The mechanism for micturition is initiated by which event?

• A. Stretch reflex of the bladder wall
• B. Voluntary contraction of the external urethral sphincter
• C. Contraction of the internal urethral sphincter
• D. Hormonal signals from the kidneys

Answer: (A)

Stretch of the bladder wall triggers the micturition reflex. When the bladder fills, stretch receptors in the detrusor send afferent signals to the spinal cord, activating parasympathetic pathways that cause the detrusor to contract and the internal urethral sphincter to relax. This automatic reflex initiates voiding. Voluntary contraction of the external urethral sphincter can delay urination but does not start it, and hormonal signals from the kidneys do not initiate micturition.