How the Bladder Expands: Detrusor Muscle Relaxation and the Filling Cycle

The bladder accommodates increasing urine volume through detrusor muscle relaxation, mediated by the parasympathetic nervous system. As the bladder fills, the detrusor stretches, activating stretch receptors that signal the spinal cord and brain. This triggers the micturition reflex: detrusor contraction and urethral sphincter relaxation, allowing urination. The cycle of relaxation (storage) and contraction (voiding) maintains continence and efficient emptying.