Bulletin of the American Physical Society
2005 72nd Annual Meeting of the Southeastern Section of the APS
Thursday–Saturday, November 10–12, 2005; Gainesville, FL
Session MA: Biophysics Invited Session |
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Chair: Stephen Hagen, University of Florida Room: Hilton Century A |
Saturday, November 12, 2005 8:30AM - 9:06AM |
MA.00001: Molecular Motors and Efficient Motion in a Viscoelastic Environment Invited Speaker: Molecular motors perform many critical functions for cells, including chromosome separation during mitosis, vesicle transport, and muscle contraction. In this talk, we will discuss the ways in which physics concepts and instrumentation are being used to determine the forces and efficiencies of two of these motors, kinesin and dynein, in cells. We will emphasize a) studies at Wake Forest University that focus on the force versus velocity curves (load curves) of kinesin in the neurites of live PC12 cells, and b) work at UNC-Chapel Hill that measures the forces developed by dynein motors within beating cilia on the outer surfaces of live lung cells during mucus transport. We will show how the viscoelastic properties of cytoplasm and mucus can be determined from the Brownian motion of vesicles and beads in these media.. We find that the load on these motors in vivo may exceed that in vitro by a factor of 1000, and that several motors can share the task of moving a single vesicle. [Preview Abstract] |
Saturday, November 12, 2005 9:06AM - 9:42AM |
MA.00002: Integrated analysis of bacterial quorum-sensing networks Invited Speaker: The regulation of gene expression is fundamental to most processes in cellular biology. At the transcriptional level, regulation occurs by the binding of specific proteins called transcription factors to DNA. Post-transcriptional regulation is often carried out by small RNAs which have become the focus of intense research activity recently. The talk will discuss the physics and biology of these two regulatory mechanisms by focusing on a specific biological system: quorum-sensing networks in bacteria. Quorum sensing is the process by which bacteria communicate to regulate gene expression in response to cell population density. Using an integrated approach which combines computational modeling, bioinformatics and experimental molecular biology, we are studying quorum-sensing pathways in bacteria. This approach led to the discovery of multiple regulatory small RNAs which are an integral part of the quorum-sensing pathway in {\it Vibrio cholerae} and {\it Vibrio harveyi}. Modeling of regulation of and by small RNAs in quorum sensing reveals the circuit characteristics controlling the transition from the low cell-density response to the high cell-density response. [Preview Abstract] |
Saturday, November 12, 2005 9:42AM - 10:18AM |
MA.00003: Genesis and Control of bursting activity in a neuronal model Invited Speaker: Neurons are observed in one of four fundamental activity modes: silence, sub-threshold oscillations, tonic spiking, and bursting. Neurons exhibit various activity regimes and regime transitions that reflect their complement of ionic channels and modulatory state. The leech presents unique opportunities for experimental and theoretical studies on the dynamics of neuronal activity. The central pattern generator controlling the leech's heartbeat contains identified pairs of mutually inhibitory neurons. Bursting activity of neurons is an oscillatory activity consisting of intervals of repetitive spiking separated by intervals of quiescence. It has been observed in neurons under normal and pathological conditions. Neurons which are capable of generating bursting activity endogenously play an important role in motor control and other brain functions. Burst duration, interburst interval and spike frequency are crucial temporal characteristics of bursting activity and thus have to be regulated. Application of the bifurcation theory of dynamical systems suggests new mechanism of how bursting activity can be generated by neurons and how burst duration can be regulated. Here we describe two mechanisms for the transition between tonic spiking and bursting. First mechanism describes a smooth, continuous and reversible transition from tonic spiking into bursting in a model neuron. The burst duration increases with no bound as 1/(\textbf{\textit{a-a}}$_{0})^{1/2}$, where \textbf{\textit{a}}$_{0}$ is a parameter determining the transition. The characteristic features of this mechanism are that (a) the burst duration can be made arbitrarily long while (b) inter-burst interval does not depend on the parameter. The second mechanism is concerned with bi-stability where simultaneous tonic spiking and bursting activities co-exist in a neuron. The mechanism is based on a saddle-node periodic orbit bifurcation with non-central homoclinic orbits. This bifurcation describes a transition between three qualitatively different types of dynamics of a neuron. If one varies the control parameter \textbf{\textit{a}} towards the critical value \textbf{\textit{a}}$_{0}$ at which the transition from the bistability region to the region where only tonic spiking is observed, the burst duration of the bursting activity becomes proportional to \textit{ln}(\textbf{\textit{a-a}}$_{0})$. The interburst interval does not correlate with the burst duration. In terms of neuron's activity these two mechanisms describe a biophysically plausible means for regulation of burst duration. We show how this bifurcation can be found in a Hodgkin-Huxley type model of a neuron and how to identify control parameters determining properties of bursting activity. The work is supported by NIH NS 43098. [Preview Abstract] |
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