<cellular biophysics- a study of the structure and function of living cells< progress report, period ending jul. 1, 1964

Cellular biophysics - ionizing radiation effect on genetics, cell mutation, and mutagenic nature of tritium deca

Twirling and Whirling: Viscous Dynamics of Rotating Elastica

Motivated by diverse phenomena in cellular biophysics, including bacterial flagellar motion and DNA transcription and replication, we study the overdamped nonlinear dynamics of a rotationally forced filament with twist and bend elasticity. Competition between twist injection, twist diffusion, and writhing instabilities is described by a novel pair of coupled PDEs for twist and bend evolution. Analytical and numerical methods elucidate the twist/bend coupling and reveal two dynamical regimes separated by a Hopf bifurcation: (i) diffusion-dominated axial rotation, or twirling, and (ii) steady-state crankshafting motion, or whirling. The consequences of these phenomena for self-propulsion are investigated, and experimental tests proposed.Comment: To be published in Physical Review Letter

A perspective on cortical layering and layer-spanning neuronal elements

This review article addresses the function of the layers of the cerebral cortex. We develop the perspective that cortical layering needs to be understood in terms of its functional anatomy, i.e., the terminations of synaptic inputs on distinct cellular compartments and their effect on cortical activity. The cortex is a hierarchical structure in which feed forward and feedback pathways have a layer-specific termination pattern. We take the view that the influence of synaptic inputs arriving at different cortical layers can only be understood in terms of their complex interaction with cellular biophysics and the subsequent computation that occurs at the cellular level. We use high-resolution fMRI, which can resolve activity across layers, as a case study for implementing this approach by describing how cognitive events arising from the laminar distribution of inputs can be interpreted by taking into account the properties of neurons that span different layers. This perspective is based on recent advances in measuring subcellular activity in distinct feed-forward and feedback axons and in dendrites as they span across layers

An improved optical method for surface plasmon resonance experiments

In this paper an inexpensive optical device is described, which is capable of measuring the optical reflectance at different angles, while keeping the laser spot stationary at one point of the surface. This is accomplished by applying cylindrical optics. Its use is demonstrated in a surface plasmon resonance (SPR) sensor. A coil-operated vibrating mirror is used to obtain an angle scan of about 4 degrees. The angle of minimum reflectance can be detected with an accuracy of approximately 2 × 10−3 degrees. Despite the use of simple optical components, disturbance of laser beam parallelism is no more than 0.02 degrees. Displacement of the laser spot at the surface during the angle scan is kept within 0.2 mm. The device eliminates disturbances due to surface irregularities in measurements.\u

Alkalinization during re-oxygenation prevents functional damage by hyperglycaemic hypoxia

HYPERGLYCAEMIA impairs recovery from transient cerebral ischaemia: the importance of tissue acidification for this phenomenon has not been clarified in detail. We investigated this issue in a less complex in vitro preparation of isolated rat dorsal spinal roots exposed for 30 min to hyperglycaemic hypoxia. Peak height of compound action potentials recovered minimally in 5 mM bicarbonate. However, recovery was greatly improved by addition of the weak base trimethylamine during re-oxygenation. Addition of the weak acid propionate had no such effect. Cytoplasmic alkalinization improved recovery in a brief time window only: application of trimethylamine after 15 min of re-oxygenation was without beneficial effect. These data emphasize the importance of cytoplasmic acidification for neurophysiological recovery from hyper-glycaemic hypoxia during the initial period of re-oxygenation

Membrane Properties and the Balance between Excitation and Inhibition Control Gamma-Frequency Oscillations Arising from Feedback Inhibition

Computational studies as well as in vivo and in vitro results have shown that many cortical neurons fire in a highly irregular manner and at low average firing rates. These patterns seem to persist even when highly rhythmic signals are recorded by local field potential electrodes or other methods that quantify the summed behavior of a local population. Models of the 30–80 Hz gamma rhythm in which network oscillations arise through ‘stochastic synchrony’ capture the variability observed in the spike output of single cells while preserving network-level organization. We extend upon these results by constructing model networks constrained by experimental measurements and using them to probe the effect of biophysical parameters on network-level activity. We find in simulations that gamma-frequency oscillations are enabled by a high level of incoherent synaptic conductance input, similar to the barrage of noisy synaptic input that cortical neurons have been shown to receive in vivo. This incoherent synaptic input increases the emergent network frequency by shortening the time scale of the membrane in excitatory neurons and by reducing the temporal separation between excitation and inhibition due to decreased spike latency in inhibitory neurons. These mechanisms are demonstrated in simulations and in vitro current-clamp and dynamic-clamp experiments. Simulation results further indicate that the membrane potential noise amplitude has a large impact on network frequency and that the balance between excitatory and inhibitory currents controls network stability and sensitivity to external inputs

Helfrich-Canham bending energy as a constrained non-linear sigma model

The Helfrich-Canham bending energy is identified with a non-linear sigma model for a unit vector. The identification, however, is dependent on one additional constraint: that the unit vector be constrained to lie orthogonal to the surface. The presence of this constraint adds a source to the divergence of the stress tensor for this vector so that it is not conserved. The stress tensor which is conserved is identified and its conservation shown to reproduce the correct shape equation.Comment: 5 page