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1

Electrical coupling and bistability in inhibitory neuronal networks

Bem T., Hallam J., Meyrand P., Rinzel J.

Biocybernetics and Biomedical Engineering

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2006

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Vol. 26, no. 2

3-14

EN

The role of gap junction-mediated electrical coupling in oscillatory networks is not yet fuIly understood. Such coupling is widespread in developing nervous systems and in many structures of the adult brain where it coexists with synaptic inhibition. Our results, both modeling and experimental, indicate that the effect of electrical coupling in networks of rhythmic inhibitory neurons is crucially dependent on the cells' duty cycle. In the Stomatogastric Nervous System, in which ceIls with large duty cycle are interconnected by reciprocal inhibition, electrical coupling may be responsible for masking adult-like properties of the embryonic network by coordinating the neuronal activity into a single rhythm with different phases. In a two-ceIl half-center oscillator model short duty cycle destabilizes antiphase activity which can be re-established by adding electrical coupling. Moreover, such a network expresses bistability of the in-phase and anti-phase patterns in some range of coupling strengths. AIso in a large-scale model network, in which ceIls are interconnected electrically and by synaptic inhibition, multistability of the in-phase and different anti-phase patterns may occur. A possible function of the multistability in the controI of movement is discussed.

2

Bistability in epileptic phenomena

Suffczyński P., Kalitzin S., da Silva F. L.

Bio-Algorithms and Med-Systems

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2005

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Vol. 1, no. 1/2

259--266

EN

It is currently believed that the mechanisms underlying spindle oscillations are related to those that generate spike and wave (SW) discharges. The mechanisms of transition between these two types of activity, however, are not well understood. In order to provide more insight into the dynamics of the neuronal networks leading to seizure generation in a rat experimental model of absence epilepsy we developed a computational model of thalamo-cortical circuits based on relevant (patho)physiological data. The model is constructed at the macroscopic level since this approach allows to investigate dynamical properties of the system and the role played by different mechanisms in the process of seizure generation, both at short and long time scales. The main results are the following: (i) SW discharges represent dynamical bifurcations that occur in a bistable neuronal network. (ii) The durations of paroxysmal and normal epochs have exponential distributions, indicating that transitions between these two stable states occur randomly over time with constant probabilities. (iii) The probabilistic nature of the onset of paroxysmal activity implies that it is not possible to predict its occurrence. (iv) The bistable nature of the dynamical system allows an ictal state to be aborted by a single counter-stimulus.

PL

Mechanizm spontanicznego powstawania oraz wygaszania napadów epileptycznych nie jest dobrze poznany. W celu lepszego zrozumienia tych mechanizmów opracowany został model symulacyjny układu wzgórzowo-korowego, odpowiedzialnego za powstawanie napadów nieświadomości u ludzi oraz zwierząt. Model wykorzystuje podejście populacyjne, co pozwoliło na zbadanie właściwości układu w skali krótko- i długoczasowej. Główne wyniki pracy to: (i) napady nieświadomości powstają w bistabilnej sieci neuronalnej, (ii) długości napadów oraz odcinków pomiędzy napadami mają rozkład eksponencjalny, co sugeruje, że przejścia pomiędzy stanem normalnym a napadem odbywają się losowo w czasie ze stałym prawdopodobieństwem (iii) probabilistyczna natura powstawania napadów sugeruje, że nie jest możliwe ich przewidywanie (iv) napady powstające w układzie bistabilnym mogą być zatrzymane poprzez stymulację pojedynczym impulsem elektrycznym.

3

Fotochromizm, czyli nowa epoka O-świecenia

Naskręcki R.

Postępy Fizyki

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2004

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T. 55, z. 4

169-174

EN

Photochromism is defined as a light-induced reversible change of color. More precisely it is a reversible transformation of a chemical species between two forms having different absorption spectra induced in one or both directions by absorption of electromagnetic radiation. Current applications of photochromism, related to the change in absorption or emission spectra, involve variable transmission optical materials, cosmetics industry, authentication systems and flowfield visualization. The most promising applications are related to optical data storage and optically controlled molecular switching, that ultimately may result in developments in molecular nanotechnology.

4

Polarization bistability in a smectic-A liquid crystal.

Pura B., Wierzbicki M., Petykiewicz J., Ratusznik G.J., Wrzesiński Z.

Optica Applicata

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2001

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Vol. 31, nr 3

619-624

EN

We report the measurements the of laser light polarization transmitted through the smectic-A phase of a liquid crystal placed inside the Fabry-Perot optical resonator. All four parameters of polarization were measured simultaneously by an ellipsometer of a special construction. Optical polarization bistability phenomenon was observed, in which polarization parameters of outgoing light depend nonlinearly on the intensity of the input beam. We propose a theoretical explanation of this phenomenon, based on the concept of orientational nonlinearity of liquid crystal's molecules

5

Bistability phenomenon in fluid dynamic

Tesar V.

Zeszyty Naukowe. Mechanika / Akademia Techniczno-Rolnicza w Bydgoszczy

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1999

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Z. 44 (221)

161-170

EN

Several interesting fluid flow fields exhibit lack of unicity: instead of a single solution, they have two stable regimes - with an unstable equilibrium between them, which however becomes stable at low Reynolds numbers. A particular example is a trapped vortex ring held by a special cavity on periphery of a fluid jet. Dynamic models of this bistability show interesting bifurcation properties, period doubling phenomenon and existence of chaotic regimes indicating that this may be an important underlying effect for turbulence.

PL

Kilka interesujących obszarów przepływu płynu wykazuje brak jednoznaczności: zamiast pojedynczego rozwiązania posiadają one dwa stabilne stany - z niestabilną równowagą między nimi, która jednakże staje się stabilna przy niskich liczbach Reynoldsa. Szczególnym przykładem jest stabilizujący pierścień wirowy otrzymany przez specjalne wgłębienie na obrzeżu strumienia płynu. Model dynamiczny tej bistabilności wykazuje interesujące rozgałęziające własności, okresowe zjawisko zdwajania i istnienie stanów chaotycznych powoduje, że może to być istotny wspomagający efekt turbulencji.