Vortex shedding as a precursor of turbulent electrical activity in cardiac muscle.

In cardiac tissue, during partial blockade of the membrane sodium channels, or at high frequencies of excitation, inexcitable obstacles with sharp edges may destabilize the propagation of electrical excitation waves, causing the formation of self-sustained vortices and turbulent cardiac electrical activity. The formation of such vortices, which visually resembles vortex shedding in hydrodynamic turbulent flows, was observed in sheep epicardial tissue using voltage-sensitive dyes in combination with video-imaging techniques. Vortex shedding is a potential mechanism leading to the spontaneous initiation of uncontrolled high-frequency excitation of the heart.     

Vortex electron dynamics in a high-current plasma lens

An analytic study is made of the following problems: the instability of a plasma against the excitation of vortex turbulence, the turbulence saturation amplitude, the types and spatial structures of the nascent vortices, and their nonlinear growth rates in an electrostatic plasma lens for focusing high-current ion beams.     

Circulation of excitation waves in a model medium containing unexcitable obstacles

Serial semithin optical sections were used for 3D reconstruction of rat ventricular tissue. At histological resolution, older animals (18 months) were found to have regions of fibrous connective tissue not observed in young animals (2 months). The 3D data were used in mathematical modeling of excitation wave propagation in a medium containing such obstacles. The values of conductance, medium excitation threshold, and pulse repetition rate were determined whereat the model predicts rhythm disorders caused by circulation of the excitation wave.     

Unpinning of a rotating wave in cardiac muscle by an electric field.

The possibility of terminating cardiac arrhythmias with electric fields of moderate intensity is a challenging problem from a fundamental point of view and an important issue for clinical applications. In an effort to understand how anatomical re-entries are affected by electric fields, we found that a weak shock, with an amplitude of an order of magnitude less than the defibrillating shock, may unpin the vortices rotating around the defects (obstacles). The unpinning results from a depolarization of the tissue near the obstacle, induced by an external electric field within a distance of order lambda approximately 1 mm. Unpinning was observed both in the FitzHugh model of excitable tissue, and in a specific Beeler-Reuter model of cardiac tissue. This theoretical observation suggests that anatomical re-entries can be transformed into functional re-entries, an effect that can be tested in experiments with cardiac muscle.     

Structure and organization of System II photosynthetic units during the greening of a dark-grown Chlorella mutant

The formation and the organization of Photosystem II photosynthetic units during the greening of a dark-grown Chlorella vulgaris, mutant 5/520, have been investigated by analysing the kinetics of the “activation” of oxygen evolution and of the fluorescence induction.

1. 1. The existence during the early stages of the greening of a stationary photosynthesis demonstrates the presence of active Photosystem II at these initial stages, which are integrated in a functional whole, leading to overall photosynthesis.

2. 2. The rise-time of oxygen evolution has been measured using far-red and green light in order to estimate the relative number of chlorophylls per unit. The amount of chlorophyll a remains relatively constant during the greening, while the progressive addition of chlorophyll b causes the size of the units to increase approx. 2-fold.

3. 3. The induction kinetics of the fluorescence are exponential during the early phases of greening and later become distinctly sigmoidal; this suggests that the first units synthesized on the surface of the membrane are isolated from each other by obstacles preventing electronic excitation transfers and that such obstacles which might correspond to some distance between such units, can disappear at later stages, allowing energy transfers to occur.

These observations suggest that the Photosystem II units represent organized functional entities. They apparently consist of a relatively constant number of chlorophyll a molecules, which during the greening is complemented progressively by the addition of chlorophyll b.     

Unidirectional block of propagation of a single autowave in a narrow gap and emergence of a two-dimensional reentry depends on geometry of the obstacle and on excitability of the media

Using mathematical simulation we show that the occurrence of excitation wave circulation (reentry) around an unexcitable obstacle depends on both the geometry of the obstacle and the excitation threshold. The reentry formation is shown to take place in a wide range of model parameters.     

Microfiltration of yeast suspensions with self-cleaning spiral vortices: possibilities for a new membrane module design

A novel method of producing controlled vortices was used to reduce both concentration polarization and membrane fouling during microfiltration of Saccharomyces cerevisiae broth suspensions. The method involves flow around a curved channel at a sufficient rate so as to produce centrifugal instabilities (called Dean vortices). These vortices depolarize the build-up of suspended particles such as yeast cells at the membrane-solution interface and allow for increased membrane permeation rates. Various operating conditions under which such vortices effectively reduced cake build-up of suspended particles such as yeast cells at the membrane-solution interface and allow for increased membrane permeation rates. Various operating conditions under which such vortices effectively reduced cake build-up during microfiltration of 0 to 0.55 dry wt% yeast broth were investigated. Flux improvements of over 60% for 0.25 dry wt% yeast broth for flow with over that without Dean vortices were observed. This beneficial effect increased with increasing retentate flow rate and increasing transmembrane pressure and decreased with increasing concentration of suspended matter. Similar behavior was observed whether the cells were viable of killed. the improvement in flux in the presence over that in the absence of vortices correlated well with centrifugal force or azimuthal velocity squared. The relative cake resistances increased with reservoir yeast concentration. These values with vortices increased from 62% to 75% of that without vortices with increasing yeast concentration. The ratio of the cake thicknesses in the limiting case (at high feed concentration) was 3.25. These results suggest that self-cleaning spiral vortices could be effective in maintaining good and steady microfiltration performance with cell suspensions other than those tested. (c) 1995 John Wiley & Sons, Inc.     

Experimental vestibulopathy produced as a result of formation of a generator of pathologically enhanced excitation in the vestibular nuclei (phenomenon of a determinative dispatch station)]

A possibility of formation of a generator of pathologically enhanced excitation in the system of the vestibular nuclei of the medulla oblongata by disturbance of their inhibitory processes (resulting in development of contralateral rotatory motions in animals) was shown. Experiments with electrical stimulation of the lateral vestibular nucleus and its coagulation showed the system of the vestibular neurons organizing the synchronous message by the vestibulo-spinal pathways to underlie the generator of the pathologically enhanced excitation. It was concluded that the generator of the pathologically enhanced excitation formed in the lateral vestibular nucleus as a result of disturbed inhibition underlied the hyperactive determinative dispatch station causing the syndrome of vestibulopathy     

A spectral-correlational analysis of the EEG of the neocortex in the rabbit in a state of thirst

Summate electrical activity of the rabbit neocortex during formation of drinking excitation was studied by means of mathematical analysis. It is shown that the change of the electrical activity depends on the level of drinking excitability created by various duration of water deprivation (24-48 h) and is expressed in a generalized lowering of potentials amplitude without frequency change. Spectro-correlative EEG analysis showed that lowering of spectrum power took place within the whole analyzed frequencies range. Besides, an increase took place of interconnections of the cortical electrical processes, estimated by coherence function. It may by suggested that the manifested reconstruction of spectro-correlative characteristics of the neocortical biopotentials reflects a formation of motivational excitation establishing optimal level of cortex functioning.     

Formation of a strong electric field resulting in the excitation of microplasma discharges at the edge of a dielectric film on a metal in a plasma flow

Results are presented from experimental and analytical studies of the processes resulting in the excitation of microplasma discharges (MPDs) on a metal surface partially covered with a thin dielectric film under the action of an external plasma flow in vacuum. It is shown experimentally that MPDs are excited at the interface between the open metal surface and the region covered by the dielectric film. The probability of MPD excitation is investigated as a function of the thickness of the dielectric film deposited on the metal. It is found that, for a film thickness of 1 μm, the probability of MPD excitation is close to unity. As the film thickness decreases below ~10 nm or increases above ~10 μm, the probability of MPD excitation is reduced by more than two orders of magnitude. A two-dimensional kinetic numerical code is developed that allows one to model the processes of Debye sheath formation and generation of a strong electric field near the edge of a finite-thickness dielectric film on a metal surface in a plasma flow for different configurations of the film edge. It is shown that the maximum value of the tangential component of the electric field is reached at the film edge and amounts to E _max ≈ |φ₀|/2d (where φ₀ < 0 is the electric potential applied to the metal and d is the film thickness), which for typical conditions of experiments on the excitation of MPDs on metal surfaces (φ₀ ≈–400 V, d ≈ 1 μm) yields E _max ≈ 2 MV/cm. The results of kinetic simulations confirm the qualitative idea about the mechanism of the formation of a strong electric field resulting in the excitation of MPDs at the edge of a dielectric film on a metal surface in a plasma flow and agree with experimental data.     

Wavelet formation in excitable cardiac tissue: the role of wavefront-obstacle interactions in initiating high-frequency fibrillatory-like arrhythmias.

High-frequency arrhythmias leading to fibrillation are often associated with the presence of inhomogeneities (obstacles) in cardiac tissue and reduced excitability of cardiac cells. Studies of antiarrhythmic drugs in patients surviving myocardial infarction revealed an increased rate of sudden cardiac death compared with untreated patients. These drugs block the cardiac sodium channel, thereby reducing excitability, which may alter wavefront-obstacle interactions. In diseased atrial tissue, excitability is reduced by diminished sodium channel availability secondary to depolarized rest potentials and cellular decoupling secondary to intercellular fibrosis. Excitability can also be reduced by incomplete recovery between successive excitations. In all of these cases, wavefront-obstacle interactions in a poorly excitable medium may reflect an arrhythmogenic process that permits formation of reentrant wavelets leading to flutter, fibrillation, and sudden cardiac death. To probe the relationship between excitability and arrhythmogenesis, we explored conditions for new wavelet formation after collision of a plane wave with an obstacle in an otherwise homogeneous excitable medium. Formulating our approach in terms of the balance between charge available in the wavefront and the excitation charge requirements of adjacent medium, we found analytically the critical medium parameters that defined conditions for wavefront-obstacle separation. Under these conditions, when a parent wavefront collided with a primitive obstacle, the resultant fragments separated from the obstacle boundaries, subsequently curled, and spawned new "daughter" wavelets. We identified spatial arrangements of obstacles such that wavefront-obstacle collisions leading to spawning of new wavelets could produce high-frequency wavelet trains similar to fibrillation-like arrhythmias.     

Role of diastolic vortices in flow and energy dynamics during systolic ejection

MRI-based computational fluid dynamics simulations were performed in the left ventricles of two adult porcine subjects with varying physiological states (before and after an induced infarction). The hypothesis that diastolic vortices store kinetic energy and assist systolic ejection was tested, by performing systolic simulations in the presence and absence of diastolic vortices. The latter was achieved by reinitializing the entire velocity field to be zero at the beginning of systole. A rudimentary prescribed motion model of a mitral valve was included in the simulations to direct the incoming mitral jet towards the apex. Results showed that the presence or absence of diastolic vortex rings had insignificant impact on the energy expended by walls of the left ventricles for systolic ejection for both the porcine subjects, under all physiological conditions. Although substantial kinetic energy was stored in diastolic vortices by end diastole, it provided no appreciable savings during systolic ejection, and most likely continued to complete dissipation during systole. The role of diastolic vortices in apical washout was investigated by studying the cumulative mass fraction of passive dye that was ejected during systole in the presence and absence of vortices. Results indicated that the diastolic vortices play a crucial role in ensuring efficient washout of apical blood during systolic ejection.     

Integrating neuromorphic action-oriented perceptual inputs to generate a navigation behaviour for a robot

We use neural networks with pointer map architectures to provide simple attentional processing in a robotic task. A pointer map comprises a map of neurons that encode a stimulus. Besides global feedback inhibition, the map receives feedback excitation via a small group of pointer neurons that encode the location of a salient stimulus on the map as a vectorial representation. The pointer neurons are able to apply selective processing to a particular region of the network. The robot uses these properties to manoeuver in relation to an attended object. We implemented a controller composed of two pointer maps, and a motor map. The first pointer map reports the direction of a salient obstacle in a one-dimensional map of distance derived from infrared sensors. The second pointer map reports the direction to potential obstacles in a two-dimensional edge-enhanced image derived from a forward looking CCD-camera. These outputs are applied to a motor map, where they bias the motor control signals issued to the robots wheels, according to navigational intentions.     

Free emboli formation in the wake of bi-leaflet mechanical heart valves and the effects of implantation techniques

The high incidence of thromboembolic complications of mechanical heart valves (MHV), primarily due to platelet activation by contact with foreign surfaces and by non-physiological flow patterns past the valve, still limits their success as permanent implants. The latter include elevated shear and turbulent stresses and shed vortices formed in the wake of the valve's leaflets during the deceleration phase, potentially entrapping activated and aggregated platelets. It is hypothesized that these flow patterns induce the formation of free emboli which are the source of cerebrovascular microemboli associated with MHV. Implicit to this hypothesis is that free emboli formation will be affected by the implantation technique employed and the valve orientation, as those will alter the flow characteristics past the valve and the interaction of the platelets with the flow. In this study, numerical simulations of turbulent pulsatile flow past a St. Jude Medical bi-leaflet MHV were conducted. Platelet shear histories were calculated along pertinent turbulent platelet trajectories, and the effect of a misaligned valve on platelet activation was quantified and compared to that of an aligned valve. It demonstrated that the combination of a tilted valve and subannularly sutured pledgets had an explicit detrimental effect on platelet activation, with the following entrapment of the platelets within the shed vortices of the wake leading to a significant increase of the thromboembolic potential of the valve. This numerical model depicted a viable course for free emboli formation, and indicated how the implantation technique may enhance the risk of cardioembolism.     

A formulation for calculating the translational velocity of a vortex ring or pair

Cephalopods, among other marine animals, use jet propulsion for swimming. A simple actuator is designed to loosely mimic pulsatile jet formation in squid and jellyfish. The actuator is comprised of a cavity with an oscillating diaphragm and an exit orifice. Periodic oscillation of the diaphragm results in the formation of an array of vortex rings and eventually could generate a periodic pulsatile jet. A general formulation for calculating the velocity of a steadily translating vortical structure in two-dimensional and axi-symmetric shear flows is presented. This technique is based on taking the variational derivative of an energetic function at its critical point. This technique is general, applicable to vortices in liquid and gas media, with no limitation on the relative size of the vortex core. The technique is then implemented to estimate the translational velocity of a vortex ring in a Helmholtz vortex ring generator.     

Photochemistry of rhodopsin and isorhodopsin investigated on a picosecond time scale.

Bovine rhodopsin and isorhodopsin were excited with a single 530-nm, 7-ps light pulse emitted by a mode-locked Nd 3+ glass laser at room temperature. Within 3 ps of excitation, absorbance changes due to formation of bathorhodopsin were observed. The difference spectra generated during and 100 ps after pulse excitation are presented. The data show that bathorhodopsin formation is completed within 3 ps for both the primary pigments and suggest that a single common bathorhodopsin is photochemically formed from both primary pigments. Our findings provide additional support for the cis-trans isomerization model of the primary event in vision. Additional absorption transients that were observed near 670 and 460 nm are discussed.     

Manifestation of a monosynaptic excitatory connection in the structure of the cross-correlation histogram

Manifestation of a monosynaptic excitatory connection in the structure of the cross-correlation histogram was studied on a mathematical model of interneuronal interaction. Specific features of the shape and position of the primary peak on the cross-correlation histogram distinguishing monosynaptic excitation of one neuron by another from monosynaptic excitation of these units by a third neuron are described. The height of the primary peak is shown to change with a change in certain parameters of the model describing mechanisms of action potential generation and synaptic transmission of excitation. The structure of the autocorrelation histogram is shown to be reflected to some extent in the structure of the cross-correlation histogram on either side of the primary peak and to take part in the formation of secondary peaks and troughs.Institute of Higher Nervous Activity and Neurophysiology, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 11, No. 4, pp. 348–354, July–August, 1979.     

Characteristics of the rhythmic activity of a normal and a damaged heart during hyperactivity of spinal cord preganglionic neurons

Experiments on white rats were made to investigate the character of rhythmical activity of normal heart and that in acute myocardial ischemia in response to electrical stimulation of preganglionic neurons (PN) of the thoracic part of the spinal cord and the formation in them of the generator of pathologically enhanced excitation induced by microinjection of tetanus toxin. In both types of PN hyperactivation, arrhythmias of different patterns developed, their severity and duration being related to the level of initial cardiac reactivity and the degree of PN excitation. It is suggested that under distress of the autonomous mechanisms responsible for regulation of the injured heart, hyperactivation of the spinal cord sympathetic apparatus might be a factor provoking arrhythmia.     

On the mechanism of the formation of magnetohydrodynamic vortices in the solar plasma

Based on the magnetohydrodynamic (MHD) equations for an incompressible conductive viscous fluid, the possible mechanism of the formation of giant MHD vortices recently discovered in the solar atmosphere (chromosphere) is analyzed. It is assumed that these vortices arise in the regions of the solar surface (photosphere) with ascending flows of hot plasma that arrives from the inner regions of the Sun as a result of thermal convection and is accelerated upward under the action of the chromospheric plasma pressure gradient. It is shown that, under the assumption of plasma incompressibility and flow continuity, the ascending plasma flows induce converging radial plasma flows, which create the convective and Coriolis nonlinear hydrodynamic forces due to the nonzero initial vorticity of the chromospheric plasma caused by Sun’s rotation. The combined action of these two forces leads to the exponential acceleration of the solid-body rotation of plasma inside the ascending flow, thereby creating a vortex that generates an axial magnetic field, in agreement with astrophysical observations.     

Analysis of analgesia induced by the generator of excitation in the dorsal raphe nucleus

In experiments on white rats the generator of excitation was created in the dorsal raphe nucleus by microinjection of tetanus toxin. After formation of the excitation generator electrical activity in this nucleus was changed as the following: the first negative component (N1) was strongly increased, general EP configuration changed and the spontaneous paroxysmal activity became more frequent. The time of the generator formation correlated with the appearance of intense and prolonged analgesia. Naloxone did not reverse the effects of analgesia described.