Heat damage of cytoskeleton in erythrocytes increases membrane roughness and cell rigidity.

The intensity of erythrocyte membrane fluctuations was studied by laser interference microscopy (LIM), which provide information about mechanical properties of the erythrocyte membrane. Atomic force microscopy (AFM) was used to study erythrocyte surface relief; it is related to the cytoskeleton structure of erythrocyte membrane. Intact human erythrocytes and erythrocytes with a destroyed cytoskeleton were used. According to the obtained results, cytoskeleton damage induced by heating up to 50 °Ð¡ results in a reduced intensity of cell membrane fluctuations compared to non-treated cells (20.6 ± 10.2 vs. 30.5 ± 5.5 nm, correspondingly), while the roughness of the membrane increases (4.5 ± 1.5 vs. 3.4 ± 0.5 nm, correspondingly).

Astrocyte calcium signaling: Interplay between structural and dynamical patterns.

Inspired by calcium activity in astrocytes, which is different in the cell body and thick branches on the one hand and thin branchlets and leaflets on the other hand, we formulate a concept of spatially partitioned oscillators. These are inhomogeneous media with regions having different excitability properties, with a global dynamics governed by spatial configuration of such regions. Due to a high surface-to-volume ratio, calcium dynamics in astrocytic leaflets is dominated by transmembrane currents, while somatic calcium dynamics relies on exchange with intracellular stores, mediated by IP 3 , which is in turn synthesized in the space nearby the plasma membrane. Reciprocal coupling via diffusion of calcium and IP 3 between the two regions makes the spatial configuration an essential contributor to overall dynamics. Due to these features, the mechanisms governing the pattern formation of calcium dynamics differ from classical excitable systems with noise or from networks of clustered oscillators. We show how geometrical inhomogeneity can play an ordering role allowing for stable scenarios for calcium wave initiation and propagation.

Excitation block in a nerve fibre model owing to potassium-dependent changes in myelin resistance.

The myelinated nerve fibre is formed by an axon and Schwann cells or oligodendrocytes that sheath the axon by winding around it in tight myelin layers. Repetitive stimulation of a fibre is known to result in accumulation of extracellular potassium ions, especially between the axon and the myelin. Uptake of potassium leads to Schwann cell swelling and myelin restructuring that impacts the electrical properties of the myelin. In order to further understand the dynamic interaction that takes place between the myelin and the axon, we have modelled submyelin potassium accumulation and related changes in myelin resistance during prolonged high-frequency stimulation. We predict that potassium-mediated decrease in myelin resistance leads to a functional excitation block with various patterns of altered spike trains. The patterns are found to depend on stimulation frequency and amplitude and to range from no block (less than 100 Hz) to a complete block (greater than 500 Hz). The transitional patterns include intermittent periodic block with interleaved spiking and non-spiking intervals of different relative duration as well as an unstable regime with chaotic switching between the spiking and non-spiking states. Intermittent conduction blocks are accompanied by oscillations of extracellular potassium. The mechanism of conductance block based on myelin restructuring complements the already known and modelled block via hyperpolarization mediated by the axonal sodium pump and potassium depolarization.

Dynamical patterns of calcium signaling in a functional model of neuron-astrocyte networks.

We propose a functional mathematical model for neuron-astrocyte networks. The model incorporates elements of the tripartite synapse and the spatial branching structure of coupled astrocytes. We consider glutamate-induced calcium signaling as a specific mode of excitability and transmission in astrocytic-neuronal networks. We reproduce local and global dynamical patterns observed experimentally.

Non-invasive study of nerve fibres using laser interference microscopy.

This paper presents the results of a laser interference microscopy study of the morphology and dynamical properties of myelinated nerve fibres. We describe the principles of operation of the phase-modulated laser interference microscope and show how this novel technique allows us to obtain information non-invasively about the internal structure of different regions of a nerve fibre. We also analyse the temporal variations in the internal optical properties in order to detect the rhythmic activity in the nerve fibre at different time scales and to shed light on the underlying biological processes. We observe pronounced frequencies in the dynamics of the optical properties and suggest that the oscillatory modes have similar origin in different regions, but different strengths and mutual modulation properties.

Giant glial cell: new insight through mechanism-based modeling.

The paper describes a detailed mechanism-based model of a tripartite synapse consisting of P- and R-neurons together with a giant glial cell in the ganglia of the medical leech (Hirudo medicinalis), which is a useful object for experimental studies in situ. We describe the two main pathways of the glial cell activation: (1) via IP(3) production and Ca(2 +) release from the endoplasmic reticulum and (2) via increase of the extracellular potassium concentration, glia depolarization, and opening of voltage-dependent Ca(2 +) channels. We suggest that the second pathway is the more significant for establishing the positive feedback in glutamate release that is critical for the self-sustained activity of the postsynaptic neuron. This mechanism differs from the mechanisms of the astrocyte-neuron signaling previously reported.

Effect of proteins from the spirochete Borrelia burgdorferi sensu lato on myelinated nerve excitability.

We studied the effect of spirochete Borrelia burgdorferi sensu lato cell membrane proteins on excitability of myelinated nerve fiber. It was found that cell surface proteins of spirochetes B. burgdorferi s. s. bind to Ranvier nodes of the axon and to Schwann cells. Binding of B. burgdorferi s. s. and B. garinii to the nerve fiber modulates the amplitude and conduction velocity of the action potential, while B. afzelii had no effect on these parameters. The decrease in the spike amplitude and conduction velocity during sorption of B. burgdorferi s. s. or cell wall proteins was accompanied by desorption of membrane-bound calcium.

Self-organized critical gating of ion channels: on the origin of long-term memory in dwell time series.

We present the model of an ion channel, operating in a regime of self-organized criticality. It is suggested that complex cooperative dynamics takes place in the protein and the overall tension of it facilitates an open or closed state of the rigid gates in the pore-making domain. For the first time multifractal spectra of ion channel dynamics are presented. Our model well reproduces the multifractal properties of ion channel dwell time series and provides an insight on the origin of the long-term correlations in these series.

Unraveling cell processes: interference imaging interwoven with data analysis.

The paper presents results on the application of interference microscopy and wavelet-analysis for cell visualization and studies of cell dynamics. We demonstrate that interference imaging of erythrocytes can reveal reorganization of the cytoskeleton and inhomogenity in the distribution of hemoglobin, and that interference imaging of neurons can show intracellular compartmentalization and submembrane structures. We investigate temporal and spatial variations of the refractive index for different cell types: isolated neurons, mast cells and erythrocytes. We show that the refractive dynamical properties differ from cell type to cell type and depend on the cellular compartment. Our results suggest that low frequency variations (0.1-0.6 Hz) result from plasma membrane processes and that higher frequency variations (20-26 Hz) are related to the movement of vesicles. Using double-wavelet analysis, we study the modulation of the 1 Hz rhythm in neurons and reveal its changes under depolarization and hyperpolarization of the plasma membrane. We conclude that interference microscopy combined with wavelet analysis is a useful technique for non-invasive cell studies, cell visualization, and investigation of plasma membrane properties.

Interference microscopy under double-wavelet analysis: a new approach to studying cell dynamics.

This Letter combines a novel experimental approach to the study of intracellular processes with a newly developed technique for multimode time-series analysis. Experiments are performed on isolated pond snail (Lymnaea stagnalis) neurons. Local variations in the cellular refractive index as detected by laser interference microscopy are related to the processes in the cell. A wavelet analysis shows the presence of several identifiable modes in the membrane and intracellular dynamics, and a double-wavelet analysis reveals nonlinear interactions between the regulatory processes in the form of mutual frequency and amplitude modulations.

[Calculation of local Hurst exponents in the Ca(2+)-activated K(+)-channel dwell time]. / Raschet lokal'nykh pokazatelei Khersta v posledovatel'nostiakh vremen zhizni Ca(2+)-aktiviruemykh K(+)-kanalov.

A novel method based on the maximum overlap wavelet transform of dwell time series is proposed. Information on local multifractal properties of the series, namely local Hurst exponents or Holder exponents, was obtained. The results confirm the presence of multifractality and intrinsic correlations in the Ca(2+)-activated K+ channel dwell time series. The data on the local multifractal structure of the series can be interpreted in terms of processes having self-organized criticality. The proposed approach allows one to widen the store of methods for the analysis of single ion channel activity.