Physiology of erection and ejaculation

• Normal sexual function in males requires:– Intact libido– Ability to achieve and maintain penile erection– Ejaculation– Detumescence

Sexual response cycle• Excitement phase– Erection and heightened sexual awareness

• Plateau phase– Intensification and body responses (HR, BP, RR, muscle

tension)• Orgasmic phase– Ejaculation and other responses that culminate in sexual

excitement• Resolution phase– Return to pre-arousal state

Innervation

• Erectile response mediated by a combination of:– Central (psychogenic) innervation• CNS stimulates or antagonises spinal pathways that

mediate erection and ejaculation– Peripheral (reflexogenic) innervation

• Sexual arousal– Tactile stimulation

• Sensory nerves from skin + glans form dorsal nerve of penis• Travel via pudendal nerve

– Psychogenic stimuli• Limbic system – transmitted via ANS

• Neural input (Spinal reflex)– Tactile stimulation reflexively triggers PSNS vasodilator activity

• These PSNS neurons arise from S2-4 and travel in pelvic splanchnic nerves• Act on endothelial cells to release NO – induces vascular smooth muscle relaxation and blood flow• Prostaglandins are released in cavernosa and aid in SMC relaxation

– ANS acts via NANC (nonadrenergic, noncholinergic) nerves that release NO post-synaptically onto SMCs• NO – activates Guanylate cyclase which converts GTP to cGMP• cGMP induces SMC relaxation• cGMP is gradually broken down by PDE-5

– SNS inhibition

• Leads to erection

PDE-5 sites of action of drugs

(Corpus spongiosum)

Erection• Vascular phenomenon – involves 3 cylindrical vascular cords running the length of

the penis• Corpus cavernosa• Corpus spongiosum

– Normally the blood vessels supplying these are constricted and thus there is little blood within these cords

– In order to achieve erection, these inflow vessels dilate, the penis becomes rigid, and thus compresses the emissary veins draining the penis, leaving the corpora as noncompressible cylinders from which blood does not escape

Ejaculation• Emission

– Mediated via SNS impulses• T11-L2 spinal segments via hypogastric + pelvic plexuses• Contraction of epididymis, ductus deferens, seminal vesicles + prostate• Internal vesical sphincter tightly closed

– Prevents semen entering bladder/urine being expelled during ejaculation

• Expulsion– Expulsion of semen from posterior urethra– Mediated by somatic motor impulses via pudendal nerve

• Rhythmic contraction of bulbospongiosus and ischiocavernosus muscles– Leads to increased pressure in penis and expulsion of semen

• Urethral smooth muscle contraction• Corpora cavernosa and corpus spongiosum contraction

• Detumescence– Mediated by:

• NA from SNS nerves• Endothelin from vascular endothelium• SMC contraction induced by postsynaptic adrenergic receptors

– Increased venous outflow – leads to restoring of flaccid state

In Summary

• Point (Erection) : PSNS (S2-4)» Pelvic splanchnic nerves

• Shoot (Emission) : SNS (T11-L2)

• Score (Somatic) : S2-4» Pudendal nerve

Pharmacotherapy

• PDE-5 inhibitor– Sildenafil (Viagra)– Vardenafil (Levitra)– Tadalafil (Cialis)