Point and Shoot: parasympathetic for erection, sympathetic for emission
Expansion
Erection: parasympathetic (S2 to S4)
- Nitric oxide released from cavernosal nerves and endothelium
- Activates guanylate cyclase, raising cyclic GMP
- Smooth muscle of the helicine arteries and trabeculae relaxes
- Blood fills the corpora, and the expanding sinusoids compress the subtunical veins, trapping blood
Emission: sympathetic (L1 to L2): contraction of the vas, seminal vesicles and prostate delivers semen to the posterior urethra, with closure of the bladder neck.
Ejaculation: rhythmic contraction of the bulbospongiosus, a somatic function through the pudendal nerve.
Pharmacology follows directly: phosphodiesterase-5 inhibitors such as sildenafil prevent breakdown of cyclic GMP, prolonging relaxation. They are contraindicated with nitrates, since both act on the same pathway and cause profound hypotension.
Clinical correlates: failure of bladder neck closure after prostate surgery or with alpha blockers causes retrograde ejaculation; autonomic neuropathy in diabetes impairs both; and priapism is failure of detumescence, a compartment syndrome requiring urgent treatment.