506 research outputs found
CONFINEMENT IN RELATIVISTIC POTENTIAL MODELS
In relativistic potential models of quarkonia based on a Dirac-type of
equation with a local potential there is a sharp distinction between a linear
potential V which is vector-like and one which is scalar-like: There are
normalizable solutions for a scalar-like V but not for a vector-like V. It is
pointed out that if instead one uses an equation of the no-pair type, which is
more natural from the viewpoint of field theory, this somewhat bizarre
difference disappears.Comment: LaTeX, 4 page
The Effects of L(+), D(-), and DL-2-amino-4-phosphonobutyrate (APB) on Electroretinogram and Ganglopn Cell Activity in the Cat Retina
L(+)-, D(-)-, or DL-2-amino-4-phosphonobutyric acid (APB; 2.5- 16 μmol) were injected into the vitreous body of anesthetized adult cats. The retina was stimulated by diffuse square wave light flashes (10- 60 ms). The flash-induced electroretinogram (ERU) and responses of single retinal ganglion cells (RGC) were recorded simultaneously. Intravitreal injection of L(+)APB led to a decrease in the ERG b-wave amplitude and the unmasking of the a-wave. The magnitude and rate of the b-wave reduction were different for the two enantiomers. The threshold dose of D(-)APB was 6 times higher than for L(+)APB. L(+)APB (8.2 ± 1.6 μmol; n=7) decreased the b-wave with an average time constant r = 88.5 min, D(-)APB (13.2 ± 1.1 μmol; n=6) with r = 357.1 min, and DL-APB (8.35 ± 1.1 μmol; n=5) with r = 101.0 min. Concomitant with the reduction of the b-wave, L(+)APB (2.7μmol) inhibited both the spontaneous and light-evoked firing in ON-center ganglion cells. The threshold doses of L(+), D(-) and DL-APB for inhibition of spontaneous adivity and the light response in ON-center cells paralleled those in reducing the ERU h-wave. Low doses of L(+)APB or DL-APB that were effective in blockng ON-center cell activity caused only very small changes in the activity of OFF-center ganglion cells. However, high doses of L(+)APB (≥8.2μmol) or DL-APB (≥13.7 μmol) also decreased the spontaneous and light-evoked activity in OFF-center RGC and first shortened than prolonged the light-induced inhibition of OFF-center RGC.Whitehall Foundation (S93-24
Ultrasound imaging of the carpal tunnel during median nerve compression
Median nerve (MN) compression is a recognized component of carpal tunnel syndrome (CTS). In order to document compressive changes in the MN during hand activity, the carpal tunnel was imaged with neuromuscular ultrasound (NMUS). Ten patients with CTS and five normal controls underwent NMUS of the MN at rest and during dynamic stress testing (DST). DST maneuvers involve sustained isometric flexion of the distal phalanges of the first three digits. During DST in the CTS patients, NMUS demonstrated MN compression between the contracting thenar muscles ventrally and the taut flexor tendons dorsally. The mean MN diameter decreased nearly 40%, with focal narrowing in the mid-distal carpal canal. Normal controls demonstrated no MN compression and a tendency towards MN enlargement, with an average diameter increase of 17%. Observing the pathologic mechanism of MN injury during common prehensile hand movements could help better understand how to treat and prevent CTS
Stability and Instability of Relativistic Electrons in Classical Electro magnetic Fields
The stability of matter composed of electrons and static nuclei is
investigated for a relativistic dynamics for the electrons given by a suitably
projected Dirac operator and with Coulomb interactions. In addition there is an
arbitrary classical magnetic field of finite energy. Despite the previously
known facts that ordinary nonrelativistic matter with magnetic fields, or
relativistic matter without magnetic fields is already unstable when the fine
structure constant, is too large it is noteworthy that the combination of the
two is still stable provided the projection onto the positive energy states of
the Dirac operator, which defines the electron, is chosen properly. A good
choice is to include the magnetic field in the definition. A bad choice, which
always leads to instability, is the usual one in which the positive energy
states are defined by the free Dirac operator. Both assertions are proved here.Comment: LaTeX fil
Molecular basis of glutamate toxicity in retinal ganglion cells
Loss of retinal ganglion cells (RGCs) is a hallmark of many ophthalmic diseases including glaucoma, retinal ischemia due to central artery occlusion, anterior ischemic optic neuropathy and may be significant in optic neuritis, optic nerve trauma, and AIDS. Recent research indicates that neurotoxicity is caused by excessive stimulation of receptors for excitatory amino acids (EAAs). In particular, the amino acid glutamate has been shown to act as a neurotoxin which exerts its toxic effect on RGCs predominantly through the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor. NMDA-receptor-mediated toxicity in RGCs is dependent on the influx of extracellular Ca2+. The increase in [Ca2+]i acts as a second messenger that sets in motion the cascade leading to eventual cell death. Glutamate stimulates its own release in a positive feedback loop by its interaction with the non-NMDA receptor subtypes. Ca2+-induced Ca2+ release and further influx of Ca2+ through voltage-gated Ca2+ channels after glutamate-induced depolarization contribute to glutamate toxicity. In vitro and in vivo studies suggest that the use of selective NMDA receptor antagonists or Ca2+ channel blockers should be useful in preventing or at least abating neuronal loss in the retina. Of particular importance for future clinical use of NMDA receptor antagonists in the treatment of acute vascular insults is the finding that some drugs can prevent glutamate-induced neurotoxicity, even when administered a few hours after the onset of retinal ischemia
Long Range Forces from Pseudoscalar Exchange
Using dispersion theoretic techniques, we consider coherent long range forces
arising from double pseudoscalar exchange among fermions. We find that Yukawa
type coupling leads to spin independent attractive potentials whereas
derivative coupling renders spin independent repulsive potentials.Comment: 27 pages, REVTeX, 3 figures included using epsfi
On the validity of the reduced Salpeter equation
We adapt a general method to solve both the full and reduced Salpeter
equations and systematically explore the conditions under which these two
equations give equivalent results in meson dynamics. The effects of constituent
mass, angular momentum state, type of interaction, and the nature of
confinement are all considered in an effort to clearly delineate the range of
validity of the reduced Salpeter approximations. We find that for
the solutions are strikingly similar for all
constituent masses. For zero angular momentum states the full and reduced
Salpeter equations give different results for small quark mass especially with
a large additive constant coordinate space potential. We also show that
corrections to heavy-light energy levels can be accurately
computed with the reduced equation.Comment: Latex (uses epsf macro), 24 pages of text, 12 postscript figures
included. Slightly revised version, to appear in Phys. Rev.
The exact Darwin Lagrangian
Darwin (1920) noted that when radiation can be neglected it should be
possible to eliminate the radiation degrees-of-freedom from the action of
classical electrodynamics and keep the discrete particle degrees-of-freedom
only. Darwin derived his well known Lagrangian by series expansion in
keeping terms up to order . Since radiation is due to acceleration the
assumption of low speed should not be necessary. A Lagrangian is suggested that
neglects radiation without assuming low speed. It cures deficiencies of the
Darwin Lagrangian in the ultra-relativistic regime.Comment: 2.5 pages, no figure
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