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1 Institute of Fundamental Sciences, Physics, Massey University, Palmerston North, New Zealand
2 The McDonald Research Laboratories/The iCAPTURE Centre, St. Paul's Hospital/Providence Health Care, University of British Columbia, Vancouver, BC, Canada
* To whom correspondence should be addressed. E-mail: cseow{at}mrl.ubc.ca.
Despite considerable investigation, the mechanisms underlying the functional properties of smooth muscle are poorly understood. This can be attributed, at least in part, to a lack of knowledge about the structure and organization of the contractile apparatus inside the muscle cell. Recent observations of the plasticity of smooth muscle and of morphometry of the cell have provided enough information for us to propose a quantitative model, though highly simplified, for the geometric arrangement of contractile units and their collective kinematic functions in smooth muscle, particularly airway smooth muscle. We propose that, to a considerable extent, contractile machinery restructures on activation and adapts to cell geometry at the time of activation. We assume that, under steady state conditions, the geometric arrangement of contractile units and the filaments within determines the kinematic characteristics of the muscle. The model successfully predicts the results of experiments on airway smooth muscle plasticity relating to maximal force generation, maximal velocity of shortening and the variation of compliance with adapted length. The model is also concordant with morphometric observations that show an increase in myosin filament density when muscle is adapted to a longer length. The model provides a framework for design of experiments to quantitatively test various aspects of smooth muscle plasticity in terms of geometric arrangement of contractile units and the muscle's mechanical properties.
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