Which urethral sphincter is voluntary?

• A. External
• B. Internal
• C. Detrusor
• D. Pelvic floor muscles

Answer: (A)

Voluntary control of the urethral opening comes from the external urethral sphincter, a ring of striated muscle surrounding the urethra in the urogenital diaphragm. Because it is skeletal muscle, its contraction is under somatic, consciously controlled innervation from the pudendal nerve, allowing you to hold urine on cue. In contrast, the internal urethral sphincter is smooth muscle near the bladder neck and is controlled autonomically; it remains closed during storage due to sympathetic tone and relaxes during voiding reflexively. The detrusor muscle is the bladder wall muscle responsible for contracting to expel urine, not a sphincter. Pelvic floor muscles support continence and contribute to stabilization, but the primary voluntary urethral sphincter is the external one.