During cystometry, a patient's bladder is filled slowly from empty to 400 mL. Despite the fourfold volume increase, intravesical pressure rises only slightly. The property of detrusor smooth muscle that accounts for this is:

• A The latch state of phosphorylated myosin
• B Stress relaxation, in which sustained stretch triggers progressive decrease in wall tension ✓
• C Recruitment of additional motor units as volume increases
• D Length-dependent activation of the thin filaments

Explanation

Visceral smooth muscle responds to maintained stretch by reducing its tone over time, a property called stress relaxation or plasticity. As the detrusor is stretched, initial tension rises, then cross-bridge rearrangement and decreased active force let the wall accommodate the larger volume with little pressure change. This permits the bladder to store increasing urine volumes at nearly constant pressure until micturition. The latch state conserves energy during sustained tonic contraction but does not explain the drop in tension with stretch, and smooth muscle lacks motor units.

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

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Written and medically reviewed by the StethoPrep medical team.