Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session Z10: Physics of Physiological Systems |
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Sponsoring Units: DBP Room: A106 |
Friday, March 19, 2010 11:15AM - 11:27AM |
Z10.00001: Quantitative Analysis of \textit{Dictyostelium Discoideum} Chemotaxis Gabriel Amselem, Matthias Theves, Albert Bae, Carsten Beta, Eberhard Bodenschatz We used microfluidic tools to expose \textit{Dictyostelium discoideum} to stationary spatial gradients of the chemoattractant cyclic adenosine 3',5' monophosphate (cAMP). At a cAMP gradient of $10^{-2}$ ${\rm nM}/\mu{\rm m}$, the chemotactic velocity reached a plateau, which continued for gradients up to 1 ${\rm nM}/\mu{\rm m}$. Our measurements agree with [Song at al, Eur. J. Cell Biol., 85(10):981]. We also found that the chemotactic velocity was highly correlated with the cell's polarization. We present a model based on a generalized Langevin equation that provides good agreement with the measured data. [Preview Abstract] |
Friday, March 19, 2010 11:27AM - 11:39AM |
Z10.00002: Memristive model of amoeba learning Yuriy V. Pershin, Steven La Fontaine, Massimiliano Di Ventra Recently, it was shown that the amoeba-like cell Physarum polycephalum when exposed to a pattern of periodic environmental changes learns and adapts its behavior in anticipation of the next stimulus to come. Here we show that such behavior can be mapped into the response of a simple electronic circuit consisting of a LC contour and a memory-resistor (a memristor) to a train of voltage pulses that mimic environment changes [1]. We also discuss a possible biological origin of the memristive behavior in the cell. These biological memory features are likely to occur in other unicellular as well as multicellular organisms, albeit in different forms. Therefore, the above memristive circuit model, which has learning properties, is useful to better understand the origins of primitive intelligence. [1] Yu. V. Pershin, S. La Fontaine, and M. Di Ventra, Phys. Rev. E 80, 021926 (2009) [Preview Abstract] |
Friday, March 19, 2010 11:39AM - 11:51AM |
Z10.00003: Dynamics of asexual reproduction in flatworms Eva-Maria Schoetz, Jared Talbot, Joern Dunkel Planarians (flatworms) are one of the simplest bilaterally symmetric organisms and famous for their extraordinary regenerative capabilities. One can cut a worm in 100 pieces and after a few weeks one obtains 100 new worms that have reconstructed their entire body, including a central nervous system. This amazing regenerative capability is due to a population of stem cells distributed throughout the planarian body. These stem cells do not only allow the worms to heal without scarring after wounding, they also allow for asexual reproduction: Planarians can split themselves in two, and then regenerate the missing body parts within about a week. Naively, one would think that this kind of asexual reproduction could be captured by simple models that describe cell growth in bacteria or other lower organisms. However, we find that there is much more to the story by monitoring $>$15 generations of many individuals, as well as the long-term behavior ($>$ 9 months) of worm populations under different environmental conditions, such as population density, temperature, and feeding frequency. Surprisingly, we observe that reproduction decreases with increasing food supply, opposite to the relationship between food and reproduction in other asexually reproducing organisms (e.g. bacteria, yeast), and causing obese worms. Finally, our data allows us to address the question of aging in an organism that is thought to be ``forever young''. [Preview Abstract] |
Friday, March 19, 2010 11:51AM - 12:03PM |
Z10.00004: \textit{E. coli} as a biological model for cancer cells David Liao, Guillaume Lambert, Robert Austin Uninhibited growth and invasion of healthy tissue characterize cancer. We co-cultured two strains of \textit{E. coli} bacteria in a microfabricated environment to model cancer. During starvation, growth-advantage-in-stationary-phase, or GASP, cells grew to a higher population than wild-type cells. GASP cells also displaced wild-type cells from nutrient-rich chambers. When we repeated the experiment with medium depleted by wild-type cells, the peak GASP population density increased 54\%, and the ``invasion,'' or displacement of wild-type cells from nutrient-rich chambers, occurred 5 hours earlier. We mathematically modeled both this increase in GASP population and this acceleration of spatial invasion by assuming that GASP cells consume detritus secreted by wild-type cells. Our experimental and model results corroborate recent caution against using tumor starvation as a cancer therapy. [Preview Abstract] |
Friday, March 19, 2010 12:03PM - 12:15PM |
Z10.00005: Hybrid Cellular Continuum Simulations of Heterogeneity in Tumor Growth H.G.E. Hentschel, Fereydoon Family, Erwin Van Meir, Hans Grossniklaus We will discuss simulations of pre-angiogenic tumor growth using a class of hybrid cellular-continuum models. A lattice site can be occupied either by a cell of a specific tumor cell population or consist of extracellular matrix. The local concentrations of oxygen is described by continuum reaction-diffusion equations. Dynamic linked lists of cells are evolved in time and contain information on cell type, position, age, concentration of oxygen at cell site. When cells proliferate via mitosis or differentiate, new cells are added to the list, if mutation occurs the cell types are altered, and if the cell dies via apoptosis the cells are removed from the linked list. The motion of individual cells consist of random walks subject to caging and chemotaxis away from regions of low oxygen concentration. We will describe the heterogenous spatial segregation of different cell types in the tumor, the development of necrotic cores as well as micronecrotic regions, and the effects of externally applied drugs on cell populations and overall tumor shape. [Preview Abstract] |
Friday, March 19, 2010 12:15PM - 12:27PM |
Z10.00006: A Molecular Model of Plant Cell Morphogenesis: The Case of Polar Growth in Pollen Tubes Enrique Rojas, Scott Hotton, Jacques Dumais The growth of plant, fungal, and bacterial cells depends critically on two processes: the deposition of new wall material at the cell surface and the mechanical deformation of this material by forces developed within the cell. To understand how these two processes contribute to cell growth, we have undertaken an experimental and theoretical investigation of polar morphogenesis in pollen tubes. The pollen tube is an ideal model system for the study of polar growth because of its rich phenomenology and its ease of experimental manipulation. We formulated an experimentally-motivated model of pollen tube morphogenesis that incorporates 1) the microscopic architecture and rheology of the polymeric wall, 2) the dynamics of intracellular calcium, a key morphogen in pollen tubes, and 3) the exocytosis of wall material. These processes constitute a feedback loop that controls growth. Our model shows two regimes corresponding to observed steady and pulsatile growth in pollen tubes. The model accounts for the frequency, amplitude and waveform of pulsatile cells, and the scaling relationships between these variables. By solving the dynamical system on a three-dimensional thin-shell geometry we can also explain the surface expansion pattern and morphology of steady-growing cells. [Preview Abstract] |
Friday, March 19, 2010 12:27PM - 12:39PM |
Z10.00007: Experimental Evidence of Strong Anomalous Diffusion in Living Cells Daphne Weihs, Naama Gal We show that transport of polymeric particles within living cancer cells exhibits strongly anomalous diffusion. Particle motion demonstrated super-diffusion, indicating active cellular transport of particles likely due to molecular motors. We also calculated a range of time-dependent displacement moments and extracted scaling exponents\textit{ $\lambda $}($q)$ for each moment order $q$. Those were non-linear with $q$, indicating non-scale-invariant motion. Also, \textit{$\lambda $}($q)$/$q$ was non-decreasing, fulfilling conditions for strong anomalous diffusion, presented here experimentally for the first time. Specifically, \textit{$\lambda $}($q)$ exhibited bi-linearity, with slopes of $\sim $0.6 and $\sim $0.8 at low and high $q$-values. That bi-linearity indicates that particle motion is composed of sub-diffusive regimes separated by active flights; those were sub-ballistic and not separable using a directionality criterion. We suggest that sub-ballistic flights are associated with the small particles used in this work (100-200 nm); those diffuse through the cytoplasm while being actively transported. Results are discussed in terms of particle interactions with their microenvironment and its dynamics. [Preview Abstract] |
Friday, March 19, 2010 12:39PM - 12:51PM |
Z10.00008: Damage and fluctuations in optimal transport networks Eleni Katifori, Gergely Szollosi, Marcelo Magnasco Leaf venation is a pervasive example of a complex biological transport network that is necessary for the survival of land plants. Transport networks optimized for efficiency have been shown to be trees, i.e. loopless. However, dicotyledon leaf venation has a large number of functional closed loops. Inspired by leaf venation, we study two possible reasons for the existence of a high density of loops in biological transport networks: resilience to damage and fluctuations in load (transpiration rate across the leaf blade). We consider optimizing functionals that account for these two criteria, and examine the topology and transport properties of the resulting networks. [Preview Abstract] |
Friday, March 19, 2010 12:51PM - 1:03PM |
Z10.00009: Terminating Ventricular Fibrillation Using Pulsed Far-Field Stimulation in Whole Rabbit Hearts Amgad Squires, Daniel Hornung, Philip Bittihn, Dong Xia Wu, Valentin Krinsky, Markus Zabel, Eberhard Bodenschatz, Stefan Luther During life-threatening cardiac fibrillation, chaotic spatio-temporal dynamics is mediated by vortex-like rotating waves. Current defibrillation strategies rely on global control through high-energy shocks, which may have severe side-effects including traumatic pain and tissue damage. Far-field antifibrillation pacing terminates fibrillation using a train of low-energy electric pulses [1,2]. Using optical mapping in isolated rabbit heart preparations, we evaluate the efficiency and robustness of this approach. We found that a series of pulses at low energies ($<$ 2.0 J) is sufficient to extinguish ventricular fibrillation with a success rate of 95{\%}. We will discuss the physical mechanisms involved.\\[4pt] [1] F.H. Fenton et al, Circulation 120 467-476 (2009).\\[0pt] [2] A. Pumir et al., Phys. Rev. Lett. 99, 208101 (2007). [Preview Abstract] |
Friday, March 19, 2010 1:03PM - 1:15PM |
Z10.00010: Propagation Dynamics of Cardiac Action Potentials on a Ring Bogomil Gerganov, Niels F. Otani, Robert F. Gilmour, Jr., Andrew Guinn, Ample Hout, Matthew Wuerffel We study the effects of the underlying ion-channel dynamics on the morphology and propagation of cardiac action potentials (AP) by investigating the stability of small perturbations to a steady-state AP rigidly propagating along a fiber of cardiac cells arranged in a ring. The Fox-McHarg-Gilmour model (FMGM) is used to describe the ion-channel dynamics, and a standard gap-junction term is used to couple neighboring cells. We compute the eigenvalues and eigenmodes of the infinitesimal evolution matrix in the moving frame and, along with numerous stable modes, find several unstable eigenmodes. Their frequencies are half-integer multiples of the fundamental frequency of action potential repetition, and represent alternans modes of increasing degree of discordance. Our results for a fiber of cells described by a physiologically realistic ion-channel model (FMGM) agree with earlier simulations for a single cardiac cell and for spiral waves in 2D cardiac tissue. The analysis provides the basis for developing more efficient electrical stimulation protocols for controlling alternans. [Preview Abstract] |
Friday, March 19, 2010 1:15PM - 1:27PM |
Z10.00011: Phase singularities in cardiac electrical activity Ilija Uzelac, Veniamin Sidorov, John Wikswo Abstract theory of topological spaces has its analogy in biological systems, one of which is the heart. The heart is an excitable medium that can be represented as a set of excitable elements (cardiomyocytes) that behave similarly to hourglasses. Excitable element needs external stimuli to be excited and after finite time goes back to its initial state, so its natural topological space is a ring. Topological space set (phases) can be simple set as \textit{``rest,'' ``excited,'' ``refractory,'' ``relatively refractory'', }but it can be as continuous as a set of angles on a 2$\pi $ circle. In topological spaces topological charge is defined by: \[ W=\frac{1}{2\pi }\oint\limits_l {d\theta } (l) \] where $l$ is the integration path and \textit{d$\theta $} is the change in phase. Non zero topological charge is called phase singularity of mapping. Practical application of topological charge analysis is a powerful method to quantify electrical dynamics during ventricular fibrillation (VF). Particularly by means of phase singularity detection it is possible to track wave breaks which relate to anatomical and electrophysiological heterogeneities. [Preview Abstract] |
Friday, March 19, 2010 1:27PM - 1:39PM |
Z10.00012: The role of resonant ear canal thermal noise pressure on the eardrum in helping to determine auditory thresholds Michael J. Harrison The influence of thermal pressure fluctuations on the tympanic membrane has been re-examined as a possible contributing determinant of the threshold of human hearing over the range of audible frequencies. The early approximate calculation of Sivian and White [1] is shown to result in higher values of thermal noise pressure on the tympanium of a model meatus than the result obtained by directly calculating the noise pressure from thermally excited resonant ear canal modes. \\[4pt] [1] L.J. Sivian and S.D. White, ``Minimum audible sound fields,'' J. Acoust. Soc. Am. \textbf{4}, 288-321 (1933). [Preview Abstract] |
Friday, March 19, 2010 1:39PM - 1:51PM |
Z10.00013: Characterization of chaotic dynamics in the human menstrual cycle Gregory Derry, Paula Derry The human menstrual cycle exhibits much unexplained variability, which is typically dismissed as random variation. Given the many delayed nonlinear feedbacks in the reproductive endocrine system, however, the menstrual cycle might well be a nonlinear dynamical system in a chaotic trajectory, and that this instead accounts for the observed variability. Here, we test this hypothesis by performing a time series analysis on data for 7438 menstrual cycles from 38 women in the 20-40 year age range, using the database maintained by the Tremin Research Program on Women's Health. Using phase space reconstruction techniques with a maximum embedding dimension of 6, we find appropriate scaling behavior in the correlation sums for this data, indicating low dimensional deterministic dynamics. A correlation dimension of $\cong $2.6 is measured in this scaling regime, and this result is confirmed by recalculation using the Takens estimator. These results may be interpreted as offering an approximation to the fractal dimension of a strange attractor governing the chaotic dynamics of the menstrual cycle. [Preview Abstract] |
Friday, March 19, 2010 1:51PM - 2:03PM |
Z10.00014: Low Velocity Waves Inside and Outside of Plants Orvin Wagner I have been reporting organizing waves in plants for many years. In 1989 I reported wave travel between plants. The waves travel at near 25 m/s horizontally through air on earth. Recently I built my own transmitters and receivers and found that the waves will penetrate mountains. Monitoring plants suggest that there is constant communication between plants with the cacophony peaking during the summer months. The location of the sun has a direct influence. I hypothesize that the observed waves are waves in dark matter as well as the other media involved. Apparently dark matter not only interacts with gravity but has much to do with the organization of nature. The velocities of waves in plants peak vertically. For example in Ponderosa pine the ratio of the vertical to horizontal velocity is 3/1 making a tall spindly tree. In apple the ratio is 4/3 making a nearly round tree. The velocity anisotropy may suggest that dark matter interacts differently with respect to the gravity direction. The penetrating qualities of the waves may provide useful communication. There appears to be a rather large velocity distribution, however, when the waves travel far through dense matter. [Preview Abstract] |
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