G. R. Ivanitsky

Negative refractoriness in excitable systems with cross-diffusion

The results of numerical experiments with mathematical models of excitable systems with cross-diffusion are presented. It was shown that the refractoriness in such systems may be negative. The effects of negative refractoriness on the propagation and interaction of waves are demonstrated.

Conditions Causing Wavefront Instability in a Growing Colony of Bacterial Cells with Chemotactic Activity

Propagation of nonlinear waves in excitable reaction–diffusion media plays a fundamental role in functioning of many biological systems [1–3]. A natural form of an excitation wave is a ring-shaped or plane front, because irregular perturbations to the front are usually dissipated with time by diffusion [4]. However, under certain conditions (e.g., at particular diffusion rate ratios between active components of the medium), instabilities may develop on the front of a running wave, leading to its breakdown and the formation of spatially inhomogeneous structures. This kind of wavefront transformation is observed, for example, in experiments with populations of motile bacteria [5–7]. Depending on the growth conditions, the population wave from a bacterial colony inoculated locally into a soft agar medium may propagate as a regular ring-shaped front (Fig. 1a) or in the form branching projections (Fig. 1b). The mechanism responsible for the formation of such unusual fractal-like structures may be instability caused by nonlinear diffusion in bacterial medium [8, 9]. If the diffusion coefficient of bacterial cells depends on their density and the nutrient concentration, noise-induced fluctuations of the front of a population wave may result in its spatially inhomogeneous propagation and the development of projections. In recent studies [8, 9], a mathematical model of bacterial population growth was analyzed, in which no allowance was made for chemotaxis [10], and fractallike structures were formed solely by virtue of nonlinear diffusion. However, microorganisms can move not only chaotically (diffuse) but also directionally, along the gradient of preferable environmental conditions (e.g., bacteria move toward increasing nutrient concentrations). This ability is a fundamental property of many microorganism communities. Therefore, the question arises as to how chemotaxis interacts with nonlinear diffusion-related instabilities capable of breaking down the population front.The purpose of this study was (1) to analyze the bacterial population fronts for stability, using a mathematical model that describes both nonlinear diffusion and chemotaxis; and (2) to predict the model conditions for transition from ringshaped waves to fractal-like structures.The known model of chemotaxis, developed by Keller and Segel [11], was modified for the case of nonlinear diffusion. The diffusion coefficient for bacterial cells was not a fixed parameter; we assumed that it is a function of the cell density and the nutrient concentration.

Biophysical basis of medical thermovision

Characteristics of the thermal relief of the human body are reviewed. It is shown that the usage of thermovision in medical diagnostics requires simultaneous and compatible consideration in the research process of three components: the contribution of the object itself, the influence of the medium through which the infrared radiation passes and the instrumental parameters of the infrared imager registering this radiation.

A Perfluorocarbon Droplet on the Water Surface: Nonstationary Motion Induced by Shot Noise

It was found that a perfluorocarbon droplet applied onto the water surface moves spontaneously over the surface. The mechanism of the motion has been determined. Physicochemical properties of various liquids can be deduced from the trajectories of motion of droplets of these liquids on the water surface [1]. The interest of biophysicists to perfluorocarbon compounds is due to the wide practical applications of perfluorocarbons in medicine. For example, these compounds are used in blood substitutes, membrane-modifying agents, wound coating, etc. [2]. It has been shown in the preceding works that the four-component water–lipid–perfluorocarbon–air system displays an unusual physical property, spontaneous mobility of the lipid [3, 4]. The mechanism of this phenomenon considered in [4] was found to be connected with the difference between the surface tensions in the three liquid components of the system. In the present study, we showed that, in the three-component perfluorocarbon droplet–water surface–air system, the perfluorocarbon droplet also moves spontaneously over the water surface. The following experiment was performed. An approximately 5-mm layer of water was poured into a Petri dish (diameter, 9 cm) at an ambient temperature of 24°ë . A droplet of perfluorodecaline ( ë 10 F 18 ) 2 μ l in volume was applied onto the water surface (Fig. 1a). Although the specific gravity of perfluorodecaline is almost twice that value of water (1.94 g/ml), the perfluorodecaline droplet remained on the water surface rather than sank. The floatage of the droplet was due to the hydrophobicity of perfluorodecaline (the perfluorodecaline solubility in water is 3 × 10 –6 vol %). The high surface-tension coefficient of water (70–72 dyne/cm) is another factor facilitating the perfluorodecaline droplet retention on the water surface. The force holding the droplet is determined by the flexing of the water surface under the gravity of the droplet itself. The diagram of the droplet position on the water surface is shown in Fig. 1b. Two modes of perfluorodecaline droplet motion were observed. The first one was a relatively fast stochastic motion over the water surface. At first glance, this mode is similar to Brownian motion. The second mode was a slow drift without pronounced breaks of the motion trajectory. The droplet motion was monitored with a Sony XC-711P 89D video camera. The coordinates were calculated using a computer. The droplet motion trajectory in the fast mode is shown in Fig. 1c. One to four-cantimeter straight segments of the motion trajectory are interrupted by random changes in the motion direction. The velocity of the droplet motion was 0.75–1 mm/s in this case. The velocity of the droplet motion in the slow mode did not exceed 0.1 mm/s, i.e., it was an order of magnitude less than in the former case (Fig. 1d). Why the droplet motion followed the fast mode in some cases and the slow mode in others? The weight of a 2μ l perfluorocarbon droplet is 4 g. Therefore, this object is too heavy to be driven to motion by random collisions with water molecules (the generally accepted explanation of Brownian motion). Our experiments showed that the relative fraction of the fast (slow) motion depended on water saturation with gases. The fast motion was observed in the case of water saturation with gases by aeration (or saturation with ëé 2 under pressure). A simple experiment demonstrated that water is indeed saturated with gases under these conditions. If a piece of a hydrophobic solid substance that does not dissolve gases (e.g., a dry particle of tea-leaf) is placed on the water surface, this particle will be covered with adsorbed microscopic bubbles of gas evolved from water after a short while. In some cases, these microscopic bubbles escape from the particle (Fig. 2). Let distilled water be poured in Petri dish and settled for several hours. The content of dissolved gases declines during this process and water attains a stationary state. The concentration of air dissolved in such water at 24°ë is less than 2–4 vol %. If a droplet of perfluorodecaline is placed onto the surface of this water, the droplet remains almost motionless. What is the mechanism of the mobility of a perfluorodecaline droplet? The molecule of perfluorodecaline does not contain free polar groups. Fluorines replace all A Perfluorocarbon Droplet on the Water Surface: Nonstationary Motion Induced by Shot Noise

Mechanisms of Wave Interaction as a Model of Dosed Substance Secretion

Mechanisms of dosed secretion of various substances (secretions) by cells and tissues of living organisms are sophisticated and insufficiently understood. According to the generally accepted classification, these mechanisms fall into three groups: merocrine, apocrine, and holocrine [1]. In the case of the merocrine mechanism, the secretion is transported through the cell membrane as a result of diffusion driven by the concentration gradient. The substances (hormones, neurotransmitters, or digestive enzymes) secreted by merocrine cells or tissue are transported with water flow. No additional energy expenditure or special contraction processes are required in this case. The apocrine mechanism involves a breaking off of the apical membrane and a part of cytoplasm. However, both the apical membrane and the cytoplasm are restored when secretion is over. The apocrine mechanism of secretion is characteristic of, e.g., mammary glands. This mechanism requires additional energy expenditure and special contraction systems. The holocrine mechanism of secretion involves complete disintegration of the secreting cell and its rejection from epithelium together with the cell contents. The holocrine mechanism of secretion is characteristic of, e.g., sebaceous glands. The contractility mechanisms of the holocrine tissue ensure the transport of liquids with high concentrations of salts (sweat, in the case of sebaceous glands) against the osmotic and electrochemical gradients.