In a patient with thoracic spinal cord injury who preserves reflexogenic erection, which two nerves form the reflex arc responsible for erection?

In a patient with thoracic spinal cord injury who preserves reflexogenic erection, which two nerves form the reflex arc responsible for erection?

• A. Pudendal nerves and pelvic splanchnic nerves
• B. Pudendal nerves and hypogastric nerves
• C. Pelvic splanchnic nerves and vagus nerve
• D. Sciatic nerve and pudendal nerves

Answer: (D)

The key idea is that reflexogenic erection is a sacral spinal reflex arc: sensory input from the genital area travels via the pudendal nerve to the sacral spinal cord, and the response is carried back out through parasympathetic fibers in the pelvic splanchnic nerves (S2–S4) to the penile vasculature, causing vasodilation and erection. Even with a thoracic injury, this sacral arc can remain intact and produce a reflex erection when the genital area is stimulated. So the nerves forming the reflex arc are the pudendal nerve (afférent/sensory input from the penis) and the pelvic splanchnic nerves (efferent parasympathetic output to the erectile tissue). The other options don’t fit because the vagus does not innervate the penis in humans, the hypogastric carries sympathetic fibers related to emission rather than erection, and the sciatic nerve has no role in penile erection.

The key idea is that reflexogenic erection is a sacral spinal reflex arc: sensory input from the genital area travels via the pudendal nerve to the sacral spinal cord, and the response is carried back out through parasympathetic fibers in the pelvic splanchnic nerves (S2–S4) to the penile vasculature, causing vasodilation and erection. Even with a thoracic injury, this sacral arc can remain intact and produce a reflex erection when the genital area is stimulated.

So the nerves forming the reflex arc are the pudendal nerve (afférent/sensory input from the penis) and the pelvic splanchnic nerves (efferent parasympathetic output to the erectile tissue). The other options don’t fit because the vagus does not innervate the penis in humans, the hypogastric carries sympathetic fibers related to emission rather than erection, and the sciatic nerve has no role in penile erection.