Phototaxis during the slug stage of Dictyostelium discoideum: a model study

During the slug stage, the cellular slime mould Dictyostelium discoideum moves towards light sources. We have modelled this phototactic behaviour using a hybrid cellular automata/partial differential equation model. In our model, individual amoebae are not able to measure the direction from which the light comes, and differences in light intensity do not lead to differentiation in motion velocity among the amoebae. Nevertheless, the whole slug orientates itself towards the light. This behaviour is mediated by a modification of the cyclic AMP (cAMP) waves. As an explanation for phototaxis we propose the following mechanism, which is basically characterized by four processes: (i) light is focused on the distal side of the slug as a result of the so-called `lens-e¡ect'; (ii) differences in luminous intensity cause differences in NH3 concentration; (iii) NH3 alters the excitability of the cell, and thereby the shape of the cAMP wave; and (iv) chemotaxis towards cAMP causes the slug to turn.We show that this mechanism can account for a number of other behaviours that have been observed in experiments, such as bidirec- tional phototaxis and the cancellation of bidirectionality by a decrease in the light intensity or the addition of charcoal to the medium

Influence of a Liquid on the Deformation Behavior of Porous Nickel and Titanium under Compression

The deformation behavior of capillary porous (CP) Ni and CP Ti is examined under uniaxial compression in air, water, and ethanol. CP metals contained inert liquids such as H20 and C2H5OH are used as the heat pipes' active elements in space applications. The samples for mechanical testing were isostatic compacting and vacuum annealing (porosity of 60%). Uniaxial compression tests were carried out in the air, water, and ethanol in Shimadzu AG-50K XD (traverse rate 0.1 mm/min). It was shown that CP Ni and CP Ti exhibit the ductile deformation behavior in all cases, which is inherent to Ni and Ti. The ethanol environment induces the increase of the compression strength and the total deformation in both materials compared to deformation behavior in air and water. Therefore, a heat pipe's failure containing CP Ni and CP Ti matrixes and ethanol as the working body is caused rather by manufacturing defects, but not the structural materials' intrinsic properties. © Published under licence by IOP Publishing Ltd.The Russian Foundation for Basic research (№ 20-48-660017)

On Malleability of Rhenium at Room Temperature

The problem of the malleability of rhenium at room temperature is discussed. It has shown that rhenium manufactured by the mean of the electron beam melting is able to the mechanical treatment by rolling at room temperature. The primary condition for mechanical treatment is the minor share of the tensile stress in the processing scheme. It suppresses the grain boundary sliding in a coarse-grained workpiece of this refractory HCP-metal. © Published under licence by IOP Publishing Ltd.The Russian Science Foundation supports this work (№ 18-19-00217)

High-frequency effects in the FitzHugh-Nagumo neuron model

The effect of a high-frequency signal on the FitzHugh-Nagumo excitable model is analyzed. We show that the firing rate is diminished as the ratio of the high-frequency amplitude to its frequency is increased. Moreover, it is demonstrated that the excitable character of the system, and consequently the firing activity, is suppressed for ratios above a given threshold value. In addition, we show that the vibrational resonance phenomenon turns up for sufficiently large noise strength values.Comment: 4 pages, 4 figures (to appear in Physical Review E

Design Works on Carcassing the Circus Big Top in the Block “A” while Reconstructing Penza Circus

AbstractThe project designers describe their experiment on carcassing the big top of the main building (Block A), the largest in Europe after the reconstruction of Penza circus. Due to the difficult conditions at a construction site, it was necessary to find an acceptable method to enlarge trusses of the big top, also to determine a method and sequence of erection of completed trusses with optimal application of erection crane's weight and high-altitude characteristics. Four options were considered for pre-assembly works of parts suitable for transportation of trusses. Assembly line technique of trusses was developed based on an agreed upon with the executive version. A small area was dedicated for this purpose. Erection and dismantling method of the big top was developed. The project is accepted for production

Mechanical Behavior of Polycrystalline Rhenium under 3-Points Bending at a Low Homological Temperature

Mechanical behaviour of polycrystalline rhenium under 3-points bending at such low homological temperature as room is discussed. Two metallurgical technologies (electron beam melting and powder metallurgy) were used for the samples preparation. Fine-grained samples (PM metal) exhibit some plasticity prior the failure, while coarse-grained ones (EBM metal) behave like a brittle solid. The intergranular fracture is the fracture mode of rhenium in both cases. Basal slip and prismatic slip of dislocations are not active in rhenium at low homological temperature, but the grain boundary sliding occurs under these conditions. Therefore, polycrystalline rhenium cannot be machined at room temperature despite the growth of grain boundary cracks are braked in the samples due to grain boundary sliding. © Published under licence by IOP Publishing Ltd.Russian Science Foundation, RSF: 18-19-00217This work is supported by the Russian Science Foundation (#18-19-00217)

Elimination of spiral waves in cardiac tissue by multiple electrical shocks

We study numerically the elimination of a spiral wave in cardiac tissue by application of multiple shocks of external current. To account for the effect of shocks we apply a recently developed theory for the interaction of the external current with cardiac tissue. We compare two possible feedback algorithms for timing of the shocks: a "local" feedback algorithm (using an external electrode placed directly on the tissue) and a "global" feedback algorithm (using the electrocardiogram). Our main results are: application of the external current causes a parametric resonant drift similar to that reported in previous model computations; the ratio of the threshold of elimination of the spiral wave by multiple shocks to the threshold of conventional single shock defibrillation in our model for cardiac tissue is about 0.5, while earlier, less realistic models predicted the value about 0.2; we show that an important factor for successful defibrillation is the location of the feedback electrode and the best results are achieved if the feedback electrode or the ECG lead is located at the boundary (or edge) of the cardiac tissue; the "local" and the "global" feedback algorithms show similar efficiency

Spatial Pattern Formation During Aggregation of the Slime Mould Dictyostelium discoideum

Stream formation and spiral wave behaviour during the aggregation of Dictyostelium discoideum (Dd) are studied in a model based on the Martiel-Goldbeter equations for cAMP relay, combined with chemotactic motion of Dd cells. The results show that stream formation occurs if the turnover rate of intracellular cAMP is increased. This increase in the turnover rate of cAMP[in] leads to a dependence of the speed of the cAMP wave on the cell density. We propose that this dependence of wave speed on cell density is the underlying mechanism for stream formation. Besides stream formation, increasing the turnover rate of cAMP[in] also results in a spiral wave period that decreases during aggregation, a phenomenon that is commonly observed in situ. Furthermore, the dependence of wave speed on cell density is measured empirically[ The speed of the cAMP wave is found to decrease as the wave travels from high to low cell density. This indicates that in situ, wave speed does depend on cell density