Complex systems in quantum technologies

124 p.

Laser-induced changes on the complex refractive indices of phase-change thin film

The refractive indices of crystalline phase-change films are usually obtained by thermal-induced crystallization. However, this is not accurate, because the crystallization of phase-change film in rewritable optical disks is laser induced. In this study, we use the initializer to crystallize the phase-change films. The dependence of the refractive index n and the extinction coefficient k of the phase-change films on the initialization conditions are studied. Remarkable changes of the refractive indices (especially k) are found when the initialization laser power density is 6.63 mW/mum(2) and the initialization velocity is 4.0 m/s. At the same time, the structure changes of the phase-change films are also studied. This dependence is explained by the structure change of the films. These results are significant in improving the accuracy of optical design and the thermal simulation of phase-change optical disks, as well as in the study of phase-change optical disks at shorter wavelengths. (C) 2003 Society of Photo-Optical Instrumentation Engineers.

Energy demands of diverse spiking cells from the neocortex, hippocampus, and thalamus

It has long been known that neurons in the brain are not physiologically homogeneous. In response to current stimulus, they can fire several distinct patterns of action potentials that are associated with different physiological classes ranging from regular-spiking cells, fast-spiking cells, intrinsically bursting cells, and low-threshold cells. In this work we show that the high degree of variability in firing characteristics of action potentials among these cells is accompanied with a significant variability in the energy demands required to restore the concentration gradients after an action potential. The values of the metabolic energy were calculated for a wide range of cell temperatures and stimulus intensities following two different approaches. The first one is based on the amount of Na+ load crossing the membrane during a single action potential, while the second one focuses on the electrochemical energy functions deduced from the dynamics of the computational neuron models. The results show that the thalamocortical relay neuron is the most energy-efficient cell consuming between 7 and 18 nJ/cm(2) for each spike generated, while both the regular and fast spiking cells from somatosensory cortex and the intrinsically-bursting cell from a cat visual cortex are the least energy-efficient, and can consume up to 100 nJ/cm(2) per spike. The lowest values of these energy demands were achieved at higher temperatures and high external stimuli.

Pivoting between Calmodulin Lobes Triggered by Calcium in the Kv7.2/Calmodulin Complex

Kv7.2 (KCNQ2) is the principal molecular component of the slow voltage gated M-channel, which strongly influences neuronal excitability. Calmodulin (CaM) binds to two intracellular C-terminal segments of Kv7.2 channels, helices A and B, and it is required for exit from the endoplasmic reticulum. However, the molecular mechanisms by which CaM controls channel trafficking are currently unknown. Here we used two complementary approaches to explore the molecular events underlying the association between CaM and Kv7.2 and their regulation by Ca2+. First, we performed a fluorometric assay using dansylated calmodulin (D-CaM) to characterize the interaction of its individual lobes to the Kv7.2 CaM binding site (Q2AB). Second, we explored the association of Q2AB with CaM by NMR spectroscopy, using N-15-labeled CaM as a reporter. The combined data highlight the interdependency of the N- and C-lobes of CaM in the interaction with Q2AB, suggesting that when CaM binds Ca2+ the binding interface pivots between the N-lobe whose interactions are dominated by helix B and the C-lobe where the predominant interaction is with helix A. In addition, Ca2+ makes CaM binding to Q2AB more difficult and, reciprocally, the channel weakens the association of CaM with Ca2+.