Penile erection is due to:-

Penile erection is associated with sexual arousal caused by sexually related psychic and/or physical stimuli. During sexual arousal, impulses from the genitalia, together with signals emanating from the limbic system, elicit motor impulses in the spinal cord. The sacral parasympathetic fibers convey these impulses from the spinal cord, the cavernous nerve branches of the prostatic plexus, into the penis. These signals cause smooth muscle relaxation of the corpora cavernosa and dilatation of the aerioles. The transmitter responsible for relaxation of smooth muscle of the cavernosa and aerioles is nitric oxide (NO).