Trajectory control in targeted force impulses

Summary This study was undertaken in order to determine the time course of the process by which information derived from a visual target is used to accurately set the amplitude of a simple motor response. We refer to this process as response specification. Separate auditory and visual cues were given to the subjects in order to independently control the moment of response initiation and the time available for processing amplitude information from the target. Six subjects initiated impulses of isometric force in synchrony with the last of predictable series of regular tones. Response amplitudes were to match one of three visual target steps occurring at random times between 0 and 400 ms before the response-synchronizing tone. Using these separate auditory and visual cues, we were able to systematically vary the time interval between target presentation and response onset, termed here Stimulus-Response or S-R interval. Target steps were presented in blocks of either predictable (simple condition) or unpredictable (choice condition) amplitudes. The peak forces and the peaks of their time derivatives were analyzed to determine how subjects achieved accuracy under the different conditions and at different S-R intervals. The trajectories of responses produced in the simple condition were independent of the S-R interval. In contrast, when targets were presented in unpredictable order, the distribution of the peak forces of the subjects' responses depended on the S-R interval. At short S-R intervals (<125 ms), subjects made responses whose peak forces were distributed around the center of the range of target steps. These responses formed a unimodal, but broad distribution which was independent of actual target amplitude. With increasing S-R interval (>125 ms), the distributions of peak forces gradually shifted toward the correct target amplitudes, with the means reaching the appropriate amplitudes at S-R intervals of 250–400 ms. At S-R intervals comparable to a reaction time, the range of peak forces was constricted to a similar extent as previously observed in a reaction time task (Hening et al. 1988). We found that the gradual improvement of accuracy was not achieved through changes in trajectory control: at all S-R intervals, subjects utilized a pulse-height control policy (Gordon and Ghez 1987a). Different peak forces were achieved by varying the rate of rise of force, while force rise time was held relatively invariant. We did find, however, that within the constraints imposed by rise time regulation, compensatory adjustments to the force trajectories (Gordon and Ghez 1987b) were greatest during the period of specification. We conclude that (1) subjects can initiate their responses independent of the degree of specification achieved and that the normal process of specification of amplitude begins earlier and continues longer than the latency of responses in a reaction time task; (2) before target presentation, subjects prepare a default response whose amplitude is biased by prior experience with the targets presented in the task. We hypothesize that the central mechanisms that specify response amplitude do so by a progressive adjustment of the default parameters.     

非特异性相互作用的断裂力与寿命测量

目的　探讨量化非特异性相互作用在特异性选择素－配体分子相互作用中的贡献。方法　利用光镊技术，对牛血清白蛋白(Bovine serum albumin, BSA)封闭的玻璃小球间的非特异性作用进行了系统测量，得出不同加载率下的断裂力以及不同外力作用下的寿命分布。结果　实验结果表明，非特异性作用同样表现出断裂力随加载率增加而增大的趋势。在较低加载率下，非特异性断裂力与选择素－配体特异性断裂力大小、增大趋势基本一致；随着加载率增加，二者的差别逐渐显著，前者的断裂力增加速率远低于后者。同样外力作用下，非特异性作用的寿命平均值比特异性作用要小；不同外力作用下，非特异性作用的寿命随外力增大仅略有下降，与特异性作用中逆锁键－滑移键转化现象有明显不同。结论　该研究结果将为正确评估非特异性相互作用对选择素－配体特异性相互作用实验结果的影响提供基础。     

A probabilistic approach to measure the strength of bone cell adhesion to chemically modified surfaces

Patterned surfaces with alternating regions of amino silanes [N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (EDS)] and alkyl silanes [dimethyldichlorosilane (DMS)] have been used to alter the kinetics of spatial distribution of cellsin vitro. In particular, we have previously observed the preferential spatial distribution of bone cells on the EDS regions of EDS/DMS patterned surfaces (10). In this study, we examined whether the mechanism of spatial distribution of cells on the EDS regions was adhesion mediated. Homogeneous layers of EDS and DMS were immobilized on quartz substrates and characterized by contact angle, X-ray photoelectron spectroscopy, and spectroscopic ellipsometry. The strength of bone cell attachment to the modified substrates was examined using a radial flow apparatus, within either 20 min or 2 hr of cell incubation in the presence of serum. A Weibull distribution was chosen to characterize the strength of cell-substratum adhesion. Within 20 min of cell exposure, the strength of adhesion was significantly larger on EDS and clean surfaces, compared with DMS surfaces (p<0.0001). Within 2 hr of cell incubation, there was no statistical difference between the strength of cell adhesion to EDS, DMS, and clean surfaces. The results of this study suggest that the surface chemistry mediates adhesion-based spatial cell arrangement through a layer of adsorbed serum proteins.     

Intracellular calcium changes in rat aortic smooth muscle cells in response to fluid flow

Vascular smooth muscle cells (VSM) are normally exposed to transmural fluid flow shear stresses, and after vascular injury, blood flow shear stresses are imposed upon them. Since Ca²⁺ is a ubiquitous intracellular signaling molecule, we examined the effects of fluid flow on intracellular Ca²⁺ concentration in rat aortic smooth muscle cells to assess VSM responsiveness to shear stress. Cells loaded with fura 2 were exposed to steady flow shear stress levels of 0.5–10.0 dyn/cm² in a parallel-plate flow chamber. The percentage of cells displaying a rise in cytosolic Ca²⁺ ion concentration ([Ca²⁺]_i) increased in response to increasing flow, but there was no effect of flow on the ([Ca²⁺]_i) amplitude of responding cells. Addition of Gd³⁺ (10 M) or thapsigargin (50 nM) significantly reduced the percentage of cells responding and the response amplitude, suggesting that influx of Ca²⁺ through ion channels and release from intracellular stores contribute to the rise in ([Ca²⁺]_i) in response to flow. The addition of nifedipine (1 or 10 M) or ryanodine (10 M) also significantly reduced the response amplitude, further defining the role of ion channels and intracellular stores in the Ca²⁺ response. © 2002 Biomedical Engineering Society. PAC2002: 8716Uv, 8719Uv, 8716Dg, 8719Ff     

Skeletal muscle contractility is preserved in COPD patients with normal fat‐free mass

Aim: Peripheral muscle dysfunction often occurs in patients with chronic obstructive pulmonary disease (COPD). The muscle dysfunction may be caused by a loss of force‐generating capacity, resulting from a loss of muscle mass, as well as by other alterations in contractile properties of skeletal muscle. Methods: The maximal isometric voluntary strength and fatigability were determined in hand‐grip and quadriceps muscles from nine male COPD patients (FEV₁ 30–50% predicted) and control subjects matched for fat‐free mass (FFM), physical activity level and age. Contractile properties and fatigability of the quadriceps muscle were also studied with electrically evoked isometric contractions. Results: The maximal voluntary force (MVC) and fatigability of the handgrip muscle did not differ between the COPD patients and control subjects. Also the MVC of the quadriceps muscle and the rate of force rise, contraction time, force–frequency relationship and fatigability, as determined with electrically evoked contractions, were similar in patients with COPD and control subjects. Conclusion: Skeletal muscle strength, contractile properties and fatigability are preserved in patients with moderate COPD and a normal FFM and activity level. This suggests that skeletal muscle dysfunction does not take place during moderate COPD until cachexia and/or a decline in physical activity occur.     

颈椎内植入固定器生物力学性能测试系统的研究

在颈椎生物力学研究中,通过离体实验对颈椎内植入固定器的生物力学性能进行评价是国际上普遍采用并认可的方法.本研究利用多靶点三维运动跟踪系统和USB数据采集卡,以LabVIEW和Matlab为软件开发平台,构建了颈椎内植入固定器生物力学性能测试系统.测试参数包括三维运动角度范围(ROM)和压力载荷值.对颈椎模型的测试结果表明,本系统能有效地用于颈椎生物力学的离体实验测试.     

低强度脉冲电磁场对大鼠骨质疏松的影响

目的：观察低强度脉冲电磁场（pulsed electromagnetic fields，PEMFs）对去卵巢诱导骨质疏松症的大鼠生化指标和骨应力的影响。方法：雌性SD大鼠30只，随机等分为3组（n=10），分别为假手术组（Sham）、骨质疏松模型组（Model）、脉冲电磁场照射组（PEMFs）。经适应4wk后，在25mg·Kg^-1戊巴比妥钠腹腔麻醉下，Model组和PEMFs组摘除双侧卵巢．Sham组找到但不切除卵巢。各实验组均在相同环境下饲养，模型制备4wk后开始治疗，由GZY型低强度低频率脉冲电磁场发生仪产生低频脉冲磁场，根据实验要求，我们使用亥姆霍兹线圈形成均匀磁场，PEMFs组经照射刺激治疗，频率14．3Hz，场强2Gs．日照8h。Model组和Sham组正常饲养。治疗8wk后，对各组大鼠血清、尿液中ALP和Ca以及骨应力进行检测。结果：（1）ALP、Ca检测结果：与Model组相比，PEMFs组ALP值、Ca值差异均有统计学意义（P〈0．05）。其中，血清中ALP值，Model组为（275．16±228，57），PEMFs组为（179．30±87．68）；Ca值，Model组为（2，66±0，13），PEMFs组为（2．52±0．05）。（2）骨应力检测结果：Model组为（923．60±34．15Pa），PEMFs组为（1152．85±118，20Pa），组间差异有统计学意义（P〈0．05）。结论：研究发现，PEMFs对于促进骨重建、提高Ca吸收和骨应力恢复具有积极作用。     

Blood Flow and Leukocyte Adhesiveness Are Reduced in the Microcirculation of a Peritoneal Disseminated Colon Carcinoma

Dynamic behavior of leukocytes in the microcirculation of solid tumor tissue was visualized using a fluorescent labeling technique combined with the use of a real-time confocal laser-scanning microscope (CLSM) system. Colon tumor cells (RCN-9) were inoculated into the peritoneal cavity of male Fischer 344 rats. Tumor-free rats were similarly injected with physiological saline (intraperitoneally). Ten days after tumor inoculation, the mesentery was exteriorized and subjected to vital microscopic observation under the CLSM system. Leukocytes were labeled with rhodamine 6G (100 g kg^–1, intravenously), and their behavior within the microvessels (10–30 m in diameter) was analyzed both in the solid tumor tissues and the normal mesentery. Wall shear rate was calculated from the measured values of vessel diameter and erythrocyte flow velocity. In tumor microvasculature of tumor-bearing rats, the centerline erythrocyte velocity (0.73 ± 0.58 mm s^–1, mean±standard deviation) and wall shear rate (210 ± 151 s^–1 were significantly lower than those of the tumor-free rats (1.27 ± 0.83 mm s^–, 344 ± 236 s^–1, respectively). Despite such reduced flow conditions, flux of the rolling leukocytes as well as density of the adhered leukocytes both decreased significantly in tumor microvasculature as compared with normal controls. The methods developed in this work show promise in improving our understanding of tumor biology and pathophysiology. © 1998 Biomedical Engineering Society. PAC98: 8722Fy, 8745Hw, 8745Ft, 8764-t, 4262Be     

The relationship between ATP hydrolysis and active force in compressed and swollen skinned muscle fibers of the rabbit

We investigated whether the inhibition of force generation observed in compressed muscle fibers is accompanied by a coupled reduction in hydrolytic activity. Isometric force and rates of ATP hydrolysis (ATPase) were measured as functions of the relative width of chemically skinned skeletal muscle fiber segments immersed in relaxing (pCa>8) and activating (pCa 4.9) salt solutions. Osmotic radial compression of the fiber segment was produced (with little or no affect on striation spacing) by adding Dextran T500 to the bathing media. ADP as a product of ATP hydrolysis in fibers undergoing 10–15 min contractions was measured using high pressure liquid chromatography. Compression of the (initially swollen) fiber segment with dextran produced a slight (4%) increase in average active force and then, with further compression, a sharp decrease (with maximum around in situ width). With compression, the average ATPase of the fiber decreased monotonically, and with extreme compression (with 0.22 g dextran per ml), ATPase fell to a fifth of its level determined in dextran-free solution while force was abolished. The time course of active force development was described by the sum of two exponential functions, the faster of which characterized the rate of rise. Fiber compression (0.14 g dextran per ml) reduced the rate of rise of force ten-fold compared to that in dextran-free solution. Hindrance of cross movement is proposed to account for the inhibition of active force generation and (coupled) ATPase in compressed fibers.     

Effects of fluid flow on elution of hydrophilic modifier from dialysis membrane surfaces

When uremic blood flows through dialyzers during hemodialysis, dialysis membrane surfaces are exposed to shear stress and internal filtration, which may affect the surface characteristics of the dialysis membranes. In the present study, we evaluated changes in the characteristics of membrane surfaces caused by shear stress and internal filtration using blood substitutes: water purified by reverse osmosis and 6.7 wt% dextran70 solution. We focused on the levels of a hydrophilic modifier, polyvinylpyrrolidone (PVP), on the membrane surface measured by attenuated total reflectance Fourier transform infrared spectroscopy. Experiments involving 4 h dialysis, 0-144 h shear-stress loading, and 4 h dead-end filtration were performed using polyester-polymer alloy (PEPA) and polysulfone (PS) membranes. After the dialysis experiments with accompanying internal filtration, average PVP retention on the PEPA membrane surface was 93.7% in all areas, whereas that on the PS membrane surface was 98.9% in all areas. After the shear-stress loading experiments, PVP retention on the PEPA membrane surface decreased as shear-stress loading time and the magnitude of shear stress increased. However, with the PS membrane, PVP retention scarcely changed. After the dead-end filtration experiments, PVP retention decreased in all areas for both PEPA and PS membranes, but PVP retention on the PEPA membrane surface was lower than that on the PS membrane surface. PVP on the PEPA membrane surface was eluted by both shear stress and internal filtration, while that on the PS membrane surface was eluted only by internal filtration.