Narrow Results

Cell motility resulting from actin polymerization is modeled on a two-dimensional square lattice. The treadmilling of actin filaments, formation of lamellipodia, protrusion and motility of the model cell are studied using Monte Carlo simulations. The grid space of the square lattice and the Monte Carlo step are related to length and time scales of the problem. The average velocity computed with this prescription from the simulations shows a remarkable agreement with the experimental velocity of a keratocyte. The model cell captures the essential aspects of treadmilling based motility. The movement of the model cell is diffusive for small times and exhibits a cross over to polymerization driven drift for large times. The studies on the parameter sensitivity of cell velocity indicated that the optimal choice of number of monomers, the number of filaments, the rate of depolymerization and the monomer diffusion leads to large velocities. The cell velocity distribution is found to be Gaussian and is in agreement with some of the experimental work.

This work deals with gliding motion of bacteria over a non-Newtonian slime layer attached to a solid substrate. The surface of a glider is approximated with a simple wavy two-dimensional sheet while the sticky slime is modeled as Carreau–Yasuda fluid. The classical Navier–Stokes equations are transformed by using Galilean transformation and dimensionless variables. Flow beneath the organism is creeping and the lubrication assumption is also valid in this scenario, hence the equation is reduced to fourth-order BVP in terms of stream function. The basic purpose is to compute the gliding speed and flow rate of the slime which are present in the boundary conditions. MATLAB built-in bvp5c solver is utilized to calculate the numerical solution of the stream function. Further, unknowns (flow rate and cell speed) are refined by using modified Newton–Raphson method (MNRT). Further, these pairs are employed in the formula of energy expended. Velocity and stream function is also plotted for these computed pairs. This study is motivated by scientific interest and the desire to understand the dynamics of gliding bacteria. The findings of this study are thought to be beneficial in the development of artificial crawlers.

The evacuation of human crowds consisting of several groups in complex venues is studied, in this paper, by the kinetic theory of active particles. We consider a complex venue that consists of several chambers which are separated by interior walls and doors, for which the geometrical effects may come from the walls both on the boundary and inside the domain, the exits, and the doors connecting the chambers. The geometrical properties are incorporated in the geometrical preferred direction, which is specified such that an individual can avoid the wall and approach the exit through the evacuation vector. The notion of group is corresponding to the concept of functional subsystem in the mathematical theory of active particles. In the present framework of several groups, the rules of interactions can be specified for both the interactions among the same group and the interactions between different groups. Furthermore, the motility is assigned for each group, which serves as the average activity of a group. Some interesting features and phenomena are shown in the numerical tests of the evacuation of a crowd with one, two or three groups.

Monitoring sphincter activity presents remarkable methodological and conceptual problems. The particular shape, structure and function of the sphincters require an accurate analysis of the objectives of diagnostic procedures and the physical concepts applied. Researchers have developed practical solutions to the description of the sphincter activity not always rigorous on a theoretical basis, and the measurement results are difficult to evaluate and compare, in the event that the contour conditions of the assumed descriptive model are not well defined. A practical solution is to "create" a constant curvature radius chamber by introducing a cylinder within the sphincter and measuring the pressure, linearly proportional to the tangential force strength of the wall. This paper analyzes the coherence between the theoretical basis and the practical solutions of the different methods of sphincter monitoring, to clarify some controversial aspects, highlight the limitations and to suggest possible alternative solutions.

The motility of cells is a multifaceted and complicated cytoskeletal process. Significant inroads can be made into gaining a more detailed understanding, however, by focusing on the smaller, more simple subunits of the motile system in an effort to isolate the essential protein components necessary to perform a certain task. Identification of such functional modules has proven to be an effective means of working towards a comprehensive understanding of complex, interacting systems. By following a bottom-up approach in studying minimal actin-related sub-systems for keratocyte motility, we revealed several fundamentally important effects ranging from an estimation of the force generated by the polymerization of a single actin filament, to assembly dynamics and the production of force and tension of composite actin networks, to the contraction of actin networks or smaller bundled structures by the motor myosin II. While even motile keratocyte fragments represent a far more complex situation than the simple reconstituted systems presented here, clear parallels can be seen between in vivo cell motility and the idealized in vitro functional modules presented here, giving more weight to their continued focus.

Disorders of gastrointestinal (GI) motility are associated with various symptoms such as nausea, vomiting, and constipation. However, the underlying causes of impaired GI motility remain unclear, which has led to variation in the efficacy of therapies to treat GI dysfunction. Optogenetics is a novel approach through which target cells can be precisely controlled by light and has shown great potential in GI motility research. Here, we summarized recent studies of GI motility patterns utilizing optogenetic devices and focused on the ability of opsins, which are genetically expressed in different types of cells in the gut, to regulate the excitability of target cells. We hope that our review of recent findings regarding optogenetic control of GI cells broadens the scope of application for optogenetics in GI motility studies.

The rhythmic movement of the microtubular axostyle in the termite flagellate, Pyrsonympha vertens, was analyzed with polarization and electron microscopy. The protozoan axostyle is birefringent as a result of the semi-crystalline alignment of ~2,000 microtubules. The birefringence of the organelle permits analysis of the beat pattern in vivo. Modifications of the beat pattern were achieved with visible and UV microbeam irradiation.

The beating axostyle is helically twisted and has two principal movements, one resembling ciliary and the other flagellar beating. The anterior portion of the beating axostyle has effective and recovery phases with each beat thereby simulating the flexural motion of a beating cilium. Undulations develop from the flexural flipping motion of the anterior segment and travel along the axostyle like flagellar waves. The shape of the waves differs from that of flagellar waves, however, and are described as sawtooth waves. The propagating sawtooth waves contain a sharp bend, ~3 μm in length, made up of two opposing flexures followed by a straight helical segment ~23 μm long. The average wavelength is ~25 μm, and three to four sawtooth waves travel along the axostyle at one time. The bends are nearly planar and can travel in either direction along the axostyle with equal velocity. At temperatures between 5° and 30°C, one sees a proportionate increase or decrease in wave propagation velocity as the temperature is raised or lowered. Beating stops below 5°C but will resume if the preparation is warmed.

A microbeam of visible light shone on a small segment of the axostyle causes the typical sawtooth waves to transform into short sine-like waves that accumulate in the area irradiated. Waves entering the affected region appear to stimulate waves already accumulated there to move, and waves that emerge take on the normal sawtooth wave pattern. The effective wavelengths of visible light capable of modifying the wave pattern is in the blue region of the spectrum. The axostyle is severed when irradiated with an intense microbeam of UV light. Short segments of axostyle produced by severing it at two places with a UV microbeam can curl upon themselves into shapes resembling lockwashers. We propose that the sawtooth waves in the axostyle of P. vertens are generated by interrow cross-bridges which are active in the straight regions.