List of symbols a = constant factor representing geometry of muscle A = coefficient of exponential crossbridge elasticity /~ = exponent of exponential crossbridge elasticity fl/lo = exponent in elastic characteristic of discrete elastic element AFt_ = corrected force decrease during stimulation and quick release (in the rising phase of the contraction) AF,+ = corrected force increase during stimulation and quick stretch (in the rising phase of the contraction) AFo_ = corrected force decrease after stimulation and during quick release (in the falling phase of the contraction) AF o + = corrected force increase after stimulation and during quick stretch (in the falling phase of the contraction) AI = extent of quick release or stretch Ax = elongation of series elastic element Eo/l o = coefficient in elastic characteristic of discrete elastic element F = force Far = force measured during a quick release or quick stretch F~o = maximum active isometric force during contraction Fzri 0 = preset force-trigger level g(X) = force as a function of length of a crossbridge h = upper limit of crossbridge lengths 1 = length of muscle strip no(t) = numbe/" of active crossbridges during an isometric contraction n(X) = spectrum of lengths of crossbridges V t = tissue volume of strip X = length of crossbridge x = macroscopic equivalent of X obtained by addition of lengths of crossbridges which are in series a~i = force measured during a quick release or quick stretch, normalised by dividing by the cross-sectional area of the strip

The series elasticity of strips of smooth muscle from pig urinary bladder was investigated by means of a series of computerised quick-release and quick-stretch measurements with and without stimulation of the muscle, and at different shortenings and force levels. The results cannot be interpreted in terms era discrete passive series-elastic element. They can, however, be interpreted in terms of the sliding-filaments model for contracting muscle.

Pig ureters were made to propagate injected fluid boluses by electrical stimulation in vitro. The propagation velocity was determined from EMG measurements made at several points along the ureter. It was found that this velocity varied both along the ureter and as a function of time, and that it was related to the contraction pressure but not to the bolus size.

In order to test whether there is a significant variation of passive mechanical properties along the length of the ureter, pig ureters were cut into eight equal segments. These were simultaneously strained stepwise and the measured stress responses were described by a viscoelastic model. It was found that, by making more than one measurement on each segment, a significant difference between the segments could nearly always be shown.

In beagles, a bladder catheter was surgically implanted and subvesical obstruction was artificially introduced. After 3 months a striking reduction in detrusor contractility measured in vitro was observed, which was associated with a presumptive reduction in the concentration of acetylcholinesterase in the muscle cells and a probable reduction in the mean cell size. In this model the contribution of the catheter to the changes is greater than that of the subvesical obstruction.