The capacity to predict future events permits a creature to detect, model, and manipulate the causal structure of its interactions with its environment. Behavioral experiments suggest that learning is driven by changes in the expectations about future salient events such as rewards and punishments. Physiological work has recently complemented these studies by identifying dopaminergic neurons in the primate whose fluctuating output apparently signals changes or errors in the predictions of future salient and rewarding events. Taken together, these findings can be understood through quantitative theories of adaptive optimizing control.

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A Neural Substrate of Prediction and Reward

Publisher Summary This chapter discusses preparation of isolated rat liver cells The early mechanical and chemical methods for liver-cell preparation were relatively successful in converting liver tissue to a suspension of isolated cells The successful preparation of intact liver cells by perfusion with collagenase is technically quite difficult The major method for preparation of nonparenchymal liver cells is based on the selective sensitivity of parenchymal cells toward proteases By incubation of rat liver minces with pronase, most of the liver parenchyma is digested, while nonparenchymal cells remain intact and can be recovered from the incubate Similar results have been reported with trypsin digestion of collagenase-dispersed liver minces, but pronase appears to be more effective The most common procedure is to perfuse the liver briefly with pronase before it is minced and incubated with the enzyme Such direct pronase methods have been used by several investigators with yields of nonparenchymal liver cells reported to be in the range 2–15 × 10 6 cells/gm liver

Preparation of isolated rat liver cells.

Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

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The hydrodynamics of swimming microorganisms

Cells employ a variety of linear motors, such as myosin, kinesin and RNA polymerase, which move along and exert force on a filamentous structure. But only one rotary motor has been investigated in detail, the bacterial flagellum (a complex of about 100 protein molecules). We now show that a single molecule of F1-ATPase acts as a rotary motor, the smallest known, by direct observation of its motion. A central rotor of radius approximately 1 nm, formed by its gamma-subunit, turns in a stator barrel of radius approximately 5nm formed by three alpha- and three beta-subunits. F1-ATPase, together with the membrane-embedded proton-conducting unit F0, forms the H+-ATP synthase that reversibly couples transmembrane proton flow to ATP synthesis/hydrolysis in respiring and photosynthetic cells. It has been suggested that the gamma-subunit of F1-ATPase rotates within the alphabeta-hexamer, a conjecture supported by structural, biochemical and spectroscopic studies. We attached a fluorescent actin filament to the gamma-subunit as a marker, which enabled us to observe this motion directly. In the presence of ATP, the filament rotated for more than 100 revolutions in an anticlockwise direction when viewed from the 'membrane' side. The rotary torque produced reached more than 40 pN nm(-1) under high load.

Direct observation of the rotation of F1-ATPase

Human skin is a remarkable organ. It consists of an integrated, stretchable network of sensors that relay information about tactile and thermal stimuli to the brain, allowing us to maneuver within our environment safely and effectively. Interest in large-area networks of electronic devices inspired by human skin is motivated by the promise of creating autonomous intelligent robots and biomimetic prosthetics, among other applications. The development of electronic networks comprised of flexible, stretchable, and robust devices that are compatible with large-area implementation and integrated with multiple functionalities is a testament to the progress in developing an electronic skin (e-skin) akin to human skin. E-skins are already capable of providing augmented performance over their organic counterpart, both in superior spatial resolution and thermal sensitivity. They could be further improved through the incorporation of additional functionalities (e.g., chemical and biological sensing) and desired properties (e.g., biodegradability and self-powering). Continued rapid progress in this area is promising for the development of a fully integrated e-skin in the near future.

25th Anniversary Article: The Evolution of Electronic Skin (E-Skin): A Brief History, Design Considerations, and Recent Progress

IT is widely agreed that bacteria swim by moving their flagella, but how this motion is generated remains obscure1,2. A flagellum has a helical filament, a proximal hook, and components at its base associated with the cell wall and the cytoplasmic membrane. If there are several flagella per cell, the filaments tend to form bundles and to move in unison. When viewed by high-speed cinematography, the bundles show a screw-like motion. It is commonly believed that each filament propagates a helical wave3. We will show here that existing evidence favours a model in which each filament rotates.

Bacteria Swim by Rotating their Flagellar Filaments

We develop a chain model of an electrorheological fluid in steady and oscillatory shear. This model is based on a balance of hydrodynamic and electrostatic forces and focuses on the mechanical stability of chains. With this model we compute the shape and orientation of an unconfined dipolar chain in steady shear as a function of the shear rate and electric field, and show that this leads to the expected shear‐thinning viscosity. We then demonstrate that chains confined by electrodes are unstable in some Mason number regimes. The chain model is extended to the case of oscillatory shear by including a kinetic equation for the aggregation and fragmentation of chains. The resultant chain dynamics is found to be strongly nonlinear, as is the rheology. Finally, we consider the effect of local field corrections and long range interactions on the bare dipolar interaction force and derive a self‐consistent pair interaction force that demonstrates that the maximum orientation of a chain in shear depends strongly on the dielectric contrast.

Chain model of electrorheology

High speed motion‐picture studies were made of the dependence of air‐water entry cavities on the relevant physical parameters—in particular, on the atmospheric pressure over the water surface. Experiments on the vertical entry of spheres ¼ in. to 1 in. in diameter, with entrance velocities between 15 ft./sec. and 100 ft./sec., and air pressures between 1/60 atmosphere and 3 atmospheres, show that surface sealing of the cavity is a major factor in controlling cavity formation, and is the factor most responsible for non‐Froude scaling of cavity phenomena. Surface sealing is a function chiefly of the atmospheric density and projectile velocity, although surface tension is also important in its effect on splash formation. Froude's scaling is found to hold true in the region of low Froude's numbers and low atmospheric pressures. The measured average drag coefficient of the projectiles is independent of cavity size and shape. Jets are found to occur regularly and with great strength at both deep and surface closures of the cavity. The observed presence of jets in finite cavities is predicted by an extension of the Kirchhoff‐Helmholtz theory of the infinite (two‐dimensional) cavity to the case of finite cavities with finite cavitation numbers. Some of the results of this theory are mentioned.

Influence of Atmospheric Pressure on the Phenomena Accompanying the Entry of Spheres into Water

Magnetic field-structured composites (FSCs) are made by structuring magnetic particle suspensions in uniaxial or biaxial (e.g., rotating) magnetic fields, while polymerizing the suspending resin. A uniaxial field produces chainlike particle structures, and a biaxial field produces sheetlike particle structures. In either case, these anisotropic structures affect the measured magnetic hysteresis loops, with the magnetic remanence and susceptibility increased significantly along the axis of the structuring field, and decreased slightly orthogonal to the structuring field, relative to the unstructured particle composite. The coercivity is essentially unaffected by structuring. We present data for FSCs of magnetically soft particles, and demonstrate that the altered magnetism can be accounted for by considering the large local fields that occur in FSCs. FSCs of magnetically hard particles show unexpectedly large anisotropies in the remanence, and this is due to the local field effects in combination with the large crystalline anisotropy of this material.

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Anisotropic magnetism in field-structured composites

We report the results of a computer simulation of the evolution of structure in a two component fluid consisting of a liquid phase and a dispersed colloidal phase subjected to a uniaxial field. Our primary objective is to understand the mechanism and kinetics of coarsening and the emergence of crystallinity. Using an efficient, linear-N simulation method we report studies of systems of N=10 000 particles over the concentration range of 10–50 vol %. We present a variety of methods of characterizing the structures that emerge, including the anisotropy of the conductivity, capacitance and dipolar interaction energy, the two-dimensional pair correlation function, principal moments of the gyration tensor, velocity correlation functions, microcrystallinity and coordination number, and the optical attenuation length. We conclude that athermal coarsening is effectively driven by the presence of defect structures and that as the concentration increases, the structures progressively lose the well-known “chain” anis...

Robert A. Anderson

Papers

Bacteria Swim by Rotating their Flagellar Filaments

Chain model of electrorheology

Influence of Atmospheric Pressure on the Phenomena Accompanying the Entry of Spheres into Water

Anisotropic magnetism in field-structured composites

Simulation of the athermal coarsening of composites structured by a biaxial field