Ionic Pulses Research Articles

Soliton-mediated ionic pulses and coupled ionic excitations in a dissipative electrical network model of microtubules

Many cellular activities are mediated by microtubules (MTs), with most electrophysiological processes depending on ionic flow through MT cylinders. This paper addresses such conductive features by representing the MT as a nonlinear electrical transmission line composed of capacitive and dissipative properties. Thanks to the semi-discrete approximation near the continuum limit, coupled ionic pulses flowing along cellular microtubules are described by a set of coupled cubic complex Ginzburg-Landau (CGL) equations whose one of the solutions is a plane wave. The stability of the latter is checked, under weak modulation, using an explicit analytical expression for the modulational instability (MI) growth rate. The parametric analysis of the instability growth rate allows detecting parameter regions where ionic conductivity, through modulated waves, in the MT network is likely to occur. Dissipative bright-bright pulse soliton solutions for the nonlinearly coupled cubic CGL equations are constructed using a modified Hirota's bilinear method. A generalized coupled mode for the discrete ionic signal is proposed and used as the initial condition to be propagated in the nonlinear electrical transmission lattice of MT under direct numerical simulations. The ionic pulse transfer, mediated by the nonlinear interaction between oscillatory modes, is manifested by the formation of trains of modulated waves whose behaviors depend on the right choice of system parameters. Those results theoretically suggest that coupled ionic signals may facilitate information processing involving MTs.

ED-BioRob: A Neuromorphic Robotic Arm With FPGA-Based Infrastructure for Bio-Inspired Spiking Motor Controllers.

Compared to classic robotics, biological nervous systems respond to stimuli in a fast and efficient way regarding the body motor actions. Decision making, once the sensory information arrives to the brain, is in the order of ms, while the whole process from sensing to movement requires tens of ms. Classic robotic systems usually require complex computational abilities. Key differences between biological systems and robotic machines lie in the way information is coded and transmitted. A neuron is the “basic” element that constitutes biological nervous systems. Neurons communicate in an event-driven way through small currents or ionic pulses (spikes). When neurons are arranged in networks, they allow not only for the processing of sensory information, but also for the actuation over the muscles in the same spiking manner. This paper presents the application of a classic motor control model (proportional-integral-derivative) developed with the biological spike processing principle, including the motor actuation with time enlarged spikes instead of the classic pulse-width-modulation. This closed-loop control model, called spike-based PID controller (sPID), was improved and adapted for a dual FPGA-based system to control the four joints of a bioinspired light robot (BioRob X5), called event-driven BioRob (ED-BioRob). The use of spiking signals allowed the system to achieve a current consumption bellow 1A for the entire 4 DoF working at the same time. Furthermore, the robot joints commands can be received from a population of silicon-neurons running on the Dynap-SE platform. Thus, our proposal aims to bridge the gap between a general purpose processing analog neuromorphic hardware and the spiking actuation of a robotic platform.

Preferential elution of ionic solutes in melting snowpacks: Improving process understanding through field observations and modeling in the Rocky Mountains

The preferential elution of ions from melting snowpacks is a complex problem that has been linked to temporary acidification of water bodies. However, the understanding of these processes in snowpacks around the world, including the polar regions that are experiencing unprecedented warming and melting, remains limited despite being instrumental in supporting climate change adaptation.In this study, data collected from a snowmelt lysimeter and snowpits at meadow and forest-gap sites in a high elevation watershed in Colorado were combined with the PULSE multi-phase snowpack chemistry model to investigate the controls of meltwater chemistry and preferential elution. The snowdepth at the meadow site was 64% of that at the forest-gap site, and the snowmelt rate was greater there (meadow snowpit) due to higher solar irradiance. Cations such as Ca2+ and NH4+ were deposited mostly within the upper layers of both the meadow and forest-gap snowpacks, and acid anions such as NO3− and SO42− were more evenly distributed. The snow ion concentrations were generally greater at the forest-gap snowpit, except for NH4+, which indicates that wind erosion of wet and dry deposited ions from the meadow may have reduced concentrations of residual snow. Furthermore, at the forest-gap site, snow interception and scavenging processes such as sublimation, ventilation, and throughfall led to particular ion enrichment of Ca2+, Mg2+, K+, Cl−, SO42− and NO3−. Model simulations and observations highlight that preferential elution is enhanced by low snowmelt rates, with the model indicating that this is due to lower dilution rates and increased contact time and area between the percolating meltwater and the snow. Results suggest that low snowmelt rates can cause multiple early meltwater ionic pulses for ions subject to lower ion exclusion. Ion exclusion rates at the grain-size level have been estimated for the first time.

A numerical model for the simulation of snowpack solute dynamics to capture runoff ionic pulses during snowmelt: The PULSE model

Early ionic pulse during spring snowmelt can account for a significant portion of the total annual nutrient load in seasonally snow-covered areas. Ionic pulses are a consequence of snow grain core to surface ion segregation during metamorphism, a process commonly referred to as ion exclusion. While numerous studies have provided quantitative measurements of this phenomenon, very few process-based mathematical models have been proposed for diagnostic and prognostic investigations. A few early modelling attempts have been successful in capturing this process assuming transport through porous media with variable porosity. However, this process is represented in models in ways that misalign with the mechanistic view of the process described in the literature. In this research, a process-based model is proposed that can simulated ionic pulses in runoff by emulating solute leaching from snow grains during melt and the subsequent vertical solute transport by meltwater through the snowpack. To facilitate its use without the need for snow-physics’ models, simplified alternative methods are proposed to estimate some of the variables required by the model. The model was applied to two regions, and a total of 4 study sites, that are subject to significantly different winter climatic and hydrological conditions. Comparison between observations and simulation results suggest that the model can capture well the overall snow melt runoff concentration pattern, including both the timing and magnitude of the early melt ionic pulse. The model enables the prediction of concentration profiles of the dry (snow) and liquid (wet) fractions within the snow matrix for the first time. Although there is a computational cost associated with the proposed modelling framework, this study demonstrates that it can provide more detailed information about the reallocation and transport of ions through snowpacks, which can ultimately be used to improve nutrient transport predictions during snowmelt.

Analysis of Tertiary Interactions between SART3 and U6 Small Nuclear RNA Using Modified Nanocapillaries

We employed modified glass nanocapillaries to investigate interactions between the RNA-binding protein, known as cell carcinoma antigen recognized by T cells-3 (SART3), and the noncoding spliceosome component, U6 small nuclear RNA (snRNA), at the single-molecule level. We functionalized the nanocapillaries with U6 snRNA fragments, which were hybridized to DNA molecules and then covalently attached to the nanocapillary surface. When transported through the modified nanocapillaries, two different SART3-derived constructs, HAT-RRM1-RRM2 and RRM1-RRM2, exhibited resistive ionic current pulses with different dwell times, which represented their different binding affinities to tethered U6 snRNAs. The dissociation constants (KD), estimated from the bias voltage dependence of translocation events, were approximately 1.9 μM and 201 μM for HAT-RRM1-RRM2 and RRM1-RRM2, respectively. These values were comparable to corresponding values obtained with isothermal titration calorimetry, demonstrating that the modified glass nanocapillaries are applicable to analyses of protein-ligand interactions at the single-molecule level.

Spring thaw ionic pulses boost nutrient availability and microbial growth in entombed Antarctic Dry Valley cryoconite holes.

The seasonal melting of ice entombed cryoconite holes on McMurdo Dry Valley glaciers provides oases for life in the harsh environmental conditions of the polar desert where surface air temperatures only occasionally exceed 0°C during the Austral summer. Here we follow temporal changes in cryoconite hole biogeochemistry on Canada Glacier from fully frozen conditions through the initial stages of spring thaw toward fully melted holes. The cryoconite holes had a mean isolation age from the glacial drainage system of 3.4 years, with an increasing mass of aqueous nutrients (dissolved organic carbon, total nitrogen, total phosphorus) with longer isolation age. During the initial melt there was a mean nine times enrichment in dissolved chloride relative to mean concentrations of the initial frozen holes indicative of an ionic pulse, with similar mean nine times enrichments in nitrite, ammonium, and dissolved organic matter. Nitrate was enriched twelve times and dissolved organic nitrogen six times, suggesting net nitrification, while lower enrichments for dissolved organic phosphorus and phosphate were consistent with net microbial phosphorus uptake. Rates of bacterial production were significantly elevated during the ionic pulse, likely due to the increased nutrient availability. There was no concomitant increase in photosynthesis rates, with a net depletion of dissolved inorganic carbon suggesting inorganic carbon limitation. Potential nitrogen fixation was detected in fully melted holes where it could be an important source of nitrogen to support microbial growth, but not during the ionic pulse where nitrogen availability was higher. This study demonstrates that ionic pulses significantly alter the timing and magnitude of microbial activity within entombed cryoconite holes, and adds credence to hypotheses that ionic enrichments during freeze-thaw can elevate rates of microbial growth and activity in other icy habitats, such as ice veins and subglacial regelation zones.

Ionic current pulses upon field desorption of cesium from graphene on iridium

Pulses of Cs+ ion current upon field desorption in constant electric field from an iridium point coated with a graphene film have been detected. Pulses with durations of less than 0.1 s were observed upon sputter deposition of cesium atoms onto graphene followed by their removal from the surface. During the pulse, one could observe a flash on the field desorption microscope display occupying all area of the images of {001} and {111} facets. Possible mechanisms of the flash formation using models of cesium intercalation in the graphene layer and the yield of cesium atoms from under the film under the action of the applied electric field of the electric field have been considered.

Ionic Pulses along Cytoskeletal Protophilaments

The experimental evidences regarding ionic waves generation and propagation along both microtubules (MTS) and actin filaments (AF3) motivated us to develop the physical models that provide the framework for the explanation and analysis of these interesting biophysical phenomena.In pertaining analysis we partly relied on some experimental as well as numerical data, but also on theoretical estimations enabling us to establish the concept of nonlinear transmission lines which could lead to reasonable clearing up of experimental facts.We are convinced that these ionic currents actually exist and serve to sustain some important biological cellular mechanisms.

Nonlinear ionic pulses along microtubules

Microtubules are cylindrically shaped cytoskeletal biopolymers that are essential for cell motility, cell division and intracellular trafficking. Here, we investigate their polyelectrolyte character that plays a very important role in ionic transport throughout the intra-cellular environment. The model we propose demonstrates an essentially nonlinear behavior of ionic currents which are guided by microtubules. These features are primarily due to the dynamics of tubulin C-terminal tails which are extended out of the surface of the microtubule cylinder. We also demonstrate that the origin of nonlinearity stems from the nonlinear capacitance of each tubulin dimer. This brings about conditions required for the creation and propagation of solitonic ionic waves along the microtubule axis. We conclude that a microtubule plays the role of a biological nonlinear transmission line for ionic currents. These currents might be of particular significance in cell division and possibly also in cognitive processes taking place in nerve cells.

Affinity of amine-functionalized plasma polymers with ionic solutions similar to those in the human body

This work presents a study on the synthesis and on the interaction of allylamine, pyrrole and ethylenglycol polymerized by plasma with solutions of ionic composition similar to those in the nervous system. These polymers are attractive substrates to interact with the ionic pulses of the spinal cord due to their electrical and biocompatible characteristics. The ionic solutions were prepared with aqueous combinations of NaCl, MgSO 4, KH 2PO 4, KCl, CaCl 2, and NHCO 3. The polymers were prepared as thin films on glass substrates. The results indicated that some of the most important physical characteristics in the hydrophilicity of the polymers, roughness, porosity and functional groups, can be controlled with the energy of polymerization. The interaction between polymers and solutions was studied measuring the contact angle at the solid–liquid interface and the electrical conductivity of the polymers wet with these solutions. The contact angles were between 8° and 38°, and the electrical conductivity was in the 10 −8 to 10 −9 S/cm interval. The general tendencies indicate that the amine-functionalized polymers of this work are good materials to interact with the spinal cord system.

A study of solute redistribution and transport in seasonal snowpack using natural and artificial tracers

Summary Solute releases from a seasonal snowpack are important features of spring runoff in temperate climate zones because meltwater accounts for a significant proportion of contaminant loading to catchments. Here we report a study that traces the movement of natural and artificial solutes in snow through the processes of snow deposition, snowpack metamorphism, and snowmelt at the Central Sierra Snow Laboratory (CSSL), Soda Springs, California, USA. Rare earth elements (REEs) were added to the snowpack to mark individual snow layers. New snow, snowpack profiles and snowmelt were sampled and analyzed for REE and anion ( SO 4 2 - , Cl− and NO 3 - ) concentrations. Correlations between NO 3 - and SO 4 2 - concentrations increased from new snow to snowpack profiles and to snow melt, suggesting significant redistributions of natural solutes in the snowpack. The maximum concentrations of NO 3 - and SO 4 2 - were both five times greater in the snow melt than in the new snow. Solute release by the snowpack was strongly controlled by weather conditions; high concentrations of natural tracers were observed at the onset of intensive melting episodes separated by relatively cold days of little melting. This weather pattern resulted in three ionic pulses rather than one. Natural solutes also showed a diurnal variation that was negatively correlated with the diurnal melting cycle. This pattern can be simulated by the standard water percolation equation linked with a mobile–immobile solute transport model (MIM). The diurnal variations of REE tracers showed positive association with the discharge, and cannot be satisfactorily simulated by the same model, suggesting that additional physical processes, such as dual or multiple flow regimes, may be needed to accurately describe solute transport in snow.

Tracer release in melting snow: Diurnal and seasonal patterns

AbstractTime sequences of tracer release from an alpine snowpack were investigated at Mammoth Mountain, California in 1989. Lysimeter discharge and conductivity were recorded at 30 minute intervals. Three separate applications of chemical tracers were added to the snow surface to provide an ionic signal with known origins in the snowpack. Grab samples of meltwater and snow from snow pits were analysed for chemical composition. There were three distinct discharge periods, each characterized by diurnal fluctuations in discharge and conductivity. An inverse relation between discharge and conductivity was interpreted as the combination of a concentrated signal from regions in the pack less subject to leaching and a relatively dilute signal from near the snow surface where the snow was actively melting Conductivity peaks were highest and diurnal changes greatest immediately following periods of freezing. Grab samples showed little correlation with either 30 minute or daily average conductivity. Relative concentrations of individual ions in meltwater were similar between samples. Non‐systematic grab sampling of snowpack meltwater is shown to be potentially misleading because of multiple ionic pulses over the ablation season and strong diurnal fluctuations in chemical concentrations. Continuous measurements of discharge conductivity are a good indicator of diurnal and seasonal changes in the rate of ion release from the snowpack, and should be used to guide sampling. Composite, or time‐integrated samples rather than grab samples may be required to estimate daily and weekly rates of ion release in melting snow.