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Force production by depolymerizing microtubules: load-velocity curves and run-pause statistics. 总被引:1,自引:0,他引:1
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Experiments indicate that depolymerization of microtubules generates sufficient force to produce the minus-end-directed transport of chromosomes during mitosis (Koshland et al., 1988). In vitro, analogous transport of kinesin-coated microspheres exhibits a paradoxical effect. Minus-end-directed transport of the microspheres driven by depolymerization is enhanced by the presence of ATP, which fuels the motor action of kinesin driving the microspheres in the opposite direction, toward the plus end of the microtubule. Here we present a mathematical model to explain this behavior. We postulate that a microsphere at the plus end of the microtubule facilitates depolymerization and hence enhances minus-end-directed transport. The force-velocity curve of the model is derived; it has the peculiar feature that velocity is maximal at some positive load (opposing the motion) rather than at zero load. The model is used to simulate the stochastic process of microsphere-facilitated depolymerization-driven transport. Simulated trajectories at low load show distinctive runs and pauses, the statistics of which are calculated from the model. The statistics of the process provide sufficient information to determine all of the model's parameters. 相似文献
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Force platforms as ergometers. 总被引:15,自引:0,他引:15
G A Cavagna 《Journal of applied physiology》1975,39(1):174-179
Walking and running on the level involves external mechanical work, even when speed averaged over a complete stride remains constant. This work must be performed by the muscles to accelerate and/or raise the center of mass of the body during parts of the stride, replacing energy which is lost as the body slows and/or falls during other parts of the stride. External work can be measured with fair approximation by means of a force plate, which records the horizontal and vertical components of the resultant force applied by the body to the ground over a complete stride. The horizontal force and the vertical force minus the body weight are integrated electronically to determine the instantaneous velocity in each plane. These velocities are squared and multiplied by one-half the mass to yield the instantaneous kinetic energy. The change in potential energy is calculated by integrating vertical velocity as a function of time to yield vertical displacement and multiplying this by body weight. The total mechanical energy as a function of time is obtained by adding the instantaneous kinetic and potential energies. The positive external mechanical work is obtained by adding the increments in total mechanical energy over an integral number of strides. 相似文献
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The coordination of digits during combined force/torque production tasks was further studied using the data presented in
the companion paper [Zatsiorsky et al. Biol Cybern this issue, Part I]. Optimization was performed using as criteria the cubic norms of (a) finger forces, (b) finger forces normalized with respect to the maximal
forces measured in single-finger tasks, (c) finger forces normalized with respect to the maximal forces measured in a four-finger
task, and (d) finger forces normalized with respect to the maximal moments that can be generated by the fingers. All four
criteria failed to predict antagonist finger moments when these moments were not imposed by the task mechanics. Reconstruction of neural commands: The vector of neural commands c was reconstructed from the equation c=W
−1
F, where W is the finger interconnection weight matrix and F is the vector of finger forces. The neural commands ranged from zero (no voluntary force production) to one (maximal voluntary
contraction). For fingers producing moments counteracting the external torque (`agonist' fingers), the intensity of the neural
commands was well correlated with the relative finger forces normalized to the maximal forces in a four-finger task. When
fingers produced moments in the direction of the external torque (`antagonist' fingers), the relative finger forces were always
larger than those expected from the intensity of the corresponding neural commands. The individual finger forces were decomposed
into forces due to `direct' commands and forces induced by enslaving effects. Optimization of the neural commands resulted
in the best correspondence between actual and predicted finger forces. The antagonist moments are, at least in part, due to
enslaving effects: strong commands to agonist fingers also activated antagonist fingers.
Received: 8 August 2001 / Accepted in revised form: 7 February 2002 相似文献
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We studied the coordinated action of fingers during static tasks involving exertion of force and torque on a handheld object.
Subjects were asked to keep a handle with an attachment that allowed for independent change of the suspended load (0.5–2.0 kg)
and external torque (0.375–1.5 N m) in a vertical position while applying minimal effort. Normal and shear forces were measured
from the thumb; normal forces only were measured from the four fingers. Experimental results: (1) the thumb shear force increased during supination efforts and decreased during pronation efforts; (2) the total moment
of the normal finger forces only counterbalanced approximately 50% of the external torque, hence shear forces accounted for
approximately one-half of the total torque exerted on the object; (3) the total normal force increased with external torque,
and the total force magnitude did not depend on the torque direction; (4) the forces of the `peripheral' (index and little)
fingers depended mainly on the torque while the forces exerted by the `central' (middle and ring) fingers depended both on
the load and torque; (5) there was a monotonic relationship between the mechanical advantage of a finger (i.e., its moment
arm during torque production) and the force produced by that finger; and (6) antagonist finger moments acting opposite to
the intended direction of the total moment were always observed – at low torques the antagonist moments were as high as 40–60%
of the agonist moments. Modeling: A three-zone model of coordinated finger action is suggested. In the first zone of load/torque combinations, activation of antagonist fingers
(i.e., fingers that generate antagonist moments) is necessary to prevent slipping. In the second zone, the activity of agonist
fingers is sufficient for preventing slips. In the third zone, the performer has freedom to choose between either activating
the antagonist fingers or redistributing activities amongst the agonist fingers. The findings of this study provide the foundation
for neural network and optimization modeling described in the companion paper [Zatsiorsky et al. (2002) Biol Cybern DOI 10.1007/s00422-002-0320-7].
Received: 8 August 2001 / Accepted in revised form: 7 February 2002 相似文献
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Hans-Günther D?bereiner Benjamin J Dubin-Thaler Gregory Giannone Michael P Sheetz 《Journal of applied physiology》2005,98(4):1542-1546
Cellular morphology is determined by motility, force sensing, and force generation that must be finely controlled in a dynamic fashion. Contractile and extensile functions are integrated with the overall cytoskeleton, including linkages from the cytoplasmic cytoskeleton to the extracellular matrix and other cells by force sensing. During development, as cells differentiate, variations in protein expression levels result in morphological changes. There are two major explanations for motile behavior: either cellular motility depends in a continuous fashion on cell composition or it exhibits phases wherein only a few protein modules are activated locally for a given time. Indeed, in support of the latter model, the quantification of cell spreading and other motile activities shows multiple distinct modes of behavior, which we term "phases" because there exist abrupt transitions between them. Cells in suspension have a basal level of motility that enables them to probe their immediate environment. After contacting a matrix-coated surface, they rapidly transition to an activated spreading phase. After the development of a significant contact area, the cells contract repeatedly to determine the rigidity of the substrate and then develop force on matrix contacts. When cells are fully spread, extension activity is significantly decreased and focal complexes start to assemble near the cell periphery. For each of these phases, there are significant differences in protein activities, which correspond to differences in function. Thus overall morphological change of a tissue is driven by chemical signals and force-dependent activation of one or more motile phases in limited cell regions for defined periods. 相似文献
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Robert?B. Best 《Biophysical journal》2013,105(12):2611-2612
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RNA polymerase (RNAP) is a processive molecular motor capable of generating forces of 25-30 pN, far in excess of any other known ATPase. This force derives from the hydrolysis free energy of nucleotides as they are incorporated into the growing RNA chain. The velocity of procession is limited by the rate of pyrophosphate release. Here we demonstrate how nucleotide triphosphate binding free energy can rectify the diffusion of RNAP, and show that this is sufficient to account for the quantitative features of the measured load-velocity curve. Predictions are made for the effect of changing pyrophosphate and nucleotide concentrations and for the statistical behavior of the system. 相似文献
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Recently, single-molecule force spectroscopy techniques have provided unprecedented opportunities to apply and to quantify forces that guide protein (un-)folding. A new study provides fascinating insights into the sophisticated mechanism by which an ATP-fueled proteolytic machine generates mechanical forces to unfold and translocate multidomain substrates. 相似文献
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