presentasi-mechanism of erection 2013

Anatomy : Mechanism of erectionDwi Cahyani Ratna SariDepartment of AnatomyFaculty of Medicine, Gadjah Mada University, Yogyakarta, Indonesia

anatomy

Penis Corpora cavernosa Fascia penis Muscles ligaments

Arterial blood supply Venous drainage Nerve system

Anatomy of penis

Key structures mediating erection are the corpora cavernosa or ‘erectile bodies’, which are fused distally for approximately threequarters of their length. They separate proximally to fuse with each ischial tuberosity of the pelvis. On their ventral surface lies the corpus spongiosum, which surrounds the urethra

Each corpus cavernosum comprises a thick fibrous sheath, the tunica albuginea, which surrounds the erectile tissue. Each corpus has a centrally running cavernosal artery, which supplies blood to the multiple lacunar spaces, which are interconnected and lined by vascular endothelium

The corpus spongiosum is firmly attached to the undersurface (ventral aspect) of the corpora cavernosa and expands distally to form the glans penis. Proximally, it forms the urethral bulb, where the urethra curves cranially to form the sphincter-active membranous urethra

Muscles of the pelvic floor surround and support the erectile bodies and corpus spongiosum. In terms of sexual function, the most important muscles are the bulbospongiosus and the ischiocavernosus. These support the erect penis and also contract rhythmically at the time of orgasm to facilitate ejaculation

Skin overlying the penis is exceptionally mobile and expandable to accommodate the considerable increase in length and girth that occurs during erection. Distally the penile skin is reflected forwards over the glans penis to form the prepuce, before folding back on itself to attach to the corona of the glans penis

The pendulous portion of the penis is supported by the suspensory ligament, a fibrous condensation which supports and stabilizes the erect penis. Division of this structure makes the penis appear longer in its flaccid state, but does not enhance the proportions of the organ when erect

Arterial blood supply

The paired internal pudendal artery is the major carrier of the blood supply to the penis

dividing into 3 branches: the bulbourethral artery the dorsal artery the cavernous artery (deep artery)

Arterial blood supply

The cavernous artery supplies the corpora cavernosa

the dorsal artery supplies the skin the subcutaneous tissue the glans penis

the bulbourethral artery supply the corpus spongiosum

Paired dorsal arteries run along the dorsal aspect of the corpora beneath Buck’s fascia, giving off multiple circumflex branches and, eventually, supplying blood to the glans penis. The cavernosal arteries run along the middle of each corpus cavernosum, giving off multiple helicine branches, which supply blood to the lacunar spaces

Cross-section of the penis showing the locations of the paired dorsal arteries and cavernosal arteries. Note the helicine arteries, which supply arterial blood to the lacunar spaces

The venous drainage

• the glans the deep dorsal vein • The corpus spongiosum :

• the circumflex• urethral • bulbar veins

• the corpora cavernosa :• the mid- and distal shaft the deep dorsal

vein the preprostatic plexus • the proximal portion the

cavernous and crural veins the preprostatic plexus and internal pudendal vein

The venous drainage

• The drainage of all 3 corpora originates in the subtunical venules, which unite to form emissary veins.

• The glans penis possesses numerous large and small veins that communicate freely with the dorsal veins

• The penile skin and subcutaneous tissue are drained by superficial dorsal veins, which then empty into the saphenous veins.

Venous drainage from the corpora cavernosa takes place mainly through the deep dorsal vein, which lies dorsally in the groove between the corpora and passes beneath the pubic arch to join the dorsal venous complex at the urethroprostatic junction

Nerves in penile erection

Three sets of peripheral nerves are involved in penile erection: two are autonomic

Parasympathetic nerves sympathetic nerves

one is somatic

Nerves in penile erection

Parasympathetic nerves : S2–S4 Sympathetic nerves : T10–L2 Somatic fibers travel in the pudendal nerves and their cell bodies are situated in the S2–S4 segments

Mechanism of erection

Cavernosal smooth muscle tone is the most important determinant of penile blood flow.

This, in turn, is critically dependent on the level of intracellular calcium ([Ca]2+), modulated by a number of mechanisms.

The most important vasodilator transmitter is nitric oxide (NO), released from both nitrergic nerve endings and vascular endothelium


NO stimulates production of cyclic guanosine monophosphate (cGMP) from guanosine triphosphate (GTP) by the enzyme guanylate cyclase (GC)

A second vasodilator mechanism involves the production of cyclic adenosine monophosphate (cAMP) from adenosine triphosphate (ATP) by adenylate cyclase (AC)


Both vasoactive intestinal polypeptide (VIP) and prostaglandin E1 (PGE1) activate AC.

Both cGMP and cAMP lower intracellular calcium, thereby triggering smooth muscle relaxation.


• NO also activates sodium (Na+)/potassium (K+)-channel ATPase, resulting in hyperpolarization of the smooth muscle cell membrane which, in turn, prevents the opening of voltage-dependent calcium channels, thereby reducing intracellular calcium

Hemodynamic of flaccidity

Tonic contraction of the walls of the helicine arteries and trabeculae allows only relatively small amounts of blood into the lacunar spaces.

Whatever blood is entering is drained through the walls of the tunica albuginea by subtunical vessels

Hemodynamic of erection

dilatation of the helicine arteries and relaxation of the trabeculae allow the lacunar spaces to fill.

Their engorgement compresses the obliquely oriented subtunical veins against the tunica albuginea.

This veno-occlusive mechanism prevents venous leakage and facilitates the development of a full and rigid erection

Hemodynamic of erection

• The key event in the induction of erection is vasodilatation of the helicine arteries, which is induced by nitric oxide and other neurotransmitters

• The veno-occlusive mechanism is a secondary event brought about by compression of the subtunical veins against the sturdy tunica albuginea

Schematic representation of the hemodynamics of flaccidity and erection

During erection, the penile vascular volume increases rapidly

Full erection is achieved when intracorporeal pressure approximates systolic blood pressure

types of erections

Three types of erections : genital-stimulated (contact or

reflexogenic) central-stimulated (noncontact or

psychogenic) central-originated (nocturnal)

Genital-stimulated erection

induced by tactile stimulation of the genital area

can be preserved in upper spinal cord lesions, although erections are usually short in duration and poorly controlled by the individual

Central-stimulated erection

• more complex• resulting from :

•memory • fantasy•visual •auditory stimuli

central-originated (nocturnal)

can occur spontaneously without stimulation or during sleep

most sleep erections occur during rapid eye movement (REM) sleep

During REM sleep, the cholinergic neurons in the lateral pontine tegmentum are activated while the adrenergic neurons in the locus ceruleus and the serotonergic neurons in the midbrain raphe are silent.

central-originated (nocturnal)

• This differential activation may be responsible for the nocturnal erections during REM sleep. Of note, the number and duration of erections for men with hypogonadism or receiving antiandrogen therapy is markedly reduced

Phases of the Erection Process

Flaccid phase Latent (filling) phase Tumescent phase Full erection phase Skeletal or rigid erection phase Detumescent phase


Flaccid phase (1) Minimal arterial and venous flow blood gas values equal those of venous blood

Latent (filling) phase (2) Increased flow in the internal pudendal artery during

both systolic and diastolic phases Decreased pressure in the internal pudendal artery unchanged intracavernous pressure Some elongation of the penis


Tumescent phase (3) Rising intracavernous pressure until full erection is

achieved Penis shows more expansion and elongation with

pulsation The arterial flow rate decreases as the pressure

rises When intracavernous pressure rises above diastolic

pressure, flow occurs only in the systolic phases


Full erection phase (4) Intracavernous pressure can rise to as much as 80–

90% of the systolic pressure Pressure in the internal pudendal artery increases

but remains slightly below systemic pressure Arterial flow is much less than in the initial filling

phase but is still higher than in the flaccid phase Although the venous channels are mostly

compressed, the venous flow rate is slightly higher than during the flaccid phase

Blood gas values approach those of arterial blood


Skeletal or rigid erection phase (5) As a result of contraction of the ischiocavernous

muscle the intracavernous pressure rises well above the

systolic pressure, resulting in rigid erection During this phase, almost no blood flows through

the cavernous artery; however, the short duration prevents the development of ischemia or tissue damage


Detumescent phase (6) After ejaculation or cessation of erotic stimuli,

sympathetic tonic discharge resumes, resulting in contraction of the smooth muscles around the sinusoids and arterioles

This effectively diminishes the arterial flow to flaccid levels, expels a large portion of blood from the sinusoidal spaces, and reopens the venous channels

The penis returns to its flaccid length and girth

KEY POINTS

PENILE SMOOTH MUCLE RELAXATION CAUSES ERECTION


Relaxation of the smooth muscle is the key to penile erection

Nitric oxide released by : nNOS contained in the terminals of

cavernous nerve initiates the erection process

eNOS in the endothelium helps maintain erection


Upon entering the smooth muscle cells, NO stimulates the production of cGMP

Cyclic GMP activates protein kinase G, which in turn opens potassium channels and closes calcium channels

Low cytosolic calcium favors smooth muscle relaxation

The smooth muscle regains its tone when the cGMP is degraded by phosphodiesterase

presentasi-mechanism of erection 2013

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