Anatomy of the penis
The penis is a specialized vascular organ composed of complex vascular tissue responsive to neurological impulses that create penile rigidity. The penis is composed of two paired corpora cavernosa and a third cylindrical corpus spongiosum that contains the urethra and is contiguous with the glans penis.
The corpora cavernosa or penile erectile bodies are surrounded by a thick fibrous sheath (tunica albuginea) that is relatively non-distensible, composed of elastic fibers and collagen that support and maintain the rigidity of erectile function.
Surrounding the tunica albuginea is a second gossamer layer of fascia called Buck's fascia. Within these functional structures courses a complex vascular sinusoidal network of spongy tissue that activates erection. While the corpora cavernosa are generally considered to be separate cylinders, there is no clear septum between the two corpora and the distal portion of the penis, and free vascular cross-connection occurs.
The corpora themselves are composed of sinusoids made up of smooth muscle tissue and lined by endothelial cells. The corpora spongiosum is composed of similar spongy tissue but is surrounded by a less rigid thinner tunica albuginea resulting in less rigidity on activation.
The blood supply of the corpora cavernosa originates from the internal iliac arteries and courses to the internal pudendal arteries that terminate in the arterioles to the penis.
These small penile arteries include the dorsal artery to the penis outside the tunica albuginea, the bulbo-urethral artery that travels within the corpus spongiosum lateral to the urethra and the central cavernosal arteries that travel in the central portion of each of the paired corpora cavernosa and supply the blood for erection.
A proximal perineal branch of the pudendal artery provides blood supply to the perineal skin and scrotum. The dorsal artery of the penis is responsible for the blood supply of the penile skin and glans penis and contributes little to erectile function.
The cavernosal arteries on the other hand enter the corpora cavernosum at the hilum of each corpus cavernosum and give rise to multiple small helicine arteries that drain directly into the vascular lacunar spaces of the corpus cavernosum. Accessory pudendal arteries may also provide blood supply to the penis.
These variable arteries may originate from the obturator
artery, inferior pudendal artery, iliac trunk, or inferior gluteal artery
and frequently lie close to the capsule of the prostate.
The superficial dorsal penile vein lies above Buck's fascia and provides drainage predominantly for the penile skin culminating in the saphenous vein.
The proximal penile shaft and proximal corpus cavernosum drains through veins exiting the crura of the corpora cavernosa termed the crural veins that join to form the internal pudendal vein. As with most venous systems, the venous drainage of the penis is variable and complex and has multiple intercommunications.
Erectile physiology: Penis in the flaccid state: Sinusoidal smooth muscle is contracted and blood flows from the internal pudendal arteries via the cavernosal arteries and the helicine arteries to the lacunar spaces and out through the open emissary veins. Penis during erection: the smooth muscle of the sinusoids relaxes, allowing blood to flow into the lacunar spaces. The resultant pressure compresses the emissary veins beneath the rigid tunica albuginea, reducing venous outflow.
The nerve supply to the penis controls not only sensation, but also vascular supply to the penis. A pair of sympathetic nerves from S2-4 nerve roots primarily control erectile function while the sympathetic nerves from T1 I-L2 control detumescence and also contribute to ejaculation and emission.
These autonomic nerve fibers form the pelvic plexus of nerves and enter the penis within the cavernous nerves that course lateral and inferior to the prostate. It is these nerves that are preserved during nerve sparing radical prostatectomy. In addition to the autonomic nervous system, peripheral nerves form sensory and motor elements through a reflex arc in the sacral spinal cord at Onuf's nucleus.
Peripheral nerves containing sensory elements are also responsible for erectile function, especially maintenance of erection. Ultimate central nervous system control is likely initiated in the hypothalamus in the medial pre-optic area that integrates psychological and tactile stimuli.
Physiology of erection
Excitatory stimuli from the central nervous system pass through the autonomic nervous system and produce erections through a variety of neurotransmitters. Many neurotransmitters including acetylcholine (ACh) and vasoactive intestinal polypeptide (VIP) contribute to erectile function.
The most important neurotransmitter in the corpora cavernosa is nitric oxide (NO), NO functions via the cyclic guanosine monophosphate (cGMP) system. It is is produced from the precursor L-arginine by the enzyme nitric oxide synthase (NOS). The secondary transmitter eGMP is ultimately responsible for smooth muscle relaxation in the corpus cavernosum and producing erection. The relaxation is mediated by calcium efflux from the smooth muscle cells.
cGMP is broken down in the corpus cavernosum smooth
muscle cell by the enzyme phosphodiesterase (PDE) with subsequent
detumescence. The most active PDE in the corpus cavernosum is type 5
(PDE-5). Additional smooth muscle relaxation occurs through a secondary
pathway mediated by cyclic adenosine monophosphate (cAMP). cAMP, which can
be stimulated by prostaglandin El (PGEI), VIP, or papaverine assists the
primary cGMP system.
As this increased flow and downstream relaxation continues, the lacunar spaces or sinusoids fill with blood, increase in size, and produce decreases in venous outflow by compressing the subtunical venous channels against the tunica albuginea producing a high-pressure rigid erection. Rigidity is increased to a level beyond that of the abdominal aorta from the contraction of the perineal muscles, especially the bulbocavernosus and ischiocavernosus muscles.
At the conclusion of sexual activity, psychological and physical stimuli diminish, smooth muscle sympathic tone increases, vasoconstriction occurs, cGMP is broken down by PDE-5 and detumescence occurs.
Useful links for information on all aspects of sexual intercourse, relationships and male sexual health including the health of the penis, include the following: major problems like control of ejaculation where a man ejaculates too quickly and delayed ejaculation where he may not ejaculate at all during sex. For those men with erectile dysfunction, the treatment of erection problems and the cure of erectile dysfunction are described here.