The distal 2/3 of the urethral sphincter is innervated by which nerve?

The distal 2/3 of the urethral sphincter is innervated by which nerve?

• A. Under involuntary control
• B. Pudendal nerve
• C. Contracts when the bladder contracts
• D. Lined with beta receptors that trigger sphincter contraction

Answer: (B)

The main idea is that the distal two-thirds of the urethral sphincter is the external urethral sphincter, which is skeletal muscle under voluntary control. This portion receives somatic motor input from the pudendal nerve, a branch of the sacral spinal nerves (S2–S4) that travels to the perineum and innervates the external urethral sphincter. Because its control is voluntary, the pudendal nerve is the correct innervation source for this part of the sphincter, enabling conscious containment of urine and coordinated voluntary micturition. It isn’t about involuntary control—that would describe autonomic pathways and doesn’t specify a nerve. Also, contracting when the bladder contracts isn’t accurate for the external sphincter, which typically relaxes during voiding. And beta receptors triggering contraction pertain to smooth muscle and internal sphincters, not the skeletal muscle of the external urethral sphincter innervated by the pudendal nerve.

The main idea is that the distal two-thirds of the urethral sphincter is the external urethral sphincter, which is skeletal muscle under voluntary control. This portion receives somatic motor input from the pudendal nerve, a branch of the sacral spinal nerves (S2–S4) that travels to the perineum and innervates the external urethral sphincter. Because its control is voluntary, the pudendal nerve is the correct innervation source for this part of the sphincter, enabling conscious containment of urine and coordinated voluntary micturition.

It isn’t about involuntary control—that would describe autonomic pathways and doesn’t specify a nerve. Also, contracting when the bladder contracts isn’t accurate for the external sphincter, which typically relaxes during voiding. And beta receptors triggering contraction pertain to smooth muscle and internal sphincters, not the skeletal muscle of the external urethral sphincter innervated by the pudendal nerve.