Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y26: Focus Session: Physics of Physiological Systems |
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Sponsoring Units: DBP Chair: Peter Jung, Ohio University Room: Baltimore Convention Center 323 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y26.00001: Electrical Wave Propagation in a Minimally Realistic Fiber Architecture Model of the Left Ventricle Xianfeng Song, Sima Setayeshgar Experimental results indicate a nested, layered geometry for the fiber surfaces of the left ventricle, where fiber directions are approximately aligned in each surface and gradually rotate through the thickness of the ventricle. Numerical and analytical results have highlighted the importance of this rotating anisotropy and its possible destabilizing role on the dynamics of scroll waves in excitable media with application to the heart. Based on the work of Peskin[1] and Peskin and McQueen[2], we present a minimally realistic model of the left ventricle that adequately captures the geometry and anisotropic properties of the heart as a conducting medium while being easily parallelizable, and computationally more tractable than fully realistic anatomical models. Complementary to fully realistic and anatomically-based computational approaches, studies using such a minimal model with the addition of successively realistic features, such as excitation-contraction coupling, should provide unique insight into the basic mechanisms of formation and obliteration of electrical wave instabilities. We describe our construction, implementation and validation of this model. [1] C.~S.~Peskin, Communications on Pure and Applied Mathematics {\bf 42}, 79 (1989). [2] C.~S.~Peskin and D.~M.~McQueen, in \textsl{Case Studies in Mathematical Modeling: Ecology, Physiology, and Cell Biology}, 309(1996) [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y26.00002: Scaling behavior and a Markov model for ventricular fibrillation generated by ectopic beats Harold Hastings, Steven Evans, Alex Zaharakis, Christian Hilaire Sudden cardiac death is a major cause of death in the industrialized world, responsible for 300,000 deaths per year in the US. Although the cardiac electrical system normally produces one ventricular activation in response to each stimulus from the sinus node, ``spontaneous'' activations, called premature ventricular contractions (PVCs), can arise in the ventricles themselves, and propagate through the ventricles. Although usually harmless in the absence of underlying disease, PVCs can generate broken wavefronts when they meet gradients of refractoriness generated by other beats. These broken wavefronts may generate spiral waves producing ventricular tachycardia and ultimately degenerate into ventricular fibrillation (VF), causing sudden cardiac death. When does a PVC lead to ventricular fibrillation ? This is a stiff problem, involving time scales from milliseconds to many years. We overcome this problem by developing universal scaling properties and using these rules to drive a Markov process. We find two significant ``amplifiers'' and discuss consequences for variability of VF rates in human populations. We thank Elizabeth Cherry, Flavio Fenton, Anna Gelzer and James Glimm for helpful discussions. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y26.00003: Indeterminacy and Image Improvement in Snake Infrared ``Vision'' J. Leo van Hemmen Many snake species have infrared sense organs located on their head that can detect warm-blooded prey even in total darkness. The physical mechanism underlying this sense is that of a pinhole camera. The infrared image is projected onto a sensory `pit membrane' of small size (of order mm$^{2}$). To get a neuronal response the energy flux per unit time has to exceed a minimum threshold; furthermore, the source of this energy, the prey, is moving at a finite speed so the pinhole substituting for a lens has to be rather large ($\sim 1$~mm). Accordingly the image is totally blurred. We have therefore done two things. First, we have determined the precise optical resolution that a snake can achieve for a given input. Second, in view of known, though still restricted, precision one may ask whether, and how, a snake can reconstruct the original image. The point is that the information needed to reconstruct the original temperature distribution in space is still available. We present an explicit mathematical model [1] allowing even high-quality reconstruction from the low-quality image on the pit membrane and indicate how a neuronal implementation might be realized. Ref: [1] A.B. Sichert, P. Friedel, and J.L. van Hemmen, TU Munich preprint (2005). [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 8:48AM |
Y26.00004: Diffusion Weighted MRI and MRS to Differentiate Radiation Necrosis and Recurrent Disease in Gliomas Lars Ewell A difficulty encountered in the diagnosis of patients with gliomas is the differentiation between recurrent disease and Radiation Induced Necrosis (RIN). Both can appear as ‘enhancing lesions’ on a typical T2 weighted MRI scan. Magnetic Resonance Spectroscopy (MRS) and Diffusion Weighted MRI (DWMRI) have the potential to be helpful regarding this differentiation. MRS has the ability to measure the concentration of brain metabolites, such as Choline, Creatin and N- Acetyl Aspartate, the ratios of which have been shown to discriminate between RIN and recurrent disease. DWMRI has been linked via a rise in the Apparent Diffusion Coefficient (ADC) to successful treatment of disease. Using both of these complimentary non-invasive imaging modalities, we intend to initiate an imaging protocol whereby we will study how best to combine metabolite ratios and ADC values to obtain the most useful information in the least amount of scan time. We will look for correlations over time between ADC values, and MRS, among different sized voxels. [Preview Abstract] |
Friday, March 17, 2006 8:48AM - 9:00AM |
Y26.00005: Modeling the statistics of elementary calcium release events Ghanim Ullah, Peter Jung Elementary $Ca^{2+}$ signals, such as '$Ca^{2+}$ puffs', which arise from the release of $Ca^{2+}$ from Endoplasmic Reticulum through small clusters of inositol 1,4,5-trisphosphate receptors, are the building blocks for intracellular $Ca^{2+}$ - signaling. The small number of release channels involved during a $Ca^{2+}$ puff renders the puffs stochastic with distributed amplitudes, durations and frequency, well characterized experimentally. We present a stochastic model that accurately describes simultaneously the statistical properties of the duration, amplitudes, frequencies, and spatial spread with a single set of parameters. [Preview Abstract] |
Friday, March 17, 2006 9:00AM - 9:12AM |
Y26.00006: Calcium Signaling enhancement during oocyte maturation Peter Jung, Ghanim Ullah, Khaled Machaca A Ca2+ signal with a special spatial and temporal characteristic universally removes cell-cycle arrest after fertilization of a mature egg cell. The Ca2+ signal is characterized by a fast rise of intracellular Ca2+ and a slow decay on the time scale of minutes. We use computational modeling of Ca2+ release on the microscale (Ca2+ puffs) and cell-scale in conjunction with experimental knowledge of the changes in the Ca2+ signaling apparatus during oocyte maturation and changing signaling patterns to explore the relationship between organization and sensitivity of IP3 receptors and SERCA pumps and the resulting signaling patterns. We hypothesize that potentiation of the IP3 receptors during oocyte maturation is the main cause for the differentiation in the signaling patterns. [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:48AM |
Y26.00007: Functional Complexity of Biological Networks Invited Speaker: It is now widely accepted that the complexity of the dynamical behavior of task-performing biological networks affords the latter adaptability to perform a wide variety of different tasks. Hence, understanding complexity is crucial for understanding the activity of biological systems. However, even in the context of simpler, man-made systems, complexity is still largely an intuitive blurry concept with no agreed-upon definition. In this lecture I will present new quantified observables of regularity and structural complexity that were first developed in the context of neural networks activity. I will then address the challenge of the complexity-function relationship via the introduction of a new quantified observable of functional complexity and will demonstrate that for neural networks this observable is connected with the capacity for information storage. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y26.00008: A model for the volume regulatory mechanism of the Airway Surface Layer Michael Lang, Michael Rubinstein, C. William Davis, Robert Tarran, Richard Boucher The airway surface layer (ASL) of a lung consists of two parts: a mucus layer with thickness of about 30 $\mu$m in contact with air and a periciliary layer (PCL) of about 7 $\mu$m below. Mucus collects dust and bacteria and is swept to throat by beating cilia, while riding on top of PCL. It is important that the thickness of PCL is matched with the length of cilia in order to optimize clearance of mucus. Decrease of PCL thickness would finally lead to an occlusion of the respiratory system. Experiments show that the height of PCL stays constant after removing mucus. When modifying height or composition of this open PCL by removing fluid or adding isotonic solution leads to the same final height of PCL. Thus, there must be a regulatory mechanism, that controls height, i.e. ASL volume. Additional experiments show that mechanical stimulus of the cells like shear leads to an increase of ASL volume, thus, the cell is able to actively adjust this volume. Based on these observations a class of models is introduced that describes the experiments and a specific minimum model for the given problem is proposed. [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y26.00009: Development of dielectric biosensors for diagnostic applications Christopher Bassey Dielectric biosensors utilize the intrinsic electrical properties (permittivity and conductivity) of materials to determine their dielectric characterization. Dielectric measurements, both in vivo and in vitro, provide information on the electrical properties of biomaterials (tissues and organs) as a function of frequency. This information can be used to determine the state of health of the subject since diseased and healthy tissues show significantly different dielectric characteristics. The dielectric properties of tissues depend largely on their water contents, hence dielectric data can provide information on the level of dehydration. In this work, the development of instrumentation and methods for determining dielectric properties of biomaterials is proposed. The principle of using dielectric data for diagnostic purposes is discussed. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y26.00010: Analytical model of induced transmembrane potentials in cells and organelles Vijayanand Vajrala , James Claycomb, John H. Miller, Jr. Oscillatory electric fields cause morphological and functional changes in biological cells by perturbing the resting potentials across the cell and organelle membranes. Although much work has been reported on ac field induced transmembrane voltages in cells, this has mainly been limited to the plasma membrane. We have developed a three-membrane analytical model describing the plasma membrane surrounding inner and outer mitochondrial membranes. Frequency dependent induced plasma and mitochondrial membrane potentials are calculated with mobile interfacial charges on either side of the membranes. The dependence of the induced membrane potentials on the membrane conductivity and permittivity is also calculated. A finite element electromagnetic model is used to calculate the induced mitochondrial membrane potentials for arbitrary geometries with axial symmetry. Modeling results are compared to nonlinear harmonic response measurements of cells and isolated mitochondria exposed to ac electric fields. [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y26.00011: Phase locking in driven integrate-and-fire neuron models Christopher Bedell, Jan R. Engelbrecht We investigate phase locking between a particular non-linear oscillator and a periodic drive. The non-linear equation we study is a reduced version of the celebrated Hodgkin-Huxley equations, which we couple to a cosine drive representing an EEG Rhythm. This model is motivated by the growing interest in the role of the exact timing of action potentials in neurons. For instance, electro-physiology experiments indicate that the phase differences between action potential times and large-scale oscillatory neuron activity (EEG rhythms) carry reliable information. We study various thresholds for phase locking and the delicate interplay between coherence and decoherence leading to chaos near these phase-locking thresholds. [Preview Abstract] |
Friday, March 17, 2006 10:36AM - 10:48AM |
Y26.00012: The Influence of Environment Geometry on Injury Outcome: I. Cervical Spine Saami J. Shaibani Previous studies with some 500 patients have indicated that the forces at particular injury sites of occupants in motor-vehicle accidents cannot be simply related to parameters for the occupant or the impact.[1-2] Another factor that might play a role is assessed in this research, namely passenger compartment geometry, which in most low-severity insults involves the seating arrangement and the restraint system. Analysis of the former is achieved here by considering the heights, lengths and angles of the seat cushion and seat back. The separate effect of geometric environment on the potential for neck injury is then found from studying only those cases with isometric occupants in isokinetic impacts. Such stringent constraints require the matching of numerous data fields, thus reducing the number of suitable candidates quite significantly. However, enough cases remain from the large population available for a proper evaluation to be undertaken. 1. Effect of occupant and impact factors on forces within neck, Bull Am Phys Soc, 45, 1018 (2000). 2. within low back, Bull Am Phys Soc, 46, 1174 (2001). [Preview Abstract] |
Friday, March 17, 2006 10:48AM - 11:00AM |
Y26.00013: Flux Analysis of Hypoxia Response Network. Yihai Yu, Rahul Simha, Frank Turano, Chen Zeng The availability of cellular oxygen regulates many physiological processes. Oxygen deficiency, i.e., hypoxia, induces expression of a set of genes that are involved in angiogenesis and metabolism. Here we report a detailed flux analysis on a theoretical model on hypoxia response network proposed by Kohn et al. The network is decomposed into smaller underlying pathways which are amenable to direct analytical calculations and capture the essence of the original larger network as observed in numerical simulations. Our analysis elucidates a generic mechanism for the switch-like response of gene up-regulation to hypoxia. [Preview Abstract] |
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