Male Reproductive System
The genital tubercle is a surface elevation which appears at the cranial end of the cloacal membrane and lengthens to form the phallus.
The glans and shaft of the penis are recognisable by the third month:
The prostatic urethra is derived from the vesico-urethral part of the cloaca whereas the remainder of the urethra is derived from the urogenital sinus.
The penis comprises three cylindrical structures of erectile tissue:
- The two dorsal corpora cavernosa
- The ventral corpus spongiosum containing the penile urethra
As illustrated, the posterior aspect of the penis is firmly attached to the pubic arch via the two crura of the corpus cavernosa and the perineal membrane via the bulb of the corpus spongiosum.
A tough, non-distensible fibrous capsule, the tunica albuginea, invests the corpora cavernosa, with a much thinner layer covering the corpus spongiosum.
Each of these three bodies is isolated from one another and acts as a separate functional unit, although there is some communication between the two corpora cavernosa.
The arterial supply to the penis is from the internal pudenal artery, a branch of the internal iliac artery, which divides into terminal branches:
- Dorsal penile artery supplying the glans penis
- Cavernosal artery supplying the corpora cavernosa
- Bulbar artery supplying the bulb and the corpus spongiosum and giving rise to the urethral artery
Emissary veins pierce the tunica albuginea and drain into the deep dorsal vein via the spongiosal, circumflex and cavernosal veins.
The parasympathetic innervation of the penis is the primary ‘erectile’ innervation, arising from the intermediolateral column of the sacral spinal cord and exits via the nerve roots of S2, S3 and S4.
Sympathetic nerves originate within the thoracolumbar cord (T11-L2). Both these nerve roots pass as the cavernous nerve to the penis, via the posterolateral aspect of the prostate to innervate the smooth muscle and endothelium of the penis directly.
The male urethra is 18-20 cm long and comprises:
- Spongiosal which can be further split into bulbar and penile portions
On its posterior wall, the prostatic part contains a midline ridge, the urethral crest. This projects into the lumen, which has a depression on either side of it known as the prostatic sinus. This is perforated by orifices of the prostatic ducts.
At the mid aspect of the length of the urethral crest the verumontanum forms an elevation on which sits the prostatic utricle. On both sides of the prostatic utricle sit the openings of the ejaculatory ducts.
The membranous part is the shortest, narrowest and least distensible part of the urethra, passing through the perineal membrane. The distal aspect ends at the external sphincter.
The spongiosum urethra ends at the external urethral orifice on the glans penis. The anterior urethra is dilated in the distal aspect within the navicular fossa.
Haemodynamics of Erection
An erection is a vascular event in which the degree of erection depends upon the balance between arterial inflow and venous outflow.
In the flaccid penis, the sympathetic nervous system is dominant with constriction of the arterioles and contraction of the cavernosal smooth muscle. Erection is brought about by parasympathetic nervous stimulation leading to arteriolar dilatation and trabecular smooth muscle relaxation.
Phases of Erection
The phases of erection are:
- Phase 0: The flaccid phase –
- The arterial flow is low at <15 cm per second and the penis is flaccid under sympathetic neural control.
- Phase 1: The filling phase –
- Parasympathetic stimulation leads to arteriolar dilatation with an increase in arterial flow to >30 cm per second.
- Phase 2: The tumescent phase –
- As pressure in the sinusoids increases, compression of the subtunical plexi occurs and arterial flow only continues in systole.
- Phase 3: The full erection phase –
- The arterial inflow falls but is still greater than the flaccid phase, with the expanding sinusoids compressing the subtunical venous plexuses.
- Phase 4: The rigid erection phase –
- Arterial inflow ceases and the emissary veins are completely closed (the veno-occlusive mechanism). This is caused by the contraction of the ischiocavernosus muscle raising intracavernosal pressure above systolic blood pressure.
- Phase 5: The initial detumescence phase –
- There is a small transient increase in intracavernosal pressure.
- Phase 6: The slow detumescence phase-
- Smooth muscle contracts, helicine arterioles constrict and the intracavernosal pressure falls.
- Phase 7: The fast detumescence phase –
- Sympathetic stimulation leads to a rapid fall in arterial inflow and an increase in venous outflow.