Male sexual dysfunction has long been known to be common. Of late, knowledge of normal male sexual function and the causes of sexual dysfunction have become better understood, and more effective treatments are available.
This topic review will provide an overview of normal and abnormal sexual function in men. Male sexual dysfunction includes erectile dysfunction, diminished libido, and abnormal ejaculation. The evaluation and treatment of men with sexual dysfunction and sexual dysfunction associated with SSRIs are discussed separately. (See "Evaluation of male sexual dysfunction" and "Treatment of male sexual dysfunction" and "Sexual dysfunction associated with selective serotonin reuptake inhibitor (SSRI) antidepressants: Management".)
PHYSIOLOGY OF MALE SEXUAL FUNCTION
Normal male sexual function requires interactions among vascular, neurologic, hormonal, and psychological systems. The initial obligatory event required for male sexual activity, the acquisition and maintenance of penile erection, is primarily a vascular phenomenon, triggered by neurologic signals and facilitated only in the presence of an appropriate hormonal milieu and psychological mindset.
- Psychogenic erections are triggered by neural impulses originating in discrete loci of the central and peripheral nervous systems . Sexual images may originate in response to erotic visual or auditory stimuli or be generated via fantasy. The centrally perceived sensual input is relayed by neural signals to a spinal cord neural center located at T-11 to L-2 (the thoracolumbar erection center). From there, neural impulses flow to the pelvic vascular bed, redirecting blood into the corpora cavernosae.
- Reflex erections are created by tactile stimulus to the penis or genital area which activates a reflex arc with sacral roots originating at S-2 to S-4 (the sacral erection center). Psychogenic erections are more common during man’s early sexually active years, whereas reflex erectile activity dominates during his mature years.
- Nonsexual, nocturnal erections, occurring three to four times nightly, start in early adolescence. Nocturnal erectile activity may go unnoticed by sleeping men, although most men will be aware of an erection when they arise in the morning. These early morning erections often fade after urination, creating the incorrect impression that they are a reflex response to a full bladder.
Nocturnal erections occur only during rapid eye movement (REM) sleep (figure 1) . Men who sleep fitfully and depressed men rarely experience REM sleep and do not have nocturnal or early morning erections. Nocturnal erections persist throughout life, although, for as yet unexplained reasons, nocturnal erectile activity is not as tightly coupled to REM sleep in older men.
Role of blood flow and nitric oxide — Normal erections require blood to flow from the hypogastric arterial system into specialized erectile chambers, the paired corpora cavernosae flanking the penile urethra, and the corpus spongiosum at the glans penis. As blood flow accelerates, the pressure within the intracavernosal spaces increases dramatically, preventing penile venous outflow from emissary veins. This combination of increased intracavernosal blood flow and reduced venous outflow allows a man to acquire and maintain a firm erection.