Arterial supply of penis
- Common penile artery – continues from the internal pudendal artery – from the internal iliac
- Common penile travels in Alcock’s canal – then splits to three branches
- Cavernosal (deep) arteries – penetrates corpora cavernosa in centre, supplying cavernous sinuses via helicine branchesDorsal arteries – travels between crus and pubis, under Buck’s fascia, one each side of the dorsal vein
- Bulbourethral artery – penetrates the perineal membrane, entering corpora spongiosum from above
- NB – significant variability – accessory pudendal arteries from inferior vesical or obturator common
- Penile skin supplied from external pudendal vessels from the femoral – running in superficial dartos fascia
Venous drainage of penis
- Superficial dorsal vein, above Buck’s in superficial dartos fascia – drains to great saphenous vein
- Deep dorsal vein – underneath Buck’s, between paired dorsal arteries – piercing suspensory ligament to drain to preprostatic plexus
- Emissary veins – run between the two layers (inner circular, outer longitudinal) of tunica albuginea
- Subtunical venous plexus – underneath tunica – drain out to emissary veins
- In the distal 2/3 of penis – a variable number of circumflex veins from the spongiosum course around the cavernosa to enter the deep dorsal vein
- In the proximal third, veins may join to form internal pudendal veins
Image from Campbell-Walsh-Wein Urology https://www.amazon.com.au/Campbell-Walsh-Wein-Urology-Partin/dp/0323546420
Corpora cavernosa
- Conglomerations of vascular sinusoids separated by smooth muscle
- Septum between which is permeable
- Microscopically – myoendothelial gap junctions connect vascular smooth muscle to endothelial cells
Tunica albuginea
- Inner circular layer
- Outer longitudinal layer
- Emissary veins run between the layers, often exiting the outer layer obliquely
- During erection – compression of the emissary veins (and subtunical veins) traps venous blood
- Tunica thins during erection – thinnest point is ventral adjacent to spongiosum – most vulnerable to fracture
- The tunica around the spongiosum itself is thinner and more elastic cf. corpora – allowing expansion of urethra during ejaculation and meaning spongiosum less prone to rupture
Innervation
Autonomic:
- Sympathetic – T11 – L2 – involved in ejaculation and detumescence
- Parasympathetic – S2 to S4 – pelvic nerves and pelvic plexus
- Cavernous nerves are branches of pelvic plexus innervating the penis
- Travel along the posterolateral prostate and SVs, then accompany membranous urethra through urogenital diaphragm
- Some fibres directly enter corpora with cavernous and bulbourethral arteries, while some fibres continue with the dorsal nerve
- Terminal cavernous nerves directly supply helicine arteries and smooth muscle in the cavernosa, releasing nitrous oxide responsible for tumescence and detumescence
Somatic:
- Sensory – abundant free nerve endings in glans -> dorsal nerves -> pudendal nerve -> S2 to S4
- Motor – Onuf’s nucleus S2 to S4 – pudendal nerve -> innervation of the ischiocavernosus and bulbospongiosus muscles
Supraspinal:
- The spinal pathways of erection can be modulated by the brain – hypothalamus, periaqueductal grey matter of midbrain, medulla
Physiology of erection
Erection is the result of a complex interplay between biochemical, neurological, vascular and psychological pathways
In short – it results from relaxation of the smooth muscle of the corpora.
Three different types of erections:
- Genital stimulated – may be preserved in spinal cord injured patients
- Centrally stimulated – more complex, non-contact arousal, from memory or visual stimuli
- Centrally originated (nocturnal) – usually during REM sleep
In resting flaccid state – there is tonic intrinsic contraction of the smooth muscle – with limited arterial inflow.
Sexual stimulation triggers release of neurotransmitters, primarily nitrous oxide, from cavernous nerve terminals. This induces smooth muscle relaxation via cGMP pathways.
Smooth muscle relaxation -> dilation of arterioles and arteries -> increased blood flow to corpora.
Expansion of the sinusoids and trapping of blood during filling -> compression of the subtunical veins and then compression of the emissary veins between layers of tunica.
Venous outflow now decreased to a minimum, and intracavernosal pressure increases up to 100 mm Hg.
Ischiocavernosus contraction then further increases the pressure.
Described phases of erection:
0. Flaccid – tonic contraction of smooth muscle
1. Latent – increasing flow, some lengthening
2. Tumescence – rising intracavernosal pressure
3. Full – intracavernosal pressure 90 – 100 % of systolic
4. Rigid – ischiocavernosus contraction, pressure well above systolic, no entry of blood
5. Detumescence – sympathetic resumption, contraction of sinusoidal smooth muscle, diminution of arterial flow, veins open
Pharmacology
Cholinergic (parasympathetic) cavernosal nerves trigger erection and smooth muscle relaxation by both releasing NO and also inhibiting adrenergic nerves which provide tonic contraction of smooth muscle.
NO diffuses into smooth muscle activating cGMP pathway.
Detumescence occurs after degradation of cGMP and cAMP to GMP and AMP achieved by phosphodiesterases – penis is rich in PDE5 which is GMP specific
Testosterone levels do not seem to correlate well with severity of ED (cf. libido).
Nocturnal erections are reduced with lower testosterone levels, but erectile response to stimulation seems preserved in hypogonad men, suggesting androgens not essential for erectile function.