoked action potential in the calyceal axon. Our results indicate that with a non-uniform distribution of Na+ and K+ channels, action potentials do not propagate uniformly into the calyx, Ca2+ influx varies across different release sites, and latency for these events varies among calyx compartments. We suggest that the electrotonic structure of the calyx of Held, which our modeling efforts indicate is very sensitive to the axial resistivity of cytoplasm, may contribute to variations in release probability within the calyx
Boundary and expansion effects on two-pion correlation functions in relativistic heavy-ion collisions
We examine the effects that a confining boundary together with hydrodynamical
expansion play on two-pion distributions in relativistic heavy-ion collisions.
We show that the effects arise from the introduction of further correlations
due both to collective motion and the system's finite size. As is well known,
the former leads to a reduction in the apparent source radius with increasing
average pair momentum K. However, for small K, the presence of the boundary
leads to a decrease of the apparent source radius with decreasing K. These two
competing effects produce a maximum for the effective source radius as a
function of K.Comment: 6 pages, 5 Eps figures, uses RevTeX and epsfi
Equilibration in Quark Gluon Plasma
The hydrodynamic expansion rate of quark gluon plasma (QGP) is evaluated and
compared with the scattering rate of quarks and gluons within the system.
Partonic scattering rates evaluated within the ambit of perturbative Quantum
Choromodynamics (pQCD) are found to be smaller than the expansion rate
evaluated with ideal equation of state (EoS) for the QGP. This indicate that
during the space-time evolution the system remains out of equilibrium.
Enhancement of pQCD cross sections and a more realistic EoS keep the partons
closer to the equilibrium.Comment: To be published in the Quark Matter 2008 poster proceeding
Flavor Phenomenology in General 5D Warped Spaces
We have considered a general 5D warped model with SM fields propagating in
the bulk and computed explicit expressions for oblique and non-oblique
electroweak observables as well as for flavor and CP violating effective
four-fermion operators. We have compared the resulting lower bounds on the
Kaluza-Klein (KK) scale in the RS model and a recently proposed model with a
metric modified towards the IR brane, which is consistent with oblique
parameters without the need for a custodial symmetry. We have randomly
generated 40,000 sets of O(1) 5D Yukawa couplings and made a fit of the quark
masses and CKM matrix elements in both models. This method allows to identify
the percentage of points consistent with a given KK mass, which in turn
provides us with a measure for the required fine-tuning. Comparison with
current experimental data on Rb, FCNC and CP violating operators exhibits an
improved behavior of our model with respect to the RS model. In particular,
allowing 10% fine-tuning the combined results point towards upper bounds on the
KK gauge boson masses around 3.3 TeV in our model as compared with 13 TeV in
the RS model. One reason for this improvement is that fermions in our model are
shifted, with respect to fermions in the RS model, towards the UV brane thus
decreasing the strength of the modifications of electroweak observables.Comment: 28 pages, 7 figures, 4 table
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