A urodynamic study shows that when the bladder is filled rapidly, intravesical pressure rises sharply at first but then falls back toward baseline despite continued filling to a larger volume. Which property of detrusor smooth muscle explains this adaptation?

• A Recruitment of additional motor units as volume increases
• B Reflex sympathetic inhibition of parasympathetic ganglia in the bladder wall
• C Activation of myosin light chain phosphatase triggered by wall stretch receptors
• D Stress-relaxation, in which tension declines over time at constant length as cross-bridges gradually slip ✓

Explanation

Stress-relaxation is a hallmark of single-unit smooth muscle: when stretched to a new length, initial tension is high, but over time some attached cross-bridges detach and reattach at lower force, so pressure falls even though volume stays high. This plasticity lets hollow viscera store large volumes with little pressure change. It is an intrinsic property of the muscle itself, not a neural reflex, which rules out options A and B.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.