Parasympathetic activation at which levels contracts the detrusor muscle and relaxes the internal sphincter?

• A. S2-S4
• B. T11-L2
• C. T1-T4
• D. L2-S2

Answer: (A)

Parasympathetic control of the bladder comes from the sacral spinal segments S2–S4 via the pelvic splanchnic nerves. When these fibers are activated, they stimulate the detrusor smooth muscle to contract and cause the internal urethral sphincter to relax, enabling voiding. The thoracic and upper lumbar levels (T11–L2) are part of the sympathetic system, which promotes storage by relaxing the detrusor and tightening the internal sphincter, not by contracting the detrusor. The sacral parasympathetic outflow is specifically S2–S4, so that set of levels best explains detrusor contraction with internal sphincter relaxation.