Final answer:
Acetylcholine is the neurotransmitter released from parasympathetic nerves to dilate penile arterioles, which, along with NO, leads to increased blood flow and erection.
Step-by-step explanation:
The neurotransmitter released from parasympathetic nerves to dilate the penile arterioles and increase blood flow to the erectile tissue is acetylcholine. Parasympathetic postganglionic fibers release acetylcholine, which causes the blood vessels in the erectile tissue of the reproductive organs to dilate, leading to engorgement of the tissue. Concurrently, nitric oxide (NO) released from nerve endings near the blood vessels within the corpora cavernosa and spongiosum activates a signaling pathway, causing further relaxation of the smooth muscles and vasodilation, resulting in erection.