Slowing of axonal regeneration is correlated with increased axonal viscosity during aging

Background  

As we age, the speed of axonal regeneration declines. At the biophysical level, why this occurs is not well understood.     

Validity of electron-temperature measurements using continuum plasma emission

In fusion plasmas, the electron temperature T_e is often deduced from the slope of the continuum spectrum. The known variation of the emissivity coefficients with $\text{h}\text{?}\text{ω}\text{kT}{}_{\mathrm{e}}$ applies rigorously for the Kramers cross-sections and a Maxwellian electron distribution. We discuss the validity of the diagnostic method for selected cases when one or both of these assumptions are unjustified. Examples are given for a hot, dense aluminum plasma. The non-Maxwellian situation corresponds to the critical density region of a laser-produced plasma. Temperatures which would be deduced from the traditional diagnostic are shown to be often erroneous by a factor of a few units.     

Automated thermal conductivity apparatus

The importance of reducing the time required for taking thermal conductivity measurements has increased due to the need to conserve energy. A method is described that reduces man-hours to a minimum while getting the maximum information from the measurements.The equipment uses a microprocessor to automatically scan the temperature, a data logger to make the required temperature measurements, and a computer to plot the results of the thermal conductivity values obtained. Early experiments produced values accurate to within 5%. With further refinements this can be improved to 1%.Already, the effect of curing thermosetting materials has been seen in measurements. Future applications can include tailoring materials to provide specific thermal conductivity properties and make it possible for some of the differences between a priori calculations and experiments to be better understood.     

Numerical decomposition of a convex function

Given then×p orthogonal matrixA and the convex functionf:R ⁿR, we find two orthogonal matricesP andQ such thatf is almost constant on the convex hull of ± the columns ofP, f is sufficiently nonconstant on the column space ofQ, and the column spaces ofP andQ provide an orthogonal direct sum decomposition of the column space ofA. This provides a numerically stable algorithm for calculating the cone of directions of constancy, at a pointx, of a convex function. Applications to convex programming are discussed.This work was supported by the National Science and Engineering Research Council of Canada (Grant No. A3388 and Summer Grant).     

On the origin of the double cellular structure of the detonation in gaseous nitromethane and its mixtures with oxygen

An experimental study of the detonation in gaseous nitromethane (NM) and nitromethane-oxygen mixtures has exhibited unambiguously the existence of a double cellular structure in the range of equivalence ratio from 1.3 to 1.75 (NM). Calculations of the reaction zone of the detonation in the same range of equivalence ratio, using a detailed chemical scheme in the ZND model, demonstrate that the chemical energy is released in two main successive distinct exothermic reaction steps characterized by their own induction length which justifies the existence of a two levels detonation cellular structure. This result strengthens the idea that the cellular detonation structure finds its origin in instabilities amplified by delayed local high energy release rate inside the reaction zone.Received: 2 February 2002, Accepted: 27 May 2004, Published online: 7 December 2004[/PUBLISHED]H.-N. Presles: Correspondence to     

Low hydrocarbon mixtures ignition delay times investigation behind reflected shock waves

Ignition delays for low alkanes/oxygen mixtures highly diluted with argon were measured behind a reflected shock wave using ultraviolet emission spectrometry in wide ranges of temperature (1200–2700 K), pressure (0.1–1.8 MPa), equivalence ratio (0.5–2) and dilution (89–99%). For each alkane (methane, ethane and propane), a correlation between ignition delay time, temperature, pressure and concentration is proposed and compared with those obtained in previous studies. This correlation enables the estimation of the delay time with an accuracy better than 20% for all measurement ranges. Results are compared with those from a recent study of the detailed kinetic modelling of the alkane oxidation. Received 12 March 2001 / Accepted 28 August 2001     

Ammonium transporters achieve charge transfer by fragmenting their substrate

Proteins of the Amt/MEP family facilitate ammonium transport across the membranes of plants, fungi, and bacteria and are essential for growth in nitrogen-poor environments. Some are known to facilitate the diffusion of the neutral NH(3), while others, notably in plants, transport the positively charged NH(4)(+). On the basis of the structural data for AmtB from Escherichia coli , we illustrate the mechanism by which proteins from the Amt family can sustain electrogenic transport. Free energy calculations show that NH(4)(+) is stable in the AmtB pore, reaching a binding site from which it can spontaneously transfer a proton to a pore-lining histidine residue (His168). The substrate diffuses down the pore in the form of NH(3), while the excess proton is cotransported through a highly conserved hydrogen-bonded His168-His318 pair. This constitutes a novel permeation mechanism that confers to the histidine dyad an essential mechanistic role that was so far unknown.     

Mechanism of aluminium spike formation and dissipation in electrothermal atomic absorption spectrometry

The mechanism of aluminium spike formation and dissipation of aluminium atoms in electrothermal atomization absorption spectrometry has been investigated using two different approaches. The first approach employs a graphite electrothermal atomizer coupled to an inductively coupled plasma mass spectrometer (ICP-MS) in a configuration that allows simultaneous measurement of atomic, or molecular, absorption signals and mass spectrometric signals. Aluminium sub-oxide (AlO and Al₂O) and CO(g) spikes in ICP-MS are correlated with the appearance of both Al atom spikes and Al-containing molecule spikes in absorption spectrometry. The aluminium carbide (AlC₂) signal in ICP-MS is not coincident with the appearance of either Al atom spikes or Al-containing molecule spikes in absorption spectrometry. The second approach uses two different imaging systems, i.e. shadow spectral filming (SSF) and shadow spectral digital imaging (SSDI), to provide temporally and spatially resolved absorption profiles of Al atoms and Al-containing molecules during Al spike formation and dissipation. The transverse cross-sectional distribution of Al atoms and of Al-containing molecules in the graphite furnace are complementary to one another for both wall and platform atomization. The highest concentration of Al atoms is near the graphite surface, whereas the highest concentration of Al-containing molecular species is at the centre of the graphite tube. The Al-containing molecules observed in both wall and platform atomization consist of both gaseous Al-molecules and a non-uniformly distributed cloud of finely dispersed Al₂O₃(s,1) particles. A mechanism of formation that is consistent with the above experimental observations is presented. It is proposed that Al atom spikes are formed from gaseous Al₂O precursors and that this reaction is triggered by the formation of a molten, condensed-phase Al₄C₃ melt.