Do arm postures vary with the speed of reaching?
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Do arm postures vary with the speed of reaching? For reaching movements in one plane, the hand has been observed to follow a similar path regardless of speed. Recent work on the control of more complex reaching movements raises the question of whether a similar "speed invariance" also holds for the additional degrees of freedom. Therefore we examined human arm movements involving initial and final hand locations distributed throughout the three-dimensional (3D) workspace of the arm. Despite this added complexity, arm kinematics (summarized by the spatial orientation of the "plane of the arm" and the 3D curvature of the hand path) changed very little for movements performed over a wide range of speeds. If the total force (dynamic + quasistatic) had been optimized by the control system (e.g., as in a minimization of the change in joint torques or the change in muscular forces), the optimal solution would change with speed; slow movements would reflect the minimal antigravity torques, whereas fast movements would be more strongly influenced by dynamic factors. The speed-invariant postures observed in this study are instead consistent with a hypothesized optimization of only the dynamic forces. (+info)
Progressive bradykinesia and hypokinesia of ocular pursuit in Parkinson's disease.
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OBJECTIVES: Patients with Parkinson's disease characteristically have difficulty in sustaining repetitive motor actions. The purpose of this study was to establish if parkinsonian difficulty with sustaining repetitive limb movements also applies to smooth ocular pursuit and to identify any pursuit abnormalities characteristic of Parkinson's disease. METHODS: Ocular pursuit in seven patients with moderate to severe bradykinesia predominant Parkinson's disease was compared with seven age matched controls. Predictive and non-predictive pursuit of constant velocity target ramps were examined. Subjects pursued intermittently illuminated 40(0)/s ramps sweeping to the left or right with an exposure duration of 480 ms and average interval of 1.728 s between presentations. To examine for any temporal changes in peak eye velocity, eye displacement or anticipatory smooth pursuit the 124 s duration of each record was divided into four epochs (E1, E2, E3, E4), each lasting 31 s and containing 18 ramp stimuli. Three test conditions were examined in each subject: predictive (PRD1), non-predictive (NPD), and predictive (PRD2) in that order. RESULTS: Both patients and controls initiated appropriate anticipatory pursuit before target onset in the PRD1 and PRD2 conditions that enhanced the response compared with the NPD condition. The distinctive findings in patients with Parkinson's disease were a reduction in response magnitude compared with controls and a progressive decline of response with stimulus repetition. The deficits were explained on the basis of easy fatiguability in Parkinson's disease. CONCLUSIONS: Ocular pursuit shows distinct anticipatory movements in Parkinson's disease but peak velocity and displacement are reduced and progressively decline with repetition as found with limb movements. (+info)
A new pathway for the secretion of virulence factors by bacteria: the flagellar export apparatus functions as a protein-secretion system.
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Biogenesis of the flagellum, a motive organelle of many bacterial species, is best understood for members of the Enterobacteriaceae. The flagellum is a heterooligomeric structure that protrudes from the surface of the cell. Its assembly initially involves the synthesis of a dedicated protein export apparatus that subsequently transports other flagellar proteins by a type III mechanism from the cytoplasm to the outer surface of the cell, where oligomerization occurs. In this study, the flagellum export apparatus was shown to function also as a secretion system for the transport of several extracellular proteins in the pathogenic bacterium Yersinia enterocolitica. One of the proteins exported by the flagellar secretion system was the virulence-associated phospholipase, YplA. These results suggest type III protein secretion by the flagellar system may be a general mechanism for the transport of proteins that influence bacterial-host interactions. (+info)
Path integration absent in scent-tracking fimbria-fornix rats: evidence for hippocampal involvement in "sense of direction" and "sense of distance" using self-movement cues.
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Allothetic and idiothetic navigation strategies use very different cue constellations and computational processes. Allothetic navigation requires the use of the relationships between relatively stable external (visual, olfactory, auditory) cues, whereas idiothetic navigation requires the integration of cues generated by self-movement and/or efferent copy of movement commands. The flexibility with which animals can switch between these strategies and the neural structures that support these strategies are not well understood. By capitalizing on the proclivity of foraging rats to carry large food pellets back to a refuge for eating, the present study examined the contribution of the hippocampus to the use of allothetic versus idiothetic navigation strategies. Control rats and fimbria-fornix-ablated rats were trained to follow linear, polygonal, and octagonal scent trails that led to a piece of food. The ability of the rats to return to the refuge with the food via the shortest route using allothetic cues (visual cues and/or the odor trail available) or using ideothetic cues (the odor trail removed and the rats blindfolded or tested in infrared light) was examined. Control rats "closed the polygon" by returning directly home in all cue conditions. Fimbria-fornix rats successfully used allothetic cues (closed the polygon using visual cues or tracked back on the string) but were insensitive to the direction and distance of the refuge and were lost when restricted to idiothetic cues. The results support the hypothesis that the hippocampal formation is necessary for navigation requiring the integration of idiothetic cues. (+info)
Changes in posture alter the attentional demands of voluntary movement.
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Two simple experiments reveal that the ease with which an action is performed by the neuromuscular-skeletal system determines the attentional resources devoted to the movement. Participants were required to perform a primary task, consisting of rhythmic flexion and extension movements of the index finger, while being paced by an auditory metronome, in one of two modes of coordination: flex on the beat or extend on the beat. Using a classical dual-task methodology, we demonstrated that the time taken to react to an unpredictable visual probe stimulus (the secondary task) by means of a pedal response was greater when the extension phase of the finger movement sequence was made on the beat of the metronome than when the flexion phase was coordinated with the beat. In a second experiment, the posture of the wrist was manipulated in order to alter the operating lengths of muscles that flex and extend the index finger. The attentional demands of maintaining the extend-on-the-beat pattern of coordination were altered in a systematic fashion by changes in wrist posture, even though the effector used to respond to the visual probe stimulus was unaffected. (+info)
Ocular responses to radial optic flow and single accelerated targets in humans.
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Self-movement in a structured environment induces retinal image motion called optic flow. Optic flow on one hand provides information about the direction of self-motion. On the other hand optic flow presents large field visual motion which will elicit eye movements for the purpose of image stabilization. We investigated oculomotor behavior in humans during the presentation of radial optic flow fields which simulated forward or backward self-motion. Different conditions and oculomotor tasks were compared. In one condition, subjects had to actively pursue single dots in a radial flow pattern. In a second condition, subjects had to pursue single dots over a dark background. These dots accelerated or decelerated similar to single dots in radial optic flow. In a third condition, subjects were asked to passively view the entire optic flow stimulus. Smooth pursuit eye movements with high gain were observed when dots were actively pursued. This was true for single dots moving over a homogeneous background and for single dots in the optic flow. Passive viewing of optic flow stimuli evoked eye movements that resembled an optokinetic nystagmus. Slow phase eye movements tracked the motion of elements in the optic flow. Gain was low for simulated forward self-motion (expanding optic flow) and high for simulated backward movement self-motion (contracting optic flow). Thus, voluntary pursuit and passive optokinetic responses yielded different gain for the tracking of elements of an expanding optic flow pattern. During passive viewing of the optic flow stimulus, gaze was usually at or near the focus of radial flow. Our results give insights into the oculomotor performances and needs for image stabilization during self-motion and in the role of gaze strategy for the detection of the direction of heading. (+info)
Cell patterning in migrating slugs of Dictyostelium discoideum.
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When front quarters of migrating slugs of Dictyostelium discoideum are isolated by surgery and induced to friut immediately they produce fruiting bodies with disproportionately large stalks (Raper, 1940). The data in this communication show that the 'stalky' character of fruits derived from front quarters persists even if the cells of the front quarter are disaggregated and hence to reaggregate before fruiting. The data also demonstrate that friuts derived from rear quarters of slugs have disproportionately large spore heads, and that this effect becomes more pronounced with increasing age of the slugs. These observations support the view that the cells of the front and rear of migrating slugs are to some extent committed to different fates. (+info)
Function of proline residues of MotA in torque generation by the flagellar motor of Escherichia coli.
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Bacterial flagellar motors obtain energy for rotation from the membrane gradient of protons or, in some species, sodium ions. The molecular mechanism of flagellar rotation is not understood. MotA and MotB are integral membrane proteins that function in proton conduction and are believed to form the stator of the motor. Previous mutational studies identified two conserved proline residues in MotA (Pro 173 and Pro 222 in the protein from Escherichia coli) and a conserved aspartic acid residue in MotB (Asp 32) that are important for function. Asp 32 of MotB probably forms part of the proton path through the motor. To learn more about the roles of the conserved proline residues of MotA, we examined motor function in Pro 173 and Pro 222 mutants, making measurements of torque at high load, speed at low and intermediate loads, and solvent-isotope effects (D2O versus H2O). Proton conduction by wild-type and mutant MotA-MotB channels was also assayed, by a growth defect that occurs upon overexpression. Several different mutations of Pro 173 reduced the torque of the motor under high load, and a few prevented motor rotation but still allowed proton flow through the MotA-MotB channels. These and other properties of the mutants suggest that Pro 173 has a pivotal role in coupling proton flow to motor rotation and is positioned in the channel near Asp 32 of MotB. Replacements of Pro 222 abolished function in all assays and were strongly dominant. Certain Pro 222 mutant proteins prevented swimming almost completely when expressed at moderate levels in wild-type cells. This dominance might be caused by rotor-stator jamming, because it was weaker when FliG carried a mutation believed to increase rotor-stator clearance. We propose a mechanism for torque generation, in which specific functions are suggested for the proline residues of MotA and Asp32 of MotB. (+info)