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At least 703 records · Page 39Linked to original sources
PubMed1990

Studies of respiratory stridor in young children: acoustical analyses and tests of a theoretical model.

Non-invasive procedures are proposed to aid the diagnosis of childhood laryngotracheal pathology and to monitor the course of such disease. The procedures capitalize on the one-to-one relationship which exists between the acoustic phenomena (stridors) associated with respiration and the configuration of the respiratory tract. Careful analysis of these acoustic patterns can thus assist in identifying and localizing constrictions, in diagnosis, and in monitoring disease severity. Based on the acoustical analysis of the stridor generated by children with congenital stridor, subglottal laryngitis, and trachea stenosis, the present paper demonstrates that a close relationship exists between the specific pathology and the spectrum of the associated respiratory stridor.

Acoustics↗
PubMed1992

An empirical test of two opposing theoretical models of prefrontal function.

The purpose of this study was to compare the validity of two models which contrast with each other in the manner in which they integrate neuropsychological tests into distinct prefrontal constructs. The first prefrontal model consists of five distinct functional constructs drawn from human clinical neuropsychology. The second model, elaborated by Goldman-Rakic, is based primarily on monkey research and postulates a basic prefrontal function, "on-line representational memory," which guides behavior in the absence of, or despite discriminative environmental stimuli. In the latter model, distinct prefrontal functional constructs are primarily defined in terms of various types of representational memory involved in specific tasks. Eleven "prefrontal" measures were obtained from 259 normal adults, stratified for age, education, and sex. Confirmatory factor analyses revealed that the Goldman-Rakic model "fit" the data better than the model derived from human clinical neuropsychology, while several constructs commonly used in human neuropsychology were refuted. It was concluded that new research on brain-damaged humans with a view to understanding prefrontal function might benefit from using the Goldman-Rakic model as a starting point.

Adolescent↗
PubMed1987

Blood oxygen equilibria and theoretical models. I. Effect of protons in trout (Salmo gairdneri) and human red cells, in absence of organic phosphates.

1. Oxygen equilibrium curves were measured on red cells that had been depleted of organic phosphates, for rainbow trout red cells between pH 7-9, at 15 and 20 degrees C, and for human red cells between pH 6.8-8.0, at 37 degrees C. 2. The data were fitted to the models of Adair and of Monod et al. (MWC model). Parameters were estimated by the non-linear least-squares method, from which the number of Bohr protons released during oxygenation was calculated. 3. For trout as for human red cells, the pH affects the first step of oxygenation and the overall oxygenation in nearly equal proportion. For human red cells, the association constant for binding of the last oxygen, expressed by k4 or kR, is not affected by pH, indicating that the R structure is free of constraints. For trout red cells, there is a pronounced pH dependence of this constant at low pH, which is about 10-fold increased between pH 7-9. This corresponds to the Root effect that impairs the T----R transition, and confirms previously published data for normal trout and human red cells. 4. For trout red cells, small functional heterogeneity is evidenced, despite Hb multiplicity, and effect seems linked to the level of temperature. 5. The specific effect of organic phosphates, evident for human red cells, contrasts with a lack of effect for trout red cells, when taking into account the intracellular pH values.

Algorithms↗
PubMed1982

A theoretical model of the temperature- and pressure-induced phase transition of phospholipid bilayers.

A statistical thermodynamic model of phospholipid bilayers is developed. In the model, a new concept of a closely packed system is applied, i.e., a system of hard cylinders of equal radii, the radius being a function of the average number of gauche rotations in a hydrocarbon chain. Using this concept of a closely packed system, reasonable values are obtained for the change in specific volume at the order-disorder transition of lecithin bilayers. In addition to interactions between the lipid matrix and water molecules, between the head groups, themselves and between hydrocarbon chains, as well as the intramolecular energy associated with chain conformation, the Hamiltonian of the membrane also includes the energy of the pressure field. Thus, the phase transition of phospholipid membranes induced not only by temperature but also by hydrostatic pressure is described by this model simultaneously. In accordance with the experimental results, a linear relationship is obtained between the phase transition temperature and phase transition pressure. The other calculated phase transition properties of lecithin homologues, e.g., changes in enthalpy, surface area, thickness and gauche number per chain are in agreement with the available experimental data. The ratio of kink to interstitial conduction of bilayers is also estimated.

Crystallization↗
PubMed1986

A simple theoretical model of steady-state electrogenic transfer of reactive species across membranes. The case of Chara corallina.

Transfer across membranes is treated for simultaneous active pumping and passive flow of the main ionic constituents of plant cells. The chemical equilibrium conditions for H2CO3 and H2O dissociation prevailing in the external and internal solutions are taken into account. Our model assumes steady-state conditions, zero electrical current, electroneutrality inside the membrane and, in both solutions, finite space charge densities in two diffuse regions in contact with both sides of the membrane. A computer program has been developed for the integration of a set of flux equations. The results are displayed in terms of ionic flux parameters, each proportional to the ionic pump flow and to the reciprocal of the passive ionic mobility. The discussion emphasizes the role of anion inward pumping and of non-diffusible anions. The cationic pumps serve to establish different ionic balances in the cell. Transmembrane (bulk to bulk) proton pumping affects essentially the pH regulation in the cell.

Journal Article↗
PubMed1986

Oscillations in theoretical models of induction.

A two-variable model for the genetic regulatory mechanism of induction is proposed. In a feedforward step an autocatalytically accumulated substrate induces the transcription of its own degrading enzyme. The differential equations for enzyme and substrate are treated analytically and it is found that in a defined parameter range the system becomes unstable and shows structurally stable limit cycle oscillations. The system behaves like an activator-inhibitor model and instability is likely to arise if the transcription process is slow. In a slightly modified system oscillations inside a cell are generated if an external parameter (extracellular substrate concentration) exceeds a certain threshold and all other parameters are unchanged. Possible biological implications of these results are destabilization of metabolic units by transport processes and feedforward catalysis.

Biological Transport, Active↗
PubMed1983

Theoretical model of slow-wave membrane potential oscillations in molluscan neurons.

A model describing slow-wave membrane potential oscillations observed in molluscan neurons has been developed. It takes into account non-inactivating calcium and Ca-dependent potassium currents found experimentally in these neurons. This model successfully describes the form of the experimental potential wave as well as the dependence of the oscillatory behaviour of molluscan neurons on the extracellular concentration of Ca2+ and K+ ions. By changing different parameters of the model I was able to obtain three patterns of the electrical activity of molluscan neurons. These are as follows: silent neurons with the resting potential ranging from -49 to -53 mV; neurons which demonstrate a slow-wave oscillatory activity with an interval between 12 and 17 s; and neurons with repetitive firing activity. The possible interaction between these three patterns of the electrical activity is considered and its neurophysiological significance is discussed. It was shown, in the framework of the model developed, that a neuron can switch from one pattern to another by changing any parameter of the model. It is most likely that such alteration in the electrical neuronal activity is connected with a change in intracellular adenosine 3',5'-cyclic monophosphate content affecting both the magnitude of the calcium conductance and the intracellular level of Ca2+ ions.

Action Potentials↗
PubMed1984

Gene control by phosphoproteins: a theoretical model for eukarotic DNA regulation.

Although gene regulatory mechanisms in eukaryotic cells are complex, some progress is being made in understanding them. Chromosomal proteins may play a significant role in genome function and gene control. More specifically, a central role may be played by the nuclear nonhistone proteins. It appears that both viral transformation and steroid hormone action may be associated with the phosphorylation of these proteins. Recent studies have revealed that some oncogenic viruses are capable of producing viral transforming proteins with protein kinase activity. This suggests how they may subvert normal gene regulatory mechanisms. Furthermore, the effects of the steroid-receptor complex on nuclear nonhistone proteins may be similar to the effects of these viral transforming proteins, but in a controlled sense. A model of gene-regulatory, nuclear phosphorylation reactions is formulated which suggests how some oncogenic viruses may control normal gene regulatory mechanisms and how steroid hormones may interact with these same mechanisms. Such a model may reveal how disruption of these same mechanisms leads to carcinogenesis.

Animals↗
PubMed1984

Gene control by phosphoproteins: a theoretical model for eukaryotic DNA regulation.

Although gene regulatory mechanisms in eukaryotic cells are complex, some progress is being made in understanding them. Chromosomal proteins may play a significant role in genome function and gene control. More specifically, a central role may be played by the nuclear nonhistone proteins. It appears that both viral transformation and steroid hormone action may be associated with the phosphorylation of these proteins. Recent studies have revealed that some oncogenic viruses are capable of producing viral transforming proteins with protein kinase activity. This suggests how they may subvert normal gene regulatory mechanisms. Furthermore, the effects of the steroid-receptor complex on nuclear nonhistone proteins may be similar to the effects of these viral transforming proteins, but in a controlled sense. A model of gene-regulatory, nuclear phosphorylation reactions is formulated which suggests how some oncogenic viruses may control normal gene regulatory mechanisms and how steroid hormones may interact with these same mechanisms. Such a model may reveal how disruption of these same mechanisms leads to carcinogenesis.

Arteriosclerosis↗
PubMed1983

Human collagen digestion: II. Theoretical model and application to data.

A kinetic mechanism for the digestion of collagen by bacterial collagenase has been proposed which is consistent with experimental data. Arguments are presented justifying the application of a homogeneous approach to the digestion of insoluble collagen. The mechanism takes account of increased kinetic blockages with aging and enzyme complexity. It is shown to be applicable to collagen from various tissues, including relatively impure preparations. However, the mechanism is not applicable to collagen from the immature period of the lifespan. Kinetic blockages are shown to increase three- to four-fold from age 20 to 70 years, within the main helical region of the collagen molecule. There is no increase of kinetic blockages in the N-terminal region of the molecule over this age range.

Adult↗
PubMed1992

Circadian dysregulation in abused individuals: a proposed theoretical model for practice and research.

Trauma in the form of physical and sexual abuse remains a major issue confronting health professionals and society today. Despite the identification of large numbers of children at risk, no studies exist on the association between childhood abuse and the development of acute and long-term disruption of sleep/wake patterns. Furthermore, identification of the most effective nursing interventions to restore healthy sleep rhythms is sorely lacking. As a basic human process, sleep follows a circadian pattern, effecting sleep onset and stages. This article explores a possible model to link the sleep disruption seen in abuse children with circadian and rhythmic theories.

Child↗
PubMedDate not supplied

Postural stability in the elderly: empirical confirmation of a theoretical model.

The study investigated whether physiological factors related to specific body systems can explain postural control as measured by the scalar distance at a given time between the center of pressure (COP) and the center of mass (COM), the COP-COM variable. The data from 46 healthy subjects and 29 subjects with disabilities due to stroke or diabetic peripheral neuropathy were analyzed. The biomechanical variable COP-COM was determined using two force platforms and an optoelectric system. Three systems were considered as possible predictors of the COP-COM amplitude: sensory (somatosensory and vision), musculoskeletal and central processor. A confirmatory analysis was done using structural equation modeling. Strength explained 23.74% of the COP-COM amplitude in the antero-posterior (A/P) with eyes open (EO) condition, and in the medio-lateral (M/L) direction strength explained 40.73 and 28.75% in the EO and eyes closed (EC) respectively. In the A/P direction with EC, 51.75% of the COP-COM amplitude variance was explained mainly by the somatosensory system. This study highlight the role of peripheral somatosensory input and muscle strength in the maintenance of postural stability during quiet stance in the elderly. The indirect action of the different systems on the COP-COM amplitude supports the systems theory.

Aged↗
PubMed2004

A theoretical model for electron and proton coupling at quinone-binding site of photosystem II of higher plants.

In this work we consider the coupling of electron and proton transfer near Q(B) in the reaction center (RC) of photosystem II (PS2). We have carried out the calculations of the energy levels and proton density in the system Q(B)(-) Histidine L190. It is shown that the proton of the histidine forms the H-bond with twice-reduced Q(B)(2-). Based on these calculations, we propose a new explanation of the coupling between the electron and proton transfer.

Benzoquinones↗
PubMed2004

Modest catalysis of the decarboxylation of orotate by hydrogen bonding: a theoretical model for orotidine- 5' -monophosphate decarboxylase.

As a model for interactions present in the active site of orotidine-5'-monophosphate decarboxylase (ODCase), the effect of hydrogen bonds to the carbonyl groups (O-2 and O-4) of orotic acid and its decarboxylation product was probed with ab initio calculations. We have found that the transition state/carbanion intermediate is a better proton receptor and therefore, the hydrogen bonds can be a modest source of catalysis. Comparison of the calculated data with results from site-directed mutagenesis provides some insights into the polarity of the active site.

Catalysis↗
PubMed2006

A game-theoretic model for punctuated equilibrium: species invasion and stasis through coevolution.

A general theory of coevolution is developed that combines the ecological effects of species' densities with the evolutionary effects of changing phenotypes. Our approach also treats the evolutionary changes between coevolving species with discreet traits after the appearance of a new species. We apply this approach to habitat selection models where new species first emerge through competitive selection in an isolated habitat. This successful invasion is quickly followed by evolutionary changes in behavior when this species discovers the other habitat, leading to punctuated equilibrium as the final outcome.

Biological Evolution↗
PubMed2005

Theoretical models of possible compact nucleosome structures.

Chromatin structure seems related to the DNA linker length. This paper presents a systematic search of the possible chromatin structure as a function of the linker lengths, starting from three different low-resolution molecular models of the nucleosome. Gay-Berne potential was used to evaluate the relative nucleosome packing energy. Results suggest that linker DNAs, which bridges and orientate nucleosomes, affect both the geometry and the rigidity of the global chromatin structure.

DNA↗