What is the correct sequence of events for normal voiding?

What is the correct sequence of events for normal voiding?

• A. Detrusor contraction, internal sphincter relaxation, external sphincter relaxation
• B. Detrusor relaxation, internal sphincter contraction, external sphincter contraction
• C. Detrusor contraction, internal sphincter contraction, external sphincter relaxation
• D. Detrusor relaxation, internal sphincter relaxation, external sphincter relaxation

Answer: (A)

Normal voiding requires a coordinated sequence where pressure is generated first and then the outlets open in order. The detrusor muscle contracts to build bladder pressure and propel urine toward the outlet. This contraction is followed by relaxation of the internal urethral sphincter (the smooth muscle at the bladder neck), which opens the bladder outlet for flow. Finally, the external urethral sphincter (the striated muscle under somatic control) relaxes to allow urine to pass through the urethra. If the detrusor doesn’t contract, there’s no pressure to push urine out. If the internal sphincter doesn’t relax, the outlet remains closed despite bladder pressure. If the external sphincter doesn’t relax, urine cannot exit even when the outlet is opened. Hence the correct sequence is detrusor contraction, internal sphincter relaxation, then external sphincter relaxation.

Normal voiding requires a coordinated sequence where pressure is generated first and then the outlets open in order. The detrusor muscle contracts to build bladder pressure and propel urine toward the outlet. This contraction is followed by relaxation of the internal urethral sphincter (the smooth muscle at the bladder neck), which opens the bladder outlet for flow. Finally, the external urethral sphincter (the striated muscle under somatic control) relaxes to allow urine to pass through the urethra. If the detrusor doesn’t contract, there’s no pressure to push urine out. If the internal sphincter doesn’t relax, the outlet remains closed despite bladder pressure. If the external sphincter doesn’t relax, urine cannot exit even when the outlet is opened. Hence the correct sequence is detrusor contraction, internal sphincter relaxation, then external sphincter relaxation.