Learned changes in the complexity of movement organization during multijoint, standing pulls

 This paper tests the hypothesis that the central nervous system (CNS) learns to organize multijoint movements during a multijoint ‘bouncing pull’ task such that, after practice, motion of the anterior-posterior center of mass (CM_AP) more closely resembles that of a conservative, one degree of freedom (DF), inverted pendulum model. The task requires standing human subjects to produce precise peak pulling forces on a handle while maintaining balance – goals that can be easily accomplished if movement is organized as in the model. Ten freely standing subjects practiced making brief, bouncing pulls in the horizontal direction to target forces (20–80% of maximum) for 5 days. Pulling force, body kinematic and force plate data were recorded. An eight-segment analysis determined sagittal-plane CM motion. We compared the effects of practice on the regression-based fit between actual and model-simulated CM_AP trajectories, and on measures of CM_AP phase plane symmetry and parameter constancy that the model predicts. If the CNS learns to organize movements like the inverted pendulum model, then model fit should improve and all other measures should approach zero after practice. The fit between modeled and actual CM_AP motion did not improve significantly with practice, except for moderate force pulls. Nor did practice increase phase plane symmetry or parameter constancy. Specifically, practice did not decrease the differences between the pre-impact and rebound positions or speeds of the CM_AP, although speed difference increased with pulling force. CM_AP at the end of the movement was anterior to its initial position; the anterior shift increased after practice. Differences between the pre-pull and balance-recovery ankle torque (T _A) impulses were greater on day 5 and correlated with the anterior shift in CM_AP. These results suggest that practice separately influenced the force production and balance recovery phases. A modified model with damping could not explain the observed behaviors. A modified model using the actual time-varying T_A profiles improved fit at lower force levels, but did not explain the increased postural shift after practice. We conclude that the CNS does not learn to organize movements like the conservative, inverted pendulum model, but rather learned a more complex form of organization that capitalized on more time-varying controls and more intersegmental dynamics. We hypothesize that at least one additional DF and at least one time-varying parameter will be needed to explain fully how the CNS learns to organize multijoint, bouncing pulls made while standing. Received: 9 January 1997 / Accepted in revised form: 27 May 1997     

ENVIRONMENTAL RISK ASSESSMENT: TASKS AND OBLIGATIONS

Synthesizing, characterizing, and communicating the risk science information used in environmental decision-making depends in the first instance on the nature and quality of the technical analysis. At the same time, other important features of the risk analysis, features that require special attention to provide context for the analysis as a whole, are frequently overlooked in practice or in presentation. Now, as the field expands to meet new challenges and to include new participants, all practitioners — government, academics, industry, and interest groups — must give renewed emphasis to certain hall marks of sound risk assessment: identifying incomplete information and its influence on the risk assessment process, articulating alternative assumptions and the scientific or policy reasons for choices made among alternatives, describing process considerations and limitations as well as numerical results, and fully informing decision makers, the press, and the interested public. The resulting greater clarity and transparency in the scientific analyses that under lie environmental decision making can enhance credibility and public confidence in the scientific foundation for those decisions.     

Solvent focusing for rapid and sensitive quantification of total lipids on Chromarods

A method for the measurement of total lipid weight in biological and geological lipid samples using the latroscan TH-10 analyzer is described. The method involves the application of small (5 μl) volumes to Chromarods, the focusing of the sample at one point by partial development in chloroform-methanol (1:1) or methanol, and the quantification by flame ionization detection. The small response variation between different sample types did not affect the linearity of the response. The method exhibited a reproducibility of ± 10% of the mean or better for samples ranging from 0.5 to 32 μg. The method, at least as sensitive and precise as microgravimetric procedures for total lipid determinations, allows total lipid measurement of 10 samples in 30 min.     

Laminin-11.

Laminins are a family of glycoproteins which are ubiquitous components of basement membranes and play key structural and functional roles. Eleven isoforms have been identified to date; each is an alpha beta gamma heterotrimer assembled from a repertoire of five alpha, three beta and two gamma chains. Studies of laminin-11 (alpha 5 beta 2 gamma 1) illustrate the unique expression patterns and distinct functions that can be displayed by laminin isoforms. Laminin-11 is found in the glomerular basement membrane in kidney, in the neuromuscular synaptic cleft in skeletal muscle and in other tissues such as placenta and lung. Mice lacking laminin-11 exhibit defective glomerular filtration and developmental defects in neuromuscular synapse formation, with Schwann cells invading the synaptic cleft. Consistent with these observations, both motoneurons and Schwann cells distinguish laminin-11 from other isoforms in vitro. These results suggest that laminin-11 is a structural component of the basement membrane which influences cell behavior in physiologically relevant ways. A greater understanding of laminin-11 assembly and basement membrane incorporation could provide a logical basis for therapy in merosin-deficient congenital muscular dystrophy.