Final answer:
Erection involves decreased sympathetic and increased parasympathetic activity, resulting in arteriolar vasodilation in the pe nis.
Step-by-step explanation:
Erection is a vascular phenomenon that involves decreased sympathetic stimulation and increased parasympathetic stimulation to the penile arterioles, resulting in arteriolar vasodilation and filling of the erectile tissue with blood. The correct group of answer choices is therefore 'sympathetic; parasympathetic; vasodilation'.
During sexual arousal, release of nitric oxide (NO) leads to relaxation of smooth muscles in the penile arteries, allowing them to dilate and increasing blood flow. As a result, the blood fills the erectile chambers and the pressure compresses penile venules, reducing blood outflow and maintaining an erection. Acetylcholine, released by postganglionic parasympathetic fibers, also causes the blood vessels in erectile tissue to dilate, further contributing to the engorgement of the tissue with blood.