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At least 523 records · Page 29Linked to original sources
PubMed2002

Population dynamics of interacting spiking neurons.

A dynamical equation is derived for the spike emission rate nu(t) of a homogeneous network of integrate-and-fire (IF) neurons in a mean-field theoretical framework, where the activity of the single cell depends both on the mean afferent current (the "field") and on its fluctuations. Finite-size effects are taken into account, by a stochastic extension of the dynamical equation for the nu; their effect on the collective activity is studied in detail. Conditions for the local stability of the collective activity are shown to be naturally and simply expressed in terms of (the slope of) the single neuron, static, current-to-rate transfer function. In the framework of the local analysis, we studied the spectral properties of the time-dependent collective activity of the finite network in an asynchronous state; finite-size fluctuations act as an ongoing self-stimulation, which probes the spectral structure of the system on a wide frequency range. The power spectrum of nu exhibits modes ranging from very high frequency (depending on spike transmission delays), which are responsible for instability, to oscillations at a few Hz, direct expression of the diffusion process describing the population dynamics. The latter "diffusion" slow modes do not contribute to the stability conditions. Their characteristic times govern the transient response of the network; these reaction times also exhibit a simple dependence on the slope of the neuron transfer function. We speculate on the possible relevance of our results for the change in the characteristic response time of a neural population during the learning process which shapes the synaptic couplings, thereby affecting the slope of the transfer function. There is remarkable agreement of the theoretical predictions with simulations of a network of IF neurons with a constant leakage term for the membrane potential.

Action Potentials↗
PubMed1986

Population dynamics of bone marrow B lymphocytes.

The dynamic concepts of lymphocyte populations which heralded the era of modern cellular immunobiology have been generally substantiated by recent studies and are still being correlated with functional properties. B lineage cells in the bone marrow are dynamically heterogeneous: A large majority are newly-formed, rapidly renewed cells, continuously produced from precursor cells within the bone marrow and disseminated during a terminal maturation phase via the blood stream. These cells develop low densities of sIgM in the extravascular bone marrow parenchyma and may undergo some further maturation within bone marrow sinusoids. The rate of production of bone marrow B cells appears to depend partly on the total load of exogenous agents to which the individual is exposed. Bone marrow lymphocyte production maintains a population of rapidly renewed virgin B cells in the peripheral lymphoid tissues. A small proportion of these cells apparently may be selected to enter a long-lived pool of B cells if suitably activated. By continuously creating novel clonotypes this process potentially can anticipate new antigen challenges and allow the immune system to build up a repertoire of antigen specificities most appropriate to the individual's changing environment throughout life. A minority of B lymphocytes in the bone marrow comprises slowly renewed, long-lived cells which enter and leave the bone marrow parenchyma as a selective part of the recirculating lymphocyte pool in the blood stream. Their role in the bone marrow is unknown. They include antigen-specific B memory cells, yet these are not activated within the bone marrow itself. No regulatory role has yet been directly demonstrated. Recently activated B cells enter from the spleen after secondary antigenic stimulation to develop into antibody-producing cells within the bone marrow. In assessing the significance of any phenotypically or functionally distinct B cell subset in the bone marrow, a basic consideration is to assign the subset to one of the foregoing dynamic categories. Within a given category cells may represent one stage in a time sequence of development. The bone marrow also produces lymphocytes of as yet uncertain lineage and contains selected subsets of T cells. The roles of these cells in cytotoxic, regulatory, or other events remain to be elucidated.

Animals↗
PubMed2005

Diverse evolutionary trajectories characterize a community of RNA-cleaving deoxyribozymes: a case study into the population dynamics of in vitro selection.

Two parallel in vitro selections (denoted Selection A and Selection B) were conducted under different selection-pressure regimes, yielding a diverse community of RNA-cleaving deoxyribozymes. In Selection A, the reaction time was reduced four times (from 5 h to 5 s) over the course of 24 generations, while in Selection B the reaction time was maintained at 5 h for 30 rounds of selective amplification. Sequence alignment was conducted on more than 800 clones assembled from 18 generations that span both selections. Many prominent catalytic sequence classes, including some that extend across both selections, were identified and used to construct fitness landscapes depicting their rise and fall over time. The landscapes from both selections exhibit similar global trends despite differences in population dynamics. Some deoxyribozymes were predominant in the early rounds of selection but gave way to other species that dominated in the middle rounds. Ultimately, these middle classes disappeared from the landscape in favor of new and presumably more fit deoxyribozyme sequence classes. The shape of these landscapes alludes to the presence of many latent deoxyribozymes in the initial library, which can only be accessed by changes in the selection pressure and/or by adaptive mutations. Basic computer simulations provide theoretical corroboration of the experimentally observed pattern of staggered sequence-class transitions across the fitness landscapes. These simulations model the influence of one or more contributing factors, including catalytic rate, folding efficiency, PCR amplification efficiency, and random mutagenesis. This is the first study which thoroughly documents the topography of a deoxyribozyme fitness landscape over many generations of in vitro selection.

Base Sequence↗
PubMed1987

Population dynamics in echinococcosis and cysticercosis: mathematical model of the life-cycles of Taenia hydatigena and T. ovis.

It is shown that under the conditions that prevailed in New Zealand in the late 1950s, Taenia hydatigena was hyperendemic, the life-cycle being regulated by a density-dependent constraint in the form of acquired immunity, and T. ovis was rare. The control measures that caused Echinococcus granulosus, which was endemic at the time, to decline towards extinction reduced T. hydatigena and T. ovis to endemic status only. A non-linear integrodifferential equation model, which was previously linearized to describe the life-cycle of E. granulosus in dogs and sheep in New Zealand, is used to describe the life-cycles of T. hydatigena and T. ovis. The model is then used to compare and contrast the population dynamics of these three species. The model is used to demonstrate that the endemic steady state is structurally unstable, and may be asymptotically unstable to small perturbations. It is also shown that despite the lower infection pressure experienced by the intermediate host in the endemic state, the numbers of larvae in sheep may be higher than in the hyperendemic state. Finally it is shown that the partial success of the control measures against T. hydatigena may have caused an increase in the numbers and prevalence of T. ovis larvae in sheep due to the reciprocal immunity between the two species.

Age Factors↗
PubMed1999

Population dynamics of chesapeake bay virioplankton: total-community analysis by pulsed-field gel electrophoresis

Recognition of viruses as the most abundant component of aquatic microbial communities has stimulated investigations of the impact of viruses on bacterio- and phytoplankton host communities. From results of field studies to date, it is concluded that in most aquatic environments, a reduction in the number of bacteria on a daily basis is caused by viral infection. However, the modest amount of in situ virus-mediated mortality may be less significant than viral infection serving to maintain clonal diversity in the host communities directly, through gene transmission (i.e., transduction), and indirectly, by elimination of numerically dominant host species. If the latter mechanism for controlling community diversity prevails, then the overall structure of aquatic viral communities would be expected to change as well over short seasonal and spatial scales. To determine whether this occurs, pulsed-field gel electrophoresis (PFGE) was used to monitor the population dynamics of Chesapeake Bay virioplankton for an annual cycle (1 year). Virioplankton in water samples collected at six stations along a transect running the length of the bay were concentrated 100-fold by ultrafiltration. Viruses were further concentrated by ultracentrifugation, and the concentrated samples were embedded in agarose. PFGE analysis of virus DNA in the agarose plugs yielded several distinct bands, ranging from 50 to 300 kb. Principal-component and cluster analyses of the virus PFGE fingerprints indicated that changes in virioplankton community structure were correlated with time, geographical location, and extent of water column stratification. From the results of this study, it is concluded that, based on the dynamic nature of the Chesapeake Bay virioplankton community structure, the clonal diversity of bacterio- and phytoplankton host communities is an important component of the virus community.

Journal Article↗
PubMed1995

Observations on worm population dynamics in calves naturally infected with Schistosoma mattheei.

The evolution of faecal egg output, worm burdens and tissue egg counts in young calves was monitored during the first year of natural exposure to Schistosoma mattheei infection on a Zambian farm. According to the duration of their stay on the farm, these calves were classified into 2 groups of 14 temporary tracers (TT calves) which were introduced on a 2-monthly basis for residential periods of 2 months, and 12 permanent tracers (PT calves) introduced either at the beginning of the experiment (Group A) or 2 months later (Group B) and gradually removed after residential periods of 2, 4, 6, 8, 10 and 12 months on the farm. Worm counts in the TT calves showed that infection occurred throughout the year on the farm and that levels of infection acquired during each period of 8 weeks correlated well with the respective infected snail densities observed at the main transmission site. Marked differences in worm population dynamics were recorded between the 2 groups of PT calves. In Group B animals which apparently were initially exposed to heavy transmission, according to the results from TT calves, much higher worm counts and greater susceptibility to reinfection were observed than in Group A animals initially exposed to lighter exposure. These results suggest that the development of resistance to natural infection with S. mattheei may depend on the initial exposure to the parasite. Low initial exposures may lead to resistance whereas high initial exposures may result in decreased immune responses resulting in susceptibility to infection.

Animals↗
PubMed1999

Increases in T cell telomere length in HIV infection after antiretroviral combination therapy for HIV-1 infection implicate distinct population dynamics in CD4+ and CD8+ T cells.

Changes in mean telomeric terminal restriction fragment (TRF) length were examined as a marker for cellular replicative history in HIV-1-infected individuals after institution of anti-retroviral therapy (ART). Increases in mean T cell TRF lengths were observed in most patients following therapy; however, the contribution of individual T cell subsets was complex. An elongation of CD8+ T cell TRF was nearly uniformly observed while changes in mean TRF length in CD4+ T cells were heterogeneous as, despite potent suppression of viral replication, CD4 cell telomeres recovered in some patients, yet continued to decline in others. Increases in CD8 cell TRF correlated with decreased memory cells, suggesting a negative selection in the periphery for CD8 cells with extensive replicative history. In contrast, increases in CD4+ T cell TRF length correlated with increases in naive cell subsets, suggesting that the CD4+ T cell TRF increase may reflect a thymic contribution in some patients. These are the first increases in somatic cell telomere length in a population of cells observed in vivo, and the findings are compatible with therapy-induced reconstitution of the lymphoid compartment with cells having a more extensive replicative potential. These findings further distinguish lymphocytes from other somatic cell populations where only decreases in TRF over time have been noted. Thus, institution of ART in persons with moderately advanced HIV-1 disease reveals distinct population dynamics of CD4 and CD8 T cell subsets and also shows that the lymphocyte replicative history is dynamic.

Adult↗
PubMed1974

Population dynamics and age grading of mosquitos.

The dynamics of a vector population follows a characteristic pattern determined by the normal or modified environmental conditions of the biotope and by vector behaviour. The wide use of the precise method of age grading developed by Detinova and Polovodova has led to the conclusion that the dynamics of the various age groups may indicate behavioural, environmental, and epidemiological trends and changes and their significance. Age grading is useful for the qualitative assessment of vector density in general and of man-mosquito contact from the operational and epidemiological points of view, but only when sampling in time and space and the processing of the material collected have been correctly performed.

Age Factors↗
PubMed1991

Population dynamics of lymphocyte subsets in the central nervous system of rats with different susceptibility to coronavirus-induced demyelinating encephalitis.

The inflammatory response in the central nervous system (CNS) of rats with differing susceptibility to demyelinating encephalitis induced by coronavirus MHV4 was characterized. Topographical maps showing the arrangement of infiltrating lymphocyte subsets in virus-infected tissue were developed by digital-image processing of immunohistologically stained CNS sections. The kinetics of the inflammatory process was evaluated by flow-cytometry on lymphocytes isolated from the CNS. Cumulative data obtained with these two techniques demonstrated the following features. In susceptible Lewis (LE) rats, viral antigens were disseminated throughout the CNS, including spinal cord. Onset as well as recovery from neurological disease was associated with a steep rise of infiltrating CD8+ T cells, which localized in close contact to virus-infected cells. Accompanying convalescence was a slight increase in B(OX33+) cells in the CNS and the accumulation of immunoglobulin-containing cells in the centre of virus-infected areas. In clinically resistant Brown Norway (BN) rats, virus-infected cells were mainly restricted to small periventricular foci and the extent of lymphocyte infiltration was never as high as that found at any time during the course of infection in LE rats. There were striking differences in the CD8+ T-cell population compared to LE rats. Cells of this phenotype were identified in virus-affected areas of BN rats only early after infection, and their infiltration profile revealed much lower quantities than in the CNS of susceptible LE rats. Although the population dynamics of B(OX33+) lymphocytes were comparable in BN and LE rats, as determined by flow-cytometry, less immunoglobulin-containing B cells were detected in virus-infected areas of BN rats.

Animals↗
PubMed1998

Population dynamics of the monogenean Anoplodiscus cirrusspiralis on the snapper, Pagrus auratus.

Populations of Anoplodiscus cirrusspiralis were monitored for 1 year on tagged individual snapper in experimental cages kept in a large on-shore pond with flow-through filtered sea water. The cages were stocked with small and large fish at either low or high initial density. Irrespective of size and density, snapper with light initial infections maintained light infections, whereas fish with heavy initial infections showed fluctuations in parasite population size throughout the year. These data indicate that some snapper have an innate resistance to infection by A. cirrusspiralis, with little evidence for acquired immunity induced by heavy infection. Parasite longevity was greater on the pectoral fin than caudal fin, and greater on large than small fish irrespective of fish density; longevity was greater on susceptible fish than on resistant fish. Recruitment and mortality rates were greater on the pectoral fin and in low density cages, but were influenced by fork length.

Animals↗
PubMed1989

Optimal birth control of population dynamics.

The authors studied optimal birth control policies for an age-structured population of McKendrick type which is a distributed parameter system involving 1st order partial differential equations with nonlocal bilinear boundary control. The functional analytic approach of Dubovitskii and Milyutin is adopted in the investigation. Maximum principles for problems with a free end condition and fixed final horizon are developed, and the time optimal control problems, the problem with target sets, and infinite planning horizon case are investigated.

Demography↗
PubMed2004

Stochastic gain in population dynamics.

We introduce an extension of the usual replicator dynamics to adaptive learning rates. We show that a population with a dynamic learning rate can gain an increased average payoff in transient phases and can also exploit external noise, leading the system away from the Nash equilibrium, in a resonancelike fashion. The payoff versus noise curve resembles the signal to noise ratio curve in stochastic resonance. Seen in this broad context, we introduce another mechanism that exploits fluctuations in order to improve properties of the system. Such a mechanism could be of particular interest in economic systems.

Algorithms↗
PubMed1986

Cell population dynamics during the differentiative phase of tissue development.

The dynamics of a cell population whose numbers are growing exponentially have been described well by a mathematical model based on the theory of age-dependent branching processes. Such a model, however, does not cover the period following exponential growth when cell differentiation curtails population size. This paper offers an extension to the branching process model to remedy this deficiency. The extended model is ideal for describing embryonic growth; its use is illustrated with data from embryonic retina. The model offers a better computational framework for the interpretation of a variety of data (growth curves of cell numbers, DNA histograms, thymidine labelling indices, FLM curves, BUdR-labelled mitoses curves) because age-distributions can be calculated at any stage of development, not just during exponential growth. Proportions of cells in the various phases of the cell cycle can be computed as growth slows. Such calculations show the gradual transition from a population dominated by cells which are young with respect to cell cycle age to one dominated by those which are old, and the effects such biases have on the proportions of cells in each phase.

Age Factors↗
PubMed1986

Parameterized multistate population dynamics and projections.

"This article reports progress on the development of a population projection process that emphasizes model selection over demographic accounting. Transparent multiregional/multistate population projections that rely on parameterized model schedules are illustrated [using data primarily from a number of developed countries, particularly Sweden], together with simple techniques that extrapolate the recent trends exhibited by the parameters of such schedules." The author notes that "the parameterized schedules condense the amount of demographic information, expressing it in a language and variables that are more readily understood by the users of the projections. In addition, they permit a concise specification of the expected temporal patterns of variation among these variables, and they allow a disaggregated focus on demographic change that otherwise would not be feasible."

Demography↗
PubMed2000

[Effect of rice variety resistance on population dynamics of Nilaparvata lugens and Sogatella furcifera].

The effect of rice varieties with different insect-resistance on dynamics of Nilaparvata lugens and Sogatella furcifera population was examined, based on the investigation of their inhabiting ratio, egg amount, survival rate of egg and nymph, nymph development, and the amount of their offspring population. Four rice varieties, TN1, N22, AsD7 and Ptb33 were investigated. Ptb33 had a significantly lower inhabiting ratio, egg amount, and survival ratio of eggs and nymphs of both parasitic populations, a prolonged development time from nymphs to adults, and an inhibited offspring population, indicating an evident resistance of this host variety. N22 and AsD7 inhibited the development of offspring S. furcifera and N. lugens, respectively. The parasitic populations of S. furcifera and N. lugens developed more rapidly on TN1, compared with N. lugens population on N22 and S. furcifera on AsD7. The relationship between the quantitative dynamics of both population and rice variety replacement was discussed, and the possibility of using resistant rice varieties to control these two insect populations was also analyzed.

Animals↗
PubMed1992

Rapidly converging numerical algorithms for models of population dynamics.

We propose algorithms for the approximation of the age distributions of populations modeled by the McKendrick-von Foerster and the Gurtin-MacCamy systems both in one- and two-sex versions. For the one-sex model methods of second and fourth order are proposed. For the two-sex model a second order method is described. In each case the convergence is demonstrated. Several numerical examples are given.

Age Factors↗