Physiology · Reproduction, pregnancy and the fetus
Erection and ejaculation !
- Mechanism
- For completeness
Erection is a blood-flow event controlled by parasympathetic nerves and nitric oxide; ejaculation is a sympathetic and somatic reflex.
Updated 2026-11-01
Also called: Sexual response, NO–cGMP pathway
See it in 3D: Corpora cavernosa · Corpus spongiosum · Pelvic plexus and cavernous nerves · Pudendal nerve · Deep artery of penis · Bulbospongiosus · Ischiocavernosus · Seminal vesicles · Prostate · Clitoris
What it is
Erection happens when parasympathetic (pelvic, S2–S4) nerves release nitric oxide, which relaxes smooth muscle in the cavernous arteries and sinusoids; blood fills the corpora cavernosa and compresses the draining veins. Emission (sympathetic, T10–L2) moves semen into the urethra, and ejaculation (pudendal nerve, bulbospongiosus) expels it. The same NO–cGMP mechanism engorges the clitoris and vestibular bulbs in women.
How it works, step by step
- Tactile or psychogenic stimuli activate sacral parasympathetic outflow (S2–S4) through the pelvic plexus.
- Nerve endings and endothelium release nitric oxide, which activates guanylyl cyclase and raises cGMP.
- cGMP lowers intracellular calcium: helicine arteries and trabecular smooth muscle relax and arterial inflow rises.
- Expanding sinusoids compress subtunical veins against the tunica albuginea, trapping blood (rigidity).
- Sympathetic discharge (T10–L2) contracts the epididymis, vas deferens, seminal vesicles and prostate (emission) and closes the bladder neck.
- Rhythmic pudendal-driven contraction of the bulbospongiosus expels semen (ejaculation).
- Sympathetic tone and PDE5 activity return; arterial smooth muscle contracts and detumescence follows.
Nerves and roles
| Phase | Pathway | Effect |
|---|---|---|
| Erection | Parasympathetic, pelvic splanchnic nerves (S2–S4) | NO release, vasodilation, engorgement |
| Emission | Sympathetic, hypogastric nerves (T10–L2) | Semen to urethra; bladder neck closes |
| Ejaculation | Somatic, pudendal nerve (S2–S4) | Bulbospongiosus contracts, semen expelled |
| Female arousal | Same pelvic parasympathetic pathway | Clitoral and vestibular bulb engorgement, vaginal lubrication |
Clinical notes
- Priapism: a prolonged erection; ischemic (low-flow) priapism traps deoxygenated blood and needs urgent care to prevent fibrosis.
- Retrograde ejaculation follows damage to the sympathetic closure of the bladder neck (diabetes, prostate surgery).
- Spinal cord injury: reflex erections may persist if the sacral segments are intact, while psychogenic ones need the thoracolumbar pathway.
Test yourself
Think of the answer, then tap the question to see it.
Why is combining a PDE5 inhibitor with a nitrate dangerous?
Both raise cGMP (nitrates by giving NO, PDE5 inhibitors by blocking its breakdown) in vascular smooth muscle; together they cause profound vasodilation and hypotension.
A diabetic man has a normal orgasm but cloudy urine afterwards and no ejaculate. What failed?
Sympathetic closure of the bladder neck during emission (autonomic neuropathy), so semen goes back into the bladder: retrograde ejaculation.
Why is erectile dysfunction considered a warning sign for coronary disease?
Erection depends on endothelial NO and small arteries; endothelial dysfunction and atherosclerosis affect the penile arteries early, often years before coronary symptoms.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
- Corpora cavernosa
- Corpus spongiosum
- Pelvic plexus and cavernous nerves
- Pudendal nerve
- Deep artery of penis
- Bulbospongiosus
- Ischiocavernosus
- Seminal vesicles
- Prostate
- Clitoris
Related topics
- Sympathetic and parasympathetic effects on each organ
- Smooth muscle contraction
- Testosterone and its actions
- Adrenergic and cholinergic receptors
Related conditions
In Biochemistry
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 27.1 Anatomy and Physiology of the Testicular Reproductive System (OpenStax (Rice University), 2022); Erectile Dysfunction (StatPearls (NCBI Bookshelf), 2024); Priapism (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 81). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.