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

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)

The bladder empties when the parasympathetic system stimulates the sacral-outflow nerves. These parasympathetic fibers come from the pelvic splanchnic nerves, originating in the spinal cord at S2–S4. Activation of these fibers causes the detrusor smooth muscle to contract, which is the primary push to expel urine, and it also relaxes the internal urethral sphincter, removing the closed neck that blocks flow. In short, sacral parasympathetic activity coordinates detrusor contraction with relaxation of the internal sphincter to enable micturition. The other levels correspond to sympathetic control (thoracic and upper lumbar), which generally promotes storage: it relaxes the detrusor and maintains internal sphincter tone, opposite of what’s asked. The external sphincter is under somatic control (pudendal nerve) and not the focus of this parasympathetic action.

The bladder empties when the parasympathetic system stimulates the sacral-outflow nerves. These parasympathetic fibers come from the pelvic splanchnic nerves, originating in the spinal cord at S2–S4. Activation of these fibers causes the detrusor smooth muscle to contract, which is the primary push to expel urine, and it also relaxes the internal urethral sphincter, removing the closed neck that blocks flow. In short, sacral parasympathetic activity coordinates detrusor contraction with relaxation of the internal sphincter to enable micturition.

The other levels correspond to sympathetic control (thoracic and upper lumbar), which generally promotes storage: it relaxes the detrusor and maintains internal sphincter tone, opposite of what’s asked. The external sphincter is under somatic control (pudendal nerve) and not the focus of this parasympathetic action.