Biomedical subjects

Antonio Politi

Publications and source records attributed to Antonio Politi.

12 recordsLinked to original sources

PubMed2006

Desynchronization in diluted neural networks.

The dynamical behavior of a weakly diluted fully inhibitory network of pulse-coupled spiking neurons is investigated. Upon increasing the coupling strength, a transition from regular to stochasticlike regime is observed. In the weak-coupling phase, a periodic dynamics is rapidly approached, with all neurons firing with the same rate and mutually phase locked. The strong-coupling phase is characterized by an irregular pattern, even though the maximum Lyapunov exponent is negative. The paradox is solved by drawing an analogy with the phenomenon of "stable chaos," i.e., by observing that the stochasticlike behavior is "limited" to an exponentially long (with the system size) transient. Remarkably, the transient dynamics turns out to be stationary.

Journal Article↗

PubMed2006

Self-consistent mode-coupling approach to one-dimensional heat transport.

In the present Rapid Communication we present an analytical and numerical solution of the self-consistent mode-coupling equations for the problem of heat conductivity in one-dimensional systems. Such a solution leads us to propose a different scenario to accommodate the known results obtained so far for this problem. More precisely, we conjecture that the universality class is determined by the leading order of the nonlinear interaction potential. Moreover, our analysis allows us to determine the memory kernel, whose expression puts on a more firm basis the previously conjectured connection between anomalous heat conductivity and anomalous diffusion.

Journal Article↗

PubMed2006

Models of IP3 and Ca2+ oscillations: frequency encoding and identification of underlying feedbacks.

Hormones that act through the calcium-releasing messenger, inositol 1,4,5-trisphosphate (IP3), cause intracellular calcium oscillations, which have been ascribed to calcium feedbacks on the IP3 receptor. Recent studies have shown that IP3 levels oscillate together with the cytoplasmic calcium concentration. To investigate the functional significance of this phenomenon, we have developed mathematical models of the interaction of both second messengers. The models account for both positive and negative feedbacks of calcium on IP3 metabolism, mediated by calcium activation of phospholipase C and IP3 3-kinase, respectively. The coupled IP3 and calcium oscillations have a greatly expanded frequency range compared to calcium fluctuations obtained with clamped IP3. Therefore the feedbacks can be physiologically important in supporting the efficient frequency encoding of hormone concentration observed in many cell types. This action of the feedbacks depends on the turnover rate of IP3. To shape the oscillations, positive feedback requires fast IP3 turnover, whereas negative feedback requires slow IP3 turnover. The ectopic expression of an IP3 binding protein has been used to decrease the rate of IP3 turnover experimentally, resulting in a dose-dependent slowing and eventual quenching of the Ca2+ oscillations. These results are consistent with a model based on positive feedback of Ca2+ on IP3 production.

Animals↗

PubMed2005

Exploring the energy landscape of model proteins: a metric criterion for the determination of dynamical connectivity.

A method to reconstruct the energy landscape of small peptides is presented with reference to a two-dimensional off-lattice model. The starting point is a statistical analysis of the configurational distances between generic minima and directly connected pairs (DCP). As the mutual distance of DCP is typically much smaller than that of generic pairs, a metric criterion can be established to identify the great majority of DCP. Advantages and limits of this approach are thoroughly analyzed for three different heteropolymeric chains. A funnel-like structure of the energy landscape is found in all of the three cases, but the escape rates clearly reveal that the native configuration is more easily accessible (and is significantly more stable) for the sequence that is expected to behave as a real protein.

Amino Acid Sequence↗

PubMed2005

Mathematical modeling of nucleotide excision repair reveals efficiency of sequential assembly strategies.

Nucleotide excision repair (NER) requires the concerted action of many different proteins that assemble at sites of damaged DNA in a sequential fashion. We have constructed a mathematical model delineating hallmarks and general characteristics for NER. We measured the assembly kinetics of the putative damage-recognition factor XPC-HR23B at sites of DNA damage in the nuclei of living cells. These and other in vivo kinetic data allowed us to scrutinize the dynamic behavior of the nucleotide excision repair process in detail. A sequential assembly mechanism appears remarkably advantageous in terms of repair efficiency. Alternative mechanisms for repairosome formation, including random assembly and preassembly, can readily become kinetically unfavorable. Based on the model, new experiments can be defined to gain further insight into this complex process and to critically test model predictions. Our work provides a kinetic framework for NER and rationalizes why many multiprotein processes within the cell nucleus show sequential assembly strategy.

Animals↗

PubMed2005

Studies of thermal conductivity in Fermi-Pasta-Ulam-like lattices.

The pioneering computer simulations of the energy relaxation mechanisms performed by Fermi, Pasta, and Ulam (FPU) can be considered as the first attempt of understanding energy relaxation and thus heat conduction in lattices of nonlinear oscillators. In this paper we describe the most recent achievements about the divergence of heat conductivity with the system size in one-dimensional (1D) and two-dimensional FPU-like lattices. The anomalous behavior is particularly evident at low energies, where it is enhanced by the quasiharmonic character of the lattice dynamics. Remarkably, anomalies persist also in the strongly chaotic region where long-time tails develop in the current autocorrelation function. A modal analysis of the 1D case is also presented in order to gain further insight about the role played by boundary conditions.

Computer Simulation↗

PubMed2003

Thermally activated processes in polymer dynamics.

Jumps between neighboring minima in the energy landscape of both homopolymeric and heteropolymeric chains are numerically investigated by determining the average escape time from different valleys. The numerical results are compared to the theoretical expression derived by Langer [J.S. Langer, Ann. Phys. (N.Y.) 54, 258 (1969)] with reference to a 2N-dimensional space. Our simulations indicate that the dynamics within the native valley is well described by a sequence of thermally activated process up to temperatures well above the folding temperature. At larger temperatures, systematic deviations from the Langer's estimate are instead observed. Several sources for such discrepancies are thoroughly discussed.

Journal Article↗

PubMed2003

Universality of anomalous one-dimensional heat conductivity.

In one and two dimensions, transport coefficients may diverge in the thermodynamic limit due to long-time correlation of the corresponding currents. The effective asymptotic behavior is addressed with reference to the problem of heat transport in one-dimensional crystals, modeled by chains of classical nonlinear oscillators. Extensive accurate equilibrium and nonequilibrium numerical simulations confirm that the finite-size thermal conductivity diverges with system size L as kappa proportional to L alpha. However, the exponent alpha deviates systematically from the theoretical prediction alpha=1/3 proposed in a recent paper [O. Narayan and S. Ramaswamy, Phys. Rev. Lett. 89, 200601 (2002)].

Journal Article↗

PubMed2003

Cardiac arrhythmia monitoring during bronchial provocation test with methacholine.

STUDY OBJECTIVES: During a bronchial provocation test (BPT), the performance of maximal inspiratory-expiratory maneuvers, causing abrupt and marked shifts in intrathoracic pressure, may increase the risk of cardiac arrhythmias. Moreover, the inhalation of methacholine (MCh), a cholinergic agonist agent, could favor the development of unwelcome cardiovascular events, namely, cardiac arrhythmias. SUBJECTS AND METHODS: We studied the number and severity of cardiac arrhythmias by ECG-Holter monitoring before, during, and after BPTs with MCh challenge in a group of 46 consecutive nonselected subjects (28 men and 18 women) with clinical indications for BPT, without preexisting cardiovascular diseases, and not receiving arrhythmogenic drugs. The subjects performed a routine pulmonary function test (PFT), followed by BPT, during ECG-Holter monitoring. Determination of the serum potassium concentration, a baseline arterial blood gas analysis, and monitoring of oxyhemoglobin saturation also were performed. RESULTS: We found no significant increase in the number of supraventricular and ventricular arrhythmias during the performance of PFTs and of BPTs with MCh in the subjects, either with or without bronchial hyperresponsiveness (BHR). However, during the performance of BPTs, we observed a significant reduction in mean heart rate. CONCLUSIONS: Our results indicate that the performance of PFTs and BPTs with MCh does not increase the cardiac arrhythmogenic risk in subjects without cardiovascular diseases, as well as in those with BHR, suggesting that these tests are safe to perform in most subjects.

Adult↗

PubMed2002

Failure time in the fiber-bundle model with thermal noise and disorder.

The average time for the onset of macroscopic fractures is analytically and numerically investigated in the fiber-bundle model with quenched disorder and thermal noise under a constant load. We find an implicit exact expression for the failure time in the low-temperature limit that is accurately confirmed by direct simulations. The effect of the disorder is to lower the energy barrier.

Journal Article↗

PubMed1997

Entropy potential and Lyapunov exponents.

According to a previous conjecture, spatial and temporal Lyapunov exponents of chaotic extended systems can be obtained from derivatives of a suitable function, the entropy potential. The validity and the consequences of this hypothesis are explored in detail. The numerical investigation of a continuous-time model provides a further confirmation to the existence of the entropy potential. Furthermore, it is shown that the knowledge of the entropy potential allows determining also Lyapunov spectra in general reference frames where the time-like and space-like axes point along generic directions in the space-time plane. Finally, the existence of an entropy potential implies that the integrated density of positive exponents (Kolmogorov-Sinai entropy) is independent of the chosen reference frame. (c) 1997 American Institute of Physics.

Journal Article↗

PubMed1992

Periodic orbits in coupled Henon maps: Lyapunov and multifractal analysis.

A powerful algorithm is implemented in a 1-d lattice of Henon maps to extract orbits which are periodic both in space and time. The method automatically yields a suitable symbolic encoding of the dynamics. The arrangement of periodic orbits allows us to elucidate the spatially chaotic structure of the invariant measure. A new family of specific Lyapunov exponents is defined, which estimate the growth rate of spatially inhomogeneous perturbations. The specific exponents are shown to be related to the comoving Lyapunov exponents. Finally, the zeta-function formalism is implemented to analyze the scaling structure of the invariant measure both in space and time.

Journal Article↗