BackgroundLens crystallines are special proteins in the eye lens. Because the epithelial basement membrane (lens capsule) completely encloses the lens, desquamation of aging cells is impossible, and due to the complete absence of blood vessels or transport of metabolites in this area, there is no subsequent remodelling of these fibers, nor removal of degraded lens fibers. Human tissue ultimately derives its 14C content from the atmospheric carbon dioxide. The 14C content of the lens proteins thus reflects the atmospheric content of 14C when the lens crystallines were formed. Precise radiocarbon dating is made possible by comparing the 14C content of the lens crystallines to the so-called bomb pulse, i.e. a plot of the atmospheric 14C content since the Second World War, when there was a significant increase due to nuclear-bomb testing. Since the change in concentration is significant even on a yearly basis this allows very accurate dating.Methodology/Principal FindingsOur results allow us to conclude that the crystalline formation in the lens nucleus almost entirely takes place around the time of birth, with a very small, and decreasing, continuous formation throughout life. The close relationship may be further expressed as a mathematical model, which takes into account the timing of the crystalline formation.Conclusions/SignificanceSuch a life-long permanence of human tissue has hitherto only been described for dental enamel. In confront to dental enamel it must be held in mind that the eye lens is a soft structure, subjected to almost continuous deformation, due to lens accommodation, yet its most important constituent, the lens crystalline, is never subject to turnover or remodelling once formed. The determination of the 14C content of various tissues may be used to assess turnover rates and degree of substitution (for example for brain cell DNA). Potential targets may be nervous tissues in terms of senile or pre-senile degradation, as well as other highly specialised structures of the eyes. The precision with which the year of birth may be calculated points to forensic uses of this technique.
Delta oscillations (1-4 Hz) associate with deep sleep and are implicated in memory consolidation and replay of cortical responses elicited during wake states. A potent local generator has been characterized in thalamus, and local generators in neocortex have been suggested. Here we demonstrate that isolated rat neocortex generates delta rhythms in conditions mimicking the neuromodulatory state during deep sleep (low cholinergic and dopaminergic tone). The rhythm originated in an NMDA receptor-driven network of intrinsic bursting (IB) neurons in layer 5, activating a source of GABA B receptor-mediated inhibition. In contrast, regular spiking (RS) neurons in layer 5 generated theta-frequency outputs. In layer 2/3 principal cells, outputs from IB cells associated with IPSPs, whereas those from layer 5 RS neurons related to nested bursts of theta-frequency EPSPs. Both interlaminar spike and field correlations revealed a sequence of events whereby sparse spiking in layer 2/3 was partially reflected back from layer 5 on each delta period. We suggest that these reciprocal, interlaminar interactions may represent a "Helmholtz machine"-like process to control synaptic rescaling during deep sleep.
The recent development of experimental photoionization cross sections of atomic ions has been reviewed. Owing to the construction of intense undulator-based photon sources it has been possible during the last ten years to perform a large number of absolute cross-section measurements using the merged-beam method. Photoionization cross sections provide a critical test of theoretical calculations, with implications for the modelling of astrophysical and laboratory plasmas for which fundamental data of this kind are required.
Absolute cross section for photodetachment of Li − ions near the 1s threshold leading to Li + formation has been measured by overlapping a beam of Li − ions with synchrotron radiation from an undulator. Multiple excitation to the coreexcited Li 1s(2s2p 3 P) 2 P and Li 1s(2p3s 3 P) 2 P states, i.e. conjugate shakeup, is found to be much stronger than observed for any neutral atom or positive ion, reflecting the dominant influence of electron correlation for negative ions. The cross section for multiple excitation is significantly larger than for single excitation.Negative ions represent a special class of atomic systems with properties markedly different from neutral atoms and positive ions. In recent years, weakly bound negative atomic ions have attracted considerable interest, and such ions are of fundamental importance in atomic physics. Their structure and dynamics form a critical test for theoretical calculations since the additional electron is bound by means of electron correlation. Recently theoretical and experimental studies of atomic negative ions have been reviewed focusing on their binding energies (Andersen et al 1999), resonances (Buckmann and Clark 1994), and photodetachment cross sections (Ivanov 1999). The experimental photodetachment studies have only been concerned with photodetachment of outer-shell electrons, with lasers being used as the photon source, whereas experimental cross sections for photodetachment of inner-shell electrons have never been reported. The reason may be a combination of the very low densities which can be obtained for negative atomic ions, the limiting photon flux available from bending magnets at synchrotron-radiation facilities and relatively small cross sections. However, with access to synchrotron-radiation beam lines equipped with insertion devices, the increased photon flux allows inner-shell photodetachment studies of atomic negative ions to be performed.This experimental study was initiated with the aim of exploring the competition between photoinduced single and multiple excitation of a weakly bound negative ion exhibiting strong correlation in the ground state. The negative ions of the alkali metals or the alkaline earth metals are suitable candidates for such an investigation (see Andersen et al 1999). We have chosen the Li − system due to its simplicity as a closed-shell ion with only 4 electrons and because the binding energy of its 1s electrons is convinient for the undulator beamline of the ASTRID storage ring. Li − is a highly correlated system-without introducing electron correlation the system is not bound. Thus, correlation is not a small perturbation on the system, rather a dominant factor of the behaviour of the system.Following the negative hydrogen ion, the negative lithium ion is the most important closedshell negative ion which is simple enough to be theoretically tractable in a rigorous manner.
The partial and total absolute cross sections for the photoionization of Xe + and Xe 2+ ions, respectively, have been measured in the 4d excitation region (50-130 eV). The experimental cross sections are compared with calculated values obtained by the random phase approximation method taking exchange and rearrangement into account. The maximum total cross section values are 27(3) Mb for both ions, a value also reported for the photoionization of the Xe atom. The reliability of the experimental cross sections has been tested by measuring the absolute photoionization cross section for the He + ion.
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