Parasympathetic activation at S2-S4 causes which of the following?

Parasympathetic activation at S2-S4 causes which of the following?

• A. Sympathetic activation at T11-L2
• B. Parasympathetic activation at S2-S4 causes detrusor contraction and internal sphincter relaxation
• C. Somatic motor activation
• D. No effect

Answer: (B)

Parasympathetic outflow from the sacral nerves (S2–S4) controls the micturition reflex. When these fibers are activated, they stimulate the detrusor muscle to contract via muscarinic receptors, and they also reduce the tone of the internal urethral sphincter, allowing it to relax. The net result is bladder contraction with internal sphincter relaxation, enabling urine to flow. The other possibilities don’t fit because sympathetic input (T11–L2) promotes storage by relaxing the detrusor and contracting the internal sphincter, somatic motor activity would affect the external sphincter rather than the internal, and there is indeed a clear effect on the bladder with sacral parasympathetic activation.

Parasympathetic outflow from the sacral nerves (S2–S4) controls the micturition reflex. When these fibers are activated, they stimulate the detrusor muscle to contract via muscarinic receptors, and they also reduce the tone of the internal urethral sphincter, allowing it to relax. The net result is bladder contraction with internal sphincter relaxation, enabling urine to flow. The other possibilities don’t fit because sympathetic input (T11–L2) promotes storage by relaxing the detrusor and contracting the internal sphincter, somatic motor activity would affect the external sphincter rather than the internal, and there is indeed a clear effect on the bladder with sacral parasympathetic activation.