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Pneumatic muscles as actuators offer several advantages due to their performance weight relation or to the passive compliance of this type of actuators. Passive compliance is an important component for the control of locomotion in rough terrain to absorb the power stroke energy. This paper presents the mechatronics of a six-legged insect-like robot AirBug with pneumatic muscles as actuators. The main focus lies on the control concept of the antagonistic actuators.

Elevator muscle activity per unit bite-force has been shown to be higher in chewing than in isometric biting. We tested the hypothesis that surplus elevator activity is evoked in response to a possible co-activation of jaw-opener muscles during the masticatory power stroke. In 32 subjects, digastric and bilateral masseter and temporalis activities were recorded during unilateral chewing of test foods, isometric biting on a force transducer, and during balancing of the jaw against maximum effort of depressor muscles. During elevator peak effort in chewing, the digastric activity was 113% higher than during peak effort in isometric biting. Comparison of balancing and chewing trials revealed that a 6% increase of elevator activity would suffice to compensate for this increased depressor action. Elevator activity in chewing, however, was up to 130% higher than in clenching. We conclude that depressor counteraction could have only a minor influence on the generation of surplus muscle activity in chewing.

Members of the myosin II class of molecular motors have been referred to as "conventional," a term used to describe their ability to form thick filaments, their low duty ratio, the ability of individual motor-containing "heads" to operate independently of each other, and their rate-limiting phosphate release. These features ensure that those motors that have completed their power stroke dissociate rapidly enough to prevent them from interfering with those motors that are beginning theirs. However, in this study, we demonstrate that myosin IIB, a cytoplasmic myosin II particularly enriched in the central nervous system and cardiac tissue, has a number of features that it shares instead with "unconventional" myosin isoforms, including myosins V and VI. These include a high duty ratio, rate-limiting ADP release, and high ADP affinity. These features imply that myosin IIB serves a set of physiologic needs different from those served by its more conventional myosin II counterparts, and this work provides a plausible basis for explaining the physiologic role of this unconventionally conventional myosin.

In the micro-world of biomedical interest, one of the primary subjects would be the single-celled protozoa, called flagellates and ciliates. The latter is similar in function and structure to ciliary organelles existing in some mammalian organs, such as the trachea, in the ear, oviduct, and in other bio-tracts. The well-being or illness of these organelles appears to be relevant to human health.

The 50K domain of the myosin cross bridge is split into two subdomains (upper and lower) by the major cleft that connects the actin binding site with the nucleotide binding site. A number of lines of experimental evidence now point to the major cleft closing on strong binding of the cross bridge to actin. In particular, our recent high resolution (14A) cryo EFTEM images show that most of the 50K upper domain is involved in this movement. However, the switch 1 element of the nucleotide binding site is anchored in the 50K upper domain so that the strong binding to actin will lead to a 5-l0A movement of switch 1. Thus strong binding to actin moves switch 1. The movement of switch 1 opens the nucleotide binding site. This movement is quite distinct from the opening of switch 2, which is connected with the movement of the lever arm during the power stroke. The possibility that switch 1 moves (opens) on actin binding suggests new mechanisms for the initiation of the power stroke and for the release of ADP and phosphate during the cross bridge cycle.

Actin and myosin interact in a cyclic series of steps linked to the hydrolysis of ATP that are representative of an ancient and widespread molecular mechanism. Spectroscopic findings are related to the analysis of the actin and myosin structures and results from kinetics, fibers, single molecules, electron microscopy, genetics, and a variety of other biophysical and biochemical studies on actin and myosin to provide an overview of the steps in this molecular process. The synthesis of the key findings from these fields reveals a highly efficient engine that amplifies subtle changes in the active site into unsurpassed molecular displacements. Recent developments in resonance energy-transfer spectroscopy and X-ray crystallography are enabling a detailed elucidation of the stages of a large power stroke that concurs with evidences from diverse lines of structural and kinetic inquiry. A complete image of actin and myosin motility appears to include twists, tilts, steps, and dynamics from both partners that could be described as a molecular dance.

The independent force generator and the power-stroke cross-bridge model have dominated the thinking on mechanisms of muscular contraction for nearly the past five decades. Here, we review the evolution of the cross-bridge theory from its origins as a two-state model to the current thinking of a multi-state mechanical model that is tightly coupled with the hydrolysis of ATP. Finally, we emphasize the role of skeletal muscle myosin II as a molecular motor whose actions are greatly influenced by Brownian motion. We briefly consider the conceptual idea of myosin II working as a ratchet rather than a power stroke model, an idea that is explored in detail in the companion paper.

We used cryo-electron microscopy and image reconstruction to investigate the structure and microtubule-binding configurations of dimeric non-claret disjunctional (ncd) motor domains under various nucleotide conditions, and applied molecular docking using ncd's dimeric X-ray structure to generate a mechanistic model for force transduction. To visualize the alpha-helical coiled-coil neck better, we engineered an SH3 domain to the N-terminal end of our ncd construct (296-700). Ncd exhibits strikingly different nucleotide-dependent three-dimensional conformations and microtubule-binding patterns from those of conventional kinesin. In the absence of nucleotide, the neck adapts a configuration close to that found in the X-ray structure with stable interactions between the neck and motor core domain. Minus-end-directed movement is based mainly on two key events: (i) the stable neck-core interactions in ncd generate a binding geometry between motor and microtubule which places the motor ahead of its cargo in the minus-end direction; and (ii) after the uptake of ATP, the two heads rearrange their position relative to each other in a way that promotes a swing of the neck in the minus-end direction.

A quantitative analysis of experimental data for posttranslational translocation into the endoplasmic reticulum is performed. This analysis reveals that translocation involves a single rate-limiting step, which is postulated to be the release of the signal sequence from the translocation channel. Next, the Brownian ratchet and power stroke models of translocation are compared against the data. The data sets are simultaneously fit using a least-squares criterion, and both models are found to accurately reproduce the experimental results. A likelihood-ratio test reveals that the optimal fit of the Brownian ratchet model, which contains one fewer free parameter, does not differ significantly from that of the power stroke model. Therefore, the data considered here cannot be used to reject this import mechanism. The models are further analyzed using the estimated parameters to make experimentally testable predictions.

The two headed motor protein kinesin appears to "walk" along the biopolymer microtubule in 8 nm steps. There is ample justification for a model where the motion of the detached head to the next docking site on the biopolymer is described as ratcheted diffusion. The forward reorientation of an attached head can be conceived of as a power stroke. A model that is based on these premises can accurately predict parameters of motor protein motion.

What do area, length, volume, work, and hydrostatic force have in common? All of these (and many other important concepts in science and engineering) can be modelled as Riemann sums (8.6) $$\sum\limits_{k = 1}^n {f\left( {{\xi _k}} \right)} {\rm{ }}\Delta {x_k},$$ and computed as integrals (8.28), $$\int\limits_{a}^{b} {f(x)dx: = \mathop{{\lim }}\limits_{{\parallel \mathcal{P}\parallel \to 0}} } \sum\limits_{{k = 1}}^{n} {f({{\xi }_{k}})\Delta {{x}_{k}}.}$$ In this chapter integrals are applied to problems of computing areas (Sects. 11.1, 11.2, and 11.6), arc lengths (Sect. 11.3), volumes (Sects. 11.4 and 11.5), moments and centroids (Sects. 11.7 and 11.8), work (Sect. 11.9), and hydrostatic force (Sect. 11.10).

The capacity for aerial maneuvering shaped the evolution of flying animals. Here we evaluate consequences of aviaian morphology for aerial performance (1,2) by quantifying static stability and control effectiveness of physical models (3) for numerous taxa sampled from within the lineage leading to birds (Paraves, 4). Results of aerodynamic testing are mapped phylogenetically (5-9) to examine how maneuvering characteristics correlate with tail shortening, fore- and hindwing elaboration, and other morphological features (10). In the evolution of the Avialae we observe shifts from static stability to inherently unstable aerial planforms; control effectiveness also migrated from tails to the forewings. These shifts suggest that some degree of aerodynamic control and and capacity for maneuvering preceded the evolution of strong power stroke. The timing of shifts also suggests some features normally considered in light of development of a power stroke may play important roles in control., Comment: 12 pages, 6 figures, 1 supplemental figures and 5 supplemental tables

The interdomain motions in myosin subfragment 1 (S1) were studied by steady-state and time-resolved fluorescence of tryptophan residues and N-(iodoacetyl)-N'-(5-sulfo-1-naphtyl)ethylenediamine (AEDANS) attached to Cys178 of alkali light chain 1 (A1) exchanged into S1. The efficiency of fluorescence resonance energy transfer (FRET) from tryptophan residues of motor domain to AEDANS at A1 decreased dramatically after addition of ATP to S1A1-AEDANS. The efficiency of FRET calculated from the crystal structure of chicken S1 corresponded to the experimental one measured in the presence of ATP. The results showed that AEDANS at Cys178 of A1 became more mobile and distant from the motor domain of S1 upon ATP binding. These findings led to the suggestion that a release of the products of ATP hydrolysis and power stroke might be associated with movement of light chain-binding domain towards the N-terminal domain of S1.

Rapid freezing and substitution with fixative prior to scanning electron microscopy was used to demonstrate the pattern of beat and recovery of the cilia of free swimming miracidia ofFasciola hepatica. There were stages of dexioplectic metachronal co-ordination and the power stroke was approximately 15° anticlockwise from the anterior–posterior axis. Around the circumference of the body of the miracidia there were approximately 12 metachronal waves of power and recovery. Free-swimming cercariae were recorded by time-lapse photography and, after conventional fixation, by scanning electron microscopy. Cercarial tail-beats were to the posterior of the body in the lateral plane at a rate of 8 Hz. The tail has paired lateral ridges positioned to act as leading edges. There is an array of 32 sensory papillae on the mid-ventral surface of the tail. The tegument of the most distal part of the tail is described: it is free of sensory endings and the surface shows a spiral pattern.

Aquatic flight, performed by rowing or flapping fins, wings or limbs, is a primary locomotor mechanism for many animals. We used a computer simulation to compare the mechanical performance of rowing and flapping appendages across a range of speeds. Flapping appendages proved to be more mechanically efficient than rowing appendages at all swimming speeds, suggesting that animals that frequently engage in locomotor behaviours that require energy conservation should employ a flapping stroke. The lower efficiency of rowing appendages across all speeds begs the question of why rowing occurs at all. One answer lies in the ability of rowing fins to generate more thrust than flapping fins during the power stroke. Large forces are necessary for manoeuvring behaviours such as accelerations, turning and braking, which suggests that rowing should be found in slow-swimming animals that frequently manoeuvre. The predictions of the model are supported by observed patterns of behavioural variation among rowing and flapping vertebrates.

The unique morphology of the toes of the great crested grebe (Podiceps cristatus), which are asymmetrically lobed with a narrower skin flap on the lateral side of the toe, enables these birds to swim very efficiently. Here we study video recordings of a diving grebe and stroboscopic pictures of its moving feet and conclude that the bird uses a hydrodynamically lift-based foot (power) stroke to propel itself underwater, with the separated toes functioning as multiple slots to increase the lift-to-drag ratio. The asymmetric lobes are an adaptation for self-stabilization of the toes during the power stroke, and the toes themselves act as separate hydrofoils, each producing lift and each being twistable individually under hydrodynamic load.

Although it is generally assumed that cranial kinesis is a plesiomorphic characteristic in squamates, experimental data tend to contradict this hypothesis. In particular, coupled kinesis (i.e. streptostyly and mesokinesis) presumably arose independently in only a limited number of highly specialised groups. In this study, we investigated cranial kinesis in one of the most specialised of these groups: geckoes. On the basis of cineradiographic and electromyographic data, the fast opening and the slow closing/power stroke phases were modelled to elucidate possible functions of the observed kinesis. The results of these analyses show that the retraction of the muzzle unit during crushing is a self-reinforcing system that increases bite force and reduces the joint forces; the active protraction of the kinetic system during jaw opening, in contrast, enhances opening speed through the coupling of the intracranial units. It can be argued that cranial kinesis in geckoes is probably not an adaptive trait as such but, instead, a consequence of the ‘Bauplan’ of the cranial system in these animals. Presumably as a result of constructional constraints on the size of the jaw musculature and eyes, the supratemporal and postorbital bars were lost, which resulted in enormous mobility in the skull. To counteract the potential negative factors associated with this (decrease in bite force, skull damage), the kinetic system may have become coupled, and thus functional.

Curve walking in two species of crayfish, Procambarus clarkii and Astacus leptodactylus, was investigated to test whether the mechanism underlying curve walking is the synchronous action of a centrally pre-programmed leg tripod or whether it is the action of one principal leg that produces the main body yaw torque. Curve walking was induced by an optomotor visual stimulus, and the yaw torque produced by the tethered animals was measured in open-loop conditions. Our main results suggest that the yaw torque oscillations in both P. clarkii and A. leptodactylus are related to the movement of outer leg 4 (i.e. leg 4 on the outside of the turn). That is, the peaks in the yaw torque occur, on average, in synchrony with the power stroke of outer leg 4. When comparing the results of this open-loop experiment on P. clarkii with results previously obtained for curve walking in untethered individuals of the same species, we found a much higher variability in leg coordination in the open-loop situation. Similarly, here we did not find the same level of synchrony in the tripod (formed by outer leg 4 and inner legs 2 and 5) observed during untethered free walking. Therefore, we suggest that tethered conditions may diminish the need for stability and thus allow outer leg 4 to produce a body rotation regardless of the leg stepping configuration. The characteristics of leg 4 are in line with its major role in turning. According to previous studies, legs 4 provide the largest force and the largest step amplitude during walking, and their force includes both a pulling and a pushing component which can facilitate the control of turning. Although it is apparent that outer leg 4 is not the only leg that can produce an inward yaw torque, its major role in modulating the yaw torque suggests that there may be a specific, centrally generated control of outer leg 4 during curve walking in crayfish.

The curve walking of freely moving crayfish trained to walk along a curved path during homing behaviour was investigated using a video-analysis system. The leg kinematics and leg phase relationships, as well as the relationship between stepping patterns and body axis rotation measured relative to external references, were studied. The anterior and posterior extreme positions of the power stroke (AEP and PEP, respectively) and step amplitudes were analysed. As in a previous study on crayfish curve walking on a treadmill, PEPs were more posterior in outer legs (the legs on the outside of the turn) than in the inner legs. As a result, outer legs showed larger step amplitudes than inner legs. Leg kinematics varied within each walking sequence. AEP leg angles (the angles between the body and leg axes at the AEP) tended to decrease over time for inner legs and increase for outer legs. This leg angle drift was present mainly in the anterior legs and it suggests that these legs did not completely compensate for the body rotation after each step. In addition, leg angle asymmetries in a direction opposite to that of leg angle drift were observed at the start of each curve-walking sequence, suggesting that the extensive training (3 weeks) may have allowed crayfish to anticipate the leg angle drift. The rotational component of curve walking showed a discontinuous pattern, with the animal’s body axis turning towards the inside of the curve only periodically. Analysis of cross-correlation functions showed that the angular acceleration of the body axis in the direction of the turn occurred during the power strokes of inner legs 2 and 5 and outer leg 4. While the tripod formed by these three legs showed in-phase relationships, the legs of the corresponding contralateral tripod (outer legs 2 and 5 and inner leg 4) were not in phase. We hypothesize that inner legs 2 and 5 and outer leg 4 act synergically causing the inward body rotation observed in curve-walking crayfish and that some of the asymmetries found in step geometry may be a passive phenomenon due to the body rotation.

Both chemical and biological actuators are capable of producing very large forces and their energy densities and efficiencies are larger than other types of actuators. Mechanisms that are used in biological systems to extract useful work from chemical reactions are quite interesting and inspiring and can be quite beneficial in the design of novel “artificial” microactuators.

Observing a Paramecium in its natural environment, it is easily recognized that this cell can behaviourally perform with a perfection and subtleness comparable to the behaviour of some of its metazoan companions, such as rotifers, oligochaetes or crustaceans. The similarities in movements between multicellular organisms, using specialized cells and a central nervous system, and unicells, are nevertheless superficial; regulation of the motor response in a ciliate is radically decentralized because it occurs, independently at the same time, in each of the thousands of active cilia. The reasons for synchronization of many independent motor organelles in a single cell have been outlined in Chapter 13 (this Vol.).

Crustacean cardioactive peptide (CCAP) elicited expression of the motor pattern that drives coordinated swimmeret beating in crayfish and modulated this pattern in a dose-dependent manner. In each ganglion that innervates swimmerets, neurons with CCAP-like immunoreactivity sent processes to the lateral neuropils, which contain branches of swimmeret motor neurons and the local pattern-generating circuits.CCAP affected each of the four functional groups of motor neurons, power-stroke excitors (PSE), return-stroke excitors (RSE), power-stroke inhibitors (PSI), and return-stroke inhibitors (RSI), that innervate each swimmeret. When CCAP was superfused, the membrane potentials of these neurons began to oscillate periodically about their mean potentials. The mean potentials of PSE and RSI neurons depolarized, and some of these neurons began to fire during each depolarization. Both intensity and durations of PSE bursts increased significantly. The mean potentials of RSE and PSI neurons hyperpolarized, and these neurons were less likely to fire during each depolarization. When CCAP was superfused in a low Ca2+saline that blocked chemical transmission, these changes in mean potential persisted, but the periodic oscillations disappeared.These results are evidence that CCAP acts at two levels: activation of local premotor circuits and direct modulation of swimmeret motor neurons. The action on motor neurons is differential; PSEs and RSIs are excited, but RSEs and PSIs are inhibited. The consequences of this selectivity are to increase intensity of bursts of impulses that excite power-stroke muscles.

The discovery of a freely swimming sperm in Ginkgo biloba at the end of the nineteenth century by Sakugoro Hirase [1–3] was one of the most significant moments in botany, because it led to the establishment of Ginkgo as the true and sole link between the non-flowering primitive plants and the advanced seed plants. Hirase, as both amateur botanist and artist, was able to beautifully depict the sperm [2] and fertilization process in Ginkgo [3], and although he was able to describe the ornamentation of the spiral, from which emanate thousands of flagella, the instruments of the day prevented him from more fully understanding the complexity of the apparatus.

Freely walking crayfish, Procambarus clarkii, were studied using a video analysis procedure adapted especially for use with crayfish. The animals were placed in a tank and their homing behaviour was filmed as they returned in a straight line to their shelter. Various sequences were studied at the two following levels. First, the trajectory of each pair of legs (from leg 2 to leg 5) during the step cycle (power stroke and return stroke) was studied to measure stride length and to analyse in detail changes in acceleration. Each leg was found to contribute in a specific manner to locomotion. Second, ipsi-and contralateral leg coordination was investigated. Ipsilateral coordination was found to involve a metachronal organization from front to back in all the walking sequences recorded, whereas contralateral coordination involved, in addition to the weak alternate coupling commonly observed in treadmill walking, another coordination pattern where the legs on each side (legs 3 and 4) are in phase. The results obtained in these free-walking sequences are discussed and compared with those obtained previously, in particular in treadmill situations.

When skeletal muscles are activated and mechanically shortened, the force that is produced by the muscle fibers decreases in two phases, marked by two changes in slope (P₁ and P₂) that happen at specific lengths (L₁ and L₂). We tested the hypothesis that these force transients are determined by the amount of myosin cross-bridges attached to actin and by changes in cross-bridge strain due to a changing fraction of cross-bridges in the pre-power-stroke state. Three separate experiments were performed, using skinned muscle fibers that were isolated and subsequently (i) activated at different Ca²⁺ concentrations (pCa²⁺ 4.5, 5.0, 5.5, 6.0) (n = 13), (ii) activated in the presence of blebbistatin (n = 16), and (iii) activated in the presence of blebbistatin at varying velocities (n = 5). In all experiments, a ramp shortening was imposed (amplitude 10%L₀, velocity 1 L₀•sarcomere length (SL)•s⁻¹), from an initial SL of 2.5 µm (except by the third group, in which velocities ranged from 0.125 to 2.0 L₀•s⁻¹). The values of P₁, P₂, L₁, and L₂ did not change with Ca²⁺ concentrations. Blebbistatin decreased P₁, and it did not alter P₂, L₁, and L₂. We developed a mathematical cross-bridge model comprising a load-dependent power-stroke transition and a pre-power-stroke cross-bridge state. The P₁ and P₂ critical points as well as the critical lengths L₁ and L₂ were explained qualitatively by the model, and the effects of blebbistatin inhibition on P₁ were also predicted. Furthermore, the results of the model suggest that the mechanism by which blebbistatin inhibits force is by interfering with the closing of the myosin upper binding cleft, biasing cross-bridges into a pre-power-stroke state.

The diversity of mammalian masticatory systems has attracted considerable attention for a long time. In the first half of this century, numerous descriptive studies appeared, including comparative ones, e.g. Fiedler (1953, Insectivora); Starck (1933, platyrrhine primates; 1935, ursids); Storch (1968, Chiroptera); later, significant studies were published by Gaspard et al. (1976), Schumacher (1961) and Turnbull (1970). Bluntschli (1929) and followers (Muller 1933; Zey 1939) published morphological papers, taking into account postnatal development.

The cross-striated muscle is the mechanical energy converter generally used in the animal in kingdom. (The Nobel Laureate Russian physicist, Dr. Kapicza, has estimated that more than half of the mechanical energy utilized on our globe originates from the animal muscle machine). Muscle research has played a prominent role in biological science, starting with Galvani’S famous experiments in 1974 (The first specific monograph about muscle was published in 1882 by A. Fick) [1]. Today the problem of muscle contraction stands in the frontline of physiology, biophysics, biochemistry and molecular biology; for example at the American Biophysical Society meeting in San Francisco, in February 1991, 15 of the 64 symposia dealt with muscle research. Great progress has been achieved over the last decades, thanks to molecular biological methods. Among the many unanswered problems that still exist in the field of muscle science, that of the energetics of muscle contraction is the most complex one.

The coupling mechanisms which coordinate the movement of ipsilateral walking legs in the crayfish have been described in earlier investigations. Concerning the coupling between contralateral legs it was only known that these influences are weaker than those acting between ipsilateral legs. The nature of these coupling mechanisms between contralateral legs of the crayfish are investigated here by running left and right legs on separate walking belts at different speeds. The results show that coordination is performed by a phase-dependent shift of the anterior extreme position of the influenced leg. This backward shift leads to a shortening of both the return stroke and the following power stroke. As the coupling influence is only weak, several steps might be necessary to retain normal coordination after a disturbance. This corresponds to v. Holst's relative coordination. The influences act in both directions, from left to right and vice versa. However, one side may be more or less dominant. A gradient was found in the way that anterior leg pairs show less strong coordination than posterior legs. In some cases the coupling between diagonally neighbouring legs was found to be stronger than between contralateral legs of the same segment. The interpretation of this result is still open.

Molecular motors transduce chemical energy obtained from hydrolizing ATP into mechanical work exerted against an external force. We calculate their efficiency at maximum power output for two simple generic models and show that the qualitative behaviour depends crucially on the position of the transition state. Specifically, we find a transition state near the initial state (sometimes characterized as a "power stroke") to be most favorable with respect to both high power output and high efficiency at maximum power. In this regime, driving the motor further out of equilibrium by applying higher chemical potential differences can even, counter-intuitively, increase the efficiency., published in EPL: http://www.iop.org/EJ/abstract/0295-5075/83/3/30005

[][1] Force analysis shows the formation of the cover neck bundle (yellow) is at the heart of kinesin's power stroke. HWANG/ELSEVIER Kinesin creates a new domain to generate its power, according to Wonmuk Hwang (Texas A&M University, College Station, TX), Matthew Lang (

A crayfish walked with the legs of each side on two separate motor-driven belts. Although the belts were driven with different speeds, the animal tried to synchronize the leg movements of both sides. The resulting effect can be described as relative coordination. The efficiency of coordination was strongly dependent on the ratio of frequencies between both sides, A model calculation is presented which is sufficient to describe the legs interactions.

Electrophysiological research in ciliates has established the central role of ionic Ca2+ in membrane excitation and ciliary electromotor coupling. Ca2+ passes depolarization-sensitive ciliary channels and is thought to bind to axonemal proteins. During hyperpolarization, the concentration of axonemally bound Ca2+ is presumably reduced. The ciliary motor response — frequency and beat direction — is a monotonous function of the intensitiy of a stimulus impinging on the cell. Intermediate steps in sensory-motor coupling: potentials of either polarity, concentration of the messenger substance Ca2+, and the binding of Ca2+ to axonemal target proteins reflect the transmission of gradedness to the ciliary motor response.

The exact mechanism of myofilamentary energy conversion and force production in skeletal muscle remains shrouded in mystery. Although a large amount of information is available on the structure and function of the tension-generating subunits of the actin and myosin filaments, the intricate processes of inter-molecular force production are still not understood. On the contrary, established theories and models of muscular contraction are now being seriously challenged on the grounds of new experimental evidence (Pollack, 1983).

Computer simulation of three-dimensional ciliary motion suggests that mechanisms controlling the activity of ciliary beating are localized at the basal region of cilia. An analysis of photographic data of the proximal shaft of a beating cilium, which had been recorded under membrane potential control, allows us to quantitatively describe motion at the level of axonemal functions. There are three parameters of ciliary beating: sliding velocity of outer doublets, transfer rate of sliding activity around the axoneme, and the steady inclination of the axis of beating.

In the experiments a crayfish walked with the legs of the left and right side on separate motor-driven belts. Even when the belts were driven with different speeds, the animal tried to synchronize the leg movements of both sides. The resulting effect can be described as relative coordination. A model calculation is presented which is sufficient to describe the interactions between the legs.

Since Reedy et al. (1965) suggested that crossbridges in relaxed insect flight muscle are at a 90° angle and those in rigor muscle are at a 45° angle, it has been widely supposed that the force of muscle contraction is caused by a transition of attached crossbridges from a 90° to a 45° state. Angled crossbridges in rigor, produced by removal of ATP from glycerinated fibres, have long served as a model for the end of the power stroke, but arrays of attached crossbridges at the expected intermediate angles have been more difficult to capture. Although an attached 90° crossbridge form is widely postulated, its existence remains to be shown. Efforts to produce stable equilibrium crossbridge states intermediate between rigor and ATP-relaxed have centred around the use of nucleotide analogues and variation of solvent conditions in order to drive the bridges backwards through the power stroke. When these static crossbridge arrays are trapped by fixation, their structure can be analysed by electron microscopy.

Two kinds of myosin motors depend on teamwork to keep their cellular cargos rolling, as Dunn et al. show. The finding explains how the molecules, which previous work suggested couldn't move forward, work as haulers. Figure 1 Myo4p concentrates labeled mRNA in a yeast bud (left), but the cargo remains dispersed if the cell lacks the myosin (right). When a yeast cell sprouts a bud, two myosin proteins help furnish the new structure with necessities. Myo2p trucks in organelles and vesicles essential for growth, whereas Myo4p hauls mRNA that helps the bud differentiate from the mother cell. Running on tracks of actin, the proteins seem to keep their cargos moving continuously. That presents a mystery, however, because evidence suggests that the individual myosins are nonprocessive—they let go of the tracks after every power stroke, instead of remaining attached and sliding along. To resolve that apparent contradiction, Dunn et al. isolated the two myosins and determined that they move differently. Molecules of Myo2p didn't always detach after the power stroke, the researchers found. They were “weakly processive.” Several Myo2p molecules latched onto each cargo. At any time, most of them might not be attached to the tracks, but one Myo2p probably will be and can nudge the cargo along. The method makes sense for what Myo2p transports. The vesicles and organelles it totes are large and have room for multiple myosins to hook on. An individual Myo4p was nonprocessive, but the molecules formed clusters that were processive. A single cluster was able to ferry one molecule of mRNA. Again, the strategy matches the cargo: mRNA has few attachment points for myosins. The researchers also wanted to determine what accounts for the differences between the two myosins. They created hybrid molecules that carried the tail end of one protein—which attaches to the cargo—fused to the motor portion of the other. A hybrid with the motor of Myo4p and the tail of Myo2p worked like Myo2p, and vice versa. The tail thus dictates myosin's behavior. The researchers now want to investigate how cells integrate pulling by separate motors. Reference: Dunn, B.D., et al. 2007. J. Cell Biol. 178:1193–1206. [PubMed]

A simple physical property based on entropy gives Hsp70 a pulling force, say Paolo De Los Rios (EPFL, Lausanne, Switzerland), Pierre Goloubinoff (University of Lausanne, Switzerland), and colleagues. The new theory resolves two deadlocked models. Figure 1 Adding Hsp70 reduces mobility and drives motion away from the membrane. Hsp70s have two major activities: they help unfold stable protein aggregates, and they pull proteins through membrane channels. In the Brownian ratchet model for protein import, Hsp70 grabs part of a protein once it is spontaneously unfolded by random thermal fluctuations, and passively prevents it from sliding backward. By contrast, the power stroke model has the Hsp70 using a channel protein as the fulcrum for a lever arm movement that yanks the protein inwards. The Lausanne group suggested that an emerging protein with a bulky Hsp70 attached would keep bumping against the nearby membrane. But if the whole complex moved further away from the membrane, this bumping no longer happened. The resulting increase in mobility and thus entropy meant a favorable change in free energy—a change that powers Hsp70's pulling force away from the membrane. Similar physics would draw Hsp70-bound protein segments away from the bulk of an aggregate and thus tease apart the tangles. “We made a chimeric theory,” says Goloubinoff. “Yes, there is active pulling, but there is no lever.” The Brownian ratchet remains partly intact also, as the unfavorable change of free energy forbids Hsp70 from being pushed back toward the membrane. “The physics behind it is very simple,” says statistical physicist De Los Rios. “It's been known that you can generate forces on polymers when you manipulate the number of conformations that the polymer can exist in.” With the Hsp70 case, he worked out how much work this change could do and found it was compatible with the forces required for protein unfolding and accelerated import. In theory, this leap in thinking could have been made at any time in the last 20 years. In fact, says De Los Rios, “the first thing I said is, this is a nice application, but I asked who must I cite. But I couldn't find anything.” Goloubinoff suggests a precedent. “When I was doing my Ph.D. we had all the tools of molecular biology,” he says. “But there was only one person who thought of PCR.” Reference: De Los Rios, P., et al. 2006. Proc. Natl. Acad. Sci. USA. doi:.10.1073/pnas.0510496103 [PubMed] [Cross Ref]