Biomedical subjects

M G Stepanov

Publications and source records attributed to M G Stepanov.

At least 19 recordsLinked to original sources

PubMed2005

Diagnosis of weaknesses in modern error correction codes: a physics approach.

One of the main obstacles to the wider use of the modern error-correction codes is that, due to the complex behavior of their decoding algorithms, no systematic method which would allow characterization of the bit-error-rate (BER) is known. This is especially true at the weak noise where many systems operate and where coding performance is difficult to estimate because of the diminishingly small number of errors. We show how the instanton method of physics allows one to solve the problem of BER analysis in the weak noise range by recasting it as a computationally tractable minimization problem.

Algorithms↗

PubMed2004

Error correction on a tree: an instanton approach.

We introduce a method that allows analytical or semianalytical estimating of the post-error correction bit error rate (BER) when a forward-error correction is utilized for transmitting information through a noisy channel. The generic method that applies to a variety of error-correction schemes in the regimes where the BER is low is illustrated using the example of a finite-size code approximated by a treelike structure. Exploring the statistical physics formulation of the problem we find that the BER decreases with the signal-to-noise ratio nonuniformly, i.e., crossing over through a sequence of phases. The higher the signal-to-noise ratio the lower the symmetry of the phase dominating BER.

Journal Article↗

PubMed2004

Growth of density inhomogeneities in a flow of wave turbulence.

We consider the flow being a superposition of random waves and describe the evolution of the spectrum of the passive scalar in the leading (fourth) order with respect to the wave amplitudes. We find that wave turbulence can produce an exponential growth of the passive scalar fluctuations when either both solenoidal and potential components are present in the flow or there are potential waves with the same frequencies but different wave numbers.

Journal Article↗

PubMed2002

Acceleration of rain initiation by cloud turbulence.

Vapour condensation in cloud cores produces small droplets that are close to one another in size. Droplets are believed to grow to raindrop size by coalescence due to collision. Air turbulence is thought to be the main cause for collisions of similar-sized droplets exceeding radii of a few micrometres, and therefore rain prediction requires a quantitative description of droplet collision in turbulence. Turbulent vortices act as small centrifuges that spin heavy droplets out, creating concentration inhomogeneities and jets of droplets, both of which increase the mean collision rate. Here we derive a formula for the collision rate of small heavy particles in a turbulent flow, using a recently developed formalism for tracing random trajectories. We describe an enhancement of inertial effects by turbulence intermittency and an interplay between turbulence and gravity that determines the collision rate. We present a new mechanism, the 'sling effect', for collisions due to jets of droplets that become detached from the air flow. We conclude that air turbulence can substantially accelerate the appearance of large droplets that trigger rain.

Journal Article↗

PubMed2001

Statistics of interacting optical solitons.

We examine statistics of two interacting optical solitons and describe timing jitter caused by spontaneous emission noise and enhanced by pulse interaction. Dynamics of phase difference is shown to be of crucial importance in determining the probability distribution function (PDF) of the distance between solitons. We find analytically the non-Gaussian tail of the PDF to be exponential. The propagation distance that corresponds to a given bit-error rate is described as a function of system parameters (filtering and noise level), initial distance, and initial phase difference between solitons. We find the interval of parameters where a larger propagation distance can be achieved for higher density of information.

Journal Article↗

PubMed2001

Large negative velocity gradients in Burgers turbulence.

We consider one-dimensional Burgers equation driven by large-scale white-in-time random force. The tails of the velocity gradients probability distribution function (PDF) are analyzed by saddle point approximation in the path integral describing the velocity statistics. The structure of the saddle-point (instanton), that is, the velocity field configuration realizing the maximum of probability, is studied numerically in details. The numerical results allow us to find analytical solution for the long-time part of the instanton. Its careful analysis confirms the result of Balkovsky et al. [Phys. Rev. Lett. 78, 1452 (1997)] based on short-time estimations that the left tail of PDF has the form ln P(u(x))infinity-/u(x)/(3/2).

Journal Article↗

PubMed2001

Laplacian growth with separately controlled noise and anisotropy.

Conformal mapping models are used to study the competition of noise and anisotropy in Laplacian growth. For this purpose, a family of models is introduced with the noise level and directional anisotropy controlled independently. Fractalization is observed in both anisotropic growth and growth with varying noise. The fractal dimension is determined from the cluster size scaling with cluster area. For isotropic growth d=1.7, at both high and low noise. For anisotropic growth with reduced noise the dimension can be as low as d=1.5 and apparently is not universal. Also, we study the fluctuations of particle areas and observe, in agreement with previous studies, that exceptionally large particles may appear during growth, leading to pathologically irregular clusters. This difficulty is circumvented by using an acceptance window for particle areas.

Journal Article↗

PubMed2001

Effect of chronic inhalation of toluene and dioxane on activity of free radical processes in rat ovaries and brain.

The effects of toluene and dioxane inhalations on the intensity of free radical oxidation in rat ovaries and brain cortex were studied. Both toxins in a dose 10-fold surpassing the maximum permissible concentration increased activity of glutathione peroxidase in brain tissue; moreover, toluene increased chemiluminescence intensity, which attested to activation of free radical processes. In ovarian tissue toluene increased activities of glutathione peroxidase and catalase and the intensity of lipid peroxidation. These changes were associated with the appearance of normally absent circadian rhythm.

Administration, Inhalation↗

PubMed2001

Disturbances of diurnal rhythms of biogenic amines contents in hypothalamic nuclei as an evidence of neurotropic effects of enterotropic carcinogen 1,2-dimethylhydrazine.

OBJECTIVES: Our data on the contents of norepinephrine (NE), dopamine (DA) and the metabolite of serotonin 5-hydroxyindoleacetic acid (5-HIAA) measured in the suprachiasmatic nuclei (SCN), preoptic area (PA) and median eminence (ME) of hypothalamus of rats after single subcutaneous injection of 1,2-dimethylhydrazine (DMH) as well as the effect of this carcinogen on formation of reactive oxygen species (ROS) in the PA are presented in this paper. RESULTS: Diurnal changes of DA in all studied brain structures and of NE in the PA have been observed in the control group. Their morning levels were higher than evening ones. Rhythms of 5-HIAA in the SCN and diurnal changes of ROS formation have been shown to have contrary changes in control. Both the morning (11 a.m.) and evening (11 p.m.) subcutaneous administration of DMH at the dose of 21 mg/kg of body weight resulted in changes of all rhythms observed in control. In some cases a phase shift was found, in others the rhythms of neurotransmitters and ROS formation disappeared entirely. CONCLUSION: The data obtained confirm the idea of dopaminergic and serotoninergic systems taking part in mechanisms of a response of the hypothalamic nuclei to non-photic stimuli. It is suggested that the effect of DMH on the content and diurnal rhythms of neurotransmitters in the hypothalamic structures under study is due to its affecting activities of the enzymes of biogenic amines synthesis, synaptic transmission, melatonin synthesis and secretion rhythms. The change in ROS formation that is caused by administration of DMH is likely to be due to a disturbance of diurnal rhythms of neurotransmitters that are one of the sources of formation of free radicals in the brain.

1,2-Dimethylhydrazine↗

PubMed2001

[Disruption of circadian rhythms of biogenic amines in rat hypothalamus upon administration of 1,2-dimethylhydrazine].

Levels of norepinephrine (NE), dopamine (DA) and the main metabolite of serotonin 5-hydroxyindoleacetic acid (5-HIAA) have been measured in the suprachiasmatic nuclei (SCN), preoptic area (PA), and median eminence (ME) of hypothalamus of rats after sole subcutaneous injection of 1,2-dimethylhydrazine (SDMH). Circadian changes of DA in all the brain structures under study as well as of NE in PA were observed in the control group, their levels in the mornings being higher than in the evenings; a circadian change of 5-HIAA in SCN had an opposite tendency. Both the evening (11 p.m.) and morning (11 a.m.) administrations of SDMH at the dose of 21 mg/kg body weight resulted in disturbances of all the circadian rhythms observed in control. In some cases only a 12 hrs circadian rhythms phase shift was found, in the others these rhythms of neurotransmitters disappeared entirely. The evening administration of SDMH, unlike the morning one, resulted in an increase in total NE content in the hypothalamic structures under study. It is suggested that the effect of SDMH on the levels and circadian rhythms of neurotransmitters in the hypothalamic structures under study is due to affecting activities of the enzymes of biogenic amines synthesis, synaptic transmission, melatonin synthesis and secretion rhythms, as well as to its genotoxic influence upon the genes controlling circadian actions.

1,2-Dimethylhydrazine↗

PubMed1995

[Early chemical sympathectomy and the function of the hypophyseal-gonadal system in sexually immature and adult female rats].

Early chemical desympathisation with 6-hydroxydopamine decreased the contents of the follicle-stimulating hormone in the blood serum of 1-month old rats. The steroidogenesis disturbances were more obvious in sexually immature rats in the general desympathisation. In sexually mature rats, these disturbances disappeared. Sympathetic innervation seems to play a major part in development of the reproductive system in rats.

Animals↗

PubMed1994

[The circadian rhythm of fluctuations in the gonadoliberin level in the rat hypothalamus and the effect on it of different xenobiotics].

An obvious circadian rhythm of the gonadoliberine levels was found in the Organum vasculosum of Lamina terminalis: the level being very low in the morning, rising 5-fold after midday and 10-fold at 5-6 p.m. The rhythm did not depend on the oestrous cycle stage and was not found in males. Xenobiotics interfere with the rhythm due, probably, to lowering of the suppressive tonus.

Animals↗

PubMed1993

[Weak androgens, their importance experimentally and clinically].

Antisera obtained for certain hormones possessed a high specific effect on homologous hormones. Radioimmunological methods were worked out for determination of these hormones in the blood serum of humans and animals. Clinical use of the above antisera for determination of dehydroepiandrosterone and androstenedione in humans revealed than it could be useful to determine dehydroepiandrosterone in some forms of hypogonadism. Androstenedione is often increased in female hirsutism.

Androstenedione↗

PubMed1992

[Effect of a new prostaglandin aroxaprostol on the contractile activity of the uterus and its abortive action in experimental studies].

Experiments with pregnant and nonpregnant rats have shown that aroxaprostol, a group F2 alpha prostaglandin, injected intravenously and intramuscularly in a dose of 50 micrograms/kg, stimulates the uterine bioelectric activity, increasing both the amplitude and frequency of biopotentials, this evidencing intensification of its contractility. Aroxaprostol has shown an abortive effect, that was related to the luteolytic effect of the drug, manifesting by reduction of progesterone level in the ovaries.

Abortifacient Agents↗

PubMed1991

[The negative and positive phases of the reaction of the hypothalamo-hypophyseal-gonadal system in female rats to the administration of sex hormones].

I.m. administration of estradiol benzoat in physiological doses decreased 3-fold the lutropin secretion and follitropin secretion by one third, in ovariectomised rats. The same dose in intact rats decreases the lutropin level in the blood and gonadoliberin contents in the medial eminence by the negative feedback mechanism. In 4-6 hrs after injection the positive effect of estradiol is at its highest level, the lutropin level in the blood increases by 80-100 times, the follitropin--by 2-3 times. Therefore, estradiol plays a decisive role in positive as well as negative effects on the lutropin secretion, whereas its effect upon follitropin secretion is less obvious. The gonadoliberin transport in negative phase of estradiol effect is mainly performed through the medial eminence whereas the positive effect of estradiol needs a participation of organum vasculosum of the lamina terminalis too.

Animals↗

PubMed1991

[The effect of aminostimil on the regulation of sexual function in rats].

The effect of aminostimil: a synthetic drug of the acylhydrazide group, was studied in white rats. Aminostimil decreased the uterus weight but did not interfere with estrogen positive action in ovariectomized (OVX) rats, although it increased the FSH level, abolished the inhibitory effect of estradiol on the FSH secretion and did not change the secretion of the LH. Aminostimil induced accumulation of GnRH in the preoptic area and median eminence area in intact female and male rats, LH concentration in the blood plasma but decreased. The data obtained suggest that the antiestrogenic effect of aminostimil is due to inhibition of the GnRH secretion from the hypothalamus into the portal blood.

Animals↗

PubMed1991

[The ratio of sex hormone production by the ovaries and the adrenal cortex in different phases of the rat estrous cycle].

The biosynthesis of estradiol and testosterone was found to occur mainly in the ovaries (4-5 and 10-20 times more than in the adrenals during diestrus and proestrus, resp.). Estron and androstendion were produced equally in both glands. The estrogens and androgens were found to act as synergists in the stimulation of biosynthetic processes in the uterus. Their level in peripheral blood reflects less fully the character of hormonal function of the ovaries than study of these steroids in the incubates of the ovaries. The biosynthesis of progesterone in adrenals exceeds 2-3-fold that in the ovaries. No changes on production of this hormone depending on the cycle phase, were found.

Adrenal Cortex↗

PubMed1990

[Physiologic mechanisms of the reaction of the reproductive system in female rats to chronic exposure to low doses of toluene].

Chronic inhalation of toluol (2-4 months) did not affect the menstrual cycle in female rats but distorted the mechanisms of the reproduction control. The level of gonadoliberin decreased during dioestrus in preoptic area, the correlation between concentrations of luteinizing and folliculo-stimulating hormones became disordered, and the prolactin secretion increased. Preovular peaks of gonadotropins secretion became lower in proestrus. The cold stress increases the prolactin secretion and decreases the gonadoliberin content in the hypothalamus of control animals. These events do not happen after toluol inhalation. After 4 months of toluol inhalation, the levels of estradiol and progesterone decrease in the blood. However, normal response to gonadotropic stimulation is preserved which suggests a secondary character of hormonal disorders, in response to primary disturbances in the hypothalamo-hypophyseal link of the reproduction control.

Administration, Inhalation↗