What physiological change triggers the initiation of micturition (voiding)?

The initiation of micturition, or voiding, is fundamentally triggered by the stretching of the bladder wall. As the bladder fills with urine, its muscular walls begin to stretch. This stretching is detected by stretch receptors located within the bladder wall. Once the bladder reaches a certain level of fullness, these receptors send signals to the brain indicating the need to void. This process is vital for the coordination between the nervous system and urinary system to safely and effectively expel urine from the body.

While contraction of the bladder wall is essential for the actual process of urination, it occurs in response to the signals from the stretch receptors activated during bladder filling. Thus, the stretching of the bladder wall is the initial physiological change that signifies readiness for micturition. The relaxation of the external urethral sphincter and contraction of the internal urethral sphincter are subsequent steps in the process of voiding rather than triggers for its initiation.