Outward Conduction Research Articles

Effect of the silicon-carbide micro- and nanoparticle size on the thermo-elastic and time-dependent creep response of a rotating Al–SiC composite cylinder

The history of stresses and creep strains of a rotating composite cylinder made of an aluminum matrix reinforced by silicon carbide particles is investigated. The effect of uniformly distributed SiC micro- and nanoparticles on the initial thermo-elastic and time-dependent creep deformation is studied. The material creep behavior is described by Sherby’s constitutive model where the creep parameters are functions of temperature and the particle sizes vary from 50 nm to 45.9 μm. Loading is composed of a temperature field due to outward steady-state heat conduction and an inertia body force due to cylinder rotation. Based on the equilibrium equation and also stress-strain and strain-displacement relations, a constitutive second-order differential equation for displacements with variable and time-dependent coefficients is obtained. By solving this differential equation together with the Prandtl–Reuss relation and the material creep constitutive model, the history of stresses and creep strains is obtained. It is found that the minimum effective stresses are reached in a material reinforced by uniformly distributed SiC particles with the volume fraction of 20% and particle size of 50 nm. It is also found that the effective and tangential stresses increase with time at the inner surface of the composite cylinder; however, their variation at the outer surface is insignificant.

Different inward and outward conduction mechanisms in NaVMs suggested by molecular dynamics simulations.

Rapid and selective ion transport is essential for the generation and regulation of electrical signaling pathways in living organisms. Here, we use molecular dynamics (MD) simulations with an applied membrane potential to investigate the ion flux of bacterial sodium channel NaVMs. 5.9 µs simulations with 500 mM NaCl suggest different mechanisms for inward and outward flux. The predicted inward conductance rate of ∼27±3 pS, agrees with experiment. The estimated outward conductance rate is 15±3 pS, which is considerably lower. Comparing inward and outward flux, the mean ion dwell time in the selectivity filter (SF) is prolonged from 13.5±0.6 ns to 20.1±1.1 ns. Analysis of the Na+ distribution revealed distinct patterns for influx and efflux events. In 32.0±5.9% of the simulation time, the E53 side chains adopted a flipped conformation during outward conduction, whereas this conformational change was rarely observed (2.7±0.5%) during influx. Further, simulations with dihedral restraints revealed that influx is less affected by the E53 conformational flexibility. In contrast, during outward conduction, our simulations indicate that the flipped E53 conformation provides direct coordination for Na+. The free energy profile (potential of mean force calculations) indicates that this conformational change lowers the putative barriers between sites SCEN and SHFS during outward conduction. We hypothesize that during an action potential, the increased Na+ outward transition propensities at depolarizing potentials might increase the probability of E53 conformational changes in the SF. Subsequently, this might be a first step towards initiating slow inactivation.

Multi-ion versus single-ion conduction mechanisms can yield current rectification in biological ion channels

There is clear evidence that the net magnitude of negative charge at the intracellular end of inwardly rectifying potassium channels helps to generate an asymmetry in the magnitude of the current that will pass in each direction. However, a complete understanding of the physical mechanism that links these charges to current rectification has yet to be obtained. Using Brownian dynamics, we compare the conduction mechanism and binding sites in rectifying and non-rectifying channel models. We find that in our models, rectification is a consequence of asymmetry in the hydrophobicity and charge of the pore lining. As a consequence, inward conduction can occur by a multi-ion conduction mechanism. However, outward conduction is restricted, since there are fewer ions at the intracellular entrance and outwardly moving ions must cross the pore on their own. We pose the question as to whether the same mechanism could be at play in inwardly rectifying potassium channels.

Numerical calculations of two-dimensional large Prandtl number convection in a box

AbstractConvection from an isolated heat source in a chamber has been previously studied numerically, experimentally and analytically. These have not covered long time spans for wide ranges of Rayleigh number Ra and Prandtl number Pr. Numerical calculations of constant viscosity convection partially fill the gap in the ranges $\mathit{Ra}= 1{0}^{3} {{\unicode{x2013}}}1{0}^{6} $ and $\mathit{Pr}= 1, 10, 100, 1000$ and $\infty $. Calculations begin with cold fluid everywhere and localized hot temperature at the centre of the bottom of a square two-dimensional chamber. For $\mathit{Ra}\gt 20\hspace{0.167em} 000$, temperature increases above the hot bottom and forms a rising plume head. The head has small internal recirculation and minor outward conduction of heat during ascent. The head approaches the top, flattens, splits and the two remnants are swept to the sidewalls and diffused away. The maximum velocity and the top centre heat flux climb to maxima during head ascent and then adjust toward constant values. Two steady cells are separated by a vertical thermal conduit. This sequence is followed for every value of $Pr$ number, although lower Pr convection lags in time. For $\mathit{Ra}\lt 20\hspace{0.167em} 000$ there is no plume head, and no streamfunction and heat flux maxima with time. For sufficiently large Ra and all values of Pr, an oscillation develops at roughly $t= 0. 2$, with the two cells alternately strengthening and weakening. This changes to a steady flow with two unequal cells that at roughly $t= 0. 5$ develops a second oscillation.

Conduction in a Biological Sodium Selective Channel

The crystal structure of NavAb, a bacterial voltage gated Na(+) channel, exhibits a selectivity filter (SF) wider than that of K(+) channels. This new structure provides the opportunity to explore the mechanism of conduction and help rationalize its selectivity for sodium. Recent molecular dynamics (MD) simulations of single- and two-ion permeation processes have revealed that a partially hydrated Na(+) permeates the channel by exploring three SF binding sites while being loosely coupled to other ions and/or water molecules; a finding that differs significantly from the behavior of K(+) selective channels. Herein, we present results derived from a combination of metadynamics and voltage-biased MD simulations that throws more light on the nature of the Na(+) conduction mechanism. Conduction under 0 mV bias explores several distinct pathways involving the binding of two ions to three possible SF sites. While these pathways are very similar to those observed in the presence of a negative potential (inward conduction), a completely different mechanism operates for outward conduction at positive potentials.

Reply to Domene and Furini: Distinguishing knock-on and vacancy diffusion mechanisms

Furini and Domene (1) suggest an “alternative [KK] mechanism for conduction of K+ in potassium channels,” which “has ions at adjacent binding sites” and “allows the possible presence of vacancies, that is, neither K+ ions nor water molecules in certain sites.” They described their mechanism in detail as follows: “Outward conduction is initiated by the ion in S1 moving toward S0 … After that, ions in S4 and S2 move to S3 and S1 … When the innermost ion reaches S4, upward movements of the ion in S3 to S2 and of the outermost ion to the extracellular milieu conclude the conduction event.” Their mechanism, illustrated here in Fig. 1, is fundamentally equivalent to the vacancy-type diffusion mechanism originally put forth by Hodgkin and Keynes (2) as one of two mechanistic possibilities for K+-channel conduction, the other being the knock-on mechanism.

Magnetars: Time Evolution, Superfluid Properties, and the Mechanism of Magnetic Field Decay

We calculate the coupled thermal evolution and magnetic field decay in relativistic model neutron stars threaded by superstrong magnetic fields (B > 1015 G). Our main goal is to evaluate how such "magnetars" evolve with time and how field decay modifies the transitions to core superfluidity and cooling dominated by surface X-ray emission. Observations of a thermal X-ray spectral component and fast timing noise place strong constraints on the presence of a superfluid core. We find that the transition to core superfluidity can be significantly delayed by magnetic field decay in the age range ~103-105 yr. The mechanism of Hall drift is related to the stability of the core magnetic field and to currents flowing outward through the crust. The heating effect is enhanced if it is continuous rather than spasmodic. Condensation of a heavy element layer at the surface is shown to cause only modest changes in the outward conduction of heat.

Probing a Potassium Channel Pore with an Engineered Protonatable Site

Blockade by intracellular cations reduces outward conduction of K+ in inward rectifier K+ channels. Mutations of residue 171 in the second transmembrane (M2) segment of the ROMK1 channel have been found to affect the affinity for blockade by intracellular Mg2+ and polyamines. In the present study, we examined the mechanism by which this residue mediates blockade by placing a proton acceptor (histidine) at this position. The results allow us to draw two conclusions. First, the side chain of residue 171 is located in the ion conduction pore about halfway across the transmembrane voltage drop. Second, its side chain comes into close contact and interacts electrostatically with a blocking ion.

On the mechanism of dissolution of porous oxide films on aluminium during anodizing

The temperature distribution along the vertical pores of the anodic oxide films on aluminium was calculated for usual conditions of anodizing in 10% sulphuric acid solution by taking into account the accumulation of both Joule heat and the heat of formation of oxide at the pore-base and the outward conduction of heat through the solution-filled pore. The results show that a steady state of distribution is established at any instant and at any film thickness during anodizing and that the temperature rise at the pore-base is always negligibly small. This conclusion was confirmed by some experiments on the dissolution behaviour of oxide films. It was demonstrated that small changes in the bulk solution temperature significantly affect the dissolution rate and anodizing current. It was also shown that the dissolution rate of oxide at the pore-base is almost constant irrespective of the film thickness and of the time of anodizing. This rate was evaluated in the manner reported previously by subtracting the dissolution rate at the side walls of the pores from the total dissolution rate. These facts are in direct opposition to the ‘thermal mechanism’ and suggest that the extra-high rate of dissolution during anodizing almost certainly results from the high electrical field applied across the barrier layer.

VII. The passage of sperms and of eggs through the oviducts in terrestrial vertebrates

The two elementary effectors, cilia and muscles, are found throughout almost the whole range of the animal kingdom. In such simple organisms as sea-anemones these two effectors are about equally developed and are usually mutually concerned in many of the common activities of these animals, such as the production of the respiratory currents, the appropriation of food, and the like. Vertebrates, on the other hand, with their growth in bulk have in large measure discarded cilia and have developed muscles as the almost exclusive means of response. Nevertheless, in parts of their bodies such as the reproductive system, both kinds of effectors, cilia and muscles, are mutually active and co-operate in carrying out certain functions of this important system. No one can observe the active oviduct of a vertebrate without being impressed by its striking resemblance to the responding coelenterate body. Peristalsis, antiperistalsis, and other co-ordinated muscular movements combined with ciliary activity, are the elements on which both the organs of coelenterates and the oviducts of vertebrates appear to rely for successful operation. The oviducts possess the remarkable property of conducting the reproductive elements in two opposite directions; sperms are conveyed by them from the exterior to the neighbourhood of the ovary and ova, or embryos are transported in the reverse direction. These two processes of inward and outward conduction are not unlike the swallowing of food by the gullet of the sea-anemone and the discharge of excrement by the same organ. Are the operations of the vertebrate oviduct performed in the same way as those of the gullet of the coelenterate or not? In other words, do the oviducts exhibit muscular movements associated with normal or reversed ciliary beat to accomplish the appropriate transfer of the vertebrate genital products, or are these responses in vertebrates based on entirely different principles? These and other related problems are discussed in the following pages.